Novel AB crosslinked polymer networks from telechelic 4-vinylbenzyl carbamate terminated polyurethanes and different vinyl monomers

Received: 8 August 2008, Revised: 1 October 2008, Accepted: 7 October 2008, Published online in Wiley InterScience: 13 January 2009

Novel AB crosslinked polymer networks fromtelechelic 4-vinylbenzyl carbamate terminatedpolyurethanes and different vinyl monomers

Sriram Venkataramania, Tharanikkarasu Kannanb*, Palash Jyoti Dasb

and Ganga Radhakrishnanc

Novel AB crosslinked polymer (ABCP) networks were synthesized from telechelic 4-vinylbenzyl carbamate terminatedpolyurethanes and monomers such as styrene, 4-vinylpyridine, methyl methacrylate and butyl acrylate. Telechelic4-vinylbenzyl carbamate terminated polyurethanes were synthesized from polypropylene glycol-based NCO-terminated polyurethane and vinylbenzyl alcohol. Effect of changing the molecular weight of polypropylene glycolon the static and dynamic mechanical properties of ABCP networks from polyurethane-polymethyl methacrylate wasstudied in detail. Dynamic mechanical thermal analysis results show that polymethyl methacrylate and polystyr-ene-based ABCPs have good damping over a broad temperature range. ABCP networks prepared from 4-vinylbenzylcarbamate terminated polyurethane and different monomers such asmethyl methacrylate, butyl acrylate and styreneexhibit single tan dmax value which implies excellent interlocking between the two polymers present in the ABCPnetworks. Static mechanical studies showed that methyl methacrylate and styrene-based ABCP networks exhibitbetter tensile properties compared to other ABCP networks from butyl acrylate and 4-vinyl pyridine monomers.Thermogravimetric analysis results revealed that the ABCP networks showed an improved thermal stability. Copyright� 2009 John Wiley & Sons, Ltd.

Keywords: polyurethanes; telechelics; thermal properties; AB crosslinked polymers; vinyl terminated polymers

INTRODUCTION

Polyurethanes are an important class of block copolymers forwhich the properties of the end product can be designedaccording to the need of the end-user.[1] The main disadvantagesof these segmented polymers are low thermal stability and poormechanical properties to some extent. Mechanical propertiesand thermal stability of polyurethane (PU) can be improved bycrosslinking amongst PU chains. Among the different type ofcrosslinking methods, AB crosslinked polymer (ABCP) network isrelatively novel and its properties are unusual from the othercrosslinked networks. In ABCP, polymer A is bonded to polymer Bat both ends or at various points along the chain.[2,3] Ideally asingle network is generated in ABCP, i.e., polymer A is bondedprimarily to polymer B and is not crosslinked itself.[4–8] ABCP isgenerally prepared by reacting functional group containingpolymer (polymer A) with vinyl monomers (source of polymer B)in the presence of a free-radical initiator.[9] The polymer networksprepared by this method have improved physical properties suchas increased tensile strength, less swelling behavior andincreased damping.Different types of ABCPs with improved properties were

reported in the literature. ABCPs based on epoxy resin/benzoxazine-functionalized poly(oxypropylene),[10] PU/poy(methyl methacrylate)(PMMA),[11–17] polynorbornene/PMMA,[18] PU/poly(N-isopropylacrylamide),[19] chitosan/poly(N-isopropyl acrylamide),[20] polystyrene/polysiloxane,[21] polyisoprene/polysiloxane,[21] polyacrylamide/PU,[22] aqueous PU/polystyrene,[17,23,24] polybenzoxazine/poly(imide-siloxane),[25] PU/novolac resin,[26–28] polycaprolactone/

