CHAPTER 4

ENCAPSULATION AND CONTROLLED RELEASE IN CROSSLINKED

POLYMERIC MATRICES AND THE SWELLING BEHAVIOUR OF

HOST' - GUEST ASSEMBLIES.

Chapter IV

ENCAPSULATION AND CONTROLLED RELEASE IN CROSSLINKED POLYMERIC MATRICES AND THE

SWELLING BEHAVIOUR OF HOST-GUEST ASSEMBLIES

4.1 Introduction

The concept of 'cavity in solution' has been put forward by Cramer in

1950's during his revolutionary work on inclusion compounds'%.'97. He believed

that molecules of suitable size and geometry can be trapped in these organized

cavities without any chemical bonding. Structure of the inclusion compounds in

the near vicinity of the entrapped molecules is exemplified by the structure of

the solvents in the environment of an ion. Physicochemists were against to this

hypothesis. A start in research of inclusion compounds was made around 1960s

and in the following years many papers were published related to the synthesis

and molecular transformation of diverse compounds which have excellent

applications in the field of agriculture, pharmaceuticals and industry.

The establishment of ;I correlation between the extent of crosslinking

and the amount of the organic substrates encapsulated within the organized

macromolecular assemblies form the subject matter for this chapter. The

encapsulation of benzoic acid in the cavities of styrene based and acrylamide

based copolymers and their cor~trolled release in presence of different solvents

are studied. These systems are cr~mpletely immobilized in the cavities. Moreover

the systems can be suggested as alternatives to chemically hctionalized polymers.

The introduction of a low-molecular weight functional species into the polymeric

backbone is usually carried out thrwgh a series of polymer-analogous reactions,

which is a laborious and time- c:or~suming process. The loading of the required

functional group might be seriously affected by this prolonged treatment even

though the initial loading capaclty of the resin is high.

The problems can be overcome if the required low molecular weight

species can be introduced directly into the polymer matrix during the process of

polymerisation as a guest molecul~:. The encapsulation of low molecular weight

organic molecules in the cavities of the three dimensional polymeric networks

without any chemical bonding could lead to the low molecular weight properties

for these molecule^^^^^^^^. At the same time the resulting polymer will have

physical properties typical of a functionalized polymer.

Crosslinked polymer colisist of infinite networks in which linear chains

are interconnected by the bifunctional monomer. In the case of styrene based

copolymer, inner spaces or cavities of definite sizes are produced during

polymerization process depending on the nature of the crosslinking agent. For

DVB crosslinked polystyrenes, these cavities have a hydrophobic environment.

Molecules can be trapped in these 'pockets' without recourse to chemical bonding

(Scheme 4.1). The method can be used for the functionalization of polymer if

the size and geometry of the guest molecules are acceptable to the geometry of

the cavities. Hexane diol dimethncrylate are also used as the crosslinking agents

in the process of encapsulation.

Monomer + benzoic acid Crosslinker -------+ ,

Scheme 4.1 Preparation of encapsulated benzoic acid.

For DVB and HDDMA crosslinked polyacrylamides the cavities have

a hydrophilic environment than styrene based polymers. By choosing these two

different polymeric support for the study of encapsulation, an understanding of

the effect of nature of monomers and crosslinking agent on the cavity size, and

hence extent of encapsulation c.m be obtained. The amount of the guest

encapsulated in the host guest system was analysed by chemical method.

By Flory-Rehner analys~s of the swelling data of the encapsulated

resin, &was calculated. These results are compared with that of the free

polymers and a better interpretatic~n of encapsulation was achieved.

4.2. The Concept of Encapsulation and Controlled Release - Theory

Microcapsules consistir~g of a core (liquid or solid) and a permeable

or non permeable wall have been widely used in release and transfer c o n t r 0 1 ~ ~ ~ ~ ~ ~ .

The capsule core may contain active compounds such as catalysts, drugs or

antifouling compounds which arc: surrounded by a suitable wall material that

regulates release. The controlled n:lease of the capsule contents strongly depends

on capsule wall thickness and porosity.

A major proportion of the published scientific and technical literature

on micro encapsulation relates lo pharmaceutical and food applications but is

often relevant to the design of other micro encapsulation process. Micro

encapsulation may be intended 10

(1) The production of a nc'vel product - the novelty may be actual, or per-

ceived.

(2) The protection of the c:nvironment from the product (where the active

core material is hazardous or toxic)

(3 Control of the rate oi' release of the core material either by 'cata-

strophic failure' by rupture of the polymer wall for timed release or by

long - acting 'sustainc:d' release (eg. by solution or diffusion)

(4) Masking the undesired properties of the active component-eg. 'odour

or taste masking' or masking the chemical properties (pH or catalytic

activity)

( 5 ) Separation of components - allowing control of the incompatibility of

components.

(6) Formation of solid systems - conversion of liquid components to free

flowing powders.

(7) Targeting of the site of release of active material (notably for pharma-

ceutically active materials.)

The properties of the microcapsules formed are governed by

(i) The nature of the polymer wall

(ii) The method of Cormation

(iii) The microcapsule wall thickness and

(iv) The integrity oi'the microcapsules.

The design of self assembling structures that contain a cavity capable

of encapsulating one or more guest molecules has attracted great attention recently

because of their potential applications including use as drug delivery devices or

as miniature reaction chambers.

A 'macroporous' copol~mer refer to materials prepared in the presence

of a pore forming agent (diluent) and having a dry porosity, characterised by a

lower density of the network due: to the voids than that of the matrix polymer.

Crosslinked copolymers prepared by FCC exhibit different structures and

properties depending on the amounts of the crosslinker and the diluent present

during reactions as well as on the: solvating power of diluent.

The total volume of the: pores inside a crosslinked polymer as well as

their size distribution can be varied by changing the independent variables of

synthesis. The main experimental parameters are the type and amount of diluent,

crosslinker concentration and polymerization temperature and the type of initiator.

The difference betweell the solubility parameters of the diluent (6,)

and the copolymer 6, ie. (6,-6,) or its square (15,-6,)~ are generally used to

represent the solvating power of a diluent in a network formation system.

According to Hildebrand theory. the solubility of a polymer in a solvent is

favoured when (6,-62)2 is minimized.

By introducing an inert diluent (a solvent or a non solvent) together

with monomers, porous structurc:~ within the particles may be obtained upon the

removal of the diluent after polymerization, (by drying)

4.3. Guest-Encapsulated Polyacrylamide Hydrogels

Molecules can interaci. with other molecules through weak interaction

(0.1 - 5 Kcal/mole) such as hydrogen bonding, van der Waals or dispersive forces

which are collectively known as non covalent interactions. More than 30 years

of research in the field of non-covalent interactions shows that this phenomenon

has an enormous potential for the construction of chemical structures exhibiting

a high degree of structural complr:xity. In the present study, we prepared a novel

host guest assembly by encapsulating benzoic acid in the cavities of

polyacrylamide hydrogel. For the preparation of hydrogel molecular design of

the building blocks is an esser~tial element in the successful formation of

thermodynamically stable non-covalent capsules. Two crosslinking agents are

used for the preparation of the h:ydrogels, one is the flexible HDDMA and the

other is comparatively rigid DVEI.

4.3.1 Benzoic Acid - Encapsulated DVB Crosslinked Polyacrylamides

Benzoic acid encapsulated polyacrylamides crosslinked with DVB (5,

10, 15 and 20 mol %) were prepared by solution polymerisation using benzoyl

peroxide as the initiator. The initiator and benzoic acid (1: 1 mole ratio with the

monomer) was dissolved in methanol - water mixture (1:2 vlv) by heating on a

water bath at 80°C. Acrylamide and crosslinking agents were added to the mixture

simultaneously, heated with stirring (80°C) till the polymerisation was completed.

