Imalka Marasinghe Arachchilage

Synthesizing Laccol Based Polymers/Copolymers and Polyurethanes;

Characterization and Their Applications

by

Imalka Marasinghe Arachchilage

A dissertation submitted in the partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

Department of Chemistry

College of Arts and Sciences

University of South Florida

Major Professor: Jianfeng Cai, Ph.D.

Julie P. Harmon, Ph.D. (late)

Randy Larsen, Ph.D.

Shengqian Ma, Ph.D.

Sylvia Thomas, Ph.D.

Date of Approval:

April 2nd

, 2021

Keywords: Lacquer, Cationic polymerization, Radiation hard, Ecofriendly

Copyright © 2021, Imalka Marasinghe Arachchilage

ACKNOWLEDGMENTS

Firstly, I would like to convey my sincere gratitude to late adviser Dr. Julie P. Harmon for her

guidance and help. Five and half years' time that I spent with her as my supervisor was

unforgettable and the things I learned from her is immense. Even though she couldn't make it to

see my last step in this PhD carrier, her unwavering consideration I received throughout my

journey is always remain in my heart as a precious memory. Secondly, I would like to convey

my sincere and heartfelt gratitude to Dr. Jianfeng Cai, who immediately received me and be my

advisor for the last semester due to the sudden departure of my previous supervisor. The support

and attention I received from him is unforgettable and it helped me to stand upright again during

the hard time. Further, I would like to convey my sincere gratitude to the committee members;

Dr. Shengqian Ma, Dr. Sylvia Thomas and Dr. Randy Larsen for their continuous guidance and

support throughout my journey.

I also would like to convey my sincere gratitude to Department of Chemistry and its affiliated

Academic & Administrative staff, Business & Fiscal staff and Laboratory staff at USF. Without

their support this success is difficult to achieve. Further, I would like thank all my past and

present lab colleagues; Dr. Ken Kull, Dr. Garrett Craft, Dr. Tamalia Julien, Dr. Alejandro Rivera

Nicholls, Taranjot Kaur and Nathaniel Johnson. Also, I would like to recognize all the

undergraduates that collaborated in the projects; Milly Patel, Joseph Basi and Mijeong Kim.

Finally, I would like to convey my sincere gratitude to my family; Rohini Marasinghe, Dilanka

Nadishani Ariyasena, Darshika Tharangani Ariyasena, Sanath Wickramasinghe, Savindu

Kankanige, Chandi Wijetunga, Ari Wijetunga, Sulochana Wijetunga and his family. Without

their support, this journey would not possible. Last but not least, I would like to thank my friends

I met in Florida and my friends in Sri Lanka, who help me throughout this long journey.

DEDICATION

I would like to dedicate this work to my dearest mother Rohini Marasinghe for her unwavering

support and also to my loving and wonderful 7 year old son Savindu Kankanige who is staying

with me all the time enduring lots of hardships.

i

TABLE OF CONTENTS

List of Tables ................................................................................................................................. iv

List of Figures ..................................................................................................................................v

List of Schemes ............................................................................................................................. vii

List of Equations .......................................................................................................................... viii

Abstract .......................................................................................................................................... ix

Chapter One: Introduction ...............................................................................................................1

1.1. Lacquer History ............................................................................................................1

1.2. Lacquer Tree .................................................................................................................2

1.2.1. Species and Distribution ................................................................................2

1.2.2. Lacquer Saps ..................................................................................................3

1.3. Cationic Polymerization................................................................................................4

1.3.1. Initiators used in Cationic Polymerization .....................................................5

1.4. Laccol based Polymers and Copolymers ......................................................................5

1.4.1. Laccol:Styrene Copolymers ...........................................................................6

1.4.2. Laccol:d-limonene Copolymers .....................................................................7

1.5. Radiation Hard Polymers/Copolymers and their Applications .....................................7

1.6. Polyurethanes ................................................................................................................8

1.6.1. Laccol based Polyurethanes ...........................................................................9

1.7. References ...................................................................................................................10

Chapter Two: Synthesizing radiation hard polymer and copolymers using laccol

monomers extracted from lacquer tree Toxicodendron succedanea via cationic

polymerization .........................................................................................................................13

Abstract ..............................................................................................................................13

2.1. Introduction .................................................................................................................14

2.2. Experimental Section ..................................................................................................17

2.2.1. Materials ......................................................................................................17

2.2.2. Preparation Procedures ................................................................................18

2.2.2.1. Extraction of laccol from lacquer sap ...........................................18

2.2.2.2. Synthesis of an Aluminum Chloride and Ethyl Acetate

[AlCl3.EtOAc] Complex ......................................................................18

2.2.2.3. Preparation of Laccol Polymer .....................................................18

2.2.2.4. Preparation of Polystyrene ............................................................20

2.2.2.5. Copolymers of laccol and Styrene ................................................20

ii

2.2.3. Characterization Methods ............................................................................22

2.2.3.1. Fourier Transform Infra-Red (FTIR) Analysis .............................22

2.2.3.2. Nuclear Magnetic Resonance Spectroscopy (NMR) ....................22

2.2.3.3. Gel Permeation Chromatography (GPC) ......................................22

2.2.3.4. Dynamic Mechanical Analysis .....................................................23

2.2.3.5. Thermal Gravimetric Analysis (TGA) ..........................................24

2.2.3.6. Shore Hardness .............................................................................24

2.2.3.7. Radiation Studies and Microscopic Analysis ...............................24

2.2.3.8. Wide Angle X-ray Scattering (WAXS) ........................................25

2.3. Results and Discussions ..............................................................................................25

2.3.1. FTIR Analysis ..............................................................................................25

2.3.2. NMR Analysis .............................................................................................29

2.3.3. GPC Analysis ...............................................................................................31

2.3.4. Dynamic Mechanical Analysis ....................................................................31

2.3.5. TGA Analysis ..............................................................................................34

2.3.6. Shore Hardness Analysis .............................................................................36

2.3.7. Microscopic Analysis...................................................................................37

2.3.8. Powder X-Ray Analysis ...............................................................................38

2.4. Conclusions .................................................................................................................39

2.5. Acknowledgment ........................................................................................................41

2.6. References ...................................................................................................................41

Chapter Three: Environmental friendly radiation hard d-limonene and laccol copolymers

synthesized via cationic copolymerization ..............................................................................45

Abstract ..............................................................................................................................45

3.1. Introduction .................................................................................................................46


3.2.1. Materials ......................................................................................................51


3.2.2.1. Copolymers of limonene and laccol .............................................51




3.2.3.3. Differential Scanning Colorimetry (DSC) ....................................53



3.2.3.6. Shore Hardness .............................................................................54

3.2.3.7. Radiation Studies ..........................................................................54

3.2.3.8. Swelling Analysis .........................................................................55


3.3.1. FTIR Analysis ..............................................................................................55

3.3.2. NMR Analysis .............................................................................................58

3.3.3. DSC Analysis ...............................................................................................61


3.3.5. TGA Analysis ..............................................................................................65

3.3.6. Shore Hardness Analysis .............................................................................67

iii

3.3.7. Swelling Analysis ........................................................................................68 3.4. Conclusions .................................................................................................................69

3.5. Acknowledgment ........................................................................................................71

3.6. References ...................................................................................................................71

Chapter Four: Synthesis and Characterization of Novel Laccol Based Polyurethanes .................76

Abstract ..............................................................................................................................76

4.1. Introduction .................................................................................................................77


4.2.1. Materials ......................................................................................................82


4.2.2.1. Synthesis of laccol and HMDI polyurethane (LH-1) ....................82

4.2.2.2. Synthesis of laccol and H12MDI polyurethane (LH-2) ................83

4.2.2.3. Synthesis of laccol and 4,4'–MDI polyurethane (LH-3) ...............83




4.2.3.3. Differential Scanning Colorimetry (DSC) ....................................84



4.2.3.6. Micro Hardness .............................................................................86

4.2.3.7. Powder X-ray Analysis (PXRD) ...................................................86

4.2.3.8. Swelling Analysis .........................................................................86


4.3.1. FTIR Analysis ..............................................................................................87

4.3.2. NMR Analysis .............................................................................................91

4.3.3. DSC Analysis ...............................................................................................93


4.3.5. TG Analysis .................................................................................................98

4.3.6. Micro hardness ...........................................................................................100

4.3.7. PXRD Analysis ..........................................................................................101

4.3.8. Swelling Analysis ......................................................................................102

4.4. Conclusions ..............................................................................................................102

4.5. Acknowledgements ...................................................................................................103

4.6. References .................................................................................................................103

Chapter Five: Conclusions ...........................................................................................................107

Appendices ...................................................................................................................................109

Appendix A: USF Fair Use Worksheet for Chapter One ................................................110

Appendix B: License of Reproduced Material in Chapter Two ......................................114

Appendix C: License of Reproduced Material in Chapter Three ....................................123

Appendix D: License of Reproduced Material in Chapter Four ......................................132

About the Author ............................................................................................................... End Page

iv

LIST OF TABLES

Table 2.1: Different monomer contents that used to prepare the copolymers ..............................20

Table 2.2: The MW of NLP and its copolymers at initial polymerization stage

(apolydispersity, PD = MW/Mn) ....................................................................................31

Table 2.3: Tg values and related activation energies obtained from rheometer ............................33

Table 2.4: Shore A hardness data for laccol polymer and copolymers;

before and after irradiation ...........................................................................................37

Table 3.1: Limonene and laccol contents in synthesized materials ..............................................52

Table 3.2: Glass transition temperatures and activation energies for synthesized

copolymers of limonene:laccol and neat laccol polymer (NLP) .................................63

Table 4.1: Tg values and corresponding ΔE values obtained from analysis .................................98

v

LIST OF FIGURES

Figure 1.1: Lacquer coated artifacts .................................................................................................1

Figure 1.2: World distribution of lacquer tree .................................................................................2

Figure 1.3: The structures of main components in different lacquer saps .......................................3

Figure 1.4: Laccol structure .............................................................................................................4

Figure 1.5: Separation of styrene and inhibitor (solvent extraction) ...............................................6

Figure 1.6: Synthesized novel PUs LH-1, LH-2 and LH-3 in film and disc form...........................9

Figure 2.1: IR spectra of (a) LE, NLP and (b) Styrene monomer ..................................................26

Figure 2.2: IR spectra of various laccol compositions; a) LE-control, a') LE-after

irradiation, b) NLP-control, b') NLP-after irradiation, c) S-90_control,

c') S-90_after irradiation, d) S-70_control, d') S-70_after irradiation,

e) S-50_control, e') S-50_after irradiation, f) NPS-control,

f') NPS-after irradiation, *NPS-Neat Polystyrene .......................................................28

Figure 2.3: NMR spectra for (a) LE and (b) NLP (*Acetone) .......................................................29

Figure 2.4: Laccol:Styrene (S-50) copolymer-control, (b) Laccol:Styrene (S-50)

copolymer-after irradiation ..........................................................................................30

Figure 2.5: (a) NLP-WLF behavior graph, (b) Master curve developed for NLP .........................33

Figure 2.6: Tan δ curves (c) for materials from temperature ramp experiment (rheometer) .........34

Figure 2.7: TG curves for NLP and S-90 samples .........................................................................35

Figure 2.8: Optical microscopy observations of polymers and copolymers

(Magnitude: 5x/0.12P) .................................................................................................38

Figure 2.9: Powder X-ray diffraction data for NLP and S-90 ........................................................39

Figure 3.1: IR spectrum of d-limonene with important functional groups .....................................56

vi

Figure 3.2: IR spectra of (a) d-limonene:laccol copolymers, (b) L-10

(10% limonene in laccol) before and after irradiation and (c) L-30

(30% limonene in laccol) before and after irradiation .................................................57

Figure 3.3: NMR spectra of (a) laccol extract and (b) L-30 copolymer, *Acetone .......................60

Figure 3.4: DSC curves for limonene:laccol copolymers ..............................................................61

Figure 3.5: Tan δ curves for limonene:laccol copolymers .............................................................64

Figure 3.6: WLF graphs for L-30 and L-50 ...................................................................................64

Figure 3.7: TG curves of L-10 and L-30; before and after gamma irradiation ..............................66

Figure 3.8: Shore A hardness data for synthesized copolymers; before and

after irradiation.............................................................................................................67

Figure 3.9: Percent weight gain of copolymers; before and after gamma irradiation ....................69

Figure 4.1: IR spectra of synthesized LH-1, LH-2 and LH-3 samples ...........................................88

Figure 4.2: IR spectra for; (a) Synthesized PUs before & after rheology (temp ramp)

experiment, (b) Synthesized PUs before & after molding ...........................................90

Figure 4.3: NMR spectra of LH-1, LH-2 and LH-3; *Acetone......................................................92

Figure 4.4: DSC data for LH-1, LH-2 and LH-3 at 30⁰C/min ramp rate .......................................94

Figure 4.5: Temp ramp experimental data for LH-2 and LH-3 to identify Tg ...............................95

Figure 4.6: LH-1 (a) WLF plot created from TTS for activation energy calculation

and (b) Master curve developed from WLF graph ......................................................96

Figure 4.7: TG curves for synthesized LH-1, LH-2 and LH-3 samples .........................................99

Figure 4.8: Micro hardness data for LH-1, LH-2 and LH-3 .........................................................100

Figure 4.9: PXRD data for synthesized PU samples (LH-1, LH-2, LH-3) ..................................101

vii

LIST OF SCHEMES

Scheme 2.1: Proposed mechanism for laccol polymerization ....................................................19

Scheme 2.2: Proposed mechanism for laccol and styrene copolymer formation .......................21

Scheme 3.1: Proposed mechanism for limonene:laccol copolymer formation ..........................50

Scheme 4.1: Generic mechanism for PU formation with diisocyanate ......................................81

viii

LIST OF EQUATIONS

Equation 1: Activation energy calculating equation .......................................................32,64,85

Equation 2: Percent weight gain calculating equation .........................................................55,87

ix

ABSTRACT

In modern world research scopes are highly favored the areas of developing new polymer

materials which are more integrated with the environment. Synthesizing eco-friendly, bio

degradable polymer materials to replace petroleum based synthetic polymers to utilize in

radiation hard applications is one of the main objectives of this study. 'Laccol' extracted from

Vietnamese lacquer tree (Toxicodendron succedanea) polymerized with other monomers namely

styrene and d-limonene (from citrus waste) respectively to develop copolymers via cationic

polymerization. Laccol contains hydroxyl phenyl groups that act as radical scavengers,

enhancing radiation resistance via their ability to convert excitation energy into non-chemistry

inducing energy in lacquer that has been cured. Further, laccol based polyurethanes were

synthesized which could possibly use as thermal insulator materials and as energy storing

materials is another objective investigated in this study. All these developments are novel and

reported firstly in the literature.

Lacquer saps are popular since ancient time periods because of its excellent toughness,

high solvent resistivity and high durability. It contains two reactive hydroxyl groups in the

phenyl ring and a C-17 unsaturated side chain at 3rd

position of the phenyl ring. Due to the

unsaturation of this C-17 side chain, it is a potent candidate for cationic polymerization process.

Firstly, the proper method was developed to homo-polymerize laccol through cationic

polymerization and procedure was further extended to produce copolymers of laccol:styrene and

x

laccol:d-limonene using AlCl3.EtOAc as the cationic co-initiator. Trans conjugated double bonds

in unsaturated side chain of laccol and terminal double bond in styrene or d-limonene was

involved prominently during the chain growth polymerization. Synthesized materials were

exposed to gamma radiation and alterations occurred due to irradiation process was studied.

Specifically the material hardness differences, FTIR information, crosslinking density

differences were investigated in addition to the other characterization techniques (DSC, TGA,

Rheology Analysis, NMR, GPC, XRD). Further, laccol based novel polyurethanes were

synthesized using three different diisocyanates groups (aliphatic; HMDI, cycloaliphatic;

H12MDI, aromatic; 4,4'-MDI) and characterized the materials using above mentioned techniques.

Deterioration of the materials after expose to gamma radiation is problematic in industrial

level operations (nuclear plants, medicinal sterilization plants) and space operations. Therefore,

development of new radiation hard materials is essential with improved properties such as

laccol:styrene copolymers we developed in this study. Though the petroleum based polymer

materials play a major role in these fields, it is high time to replace them with eco-friendly

materials such as d-limonene:laccol copolymers which possess promising radiation hard ability.