PMMA,[29] PU/polystyrene-co-acrylic acid,[30] PU/polyhydrox-yethyl acrylate,[17] PU/polyvinyl acetate,[17] and polycarbonate/polystyrene[31] are some of the examples of ABCPs which havebeen reported already. These ABCPs exhibit high damping overbroad temperature range which finds applications in controllingvibration and noise. Some thermoplastic materials have dampingproperties comparable to ABCPs, but they have the disadvantageof flow above their Tg which is not a desired property for anyengineering material. ABCPs have proved to overcome thisdisadvantage,[32–36] by not flowing above their Tg due to thepresence of crosslinking. Literature survey revealed that4-vinylbenzyl carbamate terminated PU (TVBCPU) has not beenused so far to synthesize PU/polystyrene, PU/4-vinyl pyridine and

(www.interscience.wiley.com) DOI: 10.1002/pat.1326

Research Article

* Correspondence to: Dr T. Kannan, Department of Chemistry, North Campus,

University of Delhi, Delhi 110 007, India.

E-mail: [email protected]

a S. Venkataramani

AorTech Biomaterials Pty Ltd, Melbourne, Australia

b T. Kannan, P. Das

Department of Chemistry, North Campus, University of Delhi, Delhi 110 007,

India

c G. Radhakrishnan

Advanced Centre in Polymers, Central Leather Research Institute, Adyar,

Chennai 600 020, India

Contract/grant sponsor: University Grants Commission, New Delhi, India (No.

32-291/2006 (SR) 26th February 2007).

Polym. Adv. Technol. 2009, 20 892–898 Copyright � 2009 John Wiley & Sons, Ltd.

892

PU/butyl acrylate ABCP networks. Hence, in the presentinvestigation, synthesis and characterization of novel ABCPnetworks from TVBCPUs and monomers such as styrene,4-vinylpyridine, methyl methacrylate and butyl acrylate arereported.

EXPERIMENTAL

Materials

Poly(propylene oxide) gylcol of molecular weight 1000, 2000,3000, and 4000 Dalton (PPG1000, PPG2000, PPG3000, PPG4000) werereceived from Aldrich, USA and dried under vacuum before use.Toluene diisocyanate (TDI; a mixture of 80% 2,4- and 20%2,6-isomers) and dibutyltin dilaurate (DBTDL) were purchasedfrom Aldrich, USA and used as received. Vinylbenzylchloride(VBC), styrene (Sty), and 4-vinylpyridine (4-VP) were purchasedfrom Aldrich, USA and methyl methacrylate (MMA) andn-butylacrylate (BA) were purchased from SD Fine Chem., India.All the monomers were washed with aqueous NaOH to removethe inhibitor and washed further with water to remove NaOH.After storing them over sodium sulphate overnight, they weredistilled under reduced pressure. The middle portions werestored at 0–48C until use. Dimethyl formamide (DMF) was distilledat reduced pressure and the middle portions were stored overmolecular sieves (type 4A) until use. Azobis isobutyronitrile (AIBN)was recrystallized from methanol and used as an initiator.p-Vinylbenzylalchohol (VBOH) was prepared from vinylben-zylchloride using the reported procedure.[37]

Synthesis of telechelic 4-vinylbenzyl carbamate terminatedpolyurethanes (TVBCPU)

Dried polyol (0.02M) was placed in a three-necked roundbottomed flask fitted with a mechanical stirrer, nitrogen inlet, andheated in an oil bath. When the temperature reached 658C, TDI(0.04M) was added drop-wise with stirring. Then the temperaturewas increased to 708C and the reaction was allowed to proceedtill the isocyanate content reached half of the initial quantity (asdetermined by dibutylamine titration). Then the temperature wasreduced to 508C, and VBOH (0.04M) and DBTDL (2mol % basedon TDI) were added successively. The reaction was allowed toproceed until the complete disappearance of NCO peak in theFourier transform infrared (FTIR) spectrum and the resultingviscous TVBCPU was stored at 0–48C until use. Different typesof TVBCPU were prepared by using PPG with differentmolecular weights and TVBCPU1000, TVBCPU2000, TVBCPU3000,and TVBCPU4000 are the TVBCPUs prepared from PPG1000,PPG2000, PPG3000 and PPG4000, respectively.