The guest-encapsulated polymer was then washed with water and methanol and

dried at 80°C. The product was characterised by IR spectral analysis. The initial

concentration of the monomers, crosslinking agents and the yield obtained are

listed in table 4.1

Table4.1 Preparat ion of Guest Encapsulated DVB Crosslinked

Several studies showeci that the hydrogel structure, and thus the hydrogel

properties strongly depend on '.he initial degree of dilution of the monomers.

The diluent which is a solvent (or a nonsolvent), present in the reaction mixture

act as pore forming agent and plays an important role in the design of the pore

structure of these crosslinked materials. As the amount of solvent increases, the

network structure becomes moi-e and more flexible. A continuous network is

not formed above a critical amount of solvent. The optimum concentration of

the solvent for this system was found to be 2: 1 v/v ofwater and methanol. At this

particular concentration voids formed may have appropriate size so that benzoic

acid molecules can be successfully entrapped.

Benzoic acid was strorigly entrapped in the networks of AA-DVB resins

with 5,10,15,20 mole present crosslink densities. The foreign molecules with

suitable molecular dimensions are entrapped in the well defined cavities of the

polymer matrix. These cavities are designed by the three dimensional

arrangement of the structural units in the polymer systems.

IR Spectra

Polyacrylamide hydl-ogels with benzoic acid guest moieties were

~ol~acrylamide

Crosslinking (mole %)

5

DVB (g)

0.52

Yield (g)

2.536

Acrylamide(g) -

2.698

Benzoic Acid(g)

4.636

characterized by IR analaysis The strong absorption band obtained at

1692.82 cm'l was assigned to th(: C = 0 stretching vibration of the guest acid.

The shoulder at 1653.64 cm-' corresponds to C = 0 stretching vibration of the

amide group. In the free polymer there was only one peak at 1670 cm-l(fig. 3.1).

A strong broad band at 3436.46 cin-' with a shoulder arises due to the merging of

amide N-H and 0 - H stretching vibration of the carboxylic function of benzoic

acid. In the free polymer there was only one peak around 3350 - 3400 cm-I.

brave number cm-I

Fig. 4.1 IR spectrum of benzoic acid encapsulated AA-D VB copolymer.

Scanning Electron Micrographs

Fig.4.2 shows the scanning electron micrographs of AA-DVB(IO%)

copolymer and its benzoic acid encapsulated counterpart respectively. The

surface of the crosslinked copolymer is rough with a wrinkling effect. This

shows the presence of empty space or cavities within the system. The cavities

were disappeared during encapsulation. In contrast to corrugated surface of

AA-DVB copolymer(fig. 4.2.a.), the encapsulated system shows a relatit

smooth surface(fig. 4.2.b.)

Fig. 4 .2 Scanning electron micrographs of (a) AA-D VB Copolymer (6) Benzoic acid encapsulated AA-DVB copolymer

'the

{el y

4.3.1.a Release of Benzoic Acid from Encapsulated DVB Crosslinked PA

The morphology of the polymer like pore size and pore geometry are

sensitively dependent on the polymerization conditions. With the variations in

the temperature, rate of stirring and the distribution of the monomers in the

polymerising medium the polymer produced are of variable morphological

characteristics. From release shtdies it is possible to determine the most suitable

crosslinking for encapsulation.

As the encapsulant is not covalently attached to network, we should

find out the conditions under which the release of embedded molecule from the

gel becomes negligibly slow. In addition a study of the kinetics of the release of

the encapsulant could provide useful information about the interaction of the gel

with the encapsulant during reptational diffusion as well as about the structure

of the gel.

The effects of chemical structure and monomer architecture of the host,

guest and host - guest complex on the time dependent release of the guest from

the well-defined cavities of hosi: were studied.

The benzoic acid enc:apsulated DVB crosslinked polyacrylamide

hydrogel was allowed to swell in solvents (chloroform, toluene, methanol and

water) The temperature was 1owt:red to OOc to prevent hydrolysis of amide linkage

in the polymer network. The released guest was estimated by suitable chemical

method. Benzoic acid was estimated by titrimetric methods.

The release of benzoic: acid is maximum in toluene and decreases in

the order chloroform -+ water .+ methanol for DVB crosslinked polymer. (Table

4.2 and fig 4.3).

Table 4.2. Rate of Release of Benzoic Acid from Guest Encapsulated DVB Crosslinked Polyacrylamide

I I Weight of benzoic acid (g) released per g. of the polymer 1 (M.1ute-9

1 5

10

15

30

45

60

Solvent C,H,-CH, Solvent CHCI,

Percentage of crosslinking

5%

1 .2392

,2679

.3 109

,3731

,4688

,4688

Solvent H,O

Percentage of crosslinking

Solvent CH,OH

5%

1.2057 1

,2105

,2870

,3205

,4353

,4353 I

10%

1.0903

,1003

,1204

,1254

,2006

.2006

Percentage of crosslinking P m t a g e of crosslinking

15%

1.1626

,2583

,263 1

,3253

,4209

,4209

20% 5%

1.1627

,2153

,2440

,2727

,2966

,2966

20%

1.0614

,0818

,0818

,0930

,1968

,1968

10% 5%

,1722 1

,2201

,2918

,3396

,4066

,4066 I

15%

.I431 1

,1873

,2064

,2352

,2965

,2965

15% 10%

.0702 1

,0753

,0853

,1355

,1706

.I706

20%

,0758

,0960

,1061

,1364

,1415

,1415

10%

1.0716 1

,0870

,0953

,1022

,1022

,1022

,0953 1

,1003

.I054

,1204

,1806

,1806

15%

,1483

,1579

,1674

,1961

,2248

,2248

.I789 1.0526 1

20%

1.1 121 1

,1292

,1332

,1348

,1365

,1365

,2096

,2454

,2628

,3013

,3013

.0670

,1005

,1238

,1435

,1435

Figure 4.3. Rate of release of ~ienzoic acid from benzoic acid encapsulated DVB crosslinked polyacrylamide. X axis represents time in min- utes, and the Y uxrs represents weight of benzoic acid in grams released per gram (>f the polymer

The release was over by 45 minuets for all the resins. The degree of

crosslinking, the concentration of the monomer (acrylamide) and the nature of

solvent (size of the molecule and polarity) influence the release of guest molecule

from host - guest complex. Release studies could be reasonably explained only

when the different interaction energies and interacting surface of crosslinks and

chain segments were taken in to account.

Eventhough swelling is higher in water than in toluene or chloroform,

the release is higher in toluene (fig,. 4.3). Solubility parameter is auseful quantity

for characterisation of strength oi'interaction in polymer solvent systems. The

various types of forces existing bztween polymer segments and solvents can be

obtained from three dimensional solubility parameter concept. Thus for toluene

and CHCl, dispersion force is higher (table 3.15) and polar and hydrogen bonding

interactions are negligible.

According to Quinn et a1.20' some very peculiar complicating factors

may affect the usual behaviour of hydrogels. Among them we have the following.

I. Water may act as a plasticizer or anti-plasticizer depending on con-

centration, temperature and pH. . . 11. The structural organisa~:ion of absorbed water is sensitive to polymer

mobility. . . . 111. Polymer conformational changes can accompany hydration and

N. The presence of third component such as a salt can alter the way in

which it behaves.

Thus water and methanol may not get penetrate into the cavities but

they are absorbed by means of H-bonds and polar forces. But toluene and CHCl,

can selectively penetrate into cavities by means of dispersion forces and the

release of benzoic acid is higher in these solvents. Also the mesh size and size

of solvent molecules play an important role. Aromatic solvents and guest

molecules are of comparable size and can easily displace benzoic acid molecules.

Another interesting feature observed is that the amount of benzoic acid

released (and hence encapsulated) is higher for 15 mole% than 10 mole% and

then decreases in 20 mole% sample in presence of all the four solvents

investigated. (table 4.3.& fig.4.4). These phenomena can be best explained on

the basis of Flory Rehner analysis of the swelling data of the free polymer and

guest encapsulated polymer systems(section 4.3.1 .c.).