Generation of citrus waste in industrial scale at yearly basis is becoming a major environmental

concern nowadays. Therefore, development of radiation hard d-limonene:laccol copolymers

could possibly be a good solution to this environmental concern as well. The copolymers we

developed in this study are promising candidates to serve as radiation shield or protective

coatings in the high-tech industrial setup which are dealing with radiation such as nuclear

reactors, medicinal sterilization plants and outer space operations.

xi

The synthesized novel laccol based polyurethanes were characterized to identify their

properties. Mainly, the influence of different diisocyanates, hydrogen bonding capability and

crosslinking ability was investigated apart from other attributes. These analysis could provide

broader understanding to synthesized novel PUs in future developments. Prepared novel PUs

could possibly serve as thermal insulator materials in refrigerator coolant accessories, turbines

and incinerators. Also it can be useful as an energy storing material.

1

CHAPTER ONE: INTRODUCTION

1.1 Lacquer History

The 'oriental' lacquer has a long history and it was used as protective coating material which is

durable and cost effective since ancient times. Initially this was used as adhesive for repairing

chipped porcelain, fixing gold foil and attaching arrow heads to the wooden shafts. Later with

improved awareness this was further utilized in processing bamboo, wood and other objects.

Lacquer is still being utilized in day to day arts and crafts and as protective coatings for

industrial equipments [1]. Cultural artifacts which have a history of more than 2000 years still

preserved its beautiful appearance due to lacquer coatings. This is mainly because of its excellent

physico-chemical properties such as high solvent resistance, excellent toughness and high

durability [2-4]. Figure 1.1 illustrated some lacquer coated artifacts with high durability and

beautiful appearance.

Figure 1.1: Lacquer coated artifacts

(Images: https://en.wikipedia.org/wiki/Lacquer-Appendix A: USF Fair Use Worksheet)

2

1.2 Lacquer Tree

1.2.1 Species and Distribution

Lacquer is a natural product derived from lacquer tree belongs to genus Toxicodendron (formerly

Rhus), family of Anacardiaceae which has more than 73 genera and 600 species all over the

world [5]. This is considered as a mysterious natural coating material in human lives used for

more than thousands of years. Most of these lacquer trees grow in the subtropical regions in

Southeast Asia [5,6]. Even though there are lots of species of lacquer tree, only few lacquer trees

are able to produce lacquer sap. These are Toxicodendron vernicifluum which grows in China,

Japan, and Korea, Toxicodendron succedanea which grow in Vietnam and Taiwan, and Gluta

(former Melanorrhoea) usitata which grows in Myanmar, Cambodia, Lao, and Thailand [1,4,6].

Geographical distribution of these lacquer trees is illustrated in Figure 1.2.

Figure 1.2: World distribution of lacquer tree [1,6]

3

1.2.2 Lacquer Saps

The sap is collected after tapping the bark of the lacquer tree and usually tree age ranging from 5

to 15 years [1]. Different methods are utilized to tap the lacquer tree such as horizontal type, V

type, egg type and vertical/oblique type [6]. The main components of the saps collected from

Toxicodendron vernicifluum, Toxicodendron succedanea, Gluta usitata are urishiol, laccol and

thitsiol respectively as shown in Figure 1.3.

Figure 1.3: The structures of main components in different lacquer saps

In this study Vietnamese lacquer sap (Toxicodendron succedanea) was utilized to extract laccol

in acetone medium for material processing. Laccol contains two adjacent hydroxyl groups

attached to phenyl ring and in its 3rd

position has C-17 long side chain. This side chain contains

one to three unsaturations and therefore extracted laccol is a monomer mixture. In this mixture

~40% contains laccol monomer [6] which has three unsaturations in its side chain as illustrated

in Figure 1.4. This variant prominently involved in polymer and copolymer formation through

cationic polymerization process.

Urushiol Laccol Thitsiol

4

Figure 1.4: Laccol structure [1,6]

1.3 Cationic Polymerization

Cationic polymerization is feasible with the species that have unsaturation present in their

structures. Because of these double bonds it can readily form cations which later involves in

chain propagation. Cationic polymerization reactions are considerably faster compare to anionic

and radical polymerization reactions. The reason for this is formation of stable cation and its high

selectivity during the reaction [7]. Formed cation reacts with another electron donating group (in

this case a double bond) and propagation ensues. Initiation is required at the beginning of the

reaction and high performing cationic initiators are available for this purpose.

5

1.3.1 Initiators used in Cationic Polymerization

Protonic (Bronsted) acids or Lewis acids can be utilized to initiate the cationic polymerization

reaction. HBr, HI, H2SO4 are few examples for Bronsted acids and AlCl3, BF3, SnCl4 are few

examples for Lewis acids which can act as initiators in cationic polymerization process.

Aluminum chloride (AlCl3) is one of the highly effective cationic initiator in the polymerization

processes of styrene, isobutylene and other monomers [8]. AlCl3 is very strong Lewis acid which

can lead chain propagation, transfer and termination very rapidly. Also it has very poor solubility

in organic solvents [8]. In this study AlCl3 is reacted with ethyl acetate (EtOAc) to create

AlCl3.EtOAc initiator system [9,10] which is selective for the laccol based polymer and

copolymer formation in dichloromethane (CH2Cl2).

1.4 Laccol based Polymers and Copolymers

Laccol extracted from Vietnamese lacquer sap was homo polymerized with AlCl3.EtOAc as the

co-initiator. Residual water presence in the laccol initially activated the AlCl3.EtOAc complex

and later the hydroxyl groups present in laccol phenyl ring involve in the reaction to generate the

proton. This was reacted with double bonds presence in the laccol side chain as electrophilic

addition reaction to develop an active center for polymerization. Chain propagation occurred

through this way to produce laccol polymer and laccol based copolymers. These prepared

materials were characterized using different instruments FTIR (Fourier Transform Infra-red

Spectroscopy), NMR (Nuclear magnetic Resonance Spectroscopy), GPC (Gel Permeation

Chromatography), DSC (Differential Scanning Calorimetry), TGA (Thermal Gravimetry),

PXRD (Powder X-ray Diffraction), Shore A hardness, Microscopic analysis and Swelling

analysis as described in detail in Chapter 2 and 3.

6

1.4.1 Laccol:Styrene Copolymers

Styrene was copolymerized with laccol and terminal double bond present in styrene was reacted

with trans conjugated double bonds present in laccol side chain. Commercially available styrene

is usually inhibited with 4-tert-butyl catechol (TBC) to avoid unnecessary homo polymerization

of styrene [11]. Beginning of the experiment, TBC was removed by reacting with 15% (W/V)

sodium hydroxide (NaOH) using solvent extraction process in separatory funnel. Two layers

were formed inside the separatory funnel and bottom aqueous layer was removed while

collecting the pale yellow color upper layer as illustrated in Figure 1.5. This uninhibited yellow

color solution was dried using anhydrous magnesium sulfate (MgSO4) to remove residual water

[12]. Purified styrene was reacted with laccol in various ratios to develop different laccol:styrene

formulations. Synthesized materials were characterized to identify properties of each formulation

and their potent applicability as described in Chapter 2.

Figure 1.5: Separation of styrene and inhibitor (solvent extraction)

Uninhibited

styrene monomer

(upper layer)

Aqueous layer

(15% NaOH and

TBC)

7

1.4.2 Laccol:d-limonene Copolymers

Limonene is extracted from orange peels which accumulate in the environment as citrus waste.

Citrus is the most abundant crop in the world and only one third of it is processed as food [13].

Citrus plant belongs to the family Rutaceae and includes fruits such as oranges, lemon,

mandarin, lime, sour oranges and grape fruit [14]. Compare to total citrus production, ~80% is

oranges and 50% from whole fruit mass of oranges collected as citrus waste. When consider this

in industrial level, accumulation of citrus waste is a main environmental concern [13]. Therefore,

utilizing these citrus waste products to develop different chemicals is a thriving research area

recently. Extraction collected from orange peels contain both d and l limonene. Approximately

90% from the citrus peel oil contains d-limonene [15]. D-limonene is a monocyclic terpene with

two isoprene units and abundantly produced in nature as a secondary metabolite [16,17]. In this

study, d-limonene is reacted with laccol in different monomer ratios through cationic

polymerization to produce copolymers and characterized to identify their promising properties as

described in Chapter 3.

1.5 Radiation Hard Polymers/Copolymers and their Applications

Synthesized laccol:styrene copolymers and laccol:d-limonene copolymers were treated under

gamma radiation and alterations to the materials were studied comprehensively. Radiation

treatments were carried out using a gamma irradiator in University of Florida, Gainesville. A

1.25 MeV gamma ray source was delivered about 200-250 krad/hour (~30 Gy/min) to 6-8

adjacently placed samples for 336 hours. Many alterations occur to polymer materials due to

irradiation forming radicals, anions, cations and other different species. Polymer materials can be

8

cured further by exposing to radiation and this curing achieved through forming chemical and

physical crosslinks. Chemical crosslinks occur by forming new bonds between molecules and

physical crosslinks occur due to secondary interactions such as hydrogen bonding,

ionic/hydrophobic interactions and chain entanglements [18]. A three dimensional polymer

networks can be formed because of these crosslinks and this may influence the solubility of

materials. Lower solubility of polymer materials results due to high crosslink density and also

increases the material hardness accordingly. If the polymer material undergoes chain scission

due to irradiation it will increase the solubility and decrease the hardness [19]. The properties of

laccol based polymers/copolymers identified and experimented in this study; inevitably provide

better candidates to develop as protective coatings which can withstand high gamma radiation

environments such as nuclear reactors, space operations, commercial irradiators and medicinal

sterilization plants [20].

1.6 Polyurethanes

Polyurethane (PU) synthesis and processing has undergone immense differences since its first

discovery in 1930 by Otto Bayer and his coworkers in Germany [21]. Starting with polyurea

from aliphatic diisocyanate and diamine in 1930, it came a long way for today's achievements.

Several big industries such as DuPont, BASF, Dow Chemicals and Mobay contributed massively

for this development by improving their prominent R&D (research and development) sections

[21-23]. With these new developments, it is important to consider the effect to the environment.

Therefore, as a newly emerging research area PUs are synthesized utilizing vegetable oils to

avoid the harmful impact due to usage of chloro-alkenes as blowing agents [24,25]. With this

influence, an attempt was made in this study to develop novel laccol (extracted from Vietnamese

9

lacquer sap) based PUs using three different diisocyanates and materials were characterized to

identify their unique properties.

1.6.1 Laccol based Polyurethanes

Laccol extracted from Vietnamese lacquer sap consists two adjacent hydroxyl groups attached to

phenyl ring apart from the long side chain presence in 3rd

position of phenyl ring. Used

diisocyanates are aliphatic: 1,6-hexamethylene diisocyanate (HMDI), cycloaliphatic: 4,4’-

dicyclohexylmethane diisocyanate (H12MDI) and aromatic: 4,4’-diphenylmethane diisocyanate

(4,4'-MDI). These diisocyanates were reacted separately with laccol at presence of Tin(II) 2-

ethylhexanoate to produce three different PUs namely LH-1, LH-2, LH-3 respectively as

illustrated in Figure 1.6. Hydroxyl groups of laccol reacted with isocyanate groups and resultant

PUs were characterized accordingly as described in Chapter 4. In this study, main focus was

given to identify the influence of different diisocyanates in novel PUs, hydrogen bonding

capability and crosslinking ability which can provide broader characteristic scope for future

developments.

Figure 1.6: Synthesized novel PUs LH-1, LH-2 and LH-3 in film and disc form

Disc

Film Film Film

Disc Disc

10

1.7 References

1. Lu R.; Yoshida T.; Miyakoshi T.; Polymer Reviews, 2013, 53, 153–191, DOI: 10.1080/

15583724.2013.776585

2. Tsujimoto T., Ando N., Oy-Abu H., Uy-Ama H., Kobayashi S.; 2007, Journal of

Macromolecular Science, Part A: Pure and Applied Chemistry, 44, 1055–1060. DOI:

10.1080/10601320701424503

3. Yang J., Zhu J., Liu W., Deng J., Ding Y.; 2015, International Journal of Polymer

Science, 2015, Article ID 517202, 8 pages. DOI: https://doi.org/10.1155/2015/517202.

4. Yang J., Deng J., Zhu J., Liu W., Zhou M., Li D.; 2016, Progress in Organic Coatings,

94, 41–48. DOI: http://dx.doi.org/10.1016/j.porgcoat.2016.01.021

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13

CHAPTER TWO

SYNTHESIZING RADIATION HARD POLYMER AND COPOLYMERS USING

LACCOL MONOMERS EXTRACTED FROM LACQUER TREE TOXICODENDRON

SUCCEDANEA VIA CATIONIC POLYMERIZATION1

Imalka H. A. M. Arachchilagea, Milly. K. Patel

a, Julie P. Harmon

a,*

aDepartment of Chemistry, University of South Florida

*Corresponding author email: [email protected] (Prof. J. P. Harmon)

Abstract

In recent years lacquer chemistry stimulated more attention from the scientific

community due to its environmental friendly applications. Additionally, the 'laccol' possesses

promising radiation resistance due to its highly cross-linked polymer network. In nature, lacquer

polymerization is induced via enzymatic radical initiation. Herein, as a novel approach, lacquer

polymer was synthesized via cationic polymerization with aluminum chloride-ethyl acetate

(AlCl3.EtOAc) co-initiator. This approach was further improved by polymerizing copolymers of

laccol and styrene designed for use in radiation hard materials. The Infra-Red Spectroscopic data

clearly provide evidence for reactions with trans conjugated double bonds present in long side

chain (988 cm-1

and 966 cm-1

are bending vibrations of conjugated alkenes) of laccol monomer

accompanying the polymerization. This was further verified with proton Nuclear Magnetic

Resonance analysis. An increase in hardness was observed for all the compositions; remarkably

1 This chapter has been previously published in POLYM. ENG. SCI., 59:1611–1623, 2019, and has been reproduced

with permission from John Wiley and Sons Publishing (Appendix B). Second author Milly K. Patel (undergraduate

researcher at USF in 2018) involved in sample preparation and part of the analysis. Julie P. Harmon served as the

corresponding author for this publication.

14

for neat laccol polymer, 90% and 70% laccol-styrene copolymers after the gamma irradiation

using Co-60 gamma irradiator. Properties such as thermal stability and hardness were improved

after irradiation due to the increase of physical and small amounts of chemical crosslinking.

Therefore, the developed materials are suitable candidates to produce radiation hard polymer and

copolymer coatings for aerospace operations, nuclear reactors, medicinal and automobile sectors.

2.1 Introduction

For centuries 'oriental' lacquer has been used as coatings [1-3] and paints [1,4] in Asian

countries due to its high solvent resistant [1], excellent toughness [1] and high durability

[1,3,5,6]. Lacquer trees belong to the genus Toxicodendron (formerly Rhus) and family

Anacardiaceae with more than 73 genera and 600 species all over the world. Out of these

varieties only three kinds of lacquer trees are able to produce lacquer sap. The first variety is

Toxicodendron vericifluum, grows in the regions of China, Japan and Korea with the main

component of 'urushiol'. Secondly, Toxicodendron succedanea which grows in Vietnam and

Chinese Taiwan has 'laccol' as its main component. The third variety is Gluta usitata which

grows in Myanmar, Laos, Cambodia and Thailand having the 'thitsiol' as the main liquid

component [7]. In recent years, although many synthetic polymers and coatings have been

developed, lacquer sap is still attracting the attention because of its renewable ability, eco-

friendly nature and promising physico-chemical properties [3].

The Toxicodendron succedanea tree sap from Vietnam was utilized in this study to

synthesize polymer and copolymers. This lacquer sap has slow drying speed [7,8] compared to

urushiol, under 60-80% humidity environment via enzyme-catalyzed reaction [6]. The use of

filtered lacquer sap for the experiments and processing with laccase enzyme which was involved

15

in polymerization process is widely held in reported literature [1-6,8]. In contrast to this practice,

the 'laccol' (dark brown color liquid main component) extracted with acetone from the tree sap

was utilized for all the experiments conducted herein. It has C17 unsaturated hydrocarbon chain

with 0-3 olefins at the 3-position of the catechol ring [3,7,9] and 38.7% of the monomer mixture

contains 3 olefins in the side chain [7]. The laccase enzyme which is insoluble in acetone was

removed during the extraction process as 'acetone powder' [10]. The main constituent in laccol

mixture (acetone soluble components) which has 3 olefins in the side chain was significantly

involved in the cationic polymerization process incorporated herein.

Cationic polymerization is highly feasible with species containing double bonds and

reactions are considerably faster than the anionic and radical polymerizations [11]. According to

the predicted mechanisms in Scheme 2.1 and Scheme 2.2, water (H2O) interacts with co-initiator

(AlCl3.EtOAc) to generate the proton. Then, the electrophilic addition of proton to double bond

takes place to give the active center of the polymerization. This active center then initiates

polymerization and propagation ensues. Initiation of the cationic polymerization can be

achieved by using protonic acids (Bronsted) or Lewis acids [11]. A widely used Lewis acid

which is effective in cationic polymerization of styrene, isobutylene and other monomers is

Aluminum trichloride (AlCl3). This AlCl3 initiator is involved in rapid propagation reactions in

the systems including chain transfer and termination due to its strong Lewis acid nature.