1H NMR of TVBCPU1000 (300MHz; CDCl3): d (ppm)¼ 8.5–9.5(N-H), 7.0–7.4 (aromatic 1H), 2.19 (CH3 of 2,4 TDI), 2.11(CH3 of 2,6DI), 3.3 (OCH2 of PPG), 1.1 (CH3 of PPG), 4.1 (OCH2 attached tourethane), 3.3–3.6 (OCH of PPG), 4.5 (OCH attached to urethane),4.05 (CH2 of benzyl), 6.20 (CH––CH2) 7.10 (5H ring protons andvinyl CH).

Synthesis of ABCP

To prepare ABCP, first, 50 wt% of already prepared TVBCPU wasthoroughly mixed with 50 wt% of a monomer. To thishomogenous mixture, a calculated amount of free radicalinitiator (AIBN, 0.02% w/w) was added and mixed again at room

temperature. This mixture was injected into a glass mould andplaced in a thermostated water bath at 758C for 48 hr. The glassmould (toughened glasses of thickness 4mm) is used in such away that 2mm thick sheets were produced after polymerization.The resulting films were placed in a vacuum oven at 608C for 48 hrto remove unreacted monomers and solvent. Further, to removetraces of unreacted monomers, the resulting films were soaked inDMF for 48 hr and Soxhlet extraction was carried out for 6 hr.Finally, all the films were removed from DMF washed thoroughlywith water and dried in vacuum at 508C for 48 hr.

Measurements

The FTIR spectra were recorded with Nicolet Avatar 360 infraredspectrophotometer equipped with HATR accessory. Fourier-transform nuclear magnetic resonance (FT-NMR) spectrum ofTVBCPU was recorded on a Bruker DPX-300 NMR instrumentusing deuterated dimethyl sulfoxide as the solvent andtetramethylsilane as the internal standard. Number-average(Mn) and weight-average (Mw) molecular weights and molecularweight distribution (MWD) of TVBCPU were determined by gelpermeation chromatography (GPC) using polymer laboratoriesGPC 50 integrated system equipped with a differentialrefractometer (RI Detector) and PLgel 5mm MIXED-C column.Tetrahydrofuran was used as an eluent at a flow rate of1.0mLmin�1 and the molecular weight calibrations were doneusing polystyrene standards. Differential scanning calorimetry(DSC) of TVBCPU was carried out using DSC Q200 instrument (TAinstruments, USA) at a heating rate of 108Cmin�1 under N2

atmosphere and thermogravimetric analyses (TGA) were carriedout using Du Pont 910 thermogravimetric analyser at a heatingrate of 108Cmin�1 under N2 atmosphere. Strips (20� 10� 2mm3)of each material were examined with DMA 2980, DynamicMechanical Analyser (TA instruments) in the tensile mode(tension film) in the temperature range of �1008 to þ1508C,at a heating rate of 58Cmin�1, strain amplitude of 20mm andfrequency of 1 Hz. The samples (three specimens each) forstress–strain analyses with the specification of 40mm length,10mmwidth and 2mm thickness were kept for conditioning at atemperature of 20� 28C and relative humidity of 65� 28C for24 hr before testing. The tensile testing was done using an InstronUniversal Testing machine model 4501 at an elongation rate of100mmmin�1.

RESULTS AND DISCUSSION

Synthesis of TVBCPU and ABCP networks

Scheme 1 shows synthesis of TVBCPU and ABCP networks. WhenNCO-terminated PU was reacted with VBOH, TVBCPU is formed.Table 1 gives synthesis details of different TVBCPU. As themolecular weight of PPG is increased, Mn of TVBCPU alsoincreases. This is due to the simple reason that the highmolecularweight polyol forms the high molecular weight NCO terminatedprepolymer which subsequently forms a high molecular weightTVBCPU after adding VBOH. As the molecular weight of PPGincreases, Tg of the TVBCPU decreases. As the TVBCPU molecularweight increases, the soft segment PPG content also increaseswhich increases the flexibility of the TVBCPU chain andconsequently Tg decreases.After successful synthesis of TVBCPU, it was used further to

synthesize ABCP from different monomers. Table 2 shows

Polym. Adv. Technol. 2009, 20 892–898 Copyright � 2009 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/pat