Table4.3. Weight of Benzoic Acid Released from guest encapsulated AA-DVB Copolymer with Different Crosslink Densities. - - -- --

+Toluene t (:hlorofon -A- Water t Meltland 1

Crosslining mole %

5

10

15

20

Fig. 4.4 Weight of benzoic acid released from AA-DVB copolymer with dif- ferent crosslink den:iities

Wt. of benzoic acid released per g. of polymer.

Toluene

.4688

.2006

.4209

.I968

CHCI,

,4353

.I806

.3013

.I435

yo .4066

.I706

.2965

.I415

CYOH

.2966

,1022

,2248

,1365

For DVB crosslinked polyacrylamide, the molecular weight between

crosslinks follows the same pattern as that of the amount of benzoic acid

encapsulated in differently crosslillked resins (fig. 4.4 and 4.5) confirming the -.

theoretical validity of the encapsulation studies. As MC 1s higher for 15% than

10% for the free polymer the amount of benzoic acid encapsulated also is

higher for 15% than 10% crosslinked polymer. It also proves that the formation

mechanism for both the free polymer and encapsulated polymer are same .

4.3.1.b. Swelling Behaviour of G uest -Encapsulated DVB Crosslinked Poly-

acrylamide.

The phenomenon of swel ling of the gel has been the subject of numerous

studies in polymer physics. Controlling the swelling ratio, diffusion rate and

mechanical properties of a crosslir~ked polymer is important in hydrogel design

for biomedical applications. Each of these factors depends strongly on the degree

of crosslinking. It has been demonstrated that minute changes in external

conditions such as temperature, solvent composition, ionic strength and external

electric field can induce drastic changes in the state of the swollen network.

The swelling capacity of the gel samples were measured in various

solvents and solvent mixture by gravimetric technique (table 4.4). The swelling

of the host - guest complex in a series of solvents is lesser when compared to the

swelling of free polymer (table 3.6) as expected. As some of the pores are

occupied by the encapsulant, the intake of the solvents are considerably decreased

in host-guest complex.

Table4.4 Swelling Behaviour of Benzoic Acid Encapsulated DVB Crosslinked Polyacrylamide Hydrogels

I I I Mass of swollen polymer of various crosslink densities

2. Chloroform I++

Mass of dry polymer = 0.2 g

4.3.l.c. Molecular Weight Between Crosslinks for the Guest Encapsulated AA-DVB Copolymer

Analysis of the swelling behaviour of the benzoic acid encapsulated

polymer was done on a firm tht:oretical basis of Flory-Rehner theory (table

4.6). Density of the polymer was determined by pycnometric method. To an

approximation, polymer solvent interaction parameter for the guest- encapsulated -

polymer were taken to be same as that of the free polymer. Mc is lower for

host-guest system than the free polymer as expected. Molecular weight between

crosslinks were calculated for each polymer and the result show that for the

guest - encapsulated DVB crosslinl<ed PA is higher for 10 mole % than for 5

mole % (table 4.5 and fig. 4.5).

This is due to the fact that the amount of benzoic acid encapsulated in -. 10 mole % polymer is very much lower than 5 mole % (fig. 4.4) and hence MC is

higher for 10 mole % than 5 mole %.

Table 4.5 Molecular Weight Between Crosslinks of DVB Crosslinked Poly- acrylamide Hydrogels and Host-Guest System.

U

20000 - 0 "O~E-;'~ 5' 0 0 5 10 15 20 25

Crossllnklng

Crosslinking

(mole%)

5

10

15

20

Fig. 4.5 Molecular Weight Between Crosslinks for AA-DVB polymer and Encapsulated Copolymer

Mol~zcular weight between crosslinks

Flee Polymer

1 0936 x 10'

5 9587 x lo4

8.5077 x lo4

3.5836 x lo4

Host-guest system

1.58 101

1.3426 x lo4

8.5858 x lo3

1.3214 x lo3

4.3.2 Guest - Encapsulated Hexane Diol Dimethacrylate

Crosslinked Polyacrylamide

The effect of nature of the crosslinking agents on the encapsulation

behaviour of polyacrylamide hydrogels was studied by using a more hydrophilic,

flexible crosslinking agent hexarie diol dimethacrylate.

Benzoic acid encapsulated, hexane diol dimethacrylate crosslinked

polyacrylamide hydrogels with !;,lO,l5 and 20 mole percent crosslink densities

were prepared by solution poly~nerisation in water-methanol mixture (2: 1 v/v

ratio) at 80°C. Potassium persulphate was used as the initiator. The resin was

freed from impurities by washing with water and methanol and was characterized

by spectral analysis. The results are given in table 4.6.

Table 4.6. Preparation of Guest Encapsulated HDDMA Crosslinked Poly-

IR spectra

IR spectrum of benz3ic acid encapsulated HDDMA crosslinked

acrylamide.

polyacrylamide shows a strong absorption peak at 1659.67 cm-' due to amide

carbonyl stretching coupled with carbonyl stretching of ester linkage of the

Yield (g)

2.9560

2.4276

2.7120

3.0700

Crosslinking (mol %)

5

10

15

20

crosslinking agent. The peak at (shoulder) 1724.57 cm-' is assigned due to the

Weight of monomers(g)

HDDMA

0.452

0.904

1.356

1.808

Acrylamide

2.698

2.556

2.414

2.264

Benzoic acid

4.636

4.392

4.148

3.904

carbonyl stretching of acid group of the guest moiety. This was not present in

the free polymer. (Fig. 3.4). A strong broad band at 3429.83 cm-I with a shoulder

is due to merging of amide N-H smd 0-H stretching vibration of the carboxylic

function of benzoic acid.

Fig.4.6 ZR spectrum of benzoic acid encapsulated HDDMA crosslinked

polyacrylamide.

Scanning Electron micrographs

Scanning electron micrographs of the free polymer and the encapsulated

system were recorded and compared. Benzoic acid molecules are entrapped in

the well defined cavities of AA-HDDMA copolymer. The surface properties of

the polymer and encapsulated system are different. The benzoic acid encapsulated

HDDMA crosslinked polyacrylaaide hydrogel showed a regular and smooth

surface. ( fig. 4.7(a)). The surface the free polymer is relatively rough due to

cavities (fig. 4.7.(b)).

Fig 4.7 Scanning eleclron Micrograph of (a) Benzoic acid encapsulated HDDMA crosslinked polyacrylamide. (b) HDDMA crosslinked polyacrlamide

4.3.2.a Release Studies of Guest Encapsulated HDDMA Crosslinked Poly-

acrylamide.

The stability of the encapsulated system is determined by the molecular

size, charge and geometry of the guest molecules and also by the cavity dimensions

within the polymer networks. Tlie amount of benzoic acid encapsulated within

the cavities of polymeric networks are estimated from release experiments.

The benzoic acid encal)sulated HDDMA crosslinked polyacrylarnide

was allowed to swell in presence of different solvents by stirring the solution at

a temperature of 0°C. The amouilt of acid released was estimated titrimetrically

at definite time intervals. The results are tabulated in Table 4.7 and fig 4.8.