However, AlCl3 has poor solubility in organic solvents [11,12]. To enhance the selectivity of

polymerization reaction, ethyl acetate (EtOAc) was introduced to AlCl3 for developing an

AlCl3.EtOAc complex [13,14]. This co-initiator complex was utilized herein to synthesize laccol

polymer and laccol-styrene copolymers. AlCl3.EtOAc was activated during the reaction by the

residual water in laccol mixture and carbocation was formed specifically in the conjugated

16

double bond region of the side chain of catechol ring. The two hydroxyl groups in the catechol

ring which are acidic and has able to form phenoxide anions to react with carbocation species in

the side chain. Additionally, formation of macrocyclic compounds is also possible due to

phenoxide anions and unsaturated groups (cis and trans double bonds) in side chain as discussed

by Rozentsvet et al [15]. These phenomena were investigated using IR and NMR data for laccol

polymer and laccol-styrene copolymers. The possible mechanisms for these processes are

illustrated in Scheme 2.1 and 2.2. The synthesized polymer and copolymers were exposed to

gamma radiation [16] and investigated the characteristic properties resulted from radiation

treatment.

The overall hypothesis behind this work is that the lacquer studied herein will exhibit

superior radiation resistance. This is substantiated by the fact that Toxicodendron succedanea

contains hydroxyl phenyl groups that act as radical scavengers, enhancing radiation resistance

via their ability to convert excitation energy into non-chemistry inducing energy in lacquer that

has been cured [17,18]. Further, Rogner and Langhals have shown via IR and Mass spectroscopy

that doses of 300 kGy (30 Mrad) electron beam (EB) do not result in measurable damage in qi-

lacquer (urushi) [19]. Additionally, Harmon’s group has shown that nanotubes enhance radiation

hardness of organic polymer matrices exposed to gamma rays [20-22].

Irradiated polymers could undergo various reactions forming cations, anions, gases and

other species. The two main types of reactions considered herein were crosslinking and chain

scission. The crosslinking can be physical, chemical or biological [23]. The chemical

crosslinking occurs between adjacent polymer molecules by forming new bonds and physical

crosslinks occur due to secondary interactions like hydrogen bonding, ionic interactions,

hydrophobic interactions and also due to entangle chains [24]. The biological crosslinking is also

17

a newly immerging area, however not yet developed in industrial level as chemical crosslinking

[23]. These processes could increase hardness of the polymer until it forms insoluble three-

dimensional network. Chain scission decreases the hardness and increases the solubility [25].

This makes them ideal candidates for use in radiation environments such as nuclear reactors,

machines for radiation therapy, industrial sterilization and space [26]. According to the literature,

radiation hardness of lacquer sap has not been investigated and references regarding lacquer sap

(urushiol and laccol) are limited to polymerization/curing studies primarily via UV and EB

radiation [4,6,19,27]. Therefore, in this paper the effort was made to synthesize radiation hard

laccol polymer and laccol-styrene copolymers using Toxicodendron succedanea lacquer sap

(from Vietnam) via cationic polymerization. The prepared materials were investigated in two

stages as cured control samples and irradiated samples by IR, NMR, GPC, Rheometer, TGA,

DSC, wide angle X-ray (WAXS) and shore A hardness methods.

2.2 Experimental Section

2.2.1 Materials

Raw lacquer sap was imported from Viet Lacquer Interior Co. Ltd in Vietnam. Reagent grade

acetone (CH3COCH3), dichloromethane (CH2Cl2) was used as solvents. Anhydrous Calcium

Chloride (CaCl2) granular, anhydrous Sodium Chloride (NaCl), anhydrous Magnesium Sulphate

(MgSO4), analytical grade sulphuric acid (H2SO4), hydrochloric acid (HCl), sodium hydroxide

(NaOH), 99.99% HPLC grade Tetrahydrofuran (THF) were obtained from Fischer Scientific.

Reagent Plus grade 99% Aluminum Chloride (AlCl3), 99.9% HPLC grade Ethyl Acetate

(EtOAc) and 99% Styrene monomer were purchased from Sigma Aldrich and inhibitor was

removed using 15% NaOH.

18

2.2.2 Preparation Procedures

2.2.2.1 Extraction of laccol from lacquer sap [5]:

A 100 g of filtered lacquer sap was mixed with 300.0 ml of acetone and stirred for 2 hours. The

mixture was vacuum filtered and the filtrate was collected. After much of the solvent acetone

was evaporated, two layer separations were observed due to the water content. The water layer

(bottom layer of the separatory funnel) was removed and the organic layer was further dried

using anhydrous CaCl2 granular. The vacuum evaporation was used to remove the excess

acetone. Up to 60-65% yield of laccol can be obtained from this extraction technique.

2.2.2.2 Synthesis of an Aluminum Chloride/Ethyl Acetate [AlCl3.EtOAc] Complex [13,14]:

EtOAc (2.3 ml, 2.25 x 10-2

mol) was added dropwise to slurry of AlCl3 (3 g, 2.25 x 10-2

mol) in

20.2 ml of dichloromethane (CH2Cl2) for 5 – 10 minutes. The reaction was allowed to stir for 30

– 60 minutes up to complete dissolving of AlCl3 to give a solution of complex (AlCl3.EtOAc) in

CH2Cl2 (~1 M).

2.2.2.3 Preparation of Laccol Polymer:

Polymerization reactions were carried out in erlenmeyer flasks inside an ice bath. As an example

of a typical procedure, polymerization was initiated by adding a solution of AlCl3.EtOAc in

CH2Cl2 (3.0 ml, ~1 M) to a mixture of laccol (10.0 g, ~6.07x10-2

mol) and CH2Cl2 (7.0 ml) while

stirring. After 15 minutes of stirring the sample had become more viscous and then it was

transferred to a polymer releasing paper which was folded to hold the sample. This was dried

under vacuum oven for 4 days at 60⁰C and partially cured samples were then dried under hot air

oven for 4 days at ~100⁰C. Proposed mechanism for the synthesis is illustrated in Scheme 2.1.

19

Scheme 2.1: Proposed mechanism for laccol polymerization

20

2.2.2.4 Preparation of Polystyrene:

Styrene monomer was purified by solvent extraction process with 15% (W/V) NaOH and dried

the organic layer using anhydrous magnesium sulphate (MgSO4) [28]. The polymerization was

initiated by adding AlCl3.EtOAc in CH2Cl2 (2.50 ml, ~1 M) to the mixture of Styrene (11.0 ml)

and CH2Cl2 (6.5 ml) while stirring in an ice bath under inert nitrogen atmosphere. The mixture

was stirred until it got more viscous and then it was kept under nitrogen atmosphere for

overnight. This was dried in vacuum oven for 4 days at 60⁰C.

2.2.2.5 Copolymers of laccol and Styrene:

The copolymers were prepared by mixing different monomer ratios of laccol and styrene as

illustrated in Table 2.1. S-10, S-15, S-30, S-50, S-70, S-90 sample names corresponded to 10%,

15%, 30%, 50%, 70% and 90% (w/v) laccol in styrene respectively. These samples were dried

under vacuum oven for 4 days at 60⁰C and partially cured samples were then dried under hot air

oven for 4 days at ~100⁰C.

Table 2.1: Different monomer contents that used to prepare the copolymers

Proposed mechanism for laccol-styrene copolymers formation is illustrated in Scheme 2.2.

21

Scheme 2.2: Proposed mechanism for laccol and styrene copolymer formation

22

2.2.3 Characterization Methods

Several methods were incorporated herein to identify the formation of polymer material and its

chemical, thermal and physical characteristics. Further, the effects resulted due to radiation

exposure was also investigated to identify the suitable polymer materials for applications.

2.2.3.1 Fourier Transform Infra-Red (FTIR) Analysis

For the FTIR study, a Perkin Elmer UATR Two spectrometer and solid samples were used

having the scan range of 400 – 4000 cm-1

set at a resolution 4 cm-1

and 16 average scans.

2.2.3.2 Nuclear Magnetic Resonance Spectroscopy (NMR)

NMR analysis was conducted on laccol polymer (LP) and its copolymers dissolved in

chloroform-d, using Varian INOVA 400 spectrometer. Instrumentation parameters were

employed as follows; Temperature maintained at 298K, spin set and maintained at 20Hz, 16

transients used in block sizes of 8, d1 relaxation time 2.000 second.

2.2.3.3 Gel Permeation Chromatography (GPC)

The molecular weight and molecular weight distribution of the laccol polymer and laccol-styrene

copolymers were determined by gel permeation chromatography (GPC) (PL-GPC 50 Integrated

GPC System, Polymer Laboratories). The partially cured samples were dissolved in THF to give

the concentration of 4 mg/ml and after 6 hours these samples were passed through 0.45 µm

filters separately. A 100 µl from each sample was injected at 30 ⁰C using THF as mobile phase

with a flow rate of 0.5 ml/min. A calibration curve was generated using monodispersed

23

polystyrene standards (Polymer Laboratories PS-2) to obtain the relative GPC data for the

samples.

2.2.3.4 Dynamic Mechanical Analysis [29]

Testing articles (rectangular bars with width 10.5 mm, length 50 mm and thickness 2 mm) were

molded in a heated Carver® hydraulic press with slow cooling to room temperature under

pressure. The isothermal strain sweep test was performed at -100⁰C to determine the linear

viscoelastic region (LVR) using AR 2000 (TA Instruments, USA) rheometer. Within the

measured LVR 0.3% strain was selected to characterize the samples with a temperature ramp in

oscillation mode to identify the glass transition temperature (Tg). Ramp conditions were -90ºC to

200ºC at 5 ºC/min for laccol polymer and copolymers, for synthesized polystyrene (PS) it was

0ºC to 200ºC at 10 ºC/min with liquid nitrogen for cooling. Further, frequency sweep experiment

was conducted using rectangular bars for laccol polymer and its copolymers, having the

frequency range from 0.1 – 10.0 Hz, 0.300% strain, starting temperature of -15⁰C with 5⁰C

increments each time until the sample collapse and for synthesized PS, strain was 0.200% and

starting temperature was 0ºC. Using the frequency sweep experimental data, activation energy

was calculated for each sample associated with glass transition temperature region.

Time sweep experiment was also performed for all the samples using parallel plate setup of the

rheometer with compression molded discs (diameter 1 inch and thickness 0.1 inch) having 2%

strain and 1.0 Hz frequency for 30 minutes at 150⁰C to observe the viscosity difference with time

at a specific temperature. Resulting data were analyzed with software available from TA

Instruments.

24

2.2.3.5 Thermal Gravimetric Analysis (TGA)

Thermal Gravimetric Analysis was performed on a TGA model TA Q50. A weight loss versus

temperature scans for different polymers and copolymers were recorded at a ramp rate of

10⁰C/min in air at the range between room temperature (~22⁰C) and 700⁰C.

2.2.3.6 Shore Hardness

ASTM D2240 Shore A was used to test the hardness of polymerized samples. Testing articles

(discs with diameter 1 inch and thickness 0.1 inch) were punched from sheets that were

compression molded using a Carver® laboratory press (model C) equipped with heating

elements. Hardness of the samples was measured before and after the γ-irradiation, before and

after rheometer experiments, before and after temperature treatment in dry oven at 200⁰C.

2.2.3.7 Radiation Studies and Microscopic Analysis

Co-60 Gamma Irradiator at University of Florida, Gainesville was used for the radiation studies.

A 1.25 MeV gamma rays source was utilized, delivering about 200-250 krad/hour (~30 Gy/min)

to 6-8 adjacently placed samples per each exposure. Samples were exposed to 9.8 Mega Rad in

air; this required 336 hours of exposure time.

Leica Microsystems Wetzlar GmbH instrument equipped with Leica DFC 290 camera (made in

Germany) was used to investigate the surfaces of prepared samples. Magnitude was set to PL

FLUOTAR 5x/0.12P and images were captured using Leica FireCam software.

25

2.2.3.8 Wide Angle X-ray Scattering (WAXS) [30]

To investigate the degree of crystallization of synthesized laccol polymer, laccol-styrene

copolymers and polystyrene, WAXS powder X-ray diffraction was used. X-ray diffraction

patterns were collected using Bruker D8 Focus x-ray diffractometer in the range of 2–70⁰ with a

step of 0.020⁰ at 25⁰C.

2.3 Results and Discussion

Cationic polymerization with AlCl3.EtOAc co-initiator successfully incorporated to develop

laccol polymer and laccol-styrene copolymers and this was confirmed from the results obtained

from IR, NMR, GPC and other characterization methods. These polymer and copolymers were

exposed to gamma radiation and reinvestigated the properties to identify the behavior of

materials after the radiation treatment. It was clearly indicated that NLP, S-90 and S-70

copolymers showed promising results towards the radiation hardening. Materials were further

cured and crosslinked (physically and/or chemically) due to the gamma radiation without

deteriorating and the obtained results were analyzed in detail here after.

2.3.1 FTIR Analysis

Identification of polymerizing process of lacquer based materials was frequently investigated

using FTIR data [1,4-8,23,31-33]. Figure 2.1(a) has the IR spectra of Laccol Extract (LE) and

Neat Laccol Polymer (NLP). It shows peaks at 3450 cm-1

due to hydroxyl groups (O–H), at 3010

cm-1

due to C–H stretching vibrations of unsaturated groups (cis and trans double bonds) in side

chain, at 2940 cm-1

due to methylene (C–H), at 1621 cm-1

, 1596 cm-1

and 1470 cm-1

due to

26

vibrations of phenyl ring (aromatic skeletal), at 988 cm-1

(trans) due to the bending vibrations of

conjugated alkene, at 966 cm-1

(trans) and 732 cm-1

(cis) due to alkenes in side chain [6,7,27].

Figure 2.1(b) is IR spectrum of purified styrene monomer which represents the peaks at 3010–

3083 cm-1

for C–H in aromatic group, at 1630 cm-1

for C=C, 907 and 990–1000 cm-1

for bending

vibrations of R–CH=CH2.

Figure 2.1: IR spectra of (a) LE, NLP and (b) Styrene monomer

27

The bending vibration for conjugated alkene at 988 cm-1

(trans) which is presented in the laccol

side chain was involved in the reaction via cationic polymerization process. The peak at 988 cm-1

completely disappeared after the polymerization process and the intensity of the peak at 966 cm-1

was reduced. This is possible due to the formation of laccol polymer and copolymers. This was

clearly identified with FTIR spectrum which was obtained for laccol polymer as illustrated in

Figure 2.1(a) as NLP. The peaks at 1278 cm-1

and 1184 cm-1

can be attributed as γO–H for laccol

and its copolymers. Comparing the curves LE and NLP in Figure 2.1(a), respective peaks for

γO–H significantly decreased in NLP, because of the formation of dimers and polymers [6]. This

scenario can be observed clearly from IR spectra c – e for the laccol and styrene copolymers as

well in Figure 2.2. The peak at 1352 cm-1

which is related to βO–H [6] also decreased in spectra

'b – e' compared to the curve 'a' in Figure 2.2. The peak at 3010 cm-1

which can be attributed as

the stretching vibration of the unsaturated group in the side chain was significantly reduced

possibly due to the crosslinking process occurred in the side chain. The peaks at 1621 cm-1

and

1596 cm-1

which are related to vibration of –C=C– bonds in phenyl ring were significantly

changed in Figure 2.1(a) NLP graph due to the formation of dimers and polymers. However, all

the synthesized materials are melt-processable.

Samples were reinvestigated after the irradiation process using FTIR and the peak at 966 cm-1

decreased as illustrated in Figure 2.2. The peak at 1690 cm-1

shifted to 1710 cm-1

and broadened

after the irradiation because of more laccol quinones formation [6]. The peaks at 1621 cm-1

and

1596 cm-1

were further changed after gamma irradiation due to the formation of dimers and

polymers [6]. The peaks in the region of 1350 – 1100 cm-1

also decreased in intensity after the

irradiation. These observations suggest that the laccol polymer and its copolymers were further

cured and have been crosslinked (physically and chemically) due to the gamma radiation without

28

deteriorating the material. Most promising results were observed for NLP, S-90, S-70 and S-50

samples.

Figure 2.2: IR spectra of various laccol compositions; a) LE-control, a') LE-after irradiation,

b) NLP-control, b') NLP-after irradiation, c) S-90_control, c') S-90_after irradiation,

d) S-70_control, d') S-70_after irradiation, e) S-50_control, e') S-50_after irradiation,

f) NPS-control, f') NPS-after irradiation, *NPS-Neat Polystyrene

29

2.3.2 NMR Analysis

Cationic polymerization of laccol polymer and its copolymers were further confirmed by 1H

NMR analysis. Completely cross-linked polymer and copolymers were insoluble in any organic

solvents or water, the partially dissolved samples of neat laccol polymer (NLP) and

laccol:styrene copolymers were investigated in this experiment. Laccol extract (LE) was

completely dissolved in CDCl3 and the obtained spectrum was compared to the NLP spectrum as

illustrated in Figure 2.3.

