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Progress in Organic Coatings 77 (2014) 1418–1427

Contents lists available at ScienceDirect

Progress in Organic Coatings

j o ur nal ho me pag e: www.elsev ier .com/ locate /porgcoat

Biodegradable and thermostable synthetic hyperbranchedpoly(urethane-urea)s as advanced surface coating materials

Satyabrat Gogoi, Shaswat Barua, Niranjan Karak ∗

Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, Tezpur University, Tezpur 784028, India

a r t i c l e i n f o

Article history:Received 21 June 2013Received in revised form 18 April 2014Accepted 20 April 2014Available online 25 May 2014

Keywords:Hyperbranched poly(urethane-urea)SynthesisThermal degradationBiodegradationSurface coating material

a b s t r a c t

Aliphatic hyperbranched poly(urethane-urea)s with different weight percentages of branch generatingmoiety were synthesized by a one pot A2 + BC2 approach. Isophorone diisocyanate was used as the A2

type monomer, while a tri-functional dihydroxyamine compound synthesized from �-caprolactam anddiethanol amine acted as the BC2 monomer. Evidence supporting the hyperbranched structure of thesynthesized poly(urethane-urea) was obtained from 1H NMR spectra. FTIR study confirmed the natureand extent of hydrogen bonding present in this novel macromolecule. A Gaussian band fitting procedureof the IR band at amide-I region showed that the extent of hydrogen bonding increases with the increaseof weight percentage of the tri-functional compound. The tensile strength, elongation at break, impactresistance, scratch hardness and gloss followed an increasing trend with the same. The thermal degra-dation of the hyperbranched poly(urethane-urea) was found to be dependent on the weight percentageof the BC2 type moiety. The kinetics of thermal degradation studied by the Ozawa method showed thatthe activation energy required for thermal degradation of hyperbranched polymer is higher than itslinear polyurethane analog. The synthesized polymer was found to be biodegradable by Pseudomonasaeruginosa bacteria. The study showed superiority of the hyperbranched structure over the linear one.Thus the results indicated the potential usage of the studied hyperbranched poly(urethane-urea) as anadvanced surface coating material.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Polyurethane is known as one of the most versatile polymericmaterials and was first developed in the 1930s for military andaerospace applications [1]. Due to versatility in properties and fieldof applications, it soon achieved great importance in the era ofmaterial science with applications including flexible foam in uphol-stered furniture, rigid foam as insulation in walls, roofs, appliancesand medical devices, coatings, adhesives and sealants [2,3]. One ofthe most important applications is as a binder for surface coatings.Polyurethanes are generally obtained by poly-condensation reac-tion between di/polyisocyanate and a compound that containshydroxy functionalities, such as di/polyol, although isocyanate freeroutes are also known. A wide range of chemical and physical prop-erties can be tailored by using different isocyanates, macroglycolsand chain extenders and by changing their relative proportions[4]. Many desired material properties such as toughness, flexibility,

∗ Corresponding author. Tel.: +91 3712267009; fax: +91 3712267006.E-mail addresses: karakniranjan@yahoo.com, karakniranjan@gmail.com

(N. Karak).

abrasion resistance and chemical resistance make polyurethane anattractive choice for the above coating applications [5–7].

Polyureas are closely related to polyurethane and are formed bythe reaction between di/polyisocyanate with polyamines. Polyureaconfers certain advantages to coatings, viz. high mechanicalstrength, good chemical resistance and more importantly shortercuring time [8,9]. Literature has shown that materials contain-ing urea or urethane groups within the backbone are well knownand possess high industrial importance. Thus, in order to reapthe benefits of both, inclusion of urea-urethane linkages withinthe same macromolecule is a good proposition. This also helpsto overcome the individual limitations like poor surface adhe-sion of polyurethane on certain substrates and short pot-life ofpolyurea [10,11]. Practically, this can be achieved by the reactionof di/polyisocyanate with a mixture of polyol and polyamine or apolyhydroxyamine.