Table4.8 Rate of Release of Benzoic Acid from Guest Encapsulated HDDMA Crosslinked Polyacrylamide

T I mutes)

5

10

15 - 30

4 5

60

Weight of benzoic acid (g) released per g. of the polymer

Solvent CH,OH

Percentage of crosslhkmg

Solvent CHCI,

P m t a g e of crosslinking

5%

,1744

,1876

,2395

,2483

,2509

20%

. U O ~ Y I

.0783

,0935

,1011

.lo54

,1054

5%

.UbXU

,1485

,1490

.I497

,1622

.I632

Solvent C,H,-CH,

Percentage of crosslinking

Solvent H,O

Percentage of crosslinking

,2509

10%

,1668

,1830

,2206

,2374

,2988

5%

,1403

.I806

,2014

,2353

.2408

5%

,1164

,1958

,2227

.2275

.2308

,2408

10%

. IL>lc

,1667

,1739

,1752

.I856

,1856

15%

. lUUX

.I114

,1167

,1273

,1326

,1326 ,3044

15%

,1164

,1232

,1368

,1585

. I738

10%

,1458

,1632

,1858

,2116

,2502

10%

,1250

.I406

,1771

,2139

.2408

.I659

20%

.I238

,1342

,1388

,1432

,1599

.I104 ,1738

15%

.I215

.I241

.I388

,1532

,1687

15%

.118U

,1238

,1399

,1507

,1659

20%

.0983

,1085

.I111

,1239

,1355

20%

.U76Y

,0935

,0992

,1092

. I 104

,1599 1.2481 ,2505 .I687 ,1355 1.2308

+ CHCI3 -m- Toluene -+Water t C H 3 0 H

Fig. 4.8. Rate of release of benzoic acid from benzoic acid encapsulated HDDMA crosslinked polyacrylamides (Xaxis represents Time in min. Y axis represents Wt. of benzoic acid (g;) released per g of the polymer.

The rate of release of guest from host-guest complex follow the first

order kinetics. The release of benzoic acid from the cavities of HDDMA

crosslinked PA depends on the interaction between solvent and polymer as well

as on the interaction energies of encapsulant and polymer-segments.

The release is maximum in CHC1, and decreases in the order toluene

+ water + methanol (fig.4.8). An interesting feature which is observed is that

the swelling is minimum in CHC:l, and toluene, for this host-guest system when

compared to the swelling in aqueous medium. Hence increased rate and amount

of release of benzoic acid in CHC'I, is due to high dispersion force acting between

polymer and solvent which enables the solvent to penetrate into the cavities of

polymer matrix. The polar and hydrogen bonding effect is slightly higher for

CHCI, than toluene(tab1e 3.15). Due to polar and hydrophilic nature of HDDMA

than DVB, the release is higher in CHCI, for guest encapsulated HDDMA

crosslinked polyacrylamide. The release is higher in toluene for guest

encapsulated DVB crosslinked pc~lyacrylamide. As crosslink density is increased

the pore dimensions are decreased and the amount of benzoic acid encapsulated

gets decreased correspondingly.

However, the amount of benzoic acid released and hence encapsulated

in 10 mole % guest encapsulated HDDMA crosslinked PA is higher than in 5%

guest encapsulated polymer and decreases in the order 15 mole % to 20 mole %

for all the solvents investigated ie chloroform, toluene, water and methanol (table

4.8 and fig. 4.9).

Table4.8. Weight of Benzoic Acid Released from Guest Encapsulated AA-HDDMA Copolymer with Different Crosslink Densities

Crosslinking (mole %)

Crosslining mole %

5

10

15

20

Fig. 4.9. Amount of benzoic acid released from guest encapsulated AA- HDDMA copolymer with different crosslink densities.

This phenomena can be explained on the basis of swelling analysis of

Wt. of berizoic acid(g) released per g. of polymer.

the free polymer and host-guest system by Flory-Rehner theory (section 4.3.2.c.).

Molecular weight between crosslinks for the HDDMA crosslinked PA follows

CHCl,

,2509

,3044

,1738

,1599

the same pattern; M, for the systein decreases in the order 10% >5%>15%>20%.

Y O

.2308

.2408

.I659

.I 104

Toluene

.248 1

.2505

,1687

,1355

C y o H

,1632

.I856

1326

,1054

Since M, is higher for 10% than "% the amount of benzoic acid encapsulated in

10% crosslinked polymer is higher than 5% crosslinked polymer.

4.3.2.b Swelling Behaviour of Benzoic Acid Encapsulated HDDMA

Crosslinked Polyacrylamide

Swelling studies were camed out in different solvents for the guest

encapsulated HDDMA crosslinked polyacrylamide hydrogels by gravimetric

technique and the results are given in table 4.9.

Table4.9. Swelling Behaviour of Benzoic Acid encapsulated HDDMA crosslinked polyac~ylamide Hydrogels

I I I Mass of swollen polymer of various I 1% I Solvent I crosslink densities

I 1 I

1 .2000 1 .2OOOI 2100 3. I Toluene

The swelling is lesser i n Host-guest system than the free polymer. Also

the swelling in guest - encapsulated HDDMA crosslinked polyacrylamide is

lesser than guest - encapsulated DVB- crosslinked polyacrylamides for the same

crosslink density. This indicate:; that the pore dimensions of guest encapsulated

DVB crosslinked polyacrylamide are higher than guest encapsulated HDDMA

crosslinked polyacrylamide. Also from the release studies it is clear that the

amount of benzoic acid encapsulated in guest-encapsulated DVB crosslinked

polyacrylamide is higher than in guest encapsulated HDDMA-crosslinked

acrylamide. Thus the mesh widths are suitable for DVB crosslinked polymer to

incorporate benzoic acid molecule. K.J. Shea eta1 reported that non porous

materials are resulted form p~~ymerisations with crosslinkers that contained

flexible tethers and crosslinkers with rigid hydrocarbon or aromatic tethers give

rise to macroporous materialsa0 for the polymerisation of N-methyl acrylamide

and N-methyl methacrylamide.

4.3.2.c Molecular weight between crosslinks for guest encapsulated HDDMA crosslinked polyacryl:~mide

Analysis of the swellii~g results for the system within the frame work

of Flory -Rehner hypothesis gives the theoretical validity for the experimental

results of encapsulation phenomena. Molecular weight between crosslinks for

the free polymer and guest encapsulated system are given in table 4.10 and fig.4.10.

Table4.10. Molecular Weight between Crosslinks for the HDDMA Crosslinked Polyacrylamide and the Host-Guest System.

Crosslinking I !rilolecular weight between crosslinks I (mole %)

5 2.338 x lo5 1.0213 x 104

10

300000 250000rL] 2 0 m w : ; ;Fq IS ,,, -

1 ooom -

50000

0

5 10 15 20 5 10 15 20

Crosslinking (mole%) (a)

Crosslinking (mole%) @)

Fig. 4.10. Molecular weight between crosslinks for (a) HDDMA crosslinked polyacrylamides (b) guest encapsulated system.

Since there is a phase! change occurring at 10 mole % HDDMA

crosslinked polyacrylamide in presence of solvent (H,O: methanol mixture) - Mc decreases in the order 10% > 5% >l5% > 20%. The amount of benzoic acid

encapsulated follows the same pattern (fig. 4.9). As =of free polymer

decreases the amount of benzoic acid encapsulated also decreases.

The for guest encay~sulated HDDMA crosslinked PA decreases in

the order 5 mole % > 10 mole % :> 15 mole % > 20 mole % as expected. As the

percentage of crosslinking agent increases the crosslink density increases and

k decreases as expected.

4.4 Guest Systems Encapsulated in Polystyrene Networks

The molecular character and extent of crosslinking and other important

determinants of macromolecular structure like the structure and chemical nature

of the monomers, the polymerizat~ on method and the variables of polymerisation

can be varied systematically to provide model macromolecular systems of diverse

physical properties. The rigidity, swellability, mechanical stability etc. are

important factors when we design a macromolecular matrix.

4.4.1 Guest Encapsulated D'VB Crosslinked Polystyrene

A comparatively rigid, hydrophobic and mechanically stable polymer

matrix is designed with styrene as the monomer and DVB as the crosslinking

agent. The support material is niechanically stable and not fragile in contact

with aqueous or non-aqueous so1vc:nts. No mechanical degradation was observed

during conventional magnetic or overhead stirring.