Figure 2.3: NMR spectra for (a) LE and (b) NLP (*Acetone)

The peaks relevant to the protons on the 13' to 16' carbon atoms were detected around 5.5–6.0

ppm completely disappeared after the laccol polymer formation. Also peaks relevant to cis

double bond at carbon number 10' and 11' reduced and new peaks appeared in the region of 4.0–

4.3 ppm (–C–O–C– bond formation) [2,8]. These new –C–O–C– bonds could form through the

30

side chain and also through the phenyl ring [6] due to the cationic polymerization (Scheme 2.1).

For the laccol and styrene copolymers; peaks relevant to cis double bond (10', 11') could

observed even after the polymerization process and peaks relevant to trans conjugated double

bonds (13' – 16') were completely utilized during the reaction. The –C–O–C– bond formation

was also observed as a minor product in the polymerized material (Scheme 2.2). Possibilities for

this observation are macrocyclic compound formation [15] and cross linking.

Reinvestigation of the samples was carried out after the irradiation process and it was confirmed

that the materials were further cured due to gamma radiation without deteriorating (Figure 2.4).

Peak at 4.0 ppm increased and it was evident for –C–O–C– bond formation [8] and possibility of

making macrocyclic compounds. Peaks relevant to cis double bond for copolymers were still

appearing after the gamma irradiation, but in less intensity.

Figure 2.4: Laccol:Styrene (S-50) copolymer-control, (b) Laccol:Styrene (S-50) copolymer-after

irradiation

One favorable possibility for this observation is inaccessible cis double bond due to random

orientation of molecules in the physically and chemically crosslinked three dimensional network.

31

Other possibilities are insufficient curing time or insufficient amount of styrene/laccol monomer

to allow the complete reaction.

2.3.3 GPC Analysis

Completely cured samples could not be dissolved in any solvent due to its high crosslinking

property and therefore, partially cured samples dissolved in THF were used for the experiment.

Molecular weight buildup was identified in each sample at the very beginning stage of

polymerization process. According to the results illustrated in Table 2.2, all the samples were

shown a molecular weight build up and it was significant for NLP and S-90 even in the very

beginning stage. This was again proved the successful incorporation of cationic initiator for the

polymerization.

Table 2.2: The MW

of NLP and its copolymers at initial polymerization stage

(apolydispersity, PD = M

W/M

n)

2.3.4 Dynamic Mechanical Analysis

The temperature region where the long chain segments slips is identified as the glass transition

temperature (Tg). The Tg was identified for samples using the tan δ curve from the temperature

32

ramp experiment for the rectangular shaped solid samples in rheometer. NLP showed the Tg

value of 15.40⁰C and for the copolymers, Tg values were decreased as S-30>S-50>S-90≈S-70.

According to the trend observed it was clear that the high styrene monomer consisting samples

showed the higher Tg value comparatively due to more ordered packing of the material. Also

with the increase of temperature a significant rubbery plateau was observed for S-30, S-50, S-70,

S-90 and NLP samples which are indicative of physical and/or small amounts of chemical

crosslinking (Figure 2.6).

The resulting graph (from frequency sweep data) created using Time-temperature superposition

software with shift factor versus temperature as shown in Figure 2.5(a,b) is an evidence of WLF

behavior and used to calculate the activation energy for each sample using the equation (1). C1,

C2 are material constants, R is universal gas constant (8.314 J.mol-1

.K-1

), T is temperature in

Kelvin (K) and Ea is activation energy [34,35]. Required energy to induce a large segment

slippage associated with glass transition was illustrated in Table 2.3. Highest activation energy

was observed for the S-90 sample and least was given by the S-70. NLP showed a 244.3 kJ mol-1

value in the glass transition temperature region.

∆𝐸𝑎 = (2.303) (𝐶1

𝐶2)𝑅𝑇2 (1)

The master curves were created using the above software and the storage modulus (G') always

predominant the loss modulus (G"). Figure 2.5(b) is an example for the master curve created for

NLP. Because of the high crosslinking property, completely cured samples were unable to test

using GPC to identify the molecular weights of final products as mentioned in section 2.3.3.

33

Table 2.3: Tg values and related activation energies obtained from rheometer

According to the data obtained from time sweep experiment, the viscosity increased with time at

150⁰C for all the samples and significant increment resulted from NLP, S-90 and S-70 samples.

This observation suggests a possibility of further curing of materials at a higher temperature as

reported in literature [3].

Figure 2.5: (a) NLP-WLF behavior graph, (b) Master curve developed for NLP

34

Figure 2.6: Tan δ curves (c) for materials from temperature ramp experiment (rheometer)

Tg of an amorphous polymer can be strongly influenced by molecular weight, tacticity, sample

weight, laboratory processing conditions and used ramp rate for the experiment [36-39]. The Tg

of synthesized polystyrene was identified using the temperature ramp experiment in rheometer.

The obtained experimental value was 102.8⁰C (Mw 20438) and this was inside the reported range

of Tg values from 96–106⁰C for polystyrene [37,40-43].

2.3.5 TGA Analysis

The change in sample mass is determined as a function of temperature or time in this technique.

Commonly used method is dynamic thermogravimetry where the sample is heated at a linear rate

in predetermined temperature changing environment [44]. Figure 2.7(a) illustrates the thermal

stability increment after addition of styrene. Figure 2.7(b) illustrates the results obtained for NLP

35

and S-90 samples after irradiation as a comparison to control samples. The degradation of neat

laccol polymer (NLP) could be described using two temperature stages [2,6].

Figure 2.7: TG curves for NLP and S-90 samples

36

The first stage of the NLP-control sample scan (50-150⁰C) with a weight loss of 3.63% was due

to the removal of residual water and other small molecules [2,6]. In the second stage (150-

500⁰C) with a weight loss of 83.4% can be attributed to degradation of laccol oligomer and

polymer. The irradiated NLP sample noted in the weight loss at the beginning of the scan and

after 150⁰C was less than the NLP-control sample, 2.87% and 76.6% respectively. Compared to

NLP, observed TG curves were similar for its copolymers. S-90 copolymer was chosen as an

example to demonstrate the data with NLP (laccol polymer). For S-90 control sample 2.02%

weight loss was observed in the first stage and in the second it was 80.0%. After the irradiation

process the weight loss after 200⁰C was reduced with the value of 77.4%. On set temperatures of

both irradiated NLP and S-90 samples were increased with the values of 384⁰C and 370⁰C

respectively compared to control samples. Additionally, a considerable amount of residue was

observed even after the 700⁰C for irradiated samples (NLP irradiated: 17.7%, S-90 irradiated:

17.4%) comparative to controls (NLP control: 11.8%, S-90 control: 16.5%). These observations

indicated that the irradiated samples possessed higher thermal stability possibly due to further

curing and crosslinking.

2.3.6 Shore Hardness Analysis

Shore A durometer was used for investigations due to the soft nature of synthesized materials.

Higher number of the scale from 0 – 100 indicates the greater resistance to indentation, and thus

harder materials [45]. According to the obtained results as shown in Table 2.4, addition of

styrene increased the hardness of the materials and irradiation further increased the physical and

chemical crosslinks, hence resulting harder materials. Higher laccol monomer containing

samples such as NLP, S-90, S-70 demonstrated significant increments of the hardness after

37

irradiation compared to initial readings. The effect of increased temperature during the

experimental steps and processing, to hardness property was also analyzed parallel to this study

and those data illustrated an increment of hardness for the NLP, S-90, S-70 and S-50 samples.

Table 2.4: Shore A hardness data for laccol polymer and copolymers; before and after

irradiation

2.3.7 Microscopic Analysis

The cured and processed discs were analyzed through optical microscope before and after the

gamma irradiation. Results were illustrated in Figure 2.8. It was clearly shown that more

wrinkles were appeared after the irradiation significantly for NLP, S-90, S-70 and S-50 samples.

This can be observed from the micrographs which illustrated more reflecting points than control

samples. The mechanism for the wrinkle formation and factors control the surface properties are

not clearly identified [4]. Curing process of the material could possibly lead to wrinkle formation

due to the evaporation of water and excess solvent. Exposure to gamma radiation could possibly

elicit more growing points for polymerization and remarkably increase the crosslinking density,

resulting in a hard cured material [4].

38

Figure 2.8: Optical microscopy observations of polymers and copolymers

(Magnitude: 5x/0.12P)

2.3.8 Powder X-Ray Analysis

A polymer is a macromolecule composed of many repeating sub units. The arrangement of these

sub units in a bulk polymer is controlled by molecular weight, chemical composition, spatial

orientation and processing conditions. Based on these factors polymers could show amorphous

or partially crystalline phase states. In crystalline state the arrangement of molecules are more

ordered and in the amorphous state it is more random. Amorphous polymers do not include long-

39

range order, thus consist with characteristically identifiable short-range order. As a result of this,

powder X-ray diffraction provides a diffuse peak signal [46]. According to the obtained results

as shown in Figure 2.9, controlled samples (NLP and S-90) have more crystallinity in the matrix

before exposing them to radiation. These diffused peaks are possible due to the local packing

arrangement of the molecules [47]. The distinct peak observed at 20⁰ repetitively for all the

compositions could possibly due to the main backbone chain of the polymer matrix (Scheme

2.1). After exposing the samples to gamma radiation, the peak observed at 20⁰ is preserved and

other peaks were diffused. The exact reason for the peak diffusion is not clearly identified and

possibilities could be different spatial orientation of the molecules, crosslinking, processing

conditions or chain scission.

Figure 2.9: Powder X-ray diffraction data for NLP and S-90

2.4 Conclusions

Laccol polymer and laccol-styrene copolymers were synthesized successfully incorporating

AlCl3.EtOAc cationic co-initiator. The unsaturated hydrocarbon chain in 3-position of laccol was

effectively cured with cationic co-initiator to produce physically and small amounts of

chemically crosslinked polymeric material. The changes occurred to IR peaks at 988 cm-1

, 966

40

cm-1

, 732 cm-1

(Figure 1 and 2) and the NMR peaks at 5.5–6.0 ppm range for NLP (Figure 2.3)

and copolymers after polymerization confirmed this statement. Flexibility of these amorphous

polymers was restricted below their Tg values [46]. Observed Tg values for polymer and

copolymers were ranging from 12.90⁰C to 29.60⁰C (Table 2.3). For synthesized polystyrene it

was 102.8⁰C. When the styrene monomer content increased in the copolymers, they became hard

and processability of the materials was increased, specifically for S-90, S-70, S-50, S-30

copolymers. Also, incorporating styrene to laccol monomer enhances the thermal stability of

copolymers. Exposure to gamma radiation enhances more growing points for polymerization and

significantly increases the crosslinking density which leads to increment of the hardness. The IR

and NMR data suggested the stable condition of materials after irradiation and evidences for

more physical and small amounts of chemical crosslinking. This was further justified by the data

obtained from temperature ramp experiment (tan δ curve) which illustrated a significant rubbery

plateau (Figure 2.6). The TGA and shore hardness data (Table 2.4) were evidenced the thermal

stability and hardness improvements of the materials. TG curves obtained for irradiated NLP and

S-90, clearly indicated higher onset temperatures and high residue weight even after 700⁰C.

NLP, S-90 and S-70 samples show significant hardness increments after the irradiation.

Microscopic observations also illustrated higher wrinkle formations after the irradiation process,

suggesting the fact of solvent evaporation and crosslinking. The wide angle X-ray data

exemplified significant differences due to gamma irradiation. As per the objective of the study,

synthesizing radiation hard polymer and copolymers was achieved and the prepared materials

were further cured through physical and chemical crosslinking without deteriorating after expose

to gamma radiation. Considering the factors like; processing ability, thermal stability and

radiation hard ability the neat laccol polymer (NLP) and S-90, S-70, S-50 copolymers are the

41

promising materials which illustrated the most profound outcome during this study. The

developed materials have a great potential for serving as protective layer or coating and radiation

shield against gamma radiation in the advanced high-energy radiation environments like nuclear

reactors, machines for radiation therapy, industrial sterilization and space [26].

2.5 Acknowledgement

Authors gratefully acknowledge Professor M. Luis Muga, University of Florida, Chemistry

Department, Gainesville, for performing gamma irradiation experiments and Professor Lukasz

Wojtas, University of South Florida, Chemistry Department for analyzing powder X-ray data.

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45

CHAPTER THREE

ENVIRONMENTAL FRIENDLY RADIATION HARD D-LIMONENE AND LACCOL

COPOLYMERS SYNTHESIZED VIA CATIONIC COPOLYMERIZATION2

Imalka H. A. M. Arachchilagea, Milly K. Patel

a, Joseph Basi

a, Julie P. Harmon

a,*



Abstract

In the modern world, petroleum based synthetic polymers have a great number of

applications in fields ranging from food packaging to space travel. However, the processing of

petroleum products and the resulting depletion of fossil fuels are major environmental concerns

in today’s society. As a result, the development of sustainable polymers which are made up of

renewable resources and waste products is an immerging area of research. Considering the world

food production, citrus fruit is most abundant and its contribution to waste generation is

immense. Therefore, this study focuses on offering an alternative to the use of petroleum based

polymers and also providing a regulatory pathway to manage citrus waste by developing novel

copolymers of laccol and limonene. Two environmental friendly compounds, laccol, derived

from the sap of Toxicodendron succedanea tree and limonene, extracted from orange peels, were

2 This chapter has been previously published in POLYM. ENG. SCI., 60:607–618, 2020, and has been reproduced

with permission from John Wiley and Sons Publishing (Appendix C). Second author Milly K. Patel (undergraduate

researcher at USF in 2018) involved in sample preparation and part of the analysis. Third author Joseph Basi

(undergraduate researcher at USF in 2019) involved in sample analysis. Julie P. Harmon served as the corresponding

author for this publication.

46

copolymerized via cationic polymerization to generate d-limonene:laccol copolymers with

radiation hardening capabilities which is relevant in fields such as nuclear energy generation,

medicinal sterilization, commercial irradiation and space exploration. Formation of these

copolymers was verified with Infra-Red and Nuclear Magnetic Resonance analysis. The

synthesized copolymers were characterized using different methods and exposed to Co-60

gamma radiation to identify alterations to their properties.

3.1 Introduction

Polymers play an essential role in modern society and currently the main source for the

components of synthetic polymers is fossil fuels. Monomers derived from petroleum products

can be used to synthesize polymers with extraordinary properties including remarkable

durability, conductive properties, and radiation resistance, all at a very low cost. However, the

environmental impact caused by depletion of these fossil fuels and the environmental damage

that accompanies processing petroleum products are rapidly becoming major issues with the use

of petroleum-based polymers. Therefore, in search of alternative measures, many scientists have

focused on producing environmental friendly and sustainable polymers from renewable sources

and waste products [1,2]. In particular, the incorporation of citrus waste into the next generation

of renewable polymers has recently gained considerable attention due to its appreciable

metabolites.

Out of the worldwide fruit and vegetable production, most abundant crop is Citrus and

one-third of the crop is processed [3]. In the entire industrialized crop, approximately 98%

contains oranges, lemons, grapefruits and mandarins which oranges are the most relevant with

47

82% of total. Citrus fruits are processed mainly to obtain juice. They are also used in the canning

industry to produce marmalade, segments of mandarin, and in chemical industry to extract

flavonoids and essential oils. From the whole fruit mass 50% of residual obtained as waste and it

consists of peels, seeds and fruit pulp remaining after juice and essential oil extraction [4]. When

considered on the industrial scale, this becomes a huge environmental concern. Therefore,

utilizing these waste products to develop useful chemical resources has recently become an

important topic of investigation. The citrus peel is a good source of a plethora of possibly useful

compounds, including molasses, pectin and limonene [5].

In this study, limonene extracted from citrus peels is utilized as one of two monomers

used to develop copolymers with laccol, a compound that is extracted from Vietnamese lacquer

sap. The major constituent of essential oil resulting from the citrus fruit skin is limonene and

90% of it contains d-limonene isomer [6]. This belongs to the family of terpenes [7]. Terpenes

are secondary metabolites synthesized mainly by plants, but also by a limited number of insects,

marine micro-organisms and fungi. Though these terpenes were initially considered as 'waste',

later their involvement in biosynthetic processes and ecological important role was discovered

[8]. Limonene is used in cosmetic production, foods and beverages as well as a green solvent

[2,7]. This contains double bonds which provide the necessary bifunctionality for polymerization

(Scheme 3.1-Limonene). Limonene is also an allylic monomer (CH2=CH-CH2R; R is the rest of

the molecule) which is hard to homo polymerize via free radical polymerization due to stable

radical formation and steric hindrance compared to vinyl radicals [9]. Therefore,

copolymerization of d-limonene using AlCl3.EtOAc cationic co-initiator with laccol monomer is

investigated herein as a novel approach.