Over the last two decades, highly branched polymers includingdendrimers and hyperbranched polymers have been extensivelyreported in the literature because of their unique properties. Thesenovel branched polymers possess a three dimensional structure,low intrinsic viscosity, high solubility and high surface func-tionalities [12]. Monodisperse, well defined and architecturally

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perfect dendrimers are prepared by tedious multistep poly-merization reaction and are thus expensive which limits theirindustrial importance [13]. In contrast, hyperbranched polymerscan be synthesized by a one-pot process, facilitating large-scaleproduction at reasonable cost. Despite their structural differences,the properties of many hyperbranched polymers are not far fromthose of the dendrimers. Because of the economic viability andattractive physico-chemical properties, hyperbranched polymerresearch deserves greater attention. Among different routes for thesynthesis of hyperbranched polyurethane, A2 + CB2 is attractive[14]. This approach has the advantage of incorporating a desiredchemical linkage in addition to the urethane by selection of asuitable CB2 monomer, leading to many interesting properties forsurface coating materials.

The current study therefore presents the synthesis of aliphatichyperbranched poly(urethane-urea) by an A2 + CB2 approach usingcommercially available monomers with varying weight percent-ages of the branching moiety. The structural characterization andperformance studies were conducted in comparison to a linear ana-log. Literature reveals improved properties with increase in degreeof branching; henceforth, variation of properties was studied tak-ing different weight percentages of a branch generating moiety.The thermal degradation along with its kinetics and microbialbiodegradation were also investigated. Biodegradable polymershave received much attention in recent times with many reportsfocusing on the synthesis of biodegradable polyurethanes [15,16].Bacterial degradation can be used as a demonstration tool for thestudy of biodegradation [17].

2. Experimental

2.1. Materials

Isophorone diisocyanate (IPDI; Sigma), dibutyltin dilaurate(DBTL; Aldrich) and 3-amino propan-1-ol (Sigma–Aldrich) wereused as received. �-Caprolactam (Himedia) was recrystallized fromcyclohexane and dried under vacuum over P4O10 over night atroom temperature. Poly(ethylene glycol) with MN 600 (PEG-600;Merck), diethanolamine (DEA; Merck) and 1,4-butane diol (BD;Merck) were dried in a vacuum oven for a period of minimum12 h at 55 ◦C before use. Xylene (Merck) was used after distillation,whereas dimethyl acetamide (DMAc; Himedia) was dried over CaOfor 12 h followed by distillation under reduced pressure. The dis-tilled solvents were stored in 4A type molecular sieves for furtheruse. Calcium oxide (Merck) was used as received. The biodegra-dation study of poly(urethane-urea) used Pseudomonas aeruginosa(P. aeruginosa) bacterial stain MTCC 7814 which was provided bythe Department of Molecular Biology and Biotechnology, TezpurUniversity.

2.2. Instrumentation

FTIR spectra were recorded in transmission mode on KBr pelletswith a Nicolet (Madison, USA) FTIR Impact 410 spectrometer. 1Hand 13C NMR spectra were recorded on 400 MHz FT NMR (JEOL,Japan) spectrometer using dimethyl sulfoxide-d6 (DMSO-d6) asthe solvent and Me4Si (TMS) as the internal standard. Molecularweight and polydispersity index were obtained by GPC in a Waters,USA instrument. For glass transition temperatures (Tg), differentialscanning calorimetry was done by a Perkin Elmer DSC 6000,USA instrument in the temperature range −70 to120 ◦C (startingtemperature 0 ◦C) in a cycle of heating-cooling-heating under theatmosphere of nitrogen at the scanning rate of 10 ◦C/min. Thermaldecomposition profiles of the prepared polymers were studied ina Perkin Elmer TGA 4000, USA thermal analyzer. The characteristicthermal decomposition temperatures were obtained by subjecting

Scheme 1. Synthesis of tri-functional dihydroxyamine.