Macroporous styrene-based copolymer networks are prepared mainly

by free-radical crosslinking copolymerisation (FCC) of styrene and DVB

monomers in the presence of an inert diluent. The diluent, which is a solvent,or a

nonsolvent is included in the FCC system as a pore-forming agent.

Crosslinking copolymeirization of styrene with DVB in presence of

benzoic acid in the dissolved state resulted in the formation of benzoic acid

entrapped styrene-DVB polymeric systems. Free radical initiated bulk

polymerization technique was used for the polymerization. The guest molecules

were dissolved in the monomer mixture. Benzoyl peroxide was used as the

initiator. The mixture was heated in a waterbath at 80°C with stirring. The

precipitated polymer was washed with water, methanol and benzene. DVB

crosslinked polystyrenes with different crosslink densities varying from 5 to 20

mole % were prepared in the presence of the guest molecules. The results are

given in table 4.1 1.

Table 4.11 Preparation of guest encapsulated PS-DVB copolymer

of monomers (g) Yield

DVB I Benzoic aci

ZR Spectra

The polymer obtained was characterised by IR spectra. The IR spectra

of benzoic acid- encapsulated 1's-DVB copolymer is shown in fig.4.11. The

strong peak at 1728.65 cm-' due to C = 0 stretching vibration of acid group of

guest moiety. The broad peak at 3430.20 cm-' corresponds to O-H stretching

vibration of carboxyl group. The peak at 2965.46 cm-' and 1608.00 cm-'

corresponds to C-H and C=C strel.ching vibrations respectively.

Scanning Electron Micrograph

Fig 4.12 shows the scanning electron micrographs of styrene-divinyl

benzene(lO%) copolymer and its benzoic acid encapsulated counter part

respectively. The surface of the crosslinked copolymer is rough due to cavities

and the surface of host-guest system is smooth.

0

Fig. 4.12. Scanning electron micrographs qf

(a) PS - D VB resin

@) Benzoic acid encapsulated PS-DVB resin,

By introducing an inert diluent (a solvent or non solvent) together with

monomers porous structures within the particles may be obtained upon the

removal of the diluent after polymerisation by drying.

If we add a small organic: moiety such as benzoic acid, as diluent, to

monomer mixture it is also encapsulated in the network but is not removed by

washing with hot water or upon drying. It was found that benzoic acid can be

successfully encapsulated in styrene based copolymer networks by bulk

polymerisation technique.

The foreign molecules with suitable molecular dimensions are

entrapped in the well defmed cavities of the polymer matrix. These cavities are

designed by the three dimensiqnal arrangement of the structural units in the

polymer systems. A typical situation is expressed in scheme 4.2.

CH=CHz CH=CHz COOH fJ+o+o - CH= CH2

Scheme 4.2 Synthesis of benzoic a,cid encapsulated PS-DVB resin.

The morphology of the polymer like pore size and pore geometry are

sensitively dependent on the polyn~erisation conditions. With the variations in

temperature, rate of stining and the distribution of the monomers in the suspension

medium, the polymer produced are of variable morphological characteristics.

The method of suspension polymerisation was also applied for the

preparation of benzoic acid encapsulated PS-DVB resin. But the benzoic acid

molecules entrapped in the cav~ties of resins escaped on repeated washing. If

the cavity size are not suitable to i~ccomrnodate the guest molecules, the molecules

will not be accepted in the network and no encapsulation is possible. The pore

size of network resin formed by suspension polymerisation in presence of toluene

as diluent are not suitable and hence no entrapment is possible.

It is reported that for PS-DVB copolymer (prepared by suspension

polymerisation) the pore-volunle varies as the solvating nature of diluent is

varied.75 Using a solvating diluent (SOL) (toluene, CH,Cl, etc.) a relatively low

pore volume ie.(upto about 0.8 mug) is created. Using a nonsolvating diluent

(NON SOL) (n-heptane or alcohc~ls) a large pore volume (0.6 3 2 mug) is created.

4.4.1.a Release Studies of Guest Encapsulated DVB Crosslinked Polystyrene

During polymerisatior~ process, benzoic acid is encapsulated in the

polymer network and they are held in position by non-covalent interactions

(0.19 5 K cavmole) such as hydrogen bonding, van der Waals or dispersive

forces and polar forces.

The nature of forces and bonding existing between polymer and

encapsulant,and the amount of guest which can be encapsulated in the host are

studied from release experiments.

The amount of benzoic acid encapsulated in PS-DVB resins were

measured by suitable chemical method. The resins were allowed to swell in

different solvents for definite time intervals and the amount of benzoic acid

liberated was estimated titrimetr~cally and the results are collected in Table 4.12

and fig. 4.13.

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5 10 15 30 45 €0

-n- Water -A- CHU3

t T o C w n e +m

5 10 15 30 45 60 lime

-o- Toluene -+CH3OH --I 5 10 15 30 45 60 75 90

Time

+-Water U H C I , -o- Toluene ~ H J O H I

Fig. 4.13 : Rate of Release of Benzoic Acid from Guest Encapsulated DVB crosslinked polystyrene

The release is maximilm in chloroform and decreases in the order

toluene> CH,OH>H,O. Solubility parameter is a useful quantity for

characterisation of strength of interactions in polymer solvent systems. The

various types of forces existing between polymer segments and solvents can be

obtained from three-dimensional solubility parameter concept. Thus for CHCI,

and toluene dispersion force is higher and polar and H-bonding interactions are

negligible. For water and methanol H-bonding and polar forces are higher

(table 3.15). Thus CHCI, and tcduene can selectively penetrate into cavities by

means of dispersion forces and release of benzoic acid is higher in these solvents.

Also CHCI, and toluene are solvating diluents and so they can penetrate into

micropores more easily and benzoic acid can be easily released .

The amount of benzoic acid released is maximum in 5% crosslinked

polymer and decreases in the orcler 10% > 15%. But the amount of benzoic acid

released from 20% crosslinked polymer is slightly higher than &om the 15%

crosslinked polymer in presence #of CHC1, and toluene. (Table 4.13 and fig. 4.14).

Table 4.13 Weight of Benzoic: Acid Released From Guest Encapsulated PS- DVB Resin

Solvent

CHCI,

Toluene

'%OH

Yo L

Weight of Benzoic acid released per g. of polymer

Percentage of crosslinking

5%

.4817

,4320

.2929

.2632

10%

.2979

,2632

,2185

,1738

15%

,0993

.0744

,0558

,0298

20%

,1256

,0884

.0233

.0093

Crosslinking (mole %)

Fig. 4.14 Weight of benzoic acid released from guest encapsulated PS-DVB resin

A qualitative interpretation of the results is as follows. According to

D u ~ e k ~ ~ ~ during the free radical crosslinking copolymerisation of a network, a

porous structure is formed due tc~ phase separation during the network formation.

The phase separation may occur in the form of microsyneresis, or

macro~yneres i s .~~~~~ Due to diffment mechanisms of porosity formation during

FCC the final copolymer consists of agglomerates of particles of various sizes.

Thus pores of different dimensions are formed during the network formation ie,

micropores (width upto 20°A) appear between nuclei, mesopores (width in the

range 20-500°A) appear in the interstices between microspheres and macropores

(width > 500°A). Meso and macropores appear between the agglomerates of

microspheres.

Experimental studies on pore size distribution of PS-DVB copolymer

network revealed that in the network prepared in the presence of anon solvating

diluent pores from a few tens of angstroms upto several thousands of angstroms

in radius exist inside the macroporous material.65

Okay et aI7' reported that for PS-DVB networks (prepared in the

presence of cyclohexanol as the diluent) dried fkom toluene and water, the stability

of the porous structure starts to increase as the DVB content increases from 15

to 30%. The pore size spectra of the copolymer networksa2 also show that

increasing DVB content upto 18?6 increases the number of meso and macropores.

However these pores are unstable and collapse upon drying fkom toluene. Further

increase in the DVB concentration does not change the total porosity but increases

the stability of the pores.