48

Laccol monomer is extracted from Toxicodendron succedanea tree lacquer sap which

grows mainly in Vietnam. Lacquer saps are popular since ancient time periods because of its

excellent toughness, high solvent resistivity and high durability [10-13]. It contains two reactive

hydroxyl groups in the phenyl ring and a C-17 unsaturated side chain at 3rd

position of the phenyl

ring (Scheme 3.1-Laccol). Due to the unsaturation of this C-17 side chain, it is a potent candidate

for cationic polymerization process. As described in our previously published article [14], laccol

was successfully polymerized via cationic polymerization using AlCl3.EtOAc co-initiator.

Further broadening the applicability of laccol in industrial environment, copolymerization of

laccol with limonene via cationic polymerization was studied herein to develop radiation hard

copolymers. Copolymerization process involved two or more monomers during chain growth

polymerization which can balance the properties of commercial polymers [15]. Laccol and d-

limonene was copolymerized with AlCl3.EtOAc co-initiator. Conjugated double bonds (trans) in

unsaturated side chain of laccol and terminal double bond in limonene was involved prominently

during the chain growth polymerization. Additionally, as explained by Rozentsvet et al [16]

macrocyclic compound formation is also possible due to the phenate ions and cis/trans double

bonds (unsaturated groups) in the laccol side chain. These phenomenons were clearly observed

in the IR and NMR data obtained for the synthesized copolymers. During this cationic

polymerization process 70% of limonene by weight was able to copolymerize with laccol is a

promising outcome compared to the reported literature regarding limonene copolymers [17,18].

Synthesized copolymers irradiated with gamma rays and alterations to the properties of materials

were investigated.

The Overall hypothesis for this work is to develop environmental friendly sustainable

copolymers of limonene and laccol from citrus waste and lacquer sap via cationic polymerization

49

which possess potent radiation resistivity. These synthesized copolymers will be an alternative to

petroleum based synthetic polymers with appreciable properties and also a promising pathway to

regulate citrus waste. In laccol, there are hydroxyl groups which can behave as radical

scavengers. These hydroxyl groups can convert excitation energy into non-chemistry inducing

energy to enhance the radiation resistivity of lacquer [19,20]. In addition, enhancement of

radiation hardness in nanotubes of organic polymer matrices [21-23] and laccol polymer [14]

after gamma irradiation has shown by Harmon's group. Further, this is the first article

documenting the synthesis of laccol:limonene copolymers which possess radiation hardening.

Irradiation can cause many different alterations to the polymer matrix forming radicals,

anions, cations, gases and other different species. Curing of the polymer matrixes could be

achieved by the radiation exposure due to physical and chemical crosslinks formation. Physical

crosslinks can be formed because of secondary interactions such as ionic and/or hydrophobic

interactions, hydrogen bonding and chain entanglements [24] whereas chemical crosslinks occur

through two adjacent molecules forming a new bond. Formation of three dimensional networks

due to crosslinks could lower the solubility of polymer matrix and make the material harder.

Chain scissions also possible due to radiation exposure which make lesser hardened and

solubilize polymers [25]. These properties make the materials as ideal candidates to be used in

radiation hard environments in industrial setup such as nuclear reactors, medicinal sterilization

plants, commercial irradiators and space operations [26]. Limonene:laccol copolymers

synthesized herein was analyzed as cured copolymers and irradiated copolymers with the help of

IR, NMR, DSC, TGA, Shore Hardness and Swelling analysis.

50

Scheme 3.1: Proposed mechanism for limonene:laccol copolymer formation

51


3.2.1 Materials

High purity food grade D-limonene was provided by Blubonic Industries and Viet Lacquer

Interior Co. Ltd in Vietnam provided the raw lacquer sap. Acetone (CH3COCH3) and

dichloromethane (CH2Cl2) were used as reagent grade solvents. Magnesium Sulphate anhydrous

(MgSO4), granulated anhydrous Calcium Chloride (CaCl2), HPLC grade (99.9%)

Tetrahydrofuran (THF) and sodium hydroxide (NaOH) were purchased from Fischer Scientific.

Aluminum Chloride (AlCl3) reagent plus grade 99%, Toluene (C7H8) assay 99.8% and HPLC

grade Ethyl Acetate (EtOAc) assay 99.9% were purchased from Sigma Aldrich.


Laccol extraction, laccol polymer preparation and initiator preparation were carried out

according to the procedures published in our previous article [14]. High pure food grade d-

limonene was used as received without further purification.

3.2.2.1 Copolymers of limonene and laccol:

According to the monomer ratios illustrated in Table 3.1, copolymers of limonene and laccol

were synthesized. L-10, L-30, L-50, L-70 sample names corresponded to 10%, 30%, 50% and

70% (v/w) limonene in laccol respectively. Typical preparation procedure for L-10 copolymer is

as follows. A 36.0g of laccol extract was measured to a flask containing 3.70 mL of CH2Cl2.

While stirring the reaction mixture in an ice-bath, 4.70 mL of d-limonene was added and also

11.6 mL of initiator complex (~ 1M) was added dropwise afterwards. With time (15 – 30 min.)

52

the reaction mixture became more viscous. The vacuum oven was used to dry the samples for at

60⁰C for 4 days and another 4 days in hot air oven at ~100⁰C. Scheme 3.1 illustrated the

proposed mechanism for limonene-laccol copolymer formation [27,28].

Table 3.1: Limonene and laccol contents in synthesized materials


Identification of polymer formation and its thermal, physical and chemical characteristics were

studied using various characterization methods. Additionally, the effects resulted due to gamma

irradiation was also examined to identify the proper polymer matrices for applications.


Solid films prepared from polymer materials were used with UATR Two spectrometer (Perkin

Elmer) to conduct FTIR study. Parameters were maintained as follows; resolution set at 4 cm-1

,

400 – 4000 cm-1

scan range and 16 average scans for each experiment.


Llimonene:laccol copolymers dissolved in chloroform-d was used for NMR analysis with Varian

INOVA 400 spectrometer having following instrument parameters. The temperatures was

Sample

Name

Laccol

Extract (g)

Limonene

(ml)

Initiator

Complex (ml)

CH2Cl2

Solvent (ml)

Total

Volume (ml)

L-10 36.0 4.70 11.60 3.70 20.0

L-30 28.0 14.20 10.80 5.00 30.0

L-50 20.0 24.00 10.00 6.00 40.0

L-70 12.0 33.30 9.20 7.50 50.0

53

maintained at 298K, spin set and maintained at 20Hz, 16 transients used in block sizes of 8 and

d1 relaxation time 2.000 second.

3.2.3.3 Differential Scanning Colorimetry (DSC)

The Differential Scanning Calorimeter from TA Instruments (model 2920) was used to identify

the glass transition temperature (Tg) of synthesized copolymers. Dry nitrogen gas with a flow

rate of 70 ml/min was purged through the sample cell. Cooling was accomplished with the liquid

nitrogen cooling accessory. For the temperature calibration Indium was used and three different

ramp rates were maintained (10⁰C/min, 20⁰C/min, 30⁰C/min) for proper identification of Tg

range. Samples were first cooled to -50⁰C, heated to +150⁰C, cooled again to -50⁰C and heated

again for second time to +150⁰C (two heating and cooling cycles).

3.2.3.4 Dynamic Mechanical Analysis

The rectangular bars were molded with length 50 mm, width 10.5 mm and thickness 2 mm using

heated Carver® hydraulic press with slow cooling to room temperature under pressure. The

linear viscoelastic region (LVR) was identified through isothermal strain sweep test at -100⁰C

using rheometer (TA Instruments, AR 2000, USA). The 0.3% strain was selected within the

measured LVR region to characterize the samples with a temperature ramp in oscillation mode to

identify the glass transition temperature (Tg). Conditions were maintained for temperature ramp

experiment as -60ºC to 200ºC at 10ºC/min for copolymers while cooling with liquid nitrogen.

In addition, rectangular bars were utilized to conduct frequency sweep experiment for

limonene:laccol copolymers. The parameters were adjusted for the experiment as follows;

54

0.300% strain, frequency range from 0.1 – 10.0 Hz, starting temperature of -15⁰C with 5⁰C

increments each time until the sample collapse. Activation energy associated in the glass

transition region was calculated for each sample using the frequency sweep experimental data.


A 10 mg sample of each copolymer formulations were analyzed using TA Q50 (TGA model).

Weight loss versus temperature scans were recorded at a ramp rate of 10⁰C/min in air from room

temperature (~22⁰C) to 700⁰C.

3.2.3.6 Shore Hardness

The hardness of polymerized samples was measured incorporating ASTM D2240 Shore A

durometer. Discs with diameter 2.54 cm and thickness 0.25 cm were punched from sheets that

were compression molded using a Carver® laboratory press (model C) equipped with heating

elements. Hardness of the samples was measured before and after the γ-radiation treatment

averaging eight indentations per sample.

3.2.3.7 Radiation Studies

The radiation studies were conducted using Co-60 Gamma Irradiator at University of Florida,

Gainesville. The utilized gamma ray source (1.25 MeV) was delivered about 200-250

krad/hour (~30 Gy/min) to 6-8 adjacently placed samples (discs with diameter 2.54 cm and

thickness 0.25 cm) per each exposure. Samples were exposed to 10.0 Mega Rad in air with the

required exposure time of 336 hours.

55

3.2.3.8 Swelling Analysis [29,30]

The sample flasks were prepared with ~30 ml toluene in each and limonene:laccol copolymer

specimens were placed at room temperature (~22⁰C) for 72 h. The exact weight (MI) of those

specimens was determined using a precision balance. After 72 h the solvent was decanted and

liquid solvent adhering to the sample's surface removed by short contact with filter paper. The

weight of the swollen polymer (MII) was determined immediately afterwards. Percent weight

gain was calculated as follows incorporating equation (1).

𝑊𝑒𝑖𝑔ℎ𝑡 𝑔𝑎𝑖𝑛 [%] = (𝑀𝐼𝐼

𝑀𝐼− 1) 100, 𝑀𝐼𝐼 ≥ 𝑀𝐼 (1)


Limonene and laccol copolymerized via cationic polymerization with AlCl3.EtOAc co-initiator

to produce environmental friendly sustainable copolymers which possess radiation hard ability.

Characterization of these copolymers were carried out using IR, NMR, DSC, Rheology, TGA

and various other methods. Samples were reinvestigated after the gamma radiation treatment to

identify the alterations occurred and L-10, L-30 samples illustrated the promising results. All the

samples were further cured due to crosslinks (physical and chemical) after gamma irradiation

and materials were not deteriorated. Obtained results analyzed with further details here after.

3.3.1 FTIR Analysis

Identification of important functional groups was acquired using FTIR for d-limonene and

resulted graph is illustrated in Figure 3.1. Peaks observed in the region of 3080–3020 cm-1

are for

56

stretching of unsaturated C–H group, 2925–2855 cm-1

for stretching of methylene group, 2970

and 2870 cm-1

for stretching of methyl, 1680 cm-1

for tri substituted double bond, 1646 cm-1

for

terminal methylene (–C=C–), 1438 and 1380 cm-1

for methyl and methylene bending, 888 cm-1

for out of plane bending of terminal methylene and 802 (840–800) cm-1

for out of plane bending

of tri substituted double bond [31].

Figure 3.1: IR spectrum of d-limonene with important functional groups

Peaks at 1644 cm-1

and 887 cm-1

which are relevant to terminal C=C bond were completely

utilized and peak at 802 cm-1

for out of plane bending of internal C=C bond was also involved in

the copolymerization process with laccol. A new peak at 1712 cm-1

was appeared which could

attribute as laccol quinones. In laccol monomer, peaks around 987 cm-1

and 968 cm-1

were

consumed during the reaction and this is a clear evident of polymerization which was occurred in

the side chain of laccol monomer. Obtained results were illustrated in Figure 3.2a. The peaks

related to phenyl ring vibrations were observed at 1621 cm-1

, 1596 cm-1

and 1470 cm-1

region.

57

Figure 3.2: IR spectra of (a) d-limonene:laccol copolymers, (b) L-10 (10% limonene in laccol)

before and after irradiation and (c) L-30 (30% limonene in laccol) before and after irradiation

58

The peaks at 988 cm-1

(trans) and 966 cm-1

(trans) were shown the bending vibrations of

conjugated alkene. Also cis alkene could observe in the region 732 cm-1

[13,32,33]. These

observed peaks were altered and/or completely disappeared during the copolymerization process.

According to the results maximum content of limonene in laccol was obtained as 70 weight

percentage (L-70 sample) and this is a promising outcome of the process compare to the reported

literature [17,18].

Samples were reinvestigated after the gamma radiation treatment. The peaks relevant to phenyl

ring –C=C– vibrations (1621 cm-1

, 1596 cm-1

and 1470 cm-1

) were mainly altered after the

gamma radiation treatment as illustrated in Figure 3.2(b and c) due to the formation of dimers

and polymers [13,33]. The peak at 1640 cm-1

shifted to 1710 cm-1

and broadened after gamma

irradiation due to the formation of laccol quinones [13,33].

Peaks at 987 cm-1

and 968 cm-1

were reappeared after radiation treatment. Because of the gamma

radiation feasibility of forming radicals is increased and it could favor the reactions occurring

through phenyl ring of laccol with limonene terminal double bond [13,33] compared to laccol

side chain reactions. According to the obtained results all the samples were cured further because

of radiation. Also physical and small amount of chemical crosslinks involved in the curing

process. Materials were not deteriorated after expose to gamma radiation and promising results

were observed for L-10 and L-30 copolymers.

3.3.2 NMR Analysis

Copolymer formation between d-limonene and laccol via cationic polymerization was further

confirmed using 1H NMR analysis. Cured polymers were hard to dissolve in any organic solvent,

59

therefore partially solubilize samples in deuterated chloroform were analyzed herein. The NMR

spectrum of laccol extract (Figure 3.3a) was used to identify the involvement of functional

groups in the reaction with d-limonene. Peaks appeared in the region of 5.5 to 6.0 ppm denote

the conjugated double bonds (13' – 16') in the laccol side chain. In the region of 5.0 to 5.5 ppm

denotes the cis double bond (10', 11') of the side chain and hydroxyl groups (a) in the phenyl

ring. For d-limonene peaks in the region 4.5 ppm denotes the terminal C=C bond and region 5.0

to 5.5 ppm denotes the internal C=C bond [34].

During the copolymerization process the peaks at 4.1 to 5.0 ppm region (Figure 3.3b) which

represent the terminal C=C bond of d-limonene [34] and peaks at 5.5 – 6.0 ppm region which are

relevant to conjugated double bonds in laccol side chain [35,36] were disappeared. This provides

the clear evidence of cationic polymerization which occurred through these double bonds of

limonene and laccol. Further the peaks relevant to laccol phenyl ring 4–6 were reduced and this

could be due the crosslinking reactions occurred through phenyl ring. New peak appears in the

region 4.0 to 4.3 ppm which is related to –C–O–C– bond formation during the process. The

formation of new –C–O–C– bonds could observe through phenyl ring and also through the side

chain [13] because of the cationic polymerization as illustrated in Scheme 3.1. Further this could

be due to the macrocyclic compound formation [16] as well during the copolymerization of d-

limonene and laccol. After the gamma irradiation; test samples became harder and it was

difficult to dissolve in any solvent to do the NMR analysis for data collection after radiation

treatment.

60

Figure 3.3: NMR spectra of (a) laccol extract and (b) L-30 copolymer, *Acetone

61

3.3.3 DSC Analysis

Glass transition temperature (Tg) of the materials was analyzed using the differential scanning

calorimetry. Tg value is greatly influenced by the tacticity, molecular weight, sample weight,

ramp rate and laboratory processing conditions [37-40]. To identify the proper Tg region three

different ramp rates (10⁰C/min, 20⁰C/min and 30⁰C/min) were used in the analysis and results

obtained for 30⁰C/min ramp rate was illustrated in Figure 3.4. With increment of the ramp rate,

the Tg values are also increased accordingly for each synthesized copolymer (Table 3.2). Neat

laccol polymer (NLP) and d-limonene were used as control samples during the experiment.

Figure 3.4: DSC curves for limonene:laccol copolymers

Data obtained for NLP is broad and rheology is a proper method to analyze Tg for NLP [14]. D-

limonene homo polymer was very brittle and could not use to develop rectangular bars for

62

rheology analysis. Therefore, DSC was utilized to analyze Tg and obtained value was 72.5 ⁰C at

30⁰C/min ramp rate (Table 3.2). L-10, L-30 and L-70 samples were illustrated significant change

in the heat flow inside the temperature region of 5⁰C to 55⁰C which is a broad Tg range. To

further confirm the Tg values of each copolymer and their activation energies; rheology was used

and results were analyzed in detail at section 3.3.4.