the polymers to dynamic heating at a constant scanning rateof 10 ◦C/min and under the inert atmosphere of nitrogen at theflow rate of 30 mL/min. For the study of degradation kinetics, thesample was heated under non-isothermal condition at differentscanning rates, viz. 10, 20, 30 ◦C/min. Data obtained were analyzedby Advanced Pyris® 10.1 software equipped with Perkin ElmerTGA 4000 instrument. A universal testing machine (UTM), modelZwick Z010, Germany equipped with a 10-kN load cell operating ata crosshead speed of 50 mm/min was used to measure the tensilestrength and elongation at break of samples with dimensions10 cm × 1 cm × 0.02 cm. The scratch hardness of the polymericfilms was measured by using a scratch hardness tester, ModelNo.705 (Sheen instrument limited, UK) with stylus accessory anda travel speed of 30–40 mm/s. Impact resistance was determinedby a falling weight tester (SC Dey & Co., India) in accordancewith ASTM D 1037. The chemical resistance tests were performedin accordance with ASTM D 543-67. Surface morphology of thepolymers was studied by a scanning electron microscope (SEM)model JSM-6390LV (JEOL), after platinum coating on the surface.The solution viscosity (0.5% in DMAc) was measured by using anUbbelohde viscometer at temperature 25 ± 0.1 ◦C.

2.3. Synthesis of dihydroxyamine compound

Vacuum dried DEA and recrystallized �-caprolactam in 1:1molar ratio and CaO (1% of the total reactant weight) were taken ina bottom-rounded three neck glass reactor immersed in a siliconoil bath equipped with a mechanical stirrer, a nitrogen inlet anda thermometer (Scheme 1). The temperature was maintained at140 ◦C for 4–5 h with constant mechanical agitation, under contin-uous nitrogen flow. The product obtained was stored in a reagentbottle and kept inside a vacuum desiccator.

2.4. Synthesis of hyperbranched poly(urethane-urea)

Hyperbranched poly(urethane-urea) (HPU) was prepared bya controlled pre-polymerization technique. Firstly the requiredamount of PEG-600, BD and xylene were taken in the reactor flaskunder constant mechanical agitation and nitrogen flow. A definedamount of IPDI was slowly injected into the reaction mixture atroom temperature. For different compositions, the NCO/OH ratiovaries from 1.3 to 1.8. DBTL was added as the catalyst. The temper-ature of the reaction mixture was maintained at 70 ◦C for 3 h withconstant stirring under continuous flow of nitrogen. Then reactionmixture was allowed to cool to room temperature and the requiredamount of dihydroxyamine compound in DMAc was added so thatNCO/OH functional ratio was maintained at 1.0. Temperature wasslowly raised to 50–55 ◦C and was maintained for 3–4 h. Whena sufficiently viscous polymeric solution was obtained, the reac-tion was stopped to prevent gelation. The overall solid contentof the reaction mixture was maintained at 30 wt%. Three differ-ent compositions of the polyurethane were prepared following thesame method taking different weight percentages of the branch-ing moiety (Table 1). The linear polymers (PUUL and PUL) werealso synthesized by the same procedure except that butane diol or

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Table 1Composition of the reactants for the synthesized polymers.a

Composition HPU15 HPU10 HPU05 PUUL PUL

PEG-600 (mol) 1.0 1.0 1.0 1.0 1.0BD (mol) 2.5 2.5 2.5 2.5 3.5IPDI (mol) 6.2 5.0 4.2 6.2 4.5Tri-functional moiety (mol) 1.8 1.0 0.5 0.0 0.03-Amino-1-propanol (mol) 0.0 0.0 0.0 1.7 0.0NCO/OH (Functional ratio) 1.0 1.0 1.0 1.0 1.0

a Digit of the code indicates the weight percentage of trifunctional dihydrox-yamine moiety in the polymer.

3-amino-1-propanol was added in place of trifunctional moiety inthe second step. The synthesized polymeric solution was cast onglass and galvanized tin sheet to produce polymer films of thick-ness 1–2 mm, which were dried in a forced convection oven at 65 ◦Cfor 24 h and utilized for testing.