Thus increasing the rigidity of the structure due to increasing DVB

content favours the conservatior~ of the porosity in the dry state.84

In FCC of styrene, DVB and benzoic acid during polymerisation,

benzoic acid are entrapped in the pores of network. The possibility of entrapment

is maximum in micropores ancl meso pores. The benzoic acid entrapped in

macropores may get removed or1 washing and drying.

For PS-DVB copolyn~eriation it is reported that the pore sizes lo2 -

104A" in radius formed during the polymerisation are unstable and they collapse

during the drying process. However the pores of sizes less than 10O0A in radius

that appear at the gel point, exist during the whole course of the polymerisation

reaction.

The gels formed by FCC are always inhomogeneous due to the fact that

the crosslinker has at least two \rinyl groups and if therefore one assumes equal

vinyl group reactivity, the reactivity of the crosslinker is twice that of the monovinyl

monomer. As a consequence, he crosslinker molecules are incorporated into

the growing copolymer chains niuch more rapidly than the monomer molecules

so that the final network exhibits a crosslink density distribution. The network

regions formed earlier are higher crosslinked than those formed later.

At the beginning of the copolymerisation much more DVB is

incorporated into the copolymer than is expected based on the initial composition

of the monomer mixture. Accordingly, the earlier formed and phase separated

nuclei and their agglomerates (rnicrospheres) are highly crosslinked than those

formed in a later stage of copolymerisation, when the major part of the DVB

monomers have been used up. 'The early formed gel regions will constitute the

interior of the microspheres whereas the latter formed and loosely crosslinked

regions will locate at the surface of microspheres.

Thus the pores and hence benzoic acid encapsulated in these pores inside

the first formed regions of the network remain stable during the drying process

because these regions will have higher crosslink density. Since the macropores

form the interstices of the microspheres and agglomerates that form at later stages

of the reactions, these network regions are loosely crosslinked and so the pores

in these regions collapse upon drying in the rubbery state.

The inhomogeneity in the networks increases with increasing DVB

concentration. Also the crosslii~k density of the less crosslinked regions of the

network decreases with increasing DVB concentration. Thus benzoic acid

encapsulated in the micropores are more tightly held than in the meso and

macropores due to increased crosslink density in microspheres.

As the crosslinker content increases the crosslinking density increases,

pore size also get decreased (meso and macropores) and the amount of benzoic

acid also get decreased. But as the crosslinker content increases beyond 15%

inhomogeneity and hence the number of micropores will also increase, and also

rigidity and stability of pores increases and the amount of benzoic acid entrapped

in the pores are also increased.

The studies on the sorption of aromatic solutes such as aniline and

phenol on the microporous and macroporous copolymers of styrene and DVB

show that the type of porous structure has a considerable effect on the sorption.

V.V. Azanova4' et al. reported that the maximum of solute sorption increases with

increasing amount of micropores in the polymer.

In presence of CHCI, expansion of the pores (or formation of new

pores) are occurring which must be accompanied by either deformation of the

polymer matrix or the relaxatiocl of the crosslinking network.

It should also be noted that for the guest encapsulated PS-DVB

copolymer the amount of benzclic acid released in CHIOH and water for 20%

crosslinked polymer is lesser than in 15% crosslinked polymer in contrast to the

behaviour in CHCI, (fig.4.14). It gives the clear indication that CH,OH and H20

cannot penetrate into the rigid nlicropores (which are filled by encapsulant) by

its polar and H-bonding forces and cannot extent the polymer matrix so that the

benzoic acid in the rigid micropores are released.

As the percentage of crosslinker content is increased from 10% to 15%

then to 20%, the number of mic:ropores are increased and most of the benzoic

acid encapsulated is in the micropores and hence the amount of benzoic acid

released is comparatively lesser In CHIOH and H20 (for 15 and 20% crosslinked

resin) than 5% and 10% crosslinked polymer.

4.4.1.b Swelling Studies of Guest Encapsulated DVB Crosslinked Polystyrene.

The quantitative interpretation of the experimental results are given in

terms of Flory-Rehner analysis of the swelling measurements of the free polymer

and host guest system. The theoretical results are in excellent agreement with

experimental results of release studies confirming the qualitative view given

above.

One of the basic parameters that describes the structure of non-

electrolyte hydrogels is the average molecular weight between crosslinks M, .

This describes the average molecular weight of polymer chains between two

consecutive junctions. These junctions may be chemical crosslinks, physical

entanglements, crystalline regions or even polymer complexes.

Swelling studies of the ,west encapsulated DVB crosslinked polystyrene

were carried out by placing a definite amount of dry sample in solvents such as

CHCI,, toluene, cyclohexane, mt:thanol and H,O for 48 hours. The weight of the

swollen sample were taken after removing the solvent. The results are given in

table 4.14.

Table4.14 Swelling Behaviour of Guest Encapsulated Styrene - DVB Crosslinked Resin

Mass of SP

5% 1.3210

.5 102

.42 10

.3674

,3600

No.

1.

2.

3.

4.

5 .

Solvent

CHCI,

Toluene

Cyclohexane

Water

CqOH

The extent of swelling is maximum in CHC1, and decreases in the order

toluene + cyclohexane 3 water 3 methanol similar to free PS-DVB polymer

(table 3.14). In the swollen state, polymer chains are elongated in the network,

and pore dimensions are increased and release of benzoic acid become easier.

4.4.l.c Molecular Weight beltween Crosslinks for Guest-Encapsulated

PS-DVB Crosslinked Polymer

Molecular weight between crosslinks of the guest encapsulated PS-

DVB resin were calculated using Flory-Rehner theory and are compared with

the results of the free polymer. (Table 4.15 and fig. 4.15)

Table 4.15 Molecular Weight Between Crosslinks for Guest Encapsulated PS- DVB resin

Free polymer Host-guest system

As the crosslinking density is increased &get decreased as expected.

An interesting feature which is observed is that except for 5% crosslinked polymer

the molecular weight between crosslinks for the guest encapsulated resin is higher

than the free polymer. The amount of benzoic acid encapsulated in 5% crosslinked

polymer is very much higher than in 10% crosslinked polymer. Hence is

lesser for 5% guest encapsulated resin than the free polymer.

0 L- - 0 5 10 15 20 25

Crosslinking (mole %)

t Free ,oolymer + Hostguest system] 1 - - - -

Fig. 4.15 Molecular weight bztween crosslinks for PS-DVB crosslinkedpoly- mer and guest encupsulated PS-DVB crosslinked polymer

The solvating power c~f the diluent has a critical effect on the porous

structure of macroporous copolymers. It is reported that for preparing a PS-

DVB copolymer network addition of a nonsolvating diluent results in large pore

volume. Also pore size distrib~~tion of the net work is characterized by a large

proportion of meso and macropores.

Depending on the distribution of diluent in the network structure after

its formation, networks can be classified into three groups.

1 . Expanded @re swo1lc:n) networks: Expanded network structures are

obtained if the diluen t present during the network formation remains

in the gel throughout the polymerisation. Expanded networks are thus

nonporous. During the removal of the diluent or during drying the

expanded network collapses, but reversibly. Addition of a good solvent

allows it to expand tc~ its earlier state.

Heterogeneous dry network: The diluent separates totally out of the

network phase during the ~olyrnerisation and acts only as a pore-forming

agent. The increase in the weight swelling ratio with increasing dilution

is due to the increasing volume of the pores which are filled by the

solvent. Since the diluent exists as a separate phase, during the

polymerization, the effects of cyclization and change in the contents of

the trapped entanglements on the network structure can be neglected.

Thus the volume degree of swelling does not change with the degree of

dilution.

Heterogeneous swollen networks: The diluent separates partially

out of the network phase during the polymerization. Thus it distributes

between network and diluent phases after synthesis. A part of the diluent

acts as a pore-forming .lgent, whereas the other part remains in the

network structure and increases its volume degree of swelling.