Rheometer was utilized for this analysis to identify the glass transition temperature (Tg) where

long chain segments slip at specific temperature region. Tan δ curve obtained from the

experiment of temperature ramp was used to identify the Tg values for each limonene:laccol

copolymer as illustrated in Table 3.2. Tg values obtained for copolymers were reside in the

temperature region of 17.0⁰C to 24.0⁰C with the increasing order of L-30<L-10<L-70<L-50.

According to the results more ordered packing was observed for L-50 with higher Tg value. This

copolymer contains 1:1 monomer ratio from both laccol and limonene. When deviating from the

1:1 monomer ratio, it could possibly effects the physical crosslinking of the materials hence

disturbs the proper packing of the molecules in temporary network.

The observed plateau in the rubbery region specifically for L-10, L-50 and L-70 tan δ curves as

shown in Figure 3.5 was a consequence of long molecules entanglement which led to physical

crosslinks that restrict molecular flow through the formation of temporary networks [41]. At

higher temperatures tan δ curve shows increment, possibly due to the reactions occurring inside

the materials which influenced further curing [11].

63

Table 3.2: Glass transition temperatures and activation energies for synthesized copolymers of

limonene:laccol and neat laccol polymer (NLP)

Sample Name

Tg (⁰C) values at different ramp rate

From DSC analysis From Rheometer

10⁰C/min 20⁰C/min 30⁰C/min Tg (tan δ

curve)

Activation

Energy (kJ)

L-10 14.1 18.1 25.6 20.5 243

L-30 12.6 17.9 20.3 17.1 242

L-50 13.0 34.4 35.4 23.4 258

L-70 14.1 33.0 41.2 21.8 245

NLP 14.6 36.7 45.6 15.4 244

After the gamma irradiation the test samples could not use for DMA analysis due to the fact of

the sample shapes. For the radiation studies disc shaped was utilized per the requirement and for

DMA, rectangular bar shaped was required for better analysis of Tg and activation energy. If the

samples were remolded after the gamma radiation studies, it could cause a different effect due to

temperature increment and this will alter the data. Therefore, alternative test methods were

utilized to support the crosslinking process after gamma radiation as described in sections 3.3.1,

3.3.5, 3.3.6 and 3.3.7.

Frequency sweep experiment was conducted to calculate the activation energy in the region of

glass transition temperature (Table 3.2). Resulting graphs from this experiment were analyzed

via Time-temperature superposition software with shift factor versus temperature and obtained

results were followed WLF behavior. Accordingly activation energies were calculated using the

equation (2) where R is universal gas constant (8.314 J.mol-1

.K-1

), C1, C2 are material constants,

T is temperature given in Kelvin (K) and Ea is the activation energy [42,43]. L-50 sample was

shown the highest activation energy and the least was observed for L-30 sample. For the L-50

sample higher Tg was observed hence more ordered packing and therefore required high

64

activation energy to move long chain segments in the glass transition region. As the control

sample, NLP was shown 244 kJ mol-1

value as the activation energy in the Tg region. Obtained

WLF graphs for L-30 and L-50 were illustrated in Figure 3.6.

∆𝐸𝑎 = (2.303) (𝐶1

𝐶2) 𝑅𝑇2 (2)

Figure 3.5: Tan δ curves for limonene:laccol copolymers

Figure 3.6: WLF graphs for L-30 and L-50

65

3.3.5 TGA Analysis

Weight loss of the materials was identified incorporating dynamic thermogravimeric method

where the sample is heated at a linear rate in predetermined temperature changing environment

[44]. Promising results were obtained for L-10 and L-30 copolymers which were used as

examples to describe the results herein. Figure 3.7 illustrates the TG curves of L-10 and L-30,

before and after the gamma radiation treatment. In the temperature region from 50.00⁰C to

200.0⁰C the weight loss of copolymers L-10 and L-30 was less than 2.000% for both control and

irradiated samples. This is possible due to the removal of small molecules and residual water

[13,35]. From 200.0⁰C to 450.0⁰C temperature region L-10 control sample weight loss

percentage was 61.18% and L-30 was 65.83%. This is possible due to the copolymer

degradation. After the gamma irradiation treatment the weight loss percentages were reduced for

L-10 and L-30 samples with the values of 54.69% and 64.21% respectively. Onset temperatures

for L-10 copolymer before and after irradiation were 389.1⁰C and 380.0⁰C. For L-30 copolymer

onset temperature was increased after the gamma radiation significantly with 14.72⁰C. Before

the radiation treatment the value was 379.0⁰C and after the radiation treatment it was 393.7⁰C.

These observations provide clear evidences for further curing of copolymers due to the gamma

radiation. In addition these copolymers illustrated high thermal stability after radiation treatment

leaving significant amount of residuals. After 600.0⁰C for L-10 control sample has 24.26%

residual and irradiated sample has 26.81% residual. For L-30 sample this was 18.35% (control)

and 19.12% (irradiated sample) after 650.0⁰C. These results further supporting the statement of

curing through crosslinking due to gamma radiation.

66

Figure 3.7: TG curves of L-10 and L-30; before and after gamma irradiation

67

3.3.6 Shore Hardness Analysis

Hardness of the copolymers was measured using Shore A durometer due to the soft nature of the

materials. Scale is driven through 0 – 100 in the durometer and higher numbers depicted greater

resistance to indentation; hence harder materials [45]. The hardness was increased for all the

limonene:laccol copolymers after the gamma irradiation due to further curing through

crosslinking (physical and chemical) compared to control samples according to the results

illustrated in Figure 3.8.

Figure 3.8: Shore A hardness data for synthesized copolymers; before and after irradiation

Increment of the hardness was observed for L-30, L-50 and specifically for L-70 sample which

has 70% of limonene on its weight. This is possibly due to the reactions occurring with the

radiation. Gamma radiation could induce radical formation and other reactions inside the

68

materials which could increase the ability of copolymers to undergo more crosslinking other than

the cationic polymerization. When the amount of limonene is increased these side reactions were

more feasible because of the smaller size of the limonene molecule. Smaller the size, there is a

higher possibility to penetrate the 3D polymer network by limonene to react with laccol phenyl

ring due to radiation treatment according to the results shown in Figure 3.8. Elevated

temperatures also increase the hardness of copolymers.

3.3.7 Swelling Analysis

The swelling properties depend on many factors such as polymer network density, nature of the

solvent and polymer-solvent interactions [46]. Swelling analysis was conducted in toluene

solvent to identify the crosslinking nature of the synthesized limonene:laccol copolymers

[29,30]. Percent weight gain of copolymers was calculated before and after gamma radiation

treatment as illustrated in Figure 3.9. For control samples the percent weight gain remained

mostly in the same range since the only difference in the materials were monomer mixing ratios.

When the crosslinking density increases, free volume inside the polymer network is decreased;

hence percent weight gain is reduced accordingly [47]. According to the obtained results L-10,

L-30 and L-70 copolymers were shown a significant reduction of percent weight gain after

gamma irradiation. This provides a clear evidence of more physical and chemical crosslinks

formation during the radiation treatment. With radiation radical formation is increased and this

could provide more freedom to form crosslinks inside the polymer network which ultimately led

to lesser free volume for solvent uptake. Obtained results depicted this phenomenon for all the

synthesized copolymers after gamma radiation treatment.

69

Figure 3.9: Percent weight gain of copolymers; before and after gamma irradiation

3.4 Conclusions

Synthesizing environmental friendly sustainable limonene and laccol copolymers via cationic

polymerization [48,49] was successfully achieved using AlCl3.EtOAc co-initiator.

Copolymerization was occurred through mainly trans conjugated double bonds in the laccol

monomer side chain and terminal double bond of the limonene. Internal double bond of the

limonene also involved in the reaction for lesser extent compare to major reaction as illustrated

in Scheme 3.1. Alterations occurred to the peaks 1644, 887 cm-1

which are relevant to terminal

C=C bond of limonene, 802 cm-1

for internal C=C bond of limonene, 987 cm-1

, 968 cm-1

vibrations of trans conjugated double bonds in the IR spectra (Figure 3.2) provide evidence for

the successful copolymerization process. NMR data further confirms this fact as shown in Figure

3.3. Limonene was able to copolymerize with laccol up to 70% weight out of total weight is

70

another promising outcome of this study. Glass transition temperatures and their corresponding

activation energies of the Tg region were identified for processing purposes of the materials. IR

data collected after gamma radiation was shown a new peak at 1712 cm-1

for laccol quinones

formation and similar signal pattern with some alterations compare to controls, stating the fact

that the samples were not deteriorated. Thermal stability of L-10 and L-30 copolymers was

increased after gamma irradiation comparative to controls. Significant amount of residual was

observed for L-30 specifically even after the 650.0⁰C. This is a clear evidence of further curing

through physical and chemical crosslinks due to radiation. Similarly shore A hardness was also

increased for all the copolymers after gamma radiation, supporting the fact mentioned

previously. Further, swelling analysis provided the clear evidence of crosslinks increment due to

gamma irradiation by decreasing the percent weight gain. Gamma radiation could produce

radicals which possibly induce more growing points of the polymer network ultimately led to

increase of crosslink density. As per the objective of the project synthesizing environmental

friendly sustainable limonene:laccol copolymers which possess compelling radiation hardening

was achieved. The significance of this study is the development of novel limonene:laccol

copolymers as an alternative to petroleum based synthetic polymers in radiation hard

applications and also provide a reliable solution to citrus waste problem. L-10, L-30 and L-70

samples are the most promising candidates to serve as radiation shield or protective coatings in

the high-tech industrial setup which are dealing with radiation such as nuclear reactors,

medicinal sterilization plants and outer space operations [26].

71

3.5 Acknowledgement

The gamma irradiation experiments were performed by the Professor M. Luis Muga, Chemistry

Department, University of Florida, Gainesville. Authors gratefully acknowledge Professor Muga

for the support provided towards the success of this study.

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48. Kostjuk S. V., Dubovik A. Y., Vasilenkol I. V., Mardykin V. P., Gaponik L. V., Kaputsky F. N.,

Antipin L. M.; 2004, Polymer Bulletin, 52, 227–234, DO1 10.1007/~00289-004-0280-2

49. Kostjuk S. V., Dubovik A. Y., Vasilenko I. V., Frolov A. N., Kaputsky F. N.; 2007, European

Polymer Journal, 43, 968–979, DOI:10.1016/j.eurpolymj.2006.12.011

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CHAPTER FOUR

SYNTHESIS AND CHARACTERIZATION OF NOVEL LACCOL BASED

POLYURETHANES3

Imalka H. A. M. Arachchilagea, Joseph Basi

a, Shahedul Islam

a, Julie P. Harmon

a,*



Abstract

Development of polyurethanes (PU) has come a long way from their origin in 1937 and have

unique applications in a diverse set of fields. Recent PU developments are focusing more on the

naturally derived diols in the synthesis process in an effort to make them more environmentally

friendly. In this study three different diisocyanates (aliphatic, cycloaliphatic and aromatic

diisocyanates) were combined with laccol which extracted from Vietnamese lacquer sap

(Toxicodendron succedanea) to synthesize novel PUs. Influence of the different diisocyanates in

novel PUs, hydrogen bonding capability and crosslinking ability were investigated to provide a

broader characteristic scope for future developments. Resulting materials illustrated good

thermal stability after exposed to higher temperatures and the hydrogen bonding regions

corresponding to N–H (3326 cm-1

) and C=O (1652 cm-1

) groups were shifted to higher

3 This chapter has been previously published in Polym Eng Sci. 2020;1–13. https://doi.org/10.1002/pen.25608, and

has been reproduced with permission from John Wiley and Sons Publishing (Appendix D). Second author Joseph

Basi (undergraduate researcher at USF in 2019) involved in sample preparation and analysis. Third author Shahedul

Islam helped with NMR analysis. Julie P. Harmon served as the corresponding author for this publication.

77

wavenumber according to FT-IR (Fourier Transform Infra-red Spectroscopy) analysis. Further

curing occurred with temperature treatment and improved the overall quality of novel PUs.

Powder X-ray (PXRD) analysis, micro hardness and swelling analysis were utilized to identify

molecular packing and crosslinking effects. Higher crosslink density observed for cycloaliphatic

and aromatic diisocyanate incorporated novel polyurethanes compared to aliphatic diisocyanate

incorporated polyurethane.

Keywords: Polyurethanes, FT-IR, Crosslinking, Hardness, Swelling

4.1 Introduction

Over the years from late 1930 to date, polyurethane (PU) synthesis has developed in

numerous ways with improved properties and material qualities. First discovery of PU was in

1937 by Otto Bayer and his coworkers in Germany forming polyurea from aliphatic diisocyanate

and diamine [1]. In 1952 polyisocyanate became commercially available which lead to vast

development of different polyester-polyisocyanate systems developed by Bayer group. Over time

polyester-polyols were gradually replaced by polyether-polyols due to its advantages; qualities

such as low cost and, ease of handling and improved hydrolytic stability [1,2]. DuPont, BASF

and Dow Chemicals also involved actively in the development process of new PUs. During the

past few decades PU has developed in numerous ways from flexible PU foams to rigid PU

foams. Since 1955, Mobay Cooperation has also presented a variety of monomeric and

polymeric isocyanates, polyethers, polyesters and acrylics to utilize in formulation of PUs [3].

Considering the environmental effect due to chloro-alkenes as blowing agents, development of

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PUs utilizing vegetable oils immerged as a new research area [4,5]. Currently researchers are

allotting significant amount of time, energy and resources to develop environmental friendly,

quality PU designs for applications. Therefore in this study we focused on developing and

characterizing laccol (extracted from Vietnamese lacquer sap) based PUs with different

diisocyanates which can be improved further for different applications.

Urethane linkages are formed through the reaction between alcohol and isocyanate. The

reaction between di-functional alcohol and di-functional isocyanate leads to the formation of PU.

To design PUs with superior qualities such as resistance to abrasion, chemicals, and higher

temperatures, it is necessary to have multiple functional groups in the reacting partners which

can lead to the development of three dimensional crosslinked networks [3]. Hydroxyl groups

containing substances are usually called as polyols and it is one of the major reactants in PU

synthesis. Depending on the molecular weights of polyols, rigidity of the PUs is differentiated.

Rigid PUs are produced from lower molecular weight polyols and high molecular weight long

chain polyols are utilized to synthesize flexible PUs [1]. In this work laccol extracted from

Toxicodendron succedanea (Vietnamese lacquer sap) was utilized as the source of diol which

consists of a C-17 long side chain at its 3rd

position of the ring (Scheme 4.1-Laccol). Because of

the unsaturation present in the side chain, laccol extract usually appeared as a mixture. Most

abundant component (~40%) of the extract is 3-((10Z,13E,15E)-heptadeca-10,13,15-trien-1-

yl)benzene-1,2-diol and the remainder of the extract is composed of similar diols with slight

differences in the saturation of the C-17 side chain [6-8].

As illustrated in Scheme 4.1, aliphatic: 1,6-hexamethylene diisocyanate (HMDI),

cycloaliphatic: 4,4’-dicyclohexylmethane diisocyanate (H12MDI) and aromatic: 4,4’-

diphenylmethane diisocyanate (4,4'-MDI) were utilized in this study to develop novel PUs with

79

laccol. Isocyanate group has cumulative double bonds; R-N=C=O and its reactivity is directed by

the positive character of the carbon atom (Scheme 4.1). If the R group is an aromatic substance,

the negative charge can delocalize and becomes more reactive compared to aliphatic or

cycloaliphatic isocyanates [1,3]. Therefore, aromatic isocyanate based urethane products dry

faster and developed cure properties faster than the aliphatic isocyanate based urethanes. In

contrast, aromatic isocyanate based urethanes can easily oxidize; specially after being exposed to

UV light compared to aliphatic isocyanate based urethanes, hence exterior coatings are

preferably developed using aliphatic isocyanate based urethanes [3].

Organometallic catalysts are widely used to synthesize PUs because of their strong

gelation activity. Generally these catalysts are much more effective in formation of aliphatic

based isocyanate-hydroxyl reaction compared to aromatic based isocyanate-hydroxyl reaction.

Despite of development in various metal based catalysts, inorganic and organic tin compounds

are still commonly involved in the PU synthesis. Most organotin catalysts are sensitive to

moisture and maintaining a moist free environment throughout the reaction process is very

important. In this study Tin(II) 2-ethylhexanoate was used as catalyst to facilitate our PU

synthesis because of its higher activity [9].