2.5. Broth culture technique for biodegradation

A mineral salt medium was prepared, which contained 2.0 g(NH4)2SO4, 2.0 g Na2HPO4, 4.75 g KH2PO4, 1.2 g MgSO4·7H2O,0.5 mg CaCl2·2H2O, 100 mg MnSO4·5H2O, 70 mg ZnSO4·7H2O,10 mg H3BO3·5H2O, 100 mg CuSO4·7H2O, 1 mg FeSO4·7H2O, and10 mg MoO3, all in 1.0 L of demineralized water. This mediumwas then autoclaved for 15 min at 121 ◦C under a pressure of15 lb and then allowed to cool to room temperature. P. aeruginosastrain was cultured in the prepared medium inside a shaker incu-bator for 48 h at 37 ◦C. From this culture medium, 100 �L (108

microbes/mL, as calculated by McFarland turbidity method) wasinoculated into a conical flask containing 10 mL of the preparedmineral medium. Polymeric films were sterilized by exposing themto UV light at 254 nm for 15 min. After sterilization, these filmswere incubated inside the medium under sterile conditions at 37 ◦Cfor the degradation study. Medium without polymeric films was

Scheme 2. Synthesis of HPU.

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Table 2Weight average (Mw), number average (Mn) molecular weight; polydispersity index (PDI) and solution viscosity of the polymers.

Composition HPU15 HPU10 HPU5 PUUL PUL

Mw (g/mol) 28,500 27,900 27,300 24,700 24,100Mn (g/mol) 19,900 17,800 20,800 17,000 15,000PDI 1.43 1.57 1.31 1.45 1.61Solution viscosity (dL/g) 0.229 0.256 0.265 0.358 0.345

kept as the negative control. Growth of the microorganism wasmonitored using a UV–visible spectrophotometer by measuringthe absorbance (optical density, OD) at 600 nm with respect to thecontrol [12]. The readings were taken at interval of seven days.After 6 weeks incubation, both HPU and PUL samples were ana-lyzed for changes in weight, tensile strength and elongation atbreak.

3. Results and discussion

3.1. Synthesis

The tri-functional moiety, dihydroxyamine compound was syn-thesized by a rearrangement reaction as shown in Scheme 1. HPUswith differing compositions were then prepared by a two-step,single pot process following an A2 + CB2 approach using a pre-polymerization technique. Since the ratio of NCO/OH functionalitywas between 1.3 and 1.8 and the secondary NCO group (NCOsec) ofIPDI is known to selectively prefer the urethane reaction in the pres-ence of DBTL catalyst, it is mainly the NCOsec that reacts with thehydroxyl functionality in the pre-polymerization step (Scheme 2).The risk of gelation occurring in the pre-polymerization step wasvery low as no reactant with functionality greater than two waspresent. The pre-polymer formed in this step acted as an A2 reac-tant and the synthesized dihydroxyamine was added in the secondstep as the CB2 reactant. In order to avoid competition betweendifferent polyols no other hydroxyl functionality was added in thesecond step. Furthermore, the concentration of the CB2 reactantwas kept low, because higher concentration (>15%) increases therisk of gel formation. The reaction was stopped when a sufficientlyviscous polymeric solution had formed. The completion of the reac-tion was confirmed by checking that no NCO peak remained in theFTIR spectrum. Molecular weight, polydispersity index and solutionviscosity (0.5% in DMAc) of the synthesized polymers are listed inTable 2.

3.2. FTIR study

FTIR was used here primarily to provide information about thenature and extent of hydrogen bonding present in the system [18].These factors can influence many properties of a polymer suchas thermal stability, mechanical properties, solubility and viscos-ity [19]. Since hydrogen bonding weakens the surrounding bonds,changes in the band intensity and frequency can be used as atool to detect and measure the strength of hydrogen bonds [20].IR spectral frequencies for the prepared polymers are shown inFig. 1(a) and the important bands are given in Table 3. For thestudy of hydrogen bonding in HPUs the major spectral regions ofinterest were the amide-I region; i.e. carbonyl stretching vibra-tion region near 1630–1750 cm−1 and the N H stretching vibrationregion near 3400–3500 cm−1 [18]. The bands in both regions werebroad which might reflect the summation of differently hydro-gen bonded functionalities. The presence of any hydrogen bondingcan be identified by following the shift in the bands toward lowerwave number. Since the bands present in the respective regionwere broad multiple peaks a Gaussian band fitting procedure wasemployed in order to clarify the type, amount and nature of the

Fig. 1. (a) FTIR spectra HPU15, HPU10, HPU5, PUUL and PUL; (b) deconvoluted IRfrequency for C O region; and (c) deconvoluted IR frequency for N H region.

hydrogen bonding present. After deconvolution of the IR bandin the C O region, three peaks can be assigned near 1717–1726,1641–1647 and 1572–1580 cm−1 which can be attributed towardH-bonded urethane carbonyl, hydrogen bonded urea carbonyl and

Table 3IR frequency of hyperbranched polyurethane for different functionalities.