ABCPS FROM 4-VINYLBENZYL CARBAMATE TERMINATED POLYURETHANES

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synthetic details of ABCP networks from TVBCPU andMMA. ABCPnetworks prepared from TVBCPU1000 and different monomers aregiven in Table 3. The synthesized ABCP networks were soaked inDMF and unreacted monomer was extracted using Soxhlet

extraction as discussed in the Experimental Section. During theSoxhlet extraction, all the films were insoluble in DMF and theyretained their original shape. When the extracted portion wasconcentrated by removing the solvent, it was found to be lessthan 0.05%w/w in all the cases. This result confirms the formationof ABCP networks.

Structural elucidation of TVBCPU and ABCP networks

FT-NMR spectroscopy and FTIR spectroscopy were used toconfirm the structure of TVBCPU. Figure 1 (b) shows the FTIRspectrum of PPG1000 based TVBCPU and for comparison, FTIRspectrum of VBOH is also given in Fig. 1(a). In Fig. 1(a) and (b) thecharacteristic bands of the vinyl double bond at 1630 cm�1 ispresent but this peak is not present in the FTIR spectrum of anABCP which is given in Fig. 1(c). This is the clear evidence thatvinyl double bonds are intact in TVBCPU and they disappearduring the ABCP synthesis due to the initiation of these doublebonds by AIBN. The stretching vibrations of OH groups present inVBOH are observed at 3320 cm�1 which is less intense in PPG1000

based TVBCPU (Fig. 1(b)). In the case of ABCP, the characteristicband of the urethane NH appears at 3300 cm�1 and the C––Ostretching vibration of urethane groups appears at 1725 cm�1.The absence of NCO group absorption at 2250 cm�1 and thepresence of vinyl bond stretching vibration at 1630 cm�1 inFig. 1(b) prove that TVBCPU is successfully synthesized. Aftercrosslinking with MMA, the resulting PPG1000 based ABCPs(Fig. 1(c)) show no absorption at 1640 cm�1 corresponding to C––C stretching suggesting that the crosslinking is complete. Thestructure of TVBCPU was also confirmed by NMR spectroscopyand the 1H NMR data of PPG1000 is given in the ExperimentalSection. The presence of NH, CH3 (PPG) and vinyl protons areobserved at 8.5–9.5, 1.1, and 6.2 ppm respectively, which confirmthe formation of TVBCPU.

Scheme 1. Synthesis of PU-based ABCPs.

Table 1. Synthesis and characterization of TVBCPU

Code of TVBCPU

Synthesis of TVBCPU GPC results

Tg (8C)

Polyol

TDI (Mole) VBOH (Mole) Mn � 10�3 Mw � 10�3 Mw=MnPPG Mole

TVBCPU1000 PPG1000 0.02 0.04 0.04 1.8 2.5 1.4 �28TVBCPU2000 PPG2000 0.02 0.04 0.04 2.1 3.0 1.4 �31TVBCPU3000 PPG3000 0.02 0.04 0.04 2.4 3.6 1.5 �43TVBCPU4000 PPG4000 0.02 0.04 0.04 3.0 4.1 1.4 �48

Table 2. Effect of molecular weight of polyol on the synthesis of ABCP using TVBCPU and MMA at 50/50 (w/w) composition.

Code of ABCP TVBCPU Max. tensile strength (MPa) Elongation at break (%) tan dmax Tg (8C)

PU1PMMA TVBCPU1000 11.7 279 0.906 63.5PU2PMMA TVBCPU2000 12.7 282 0.822 68.4PU3PMMA TVBCPU3000 10.7 284 0.856 89.2PU4PMMA TVBCPU4000 8.1 211 0.915 93.6

www.interscience.wiley.com/journal/pat Copyright � 2009 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2009, 20 892–898

S. VENKATARAMANI ET AL.

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Dynamic mechanical properties