Depending on the diisocyanate and the polyol involved, thermal stability and the

crystalline properties of the PUs are differentiated. PUs are thermally stable polymers and some

PUs show higher thermal degradation temperatures above 250⁰C. The onset degradation

temperature of urethane bond depends on the types of used isocyanate used and polyol [10]. In

conventional PUs have hard (HS) and soft (SS) segments. Usually HS contains diisocyanate and

a low molecular weight diol which acts as a chain extender. SS contains hydroxyl terminated

oligomers such as polyesters, polyethers or polycarbonates. Hydrogen bonding domains consist

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within the HS can act as a 'virtual' (physical) crosslinking platform. Because of this,

processability of PUs is greatly increased [2,10-12]. The N–H stretching vibration can be

observed in the IR region of 3500–3200 cm-1

and C=O stretching vibration can be observed in

1750–1600 cm-1

region [12]. Hydrogen bonding related to N–H and C=O could be observed

clearly in these regions for synthesized PUs. Different diisocyanates have different effects to the

morphology of PU. Physical cross links also changes accordingly. Hence, packing of the material

changes and these changes could be observed through Powder X-ray Analysis (PXRD), micro

hardness, and swelling tests. Thermal stability of PUs also changes with varying isocyanates and

this could be analyzed through differential scanning calorimetry (DSC), thermalgravimetry

(TGA) and rheology.

In this paper, novel laccol based PUs prepared with different diisocyanates were analyzed

and characterized. Influence of the cycloaliphatic and aromatic diisocyanate was more prominent

compared to aliphatic diisocyanate for the synthesized PUs. Temperature treatment further

improved the overall material quality by enhancing thermal stability, crosslink density and

hardness. Available long side chain of laccol can facilitate the physical crosslinking and provides

a proper platform for further modifications because of the unsaturation present. Therefore, these

novel PUs could become potent candidates for thermal insulation and energy storing applications

[11,13,14].

81

Scheme 4.1: Generic mechanism for PU formation with diisocyanate [1,3]

82


4.2.1 Materials

The three different diisocyanates namely hexa methylene diisocyanate (HMDI), 4,4'–methylene

bis(cyclohexyl isocyanate) (H12MDI), 4,4'–methylene bis(phenyl isocyanate) (4,4'–MDI), 99.8%

Toluene (C7H8) and Tin(II) 2-ethyl hexanoate (Sn catalyst) were purchased from Sigma Aldrich.

Lacquer sap was imported from Viet Lacquer Interior Co. Ltd in Vietnam. Reagent grade

dichloromethane (CH2Cl2) and acetone (CH3COCH3) were used as solvents in this study.


Laccol was extracted using acetone as described in our previously published article [15]. All the

diisocyanate samples and reagent grade solvents were used as received without further

purification. Generic reaction mechanism for PU synthesizing process with metal ion catalyst

was illustrated in Scheme 4.1 [1,3].

4.2.2.1 Synthesis of laccol and HMDI polyurethane (LH-1):

To a dichloromethane (DCM) 5.0 ml solvent, 10.0 g of laccol was added while stirring in a

~50⁰C water bath. A 5.2 g of HMDI slowly added to the mixture followed by 4 drops of Sn

catalyst and stirring was continued for half an hour. HMDI amount was calculated maintaining

the 1:1 molar ratio between laccol and HMDI. Once the sample gets considerably viscous, it was

collected and left inside the fume hood for one hour. Then the sample was transferred to vacuum

oven at ~60⁰C for 3 days and lastly to the dry oven at 100⁰C for one week (post curing).

83

4.2.2.2 Synthesis of laccol and H12MDI polyurethane (LH-2):

To a DCM 5.0 ml solvent, 10.0 g of laccol was added while stirring in a ~45⁰C water bath. A 7.4

g of H12MDI was slowly added to the mixture followed by 4 drops of Sn catalyst and stirring was

continued for 25 minutes. H12MDI amount was calculated maintaining the 1:1 molar ratio

between laccol and H12MDI. Once the sample gets considerably viscous, it was collected and left

inside the fume hood for one hour. Then the sample was transferred to vacuum oven at ~60⁰C for

3 days and lastly to the dry oven at 100⁰C for one week (post curing).

4.2.2.3 Synthesis of laccol and 4,4'–MDI polyurethane (LH-3):

To a DCM 10.0 ml solvent, 7.45 g of 4,4'–MDI was added and dissolved completely. This

mixture was then added slowly to 10.0 g of laccol while stirring in a ~45⁰C water bath. Four

drops of Sn catalyst was added lastly and stirring was continued for 15 minutes. 4,4'–MDI

amount was calculated maintaining the 1:1 molar ratio between laccol and 4,4'–MDI. Once the

sample gets considerably viscous, it was collected and left inside the fume hood for one hour.

Then the sample was transferred to vacuum oven at ~60⁰C for 3 days and lastly to the dry oven

at 100⁰C for one week (post curing).


Synthesized PUs were characterized using different techniques as described as follows.

Functional group identification, thermal and mechanical properties investigation were conducted

to understand the newly synthesized material behavior.

84


FTIR analysis was conducted on the prepared thin films of PU solid samples utilizing UATR

Two (Perkin Elmer) spectrometer with scan range from 400 cm-1

to 4000 cm-1

, 16 average scans

and at 4 cm-1

resolution.


Prepared laccol based PUs were difficult to dissolve, therefore, partially solubilized samples in

chloroform-d solvent were utilized in the NMR analysis. A Varian INOVA 400 spectrometer

was used having a spin set and maintained at 20 Hz, under 298 K controlled temperature with d1

relaxation time of 2.000 seconds and 16 transients used in block sizes of 8.

4.2.3.3 Differential Scanning Colorimetry (DSC)

Identification of glass transition temperature (Tg) of synthesized PUs was carried out using

Differential Scanning Calorimeter model 2920 of TA Instruments. The sample cell of the

instrument has controlled 70 ml/min dry nitrogen gas flow rate and liquid nitrogen cooling

accessory (LNCA) was coupled to achieve the required low temperatures. Indium was utilized

to temperature calibration of the instrument and three different ramp rates were maintained

(10⁰C/min, 20⁰C/min, 30⁰C/min) to identify the proper Tg region. Prepared PU samples were

first cooled down to -70⁰C, heated up to +300⁰C, again cooled down to -70⁰C and heated for

second time to +300⁰C (two cycles of cooling and heating).

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4.2.3.4 Dynamic Mechanical Analysis [16]

Carver® hydraulic press was utilized to mold synthesized PU samples into rectangular bars

having 10.5 mm width, 50 mm length and 2 mm thickness. This press has a slow cooling rate

from higher temperatures to room temperature with applied pressure. Isothermal strain sweep

experiment was conducted at -50⁰C utilizing AR 2000 rheometer (TA Instruments, USA) to

identify the linear viscoelastic region (LVR) for synthesized PU samples and 3.5% strain was

selected to characterize the samples. The temperature ramp experiment was conducted in

oscillation mode to identify the glass transition temperature (Tg) accurately and used ramp

conditions were -100⁰C to 300⁰C at 10⁰C/min rate with liquid nitrogen for cooling.

Further, using the rectangular bars molded from PU samples, a frequency sweep experiment was

conducted by adjusting the parameters as follows in the rheometer. Frequency range was set

from 0.1 Hz to 10.0 Hz, strained maintained at 3.5% and starting temperatures were changed

accordingly to specific samples providing 10⁰C increments per run until the sample collapse.

Starting temperatures for LH-1 sample was 0⁰C, LH-2 sample was 90⁰C and LH-3 sample was

70⁰C. For each sample, activation energy associated with glass transition region was calculated

using the frequency sweep data using equation (1).

∆𝐸𝑎 = (2.303) (𝐶1

𝐶2) 𝑅𝑇2 (1)

86


Maintaining the temperature ramp rate at 10⁰C/min in air, weight loss with respect to

temperature was measured using TA Q50 model of TGA. The temperature range was set from

room temperature (~22⁰C) to 700⁰C for testing articles.

4.2.3.6 Micro Hardness

Test samples were molded using Carver® laboratory press (model C) having the disc shaped of

2.54 cm width and 0.25 cm thickness. Prepared samples were tested utilizing Leica VMHT MOT

micro hardness tester under room temperature (~22⁰C). Eight readings were collected from both

surfaces of the sample and were averaged to obtain the final value. Indenting load was

maintained at 300 gf with 15 seconds contact time for each PU sample.

4.2.3.7 Powder X-ray Analysis (PXRD)

X-ray diffraction patterns were collected for each PU sample to identify the degree of

crystallization and packing using the Bruker D8 Focus X-ray diffractometer. Data collecting

range was set from 2⁰ to 70⁰ with a step of 0.020⁰ at 25⁰C [17].

4.2.3.8 Swelling Analysis [18,19]

From each PU sample, specimens were obtained having the exact weight (MI) for the swelling

analysis using precision balance. These specimens were immersed in ~30 ml toluene containing

flasks separately for 72 hours at room temperature (~22⁰C). After the required time frame

87

elapsed, remaining toluene solvent was decanted and the adhering solvent to the specimens was

dried by short contact with filter paper. The weight of the swollen PUs (MII) was measured

immediately after the drying process. Following the equation 2 illustrated in here, percent weight

gain was calculated.

𝑊𝑒𝑖𝑔ℎ𝑡 𝑔𝑎𝑖𝑛 [%] = (𝑀𝐼𝐼

𝑀𝐼− 1) 100, 𝑀𝐼𝐼 ≥ 𝑀𝐼 (2)


Synthesized novel laccol based PUs were characterized using FTIR, NMR, DSC, Rheology,

TGA, Micro hardness, PXRD and swelling tests as follows. Results were discussed in detail

hereafter to understand the properties of these developed materials. Resulting materials exhibited

promising attributes because of incorporating different diisocyanates to synthesize laccol based

PUs.

4.3.1 FTIR Analysis

Identification of PU synthesis process and reaction propagation was completed using IR analysis.

According to the results observed as in Figure 4.1, N–H stretching vibrations were visible near

the 3450 – 3440 cm-1

IR region and associated (hydrogen bonded) N–H stretching vibrations

observed around 3300 – 3350 cm-1

region [4,11,12,20-22].

88

Figure 4.1: IR spectra of synthesized LH-1, LH-2 and LH-3 samples

Similarly, non-associated C=O stretching vibration observed around 1720 cm-1

and associated

(hydrogen bonded) C=O stretching vibration observed around 1640 cm-1

IR region. Further the

disappearance of 2270 cm-1

peak related to isocyanate is evident of the complete use of

isocyanate in reaction and successful formation of targeted laccol based PUs namely LH-1, LH-2

and LH-3 [4,11,12,20-22]. In this synthesized PUs possible hydrogen bond formations could be

observed between urethane N–H (proton donor) and urethane C=O or –C–O–C– group (proton

acceptor) [21,23]. Also out of plane stretching vibrations corresponds to C–N bond (amide)

observed around 1520 – 1545 cm-1

IR region [21,22]. As shown in Figure 4.1, these observations

clearly are evidence of the formation of laccol based PUs and hydrogen bond formation in

synthesized PUs.

89

Additionally, the bending vibrations corresponding to trans conjugated C=C and cis C=C which

presented in C-17 side chain of laccol were also observed in the regions of 988, 968 cm-1

and

736 cm-1

[6,24-27]. Availability of this side chain is possibly involved in crosslinking process

because of its active sites (C=C) and length. Physical crosslinking occurs due to the secondary

interactions such as hydrogen bonding, hydrophobic interactions, ionic interactions, π–π stacking

(mainly in aromatic structures) and even entanglement of long chains [28]. Alterations observed

in 988, 968 cm-1

and 736 cm-1

IR regions evident the possible crosslinking in synthesized PUs.

These laccol based novel PUs were treated under higher temperature for molding purposes

(Figure 4.2-b) and molded samples were tested in rheometer (temp ramp experiment) for

characterization purposes (Figure 4.2-a). Alterations occurred because of these temperature

treatments were further studied through IR analysis to better understand the materials. According

to the obtained results as shown in Figure 4.2(a), major differences occurred in IR regions after

rheology (temp ramp) experiment. In this scenario temperature was increased from -100⁰C to

300⁰C with 10⁰C/min increments. When temperature increased the corresponding N–H and C=O

stretching vibration frequencies were shifted to higher frequencies due to the elongation of

hydrogen bonds [12,20,21] for LH-1, LH-2, LH-3 samples and became less well defined as

shown in Figure 4.2(a). After the temp ramp experiment, associated C=O stretching vibration

became more prominent in all samples and more ordered hydrogen bonding observed in LH-3

[12]. Feasibility of forming intermolecular hydrogen bonds is clearly observed in this IR analysis

after rheology experiment. Further, the IR region related to unsaturated laccol side chain

underwent significant changes after rheology experiment and this was possibly due to the

physical crosslinking.

90

Figure 4.2: IR spectra for; (a) Synthesized PUs before & after rheology (temp ramp) experiment,

(b) Synthesized PUs before & after molding

91

In the process of molding, used temperature was ~177⁰C (350⁰F). With respect to observed IR

data in Figure 4.2(b), not much difference in hydrogen bonding was detected before and after

molding. Therefore, this temperature is most appropriate to process synthesized laccol based PUs

according to our observations.

4.3.2 NMR Analysis

Cured PU samples were hardly dissolved in CDCl3. Therefore, partially solubilize PUs in CDCl3

were utilized in proton NMR analysis. According to the results obtained in Figure 4.3, urethane

linkages were formed during the reaction for all synthesized PU samples. At 5.15 ppm region

peak related to –OOCNH– was observed for LH-1, LH-2 and LH-3. For LH-1 sample, –CH2

next to –NH group was observed around 3.3 ppm region and methylene groups denoted as d-g

observed around 1.0 ppm. Signature region for laccol could be seen in the region between 1.0

ppm to 2.5 ppm with some overlapping peaks for PU as well. Unsaturation present in laccol side

chain clearly observed in the region between 5.25 ppm to 5.75 ppm [15,29-31]. The peak related

to –CH2 group present in LH-2 and LH-3 denoted as g and e was observed in the region of 1.0

ppm respectively. The –CH group (in the phenyl ring of 4,4'–MDI) denoted as c and d in LH-3

was observed in 6.6 ppm region with overlapping peaks related to laccol phenyl group. A small

amount of acetone was observed as an impurity in these samples which was used to extract

laccol from Vietnamese lacquer sap. These observations presented in Figure 4.3, are further

evidence of the successful formation of laccol based PUs with different diisocyanate groups.

92

Figure 4.3: NMR spectra of LH-1, LH-2 and LH-3; *Acetone

93

4.3.3 DSC Analysis

Synthesized PUs were analyzed to identify the glass transition temperature (Tg) and all samples

were characterized using three different ramp rates (10⁰C/min, 20⁰C/min, 30⁰C/min) to elucidate

the Tg region. Observed data for LH-1 is more accurately represented compared to other two

materials as illustrated in Figure 4.4 and scale of the graph is manually adjusted from -60⁰C to

100⁰C to avoid disturbances. After the 100⁰C temperature mark, data curves representing LH-2

and LH-3 samples were somewhat distorted and several peaks (broad/narrow) overlapped the Tg

region. No crystallinity peaks were observed for samples, even though the powder X-ray data

show some narrow peaks as described in section 3.7. With the increment of ramp rate, the Tg

region was shifted to higher temperature. This phenomenon was used to identify the proper Tg

region for novel PUs. From the DSC data obtained, proper operating window for temperature

was identified and this was utilized in rheometer to further analyze these PU samples and results

were described in section 4.3.4. LH-1 consists of linear diisocyanate group which can possibly

facilitate less hindrance movement of polymer molecules at low glass transition temperature

(52.51⁰C). Alterations occurred in DSC curves for LH-2 and LH-3 samples (contained cyclic and

aromatic isocyanate groups respectively) in the temperature range of 100⁰C to 300⁰C may

possibly due to the hydrogen bond formation. This possibility could further confirmed from the

IR data illustrated in Figure 4.2. It is stipulated that hydrogen bond formation is an exothermic

process [10,11,21].

94

Figure 4.4: DSC data for LH-1, LH-2 and LH-3 at 30⁰C/min ramp rate


Prepared PU samples were molded to rectangular bars as described in section 4.2.4 and analyzed

utilizing rheometer. Linear viscoelastic region (LVR) was identified for PU samples by

conducting strain sweep experiment and obtained value 3.5% was maintained throughout other

experiments. Using temp ramp experiment Tg values for samples were obtained and

corresponding activation energies at Tg region was calculated with equation 1. Frequency sweep

data obtained for samples were incorporated to Time Temperature Superposition software to

create WLF graph which utilized later to calculate activation energies. In equation (1), C1 and C2

95

represent material constants which obtained from WLF graph, R is universal gas constant (8.314

Jmol-1

K-1

), T is temperature in Kelvin and Ea is activation energy.