Thus in the encapsulated polystyrene networks heterogeneous swollen

networks may be formed. For the guest encapsulated DVB crosslinked

polystyrene is higher for 15% than for 20%. (Fig.4.16). This is in excellent

agreement with what we observe in release studies. The amount of benzoic acid

encapsulated in 20% crosslinked polymer is higher than in 15% crosslinked

polymer. (Fig.4.14). Since the i~mount of benzoic acid encapsulated in 15%

crosslinked polymer is very low when compared with 10% and 20% the molecular

weight between crosslinks will be higher for it because free pores are present in

the system.

4.4.2 Guest Encapsulated HI)DMA Crosslinked Polystyrene

The total volume of the pores inside a crosslinked polymer as well as

their size distribution can be varied by changing the independent variables of

synthesis. The sensitive dependence of the properties of the porous structure on

the synthesis parameters allows one to design a tailor made macroporous material

for a specific application. The main experimental parameters are the type and

amount of diluent, crosslinker corcentration and polymerization temperature and

the type of initiator.

The molecular character and cavity dimensions of the encapsulated

polymeric system were drastically changed with changes in crosslinking agent.

The behaviour of these polymenc systems towards encapsulation process and

the stability of the encapsulated !;ystems are different for the different polymer

system.

Benzoic acid encapsulated crosslinked polystyrene was prepared by

using hexane diol dimethacrylatc: as the crosslinking agent. Benzoic acid was

dissolved in the monomer mixture, benzoyl peroxide was added as the initiator

and polymerization were canietl out by bulk polymerization technique. The

precipitated polymer was washzd with water and CH,OH. 5,10,15 and 20

mole % crosslinked resins were prepared and are characterized by IR spectral

analysis and scanning electron microscopic studies. The results are given in

table 4.16.

Table4.16. Preparation of Gruest Encapsulated HDDMA Crosslinked Polystyrene

ZR Spectra

The IR Spectrum of HI)DMA crosslinked PS is shown in figure 4.16.

The strong peak at 17 19.34 cm-I is due to mixing of carbonyl stretching vibrations

of ester linkage of crosslinks and carboxylic function of the encapsulant, benzoic

acid. The broad band at 3423.20 cm-' is due to 0 -H stretching vibrations of

Crosslinking mole %

5

10

15

20

Fig. 4.16 IR spectrum of ber;!zoic acid encapsulated HDDMA crosslinked polystyrene

Yield (9)

2.756

2.906

3.184

3.3712

Weight of monomers (g)

Benzoic acid

4.636

4.392

4.148

3.904

Styrene

3.952

3.744

3.456

3.328

HDDMA

0.452

0.904

1.356

1.808

carboxylic functions. The peaks at 2932.60 and 1600 cm-I are characteristic of

C-H and C = C stretching vibrations respectively

Scanni~zg Electron Micrographs

The surface properties of the free polymer and Host-gucst system are

@) Fig.4.17 Scanning electron micrographs of (a) HDDMA crosslinked

polystyrene and (b) Host- guest system

compared and analyzed with sc.mning election micrograph. (fig.4.17(a) and

(b)). The surface of the free po~lymer is relatively irregular due to cavities and

that of the host - guest system is smooth and regular.

The suspension poly~nerisation technique was also tried for the

encapsulation in presence of toluene as diluent. But benzoic acid was not

entrapped in the cavities. This is; because by suspension polymerization using

toluene as diluent the pore size formed are not of suitable size to encapsulate

benzoic acid.

4.4.2.a Release Studies of Guest Encapsulated PS -HDDMA Resin

The release of guest froin host-guest complex were studied in presence

of four different solvents CHCl,, toluene, methanol and water. Since the nature

of crosslinking agent is different, the amount of benzoic acid encapsulated are

also different than DVB crosslinked PS.

The rate of release of' benzoic acid from the guest encapsulated

PS-HDDMA crosslinked resin follows the same pattern as that of the guest

encapsulated PS-DVB crosslinked resin. The release is maximum in CHCI, and

decreases in the order toluene > C:H,OH > Water. (table 4.17 and fig.4.18)

Table 4.17 Rate of Release of Benzoic Acid from Guest Encapsulated HDDMA, Crosslinked Polystyrene

Tm W U ~ )

5

10

15

30

4 5

60

90

Weight of benzoic acid (g) released per g. of the polymer

Solvent CHCI,

Percentage of crosslinlung

Solvent C,H,-CH,

Percentage of crosslinking

Solvent CH,OH

Percentage of crosslinking

5%

0.27

0.288

0.326

0.335

0.340

0.340

0.340

Solvent H,O

Percentage of crosslinking

20%

0

0

0

0

0

0

0

10%

0.019

0.024

0.029

0.038

0.057

0.057

0.057

5%

0.1860

0.2279

0.2651

0.2837

0.3163

0.3163

0.3163

5%

0.0419

0.0465

0.0511

0.0651

0.0791

0.0791

0.0791

5%

0

0

0

0

0

0

0

10%

0.0196

0.0238

0.0286

0.0381

0.0572

0.0572

0.0572

15%

0

0

0

0.0095

0.0143

15%

0

0

0

0

0.014

0.024

0.024

20%

0.0048

0.0095

0.0143

0.00480.0238

0.0333

0.014j0.0524

0.0524

10%

0.0048

0.0095

0.0143

0.0381

0.0429

0.0429

0.0429

10%

0

0

0

0

0

0

0

20%

0.048

0.048

0.052

0.057

0.067

0.076

0.076

15%

0

0

0

0

0.0048

0.0096

0.0096

15%

0

0

0

0

0

0

0

20%

0

0

0

0

0

0

0

0.025

I

0.015

5 8 0.01 0 0.04

s 0.005 -- 0.02

0.01

0 4

5 10 15 30 45 €0 90 5 10 15 30 45 60 90

Time +Water -B-CHCI, -ToClene -0-CHjOH

Fig. 4.18 : Rate of release of henzoic acid from guest encapsulated PS- HDDUA resin

When the encapsulated PS-HDDMA polymer is allowed to interact

with NaOH in presence of different solvents, solvents can selectively penetrate

into the pores depending upon the solvating power of the diluent. Chloroform

and toluene are solvating diluents and so they can penetrate into small micropores

more easily and can release benzoi~: acid than methanol and water. The dispersion

force is higher for CHCl, and toluene than polar and H-bonding. Thus CHCI, and

toluene can selectively penetratt: into cavities by dispersion forces and hence

release of benzoic acid is easier in these solvents.

One striking feature which is observed is that for 15 and 20 %

crosslinkedpolymer the amount of benzoic acid released is negligible in CH,OH.

Also for all the resins (5,10,15,20 mole%), benzoic acid is not released at all in

aqueous medium. (table 4.17 and fig. 4.18).

The explanation which can be given for this is that, the benzoic acid

encapsulated in PS-HDDMA a:sin is negligibly small especially for higher

crosslink densities and the benzc~ic acid encapsulated at these percentages are at

micropores (which are formed in the nuclei of microspheres). The relatively

polar CH,OH and water cannot penetrate into these smaller micropores (also

micropores are more rigid and are tightly filled by the encapsulant) and release

of benzoic acid is not possible.

The amount of benzoic acid released is higher in 5% crosslinked

polymer and decreases upto 15%). The amount of benzoic acid released for 20%

crosslinked polymer is slightly higher than 15% in CHCl, and toluene. However

the amount of benzoic acid released for 20% crosslinked resin is lesser than

15% crosslinked polymer in CII,OH and water. (table 4.18 and fig. 4.19) A

similar pattern was observed for guest encapsulated DVB crosslinked polymer.