Functionality Wave number (cm−1)

N H (Free) 3440–3490N H (N H· · ·N H) 3410–3421N H (N H· · ·O C O) 3368–3378C O, Amide I 1630–1750C O (urethane, C O· · ·H N) 1717–1726C O (urea, C O· · ·H N) 1641–1647NH C O Amide II 1580–1572CH2 scissoring, CH3 deformation, CH2 bending 1436–1465C N and in-plane, N H deformation amide III 1108–1115Amide IV (In and out deformation) 773–779Amide V (In and out deformation) 631–635

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Fig. 2. 1H NMR and 13C NMR spectra of dihydroxyamine moiety.

N H bending (amide II) frequency respectively (Fig. 1(b)). Decon-volution of the IR band in the 3300–3500 cm−1 region yieldedthree peaks at 3452–3485, 3410–3421 and 3368–3378 cm−1 whichcan be assigned to free N H and O H, hydrogen bonded N H(N H· · ·N H) and hydrogen bonded N H (C O· · ·H N) respec-tively (Fig. 1(c)). The study of the peak positions of deconvolutedspectra suggests that incorporation of urea and amide linkagescauses a shift of the IR absorption peaks toward lower wave numberregion. Compared to the polymer based on the diol chain extender,which only contains urethane bonds, the amine containing moiety(linear or branching) additionally inserts polar urea linkages, whichcauses enhanced inter-chain association between the macromolec-ular chains by hydrogen bonding. In HPUs, the shift is consistentwith the increase in weight percentage of the branching moiety.Such shift in the IR spectra indicates enhanced inter-chain asso-ciation through hydrogen bonding between the macromolecularchains which get strengthened by the increasing amount of branch-ing moiety in the polymer.

3.3. NMR spectroscopy

The structure of the synthesized tri-functional moiety was con-firmed by 1H and 13C NMR analyses (Fig. 2) with details as follows:

1H NMR spectroscopy data (ppm): � 1.46- [ CH2 ]; � 1.58-[ CH2 CH2 NH2, CH2 CH2 C O]; � 2.24- [ CH2 C O]; � 2.98-[ CH2 NH2]; � 2.52- [ N(CH2)2]; � 3.48- [ CH2 OH]; � 5.10–5.12[amine and hydroxyl protons].

13C NMR spectroscopy data (ppm): � 20.2-[ CH2 ]; �30.2-[ CH2 CH2 NH2]; � 30.5- [ CH2 CH2 C O]; � 36.8-[ CH2 C O]; � 41.9- [ CH2 NH2]; � 52.1- [ N(CH2)2]; �60.9-[ CH2 OH], � 172- [carbonyl carbon C O]

Analysis of the NMR spectrum of IPDI derived HPU is rela-tively complex since the two isocyanate groups show variableselectivity toward the urethane reaction. IPDI is composed of twodifferent isomers viz. cis (Z) and trans (E) in 1: 3 ratio [21–23].Moreover, the two isocyanate groups within the same moleculeare in chemically different environments, one is directly attachedto the cyclohexyl ring system (NCOsec) and the other is attachedto the cyclohexyl ring through a methylene linkage (NCOprim). Asa result, the reactivity of the two isocyanate groups differs [24].In the pre-polymerization step excess IPDI (NCO > OH) was used.In presence of DBTL catalyst, IPDI is known to have selectivitytoward NCOsec [25]. Therefore, in the pre-polymerization step onlythe NCOsec is expected to react with the hydroxyl compounds toform urethane linkage (as shown in Scheme 2). In the second step,the remaining isocyanate, mostly NCOprim, reacts with hydroxyl