Figure 4.5: Temp ramp experimental data for LH-2 and LH-3 to identify Tg

96

Figure 4.6: LH-1 (a) WLF plot created from TTS for activation energy calculation and (b)

Master curve developed from WLF graph

97

Tg values of three samples are arranged in increasing order as follows; LH-1<LH-3<LH-2 and

activation energies are increased as LH-1<LH-2<LH-3 (Table 4.1). Glass transition temperature

(Tg) values obtained for synthesized novel PUs are generally high specifically LH-2 and LH-3

compared to reported literature. This is possible due to the used laccol and the diisocyanate

formulations. Freely available side chain of laccol can possibly enhance the crosslinking which

lead to higher Tg values as a result. Also the increase amount of hydrogen bonding can elevate

the Tg of samples because of the restriction of free molecular movement [11]. Tg for LH-2 and

LH-3 are higher than the LH-1. LH-2 and LH-3 PU samples contain cyclic H12MDI and aromatic

4,4'-MDI diisocyanate groups respectively. According to the results illustrated in Figure 4.5, Tg

for LH-2 is 162.0⁰C and LH-3 is 138.9⁰C. The ring structures present in the diisocyanate groups

of LH-2 and LH-3 contributed to higher Tg values because of the physical crosslinking.

Hydrogen bonding, π – π stacking (aromatic structure) and even long side chain (in laccol)

entanglements as physical crosslinks can involve to increase the Tg value. Hence more energy is

required to initiate a slippage in polymer material and it leads to increase of activation energy

related to Tg. In Figure 4.5, G' is storage modulus, G'' is loss modulus and tan(δ) is loss factor are

plotted as function of temperature. With high temperature treatment (after 200⁰C), tan(δ) curve is

changing in the rubbery region due to the further curing of materials. This phenomenon was

observed for all PU samples. Further curing occurs through crosslink formation at higher

temperatures.

98

Table 4.1: Tg values and corresponding ΔE values obtained from analysis

However, comparing the results illustrated in Table 4.1, Tg of LH-3 is lower than LH-2, but

activation energy of LH-3 is higher than LH-2. This could possibly be due to the formation of

smaller molecular weight polymer molecules in large amounts compared to LH-2. Another

possibility could be due to the repulsion between aromatic ring in 4,4'-MDI and the available

long side chain of laccol, hinders the formation of long polymer molecule. Therefore, the chain

slippage occurs at lower temperature in LH-3, but collectively required higher activation energy

due to large amount for the whole material. Developed WLF curves from time temperature

superposition (TTS) software which utilized to calculate activation energy for LH-1 is illustrated

in Figure 4.6 as a reference. Master curve was also constructed accordingly after the assignment

of WLF curve. For all PU formulations, the obtained graphs have similar pattern as Figure 4.6

and only one formulation is demonstrated (Figure 4.6) here as an example.

4.3.5 TG Analysis

Thermal stability of synthesized laccol based PU materials were analyzed through TGA at

10⁰C/min ramp rate. According to the results illustrated in Figure 4.7, degradation of PU

materials were divided mainly into two stages. In first stage urethane bonds degraded and later

99

the ester groups related to polyol degraded. It is stated that urethane bonds are relatively

thermally unstable [10,22,32-35]. LH-1 has the highest thermal decomposition temperature

(260⁰C) compared to LH-2 and LH-3 (200⁰C and 212⁰C respectively). Linear diisocyanate group

present in LH-1 could possibly facilitate the ordered packing of polymer molecules with physical

crosslinks which can lead to the higher thermal stability overall. According to the results

increasing order of initial degradation temperature for PU materials can arrange as LH-3<LH-

2<LH-1. The amount of residue remained after 600⁰C for LH-1, LH-2 and LH-3 are 15%, 7.5%

and 18% respectively. Highest residue obtained for LH-3 which has an aromatic diisocyanate

groups. Presence of aromatic diisocyanate groups can lead to the possible occurrence of π – π

stacking apart from other physical crosslinks. This may influence the thermal stability of

materials at higher temperatures.

Figure 4.7: TG curves for synthesized LH-1, LH-2 and LH-3 samples

100

4.3.6 Micro hardness

Micro hardness of the synthesized laccol based PUs were analyzed using disc/rectangular shaped

molds. According to the results illustrated in Figure 4.8, hardness increases with the change of

diisocyanate used to synthesize PUs. Highest hardness value (26.55) observed for LH-3

consisting aromatic isocyanate group. Possibility of forming different types of physical

crosslinks such as hydrogen bonding, π – π stacking and chain entanglements could increase

hardness of this LH-3 material. After rheology temp ramp experiment, micro hardness of the

resulting samples was analyzed. Obtained data shows the increment of hardness for all PUs.

These results suggest the formation of more physical crosslinks after temperature increments.

This phenomenon was further supported by the IR analysis data discussed earlier in section 4.3.1

specifically related to Figure 4.2.

Figure 4.8: Micro hardness data for LH-1, LH-2 and LH-3

101

4.3.7 PXRD Analysis

Morphology of synthesized PUs was analyzed using powder X-ray diffraction technique. This

technique determines the long range order of material with respect to short range interactions

[2,11,12,22,32]. According to the results shown in Figure 4.9, all PUs show broad peaks at 2θ

angles indicating some degree of crystallinity [32]. LH-1 shows broad peaks around 21⁰ and 24⁰

compared to LH-2 (18⁰) and LH-3 (19⁰). It is reasonable to stipulate that linear diisocyanate in

LH-1 possibly enhance the more ordered packing compared to other two PU samples.

Synthesized PU samples in this study show more ordered packing as illustrated in Figure 4.9.

This is possibly due to the physical crosslinks such as hydrogen bonding, π – π stacking and long

chain entanglements appeared in the synthesized PU materials.

Figure 4.9: PXRD data for synthesized PU samples (LH-1, LH-2, LH-3)

102

4.3.8 Swelling Analysis

Formation of physical crosslinks enhances the properties of synthesized PUs and this was

analyzed through swelling of the polymers. Equation 2 illustrated in section 2.3.8 utilized to

calculate percent weight gain for each PU material. If the solvent uptake of the PU materials are

higher, this will lead to less crosslinks formation and vice versa [18,19]. According to the

obtained results, LH-1 shows the higher %weight gain (12.96) hence lesser number of crosslinks.

Even this material has the more ordered packing, amount of physical crosslinks present in the

material are lower compared to LH-2 and LH-3. The latter consists of cycloaliphatic and

aromatic diisocyanate groups in the PUs and density of physical crosslinks are higher. Percent

weight gains of LH-2 and LH-3 are 3.18 and 4.70 respectively. Because of the ring structures

present in these materials, π – π stacking and other physical interactions are greatly increased

which influenced the overall material quality.

4.4 Conclusions

Novel laccol based PUs were successfully synthesized and characterized utilizing different

methods to identify their promising properties. The laccol extracted from Vietnamese lacquer sap

acted as the polyol component and reacted with three different diisocyanates (aliphatic,

cycloaliphatic and aromatic) in the presence of Tin(II) 2-ethylhexanoate catalyst. IR data

obtained for resulting PUs clearly indicated the formation of hydrogen bonding in the regions

depicted to N–H and C=O vibrations. With increase of temperature, it shows the peaks related to

N–H (3326 cm-1

) and C=O (1652 cm-1

) shifting to higher wave numbers and molding

temperature (~177⁰C) used in this study does not affect the overall quality of the material. NMR

data also confirmed the formation of urethane linkages and this data is evidence of the successful

103

development of laccol based PUs. Glass transition temperature (Tg) of the PUs were identified

through DSC and rheology analysis. Highest activation energy related to Tg was observed for

LH-3 and physical crosslinks formation mainly contributed to this fact. Ring structures present in

LH-2 and LH-3 enhance the physical crosslinking (hydrogen bonding, π – π stacking and long

side chain entanglements) and this phenomenon confirmed through the analysis of PXRD, micro

hardness and swelling techniques. According to observed results through characterization

techniques, the properties of synthesized laccol based novel PUs were influenced by used

diisocyanates (aliphatic HMDI, cycloaliphatic H12MDI and aromatic 4,4'-MDI) groups and high

temperature treatment. Also the side chain in laccol structure (meta position in ring) more or less

is involved with physical crosslinking and also provides a better platform to further

modifications due to unsaturation presence. Successful formation and characterization of novel

laccol based PUs were achieved in this study. Promising properties observed for these materials

gain qualification as potent candidates of industrial applications such as thermal insulation and

energy storing.

4.5 Acknowledgement

Authors gratefully acknowledge Professor Lukasz Wojtas in Chemistry Department of

University of South Florida, for analyzing powder X-ray data.

4.6 References

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104

2. Das S., Cox D. F., Wilkes G. L., Klinedinst D. B., Yilgor I., Yilgor E., Beyer F. L.; 2007;

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00222340701388805

3. Bayer Material Science LLC; The Chemistry of Polyurethane Coating: A General

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Sablier M.; 2012, Analytica Chimica Acta, 710, 9–16, DOI: 10.1016/j.aca.2011.10.024

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107

CHAPTER FIVE

CONCLUSIONS

Laccol extracted from Vietnamese lacquer sap was successfully polymerized via cationic

polymerization process. The initiator used, AlCl3.EtOAc is identified as most suitable for this

process. Conjugated double bonds present in the laccol side chain mainly involve in the cationic

polymerization process and crosslinks also formed because of available active sites (hydroxyl

groups, cis double bond, reactive protons available in laccol phenyl ring). These crosslinks

promoted to form three dimensional polymer networks which are hard in nature and less soluble.

Obtained IR and NMR data confirmed the formation of these polymers. Laccol based

copolymers were also produced utilizing styrene and d-limonene. Cationic polymerization is

utilized for this process and synthesized materials were high in quality with promising attributes.

Developed laccol based polymers and copolymers were exposed to gamma radiation and studied

their alterations. According to the results after gamma irradiation, materials were further cured

without deteriorating. IR data, rheology data, shore hardness data, PXRD data and swelling

analysis data confirmed the above statement. Hardness of the materials was improved

significantly after gamma irradiation and solubility of materials was reduced accordingly. D-

limonene extracted from orange peels was utilized in this study to produce laccol:d-limonene

copolymers. This is an environmental friendly approach to develop sustainable copolymer

materials. Apart from the above, this attempt could somewhat help to regulate citrus waste

accumulation due to use of waste products in new material synthesis. Developed laccol based

108

polymers and copolymers are prominent candidates to use as protective coatings/shields against

gamma radiation rich environments such as nuclear reactors, outer space operations, medicinal

sterilization plants and commercial irradiators.

Laccol based polyurethanes (PUs) were able to produced successfully with the use of Tin (II)

catalyst. Amide linkages were formed between hydroxyl groups present in laccol and isocyanate

groups and this was confirmed by the results obtained from IR and NMR data. Three different

diisocyanates were utilized in this process to develop polyurethanes and materials were

characterized to identify their properties. Hydrogen bonds formed through N–H and C=O

between polymer molecules were clearly observed in IR data. Physical and chemical crosslinks

were also present in this 3D polymer networks as shown in the results of swelling analysis, micro

hardness and PXRD. Further, neat packing arrangements of polymer molecules were observed

for all PUs with narrow peaks visible in PXRD data compare to amorphous regions obtained for

usual polymers. All the results and observations confirmed the fact that synthesis of laccol based

novel PUs was successful. This novel PUs could be potent candidates to use as thermal insulator

materials and as energy storing materials.

109

APPENDICES

110

Appendix A: USF Fair Use Worksheet for Chapter One

University of South Florida

USF Fair Use Worksheet

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Before you begin your fair use determination, ask yourself the following questions:

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None of these factors are independently determinative of whether or not a use is likely to be

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LeEtta Schmidt, [email protected] and Drew Smith [email protected]

Reviewed by USF General Counsel 08/11/2015

mailto:[email protected]


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INSTRUCTIONS

Check all boxes that apply, and keep a copy of this form for your records. If you have questions,

please contact the USF General Counsel or your USF Tampa Library Copyright Librarian.

Name: Imalka Marasinghe Arachchilage Date: 3/23/2021

Class or Project: PhD Dissertation

Title of Copyrighted Work: Lacquer (https://en.wikipedia.org/wiki/Lacquer)

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Appendix C: License of Reproduced Material in Chapter Three


Jan 17, 2021








Licensed Content Title Environmentally Friendly Radiation Hard d‐

Limonene and Laccol Copolymers Synthesized via

Cationic Copolymerization

Licensed Content Author Imalka H. A. M. Arachchilage, Milly K. Patel,

Joseph Basi, et al

Licensed Content Date Dec 30, 2019





124






polyurethanes; characterization and their

applications





TAMPA, FL 33613

United States



Total 0.00 USD





Wiley Company has exclusive publishing rights in relation to a particular work(collectively


125

agree that the following terms and conditions apply to this transaction (along with the billing and







You are hereby granted a personal, non-exclusive, non-sub licensable (on a stand-alone














permission.

http://myaccount.copyright.com/

126














other person.








license or interest to any trademark, trade name, service mark or other branding

("Marks") of WILEY or its licensors is granted hereunder, and you agree that you shall

127

not assert any such right, license or interest with respect thereto

NEITHER WILEY NOR ITS LICENSORS MAKES ANY WARRANTY

ORREPRESENTATION OF ANY KIND TO YOU OR ANY THIRD PARTY,

EXPRESS, IMPLIED OR STATUTORY, WITH RESPECT TO THE MATERIALS OR

THE ACCURACY OF ANY INFORMATION CONTAINED IN THEMATERIALS,

INCLUDING, WITHOUT LIMITATION, ANY IMPLIEDWARRANTY OF


PARTICULAR PURPOSE, USABILITY,INTEGRATION OR NON-INFRINGEMENT




Agreement by you.











CONTRACT, BREACH OF WARRANTY, TORT, NEGLIGENCE, INFRINGEMENT

128






OF ANY LIMITED REMEDY PROVIDED HEREIN.
















receipt by the CCC.


129






authorized assigns.

















130














not used for commercial purposes. (see below)





below)

131









646-2777.



132

Appendix D: License of Reproduced Material in Chapter Four


Jan 17, 2021








Licensed Content Title Synthesis and characterization of novel laccol based

polyurethanes

Licensed Content Author Imalka H. A. M. Arachchilage, Joseph Basi, Shahedul

Islam, et al

Licensed Content Date Dec 14, 2020






133





polyurethanes; characterization and their applications





TAMPA, FL 33613

United States



Total 0.00 USD





Wiley Company has exclusive publishing rights in relation to a particular work(collectively


agree that the following terms and conditions apply to this transaction(along with the billing and


134






You are hereby granted a personal, non-exclusive, non-sub licensable (on a stand-alone














permission.



http://myaccount.copyright.com/

135












other person.








license or interest to any trademark, trade name, service mark or other branding("Marks")

of WILEY or its licensors is granted hereunder, and you agree that you shall not assert

any such right, license or interest with respect thereto

NEITHER WILEY NOR ITS LICENSORS MAKES ANY WARRANTY OR

136

REPRESENTATION OF ANY KIND TO YOU OR ANY THIRD PARTY, EXPRESS,

IMPLIED OR STATUTORY, WITH RESPECT TO THE MATERIALS OR THE

ACCURACY OF ANY INFORMATION CONTAINED IN THEMATERIALS,

INCLUDING, WITHOUT LIMITATION, ANY IMPLIED WARRANTY OF


PARTICULAR PURPOSE, USABILITY,INTEGRATION OR NON-INFRINGEMENT




Agreement by you.











CONTRACT, BREACH OF WARRANTY, TORT,NEGLIGENCE, INFRINGEMENT



137




OF ANY LIMITED REMEDY PROVIDEDHEREIN.
















receipt by the CCC.




138




authorized assigns.



















139












not used for commercial purposes.(see below)





below)



140







646-2777.



ABOUT THE AUTHOR

Imalka Marasinghe Arachchilage; a Sri Lankan national, obtained the BSc degree in Chemistry

from University of Colombo (UOC), Sri Lanka in 2010. Afterwards, she worked as a research

assistant for more than two years in Center of Analytical Research & Development (CARD) at

Department of Chemistry, UOC. During the time frame from 2014 to July 2015, she worked as a

scientific officer in National Dangerous Drugs Control Board (NDDCB) and as a research officer

in Medical Research Institute (MRI). In August 2015, she came to USA after obtaining an

opportunity to pursue PhD in Chemistry at University of South Florida (USF). Since then she is

staying in Tampa, Florida while working on her degree. She participated several in-campus and

out of campus meetings to present her research data. She is married and have a seven year old

son; Savindu Kankanige who is studying in second grade at Maniscalco Elementary School,

Lutz.

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Imalka Marasinghe Arachchilage