(fig. 4.16)

Table4.18. Amount of Benzoic Acid Released with % Crosslinker for Guest Encapsulated PSHDDMA Resin

0 5 10 15

Crosslinking (mole %) --

~ C I , +Tduencl +CHIOH --tH>O I

Crosslinking mole %

5

10

15

20

Fig. 4.1 9 Benzoic Acid Released with % Crosslinker for Guest Encapsulated PS-HDDMA Resin

As the percentage of c:rosslinking agent is increased, the pore size get

decreased (mesopores) and heme the amount of benzoic acid encapsulated also

Weight of be~lzoic acid(g) released per g. of the polymer

Solvents

To 0

0

0

0

CH,OH

0.0791

0.0429

0.0095

0

CHC!,

0.340

0.057

0.048

0.076

C6HsCH~

0.3 1625

0.05717

0.01430

0.05240

get decreased upto 15%. Beyond 15% the number and rigidity of the micropores

are increased and more amoun: of benzoic acid is encapsulated in 20% than

15% crosslinked resin. This idea is confirmed by the fact that CH,OH and H,O

cannot penetrate into the micropores by their polar and H-bonding forces and

hence release of benzoic acid is very difficult in presence of these solvents

especially at higher percentages.

The amount of benzoic acid encapsulated in PS-HDDMA crosslinked

resin is much lesser than in the P!I-DVB crosslinked resin for the same percentage

of crosslinker. Hexane diol dilnethacrylate is more flexible, hydrophilic and

length of the crosslinker unit is higher than DVB crosslinker. Hence the pore

size of (macropore) of PS-HDDMA polymer network will be higher than PS-

DVB network and hence benzt~ic acid cannot be successfully encapsulated in

the macropores of PS-HDDMA resin. Thus benzoic acid in macropores may not

be fitted and most of the benzoi;: acid encapsulated is in micro and mesopores.

Hence the amount of benzoic acid released in presence of CH,OH and H,O is

very low when compared with PS-DVB resin.

4.4.2.b Swelling behaviour of guest encapsulated PS-HDDMA resin

Swelling studies of guest encapsulated HDDMA crosslinked polystyrene

were carried out by placing a definite amount of dry polymer in different solvents

for 48 hours. The weight of swollen sample were determined. The results are

given in table 4.19.

148

Table 4.19 Swelling Behaviour of Guest Encapsulated PS-HDDMA Resin

Swelling is maximum in CHCI, and decreases in the order toluene >

cyclohexane > methanol > water. However extent of swelling is higher for guest

encapsulated PS-HDDMA resin than for guest encapsulated PS-DVB resin.

[Reverse is the case for free polymer]. The reason is that the amount of benzoic

acid encapsulated in PS-DVB resm in higher than in PS-HDDMA resin. Hence

the voids available in guest enca~osulated PS-HDDMA resin is higher than for

guest encapsulated PS-DVB resin.

No.

1.

2.

3.

4.

5.

4.4.2.c Quantitative Interpretation of the Results

Quantitative interpretation of the swelling results of PS-HDDMA resin

and the host-guest complex are in excellent agreement with the experimental

results of release studies. Mo1ec:ular weight between crosslinks of the Host-

guest system are calculated using Flory-Rehner theory and are compared with

the molecular weight between crosslinks for the free polymer (table 4.20 and

fig. 4.20).

Solvent

CHCl,

C,H,CH,

Cyclohexane

CYOH

40

Mass of sw

5%

1.2760

0.8958

0.3714

0.3 112

0.2940

Table 4.20 Molecular Weight Between Crosslinks for PSHDDMA Resin and Host-Guest System

Crosslinking Molecular weight between crosslinks

(mole %)

Similar to PS-DVB system, molecular weight between crosslinks -. M~ IS higher far encapsulated polymer than for the free polymer.

6000 5000E ;i\--;i 4000

IS -- 12 20000 3000

2000

1000

0 0 5 10 15 20 25 0 5 10 15 20 25

Crosslinking (mole %) Crosslinking (mole%)

(a) (b)

Fig.4.22 Molecular weight between crosslinks for (a) PS-HDDMA co-polymer and (b) Host-guest systerrn

In the preparation of the flee polymer by bulk polymerisation, no diluent

was used. But in preparation of berlzoic acid encapsulated PS-HDDMA system,

benzoic acid acts as the diluent and heterogeneous swollen network is formed.

Here the diluent (benzoic acid) separates partially out of the network phase during

the polymerisation. Thus a part of the diluent acts as a pore-forming agent, where

as the other part remains in the network structure and increases the volume degree

of swelling. - Mc for the host-guest system decreases in the order 5% > 10% > 20%

> 15% which is the same result as obtained from release studies. Since the

amount of benzoic acid encapsulated in 15% sample is low, than for 20% and

lo%, will be higher for it. As the percentage of crosslinking agent increases

from 15% to 20% the number of' rigid micropores are increased and hence the

amount of benzoic acid entrappe'd in these pores are also increased.

4.5. Thermal Analysis of Crosslinked Copolymers and the Host-Guest System The DSC traces of the crosslinked polymers and the host-guest system

are performed on a Metler Toledo, DSC 822e STARe scanning calorimeter at

20°C/min. DSCcurves from -40°C to 200°C are shown in fig. 4.23 and 4.24. Tg

and peak temperatures of endothem for the different crosslinked polymers and

the host-guest system( 10% crosslinked) are shown in table 4.21.

Table431 Tg and Peak Temperature of Phase Change for Acrylamide and Styrene Based Polymers.

Crosslinked Polymer (10%)

AA-DVB

AA-DVBC&COOH

AA-HDDMA

AA-HDDMAC6H,COOH

S-DW

S-DW-C6H,COOH

S-HDDMA

r~-. AA-HDDMA

.,. a,.

1 . S93.07d lid - 2 J 8 . 4 l J r '

btcEr.1 - M9.63d Normsl id - 234.79Je1 hurt - JO74.c Put - 8611C E n M - 137.52%

Fig. 4.23. DSC Curves r f l A hydrogels (10% crosslinked) and guesf encapsulated PA copolymer

Fig. 4.24. DSC Curves of PS copolymers (10% crosslinked) and guest encapsulated PS networks

The free polymer and the host -guest system has the same value of Tg.

This shows that at lower temperature the mobility of chain segments in crosslinked

polymeric systems is not strongly dependent on the presence of small guest

stystem. The thermal stability of the host-guest complex is also evident from

these studies. The polymer is stable even upto 200°C and the guest molecules

are still within the cavities of the host.

Each of the resin show a broad endotherm with peak temperatures

varying from 75.47OC to 98.07OC For acrylamide polymers and from 1 10.91°C to

152°C for the styrene polymers. 'llese endotherms may be due to temperature

dependant phase changes. The peak temperature for the host-guest system are

lower than the free polymer. Due to the encapsulated guest moieties in the host

guest system, the phase change will be easier for host-guest system and occurs at

a lower temperature than the free polymer.

The difference in the ertdothermic peak temperature for PS-HDDMA

resin and guest encapsulated PS-HDDMA system is very low. From release

studies it was clear that for the benzoic acid encapsulated HDDMA crosslinked

polystyrenes the amount of benzoic acid encapsulated is very low compared to

the DVB crosslinked PS. Hence ibr the two systems the phase change occurs at

almost same temperature. For all the resins, decomposition temperature Td is

greater than 200°C.

The foregoing studies reveal that the entrapment of suitable guest

molecules in the cavities of crossilinked polymers like PS-DVB, PS-HDDMA,

AA-DVB and AA-HDDMA are p~ssible. The rate of release of encapsulant from

the host-guest complex in presence of different solvents depends on the different

interaction energies acting between polymer segments, host and guest and host-

guest complexes and solvent. The structural architecture of the network polymers

and its solvophobic - solvophilic interactions with various solvents and solvent

mixtures are the decisive factors which determine the extent of entrapment of a

foreign organic molecule in the well defined cavities of styrene and acrylamide

based polymers.