and amine groups of the tri-functional moiety to form both uret-hane and urea linkages. The formation of two different types ofdiurethane (Z or E) was confirmed by 1H NMR spectroscopy (Fig. 3).Comparison of the spectra of the hyperbranched (Fig. 3(a)) andlinear (Fig. 3(b) and (c)) polymers shows several peaks that arepresent for the hyperbranched polymer are missing for the lin-ear analog. The missing peaks, at � 6.0–6.4 ppm (urea proton), �2.92 ppm (NCH2 ), � 2.76 ppm (N C(O)CH2 CH2 ) and � 3.31 ppm( CH2 NH CO NH ), together with the increased peak inten-sity of CH2 O C O compared to CH2 OH signal supports thehyperbranched structure for the HPUs. However a quantitative esti-mation of the degree of branching for HPU is difficult as the peaksof interest overlap and calculation of the integral area is difficult[13].

3.4. Surface morphology

The morphological study of the prepared HPUs was carried outby analyzing the SEM images (Fig. 4). From the images, it is clear thatthe polymer surfaces were not uniform; rather there was a degreeof inhomogeneity in the distribution. It is the intrinsic incompatibil-ity or thermodynamic immiscibility of the hard and soft segmentswhich is responsible for phase separation. Due to their polar nature,the hard segments can form hydrogen bonds between carbonyl andamine linkages. Such interactions result in the clustering or aggre-gation of the hard segments into ordered hard domains [26]. On theother hand soft segments do not give rise to polar interactions andtherefore they form amorphous domains. This results in phase sep-aration. However, the light reflecting and transmitting properties ofthe polymers seem to be unaffected by the degree of inhomogeneityas indicated by the good gloss and clarity of the films.

3.5. Thermal properties

The thermal stability of the synthesized HPUs was studied bythermo gravimetric analysis (TGA). The thermal stability of a poly-mer depends on many factors, such as chemical structure, typeof chemical linkages, reactant composition, molecular weight andintra/intermolecular forces [27]. All the studied polymers showeda single step thermal degradation profile (Fig. 5). The TG thermo-grams show characteristic thermal decomposition temperaturesviz. onset decomposition (TON) and end set decomposition (TEND)temperature of the polymer samples. The first derivative of the TGcurve (DTG) is proportional to the rate of decomposition and rep-resents the temperature corresponding to the maximum rate ofweight loss (TMAX). The data in Table 4 show the TON, TMAX, TEND

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Fig. 3. 1H NMR spectrum of (a) HPU15, (b) PUUL and (c) PUL.

values, temperature corresponding to 50% and 90% weight lossand weight residue at 500 ◦C for the different polymer composi-tions. The thermal degradation data clearly show the incrementin the degradation temperature in presence of urea linkages inthe polymers. The increment is consistent with increase in weightpercentage of the branching moiety. This can be attributed to theincrease in the amount of hydrogen bonding arising from the pres-ence of highly polar urea functionality as well as the structuraluniqueness, which makes the thermal degradation process moreheat expensive. Tg values in Table 4 shows increment with theweight percent of branching moiety, reflecting the dependence onhydrogen bonding between the polar groups.

3.6. Kinetics of thermal degradation

For calculating the kinetic parameters from TGA data the Ozawamethod was employed [28–30]. The general expression for kineticdegradation is given by

d˛

dt= ˇ

d˛

dT= K(T)f (˛) (1)

where is conversion rate which can be defined as the ratio ofactual weight loss to the total weight loss, K(T) is the reaction rate

Table 4Thermal data for the synthesized polymers.

Polymer code TON (◦C) TEND (◦C) TMAX (◦C) T50% (◦C) T90% (◦C) Weight residue @ 500(◦C) (%) Tg (◦C)

PUL 222 367 325 327 410 04.80 −31.2PUUL 225 380 330 335 450 5.01 −30.4HPU5 236 408 334 340 462 07.80 −28.8HPU10 250 432 339 355 489 09.36 −26.7HPU15 267 445 360 367 512 10.27 −25.2

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Fig. 4. SEM micrographs of (a) HPU15, (b) HPU10, (c) HPU5 (d) PUUL and (e) PUL.

constant and f(˛) is the kinetic model function. is the rate ofheating. Mathematically ˛, and f(˛) can be defined as

= w0 − w

w0 − wf

= dT

dt

and

f (˛) = (1 − ˛)n

where w is the weight of the sample and w0 and wf are the initial andfinal weight of the sample during the weight loss event of interest.In the latter equation, n denotes the overall order that the pro-cess had followed. For a non-isothermal process the temperature

Fig. 5. TGA thermograms of HPU15, HPU10, HPU5, PUUL and PUL.

dependence of weight loss process can be related to the Arrheniusequation

K(T) = A exp(

− Ea

RT

)(2)

where A is the pre-exponential factor, Ea is the activation energy ofdegradation; R is the universal gas constant. From Eqs. (1) and (2)the following correlation can be made

d˛

dt= ˇ

d˛

dT= f (˛) = (1 − ˛)nA exp

(− Ea

RT

)(3)

The above expression forms the base equation for both deriva-tive and integral method to study degradation kinetics.

The practical method to study the degradation kinetics requiresseveral measurements at different heating rates (ˇ). For a givendegree of conversion (i.e. at constant ˛) the activation energy canbe obtained by plotting the logarithm of heating rate (ˇ) vs 1/T (Tin absolute scale) which is expected to give straight line; the slopegiven by

m = 1.052(

Ea

R

)(4)

From the above equation, the apparent activation energy fora given degree of conversion can be calculated. All the dynamicdegradation studies were done at the scanning rate of 10, 20 and30 ◦C/min, between 25 ◦C and 700 ◦C under nitrogen. All the curveswere slightly shifted toward higher temperature region due to theheat transfer lag with the increasing heating rate. The plots oflogarithm of heating rate against the inverse of the absolute tem-perature for different degradation factors were obtained as straightlines as shown in Fig. 6. For each degree of conversion the activa-tion energy can be obtained from the slope of the straight line usingEq. (4). The kinetic energy vs. degree of degradation plot illustratesthat activation energy increases with the degree of degradation.The thermal stability of polymer increases with the degree of degra-dation, because the char formed during the initial phase protectsthe polymer from further degradation [31]. Moreover, degradationof HPU15 requires greater energy of activation compared to PUL

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Table 5Mechanical properties of the synthesized polymers.

Mechanical properties Composition

PUL PUUL HPU5 HPU10 HPU15

Tensile strength (MPa) 9.42 ± 0.3 11.32 ± 0.9 12.9 ± 0.9 14.31 ± 0.6 16.09 ± 0.7Elongation at break (%) 404 ± 3 452 ± 8 456 ± 7 495 ± 5 528 ± 2Scratch hardness (kg) 4 ± 0.1 5 ± 0.1 5 ± 0.1 6 ± 0.2 8 ± 0.1Impact resistance(cm) 85 90 90 95 95Gloss (60◦) 97.4 ± 0.9 97.8 ± 0.6 98.0 ± 0.7 98.0 ± 0.3 103 ± 0.5

Fig. 6. Ozawa plots for HPU15 and PUL.

(Fig. 7). An accurate kinetic degradation study is not easy to per-form as the degradation mechanism is not certain; hence, the datashown may not always fit for the practical process. Nevertheless itis sufficient to give a probable picture of the degradation process.

3.7. Mechanical properties

Mechanical properties of HPUs depend on many factors viz. thepresence of inter and intra molecular interactions, presence of polargroups within the polymeric chains, entanglement of chains, natureand type of reactants and molecular chain length of the polymer[27]. Synthesized HPUs possess high tensile strength along withgood other mechanical properties viz. scratch hardness, impactresistance and gloss (Table 5). All the polymers showed bending<1 mm. In general, the performance in these tests improved withincreasing amount of tri-functional moiety in the polymer. Also,

Fig. 7. Energy of activation at different degrees of degradation for HPU15 and PUL.

PUUL exhibited slightly better performance characteristics thanPUL. This can be attributed to the presence of extensive intra- andinter molecular hydrogen bonding arising from the presence ofurethane and urea linkages. The improved mechanical propertiesof HPUs arise from the confined structural geometry.

3.8. Chemical resistance

All the polymer compositions showed very good chemicalresistance under the experimental chemical environments of 10%aqueous sodium hydroxide (w/v), 10% aqueous sodium chloride(w/v) and 5% aqueous hydrochloric acid (v/v) solutions. No weightloss was recorded in distilled water. The weight losses (%) after 10days of testing are shown in Table 6. From the data it is clear thatno significant difference in terms of weight loss was observed forany of the polymer compositions. However HPUs and PUUL wereslightly superior to PUL which may be due to the presence of urealinkages in the former which are known to possess good chemicalresistance under severe conditions [9]. No effect was observed fordifferent solvents like methanol, ethanol, xylene and toluene.

3.9. Biodegradation

Biodegradation profile of the films was analyzed by determiningthe weight loss due to bacterial exposure. The retention of weightpercentage plotted against the incubation period of the films isshown in Fig. 8(a). It is quite clear that PUL showed less weight loss

Table 6Weight changes (%) under different chemical media for the polymers.

Polymer code 10% aq. NaOH 10% aq. NaCl 5% aq. HCl

PUL 0.098 0.0105 0.0108PUUL 0.008 0.0090 0.0078HPU5 0.004 0.0097 0.0079HPU10 0.002 0.0094 0.0080HPU15 0.000 0.0098 0.0078

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1426 S. Gogoi et al. / Progress in Organic Coatings 77 (2014) 1418–1427

Fig. 8. (a) Weight loss profiles for HPU15 and PUL, (b) bacterial growth curves of P. aeruginosa on HPU15 and PUL, and (c) Representative SEM images for the biodegradedHPU15 and PUL by P. aeruginosa after 6 weeks.

Table 7Mechanical properties of HPU15 and PUL after biodegradation.

Mechanical properties HPU15 PUL

Before After % change Before After % change

Tensile strength (MPa) 16.09 9.28 42.32 9.42 6.51 30.89Elongation at break (%) 528 205 61.17 404 180 55.44

compared to HPU15. This is due to the presence of biodegradableamide and urea linkages present in the hyperbranched structure.The growth profile shows a linear increment of bacterial populationwith time of incubation as evident from the optical densities (OD)data (Fig. 8(b)). OD values were higher for HPU15 than PUL. Thisis due to the better adherence of the microbes on the polymericsurface due to the unique structural architecture of the HPU. SEMmicrographs (Fig. 8(c)) further justified this observation. The tensilestrength of the samples was measured after 6 weeks of biodegra-dation. Table 7 shows a clear decrease in the tensile strengths ofthe tested films (HPU15 and PUL) with time of bacterial exposure.The elongation at break also decreased to a considerable extent dueto degradation within the segments of the polymer. However, theextent of degradation was greater for the HPU compared to PUL.

4. Conclusion

HPU and its linear analog were prepared successfully by usingcommercially available monomers. Comparison of PUL and PUULshowed that presence of both urethane and urea linkages in thelatter resulted in more secondary interactions like hydrogen bond-ing. Such interactions conferred superior mechanical and thermalproperties to the polymer. The study also reveals the architecturaleffect of the studied polymers on their properties. The architec-tural characteristics of the HPU convey superior mechanical and

thermal properties compared to the linear analog. Furthermore thehyperbranched polymer was more biodegradable compared to thelinear one that did not contain any urea linkages. Thus the superiorproperties of HPU are due to the combined effect of the interactionsarising from the urea/urethane units within the backbone and theunique structural architecture of the polymers. Based on these find-ings the synthesized HPU is a potential candidate for use as a surfacecoating material.

Acknowledgements

The authors express their gratitude to the research project assis-tance given by DBT, India through Grant No. BT/235/NE/TBP/2011,dated April 30, 2012 and SAP (UGC), India through Grant No. F.3-30/2009 (SAP-II) and FIST program- 2009 (DST), India through theGrant No. SR/FST/CSI-203/209/1 dated 06.05.2010. The authors sin-cerely thank Professor Alak K. Buragohain and Mrs. Lipika Aidew,Department of Molecular Biology and Biotechnology, Tezpur Uni-versity for their kind help during the biodegradation study.

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