Figure 2 shows tan d versus temperature curves for PMMA-basedABCP networks prepared from PPG of different molecular weightsand the corresponding Tg and tan dmax values are given in Table 2.It is interesting to note that Tg of PPG is seen in TVBCPU (cf.Table 1) but there is no separate Tg for PPG block in ABCPnetworks. This is due to the crosslinking that restricts segmentalmobility and increases phase mixing in ABCP networks. Anotherinteresting result is that each PU-PMMA ABCP network exhibitssingle tan dmax, though two different polymers are present in thenetwork. This is due to a forced compatibility that developsduring chemical crosslinking and interlocking of the entangledchains that confines phase separation. The broader peaks in Fig. 2reflect a good damping behavior of PMMA-based ABCP networks.It is well known that the tan dmax value decreases with increase incrosslinking.[11] But in the present case, as given in Table 2, tandmax value decreases initially and then increases as the molecularweight of PPG increases. According to this result, crosslinkingincreases initially up to PPG 2000 and then decreases when thePPGmolecular weight increases from 2000 to 4000 but the actualfact is that crosslinking decreases as the molecular weight of PPGincreases from 1000 to 4000. To understand this, change in thefree volume of PMMA chains due to change in PPG molecularweight should also be taken into account. It is explained in theliterature[29] that the crosslinking density at the constant weightratio of vinyl terminated polyol/PMMA decreases on increasingthe molecular weight of polyol. At high crosslinking densities(applicable to PPG1000 based ABCP), the free volume of PMMA inthe ABCP network is less and at low crosslinking densities(applicable to PPG4000 based ABCP), the free volume of PMMA is

Table 3. Effect of monomer type on the synthesis of ABCP using 50/50 (w/w) of TVBCPU1000 and different monomers

Code of ABCP Monomer Max. tensile strength (MPa) Elongation at break (%) tan dmax1 Tg1 (8C) tan dmax2 Tg2 (8C)

PU-PMMA MMA 11.7 279 0.906 63.5 — —PU-PBA BA 0.3 51 1.102 4.0 — —PU-PSty Sty 16.0 190 1.303 60.2 — —PU-PVP 4-VP 32.0 20 0.357 88.0 0.081 �18.0

Figure 1. FTIR spectra of (a) VBOH, (b) TVBCPU1000, and (c) PU1PMMA.

Figure 2. Dynamic mechanical analysis curves of PU-PMMA ABCPs:

(a) PU1PMMA, (b) PU2PMMA, (c) PU3PMMA, and (d) PU4PMMA.



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more.[29] Based on these literature results, it can be explained thatinteraction between PPG and PPMA chains, and free volume ofPMMA chains in ABCP play a major role in deciding themechanical properties. Wherever interaction between PPG andPMMA is good (observed in PPG 2000 and PPG 3000 cases), thena lower tan dmax value is expected and whenever a free volume ofPMMA chains is increased, tan dmax value is expected to increase(observed in PPG 1000 and PPG 4000 cases). Though tan dmax

values are not following any trend, there is an increase in Tg as themolecular weight of PPG increases. As the molecular weight ofPPG increases, the number of vinyl double bonds decreases andas a result the crosslinking density also decreases. The decreasein crosslinking density leads to less compatibility and lessinterlocking of the entangled chains that lead to increasedmicro-phase separation. This increase in micro-phase separationbetween PPG and PMMA increases with increasing molecularweight of PPG. As the micro-phase separation increases, bothPPG and PMMA chains try to retain their original nature. Hence Tgof PMMA chain shifts toward 1048C (Tg of PMMA homopoly-mer[38]) as the molecular weight of PPG increases. Thoughmicro-phase separation is increased as the molecular weight ofPPG increases, there is no macro-phase separation between thePPG and PMMA chains in ABCP networks which is proved by thepresence of a single tan dmax value for each ABCP network inFig. 2.Damping curves of ABCPs prepared using different vinyl

monomers are given in Fig. 3 and the corresponding Tg values arecompiled in Table 3. The ABCP networks prepared from MMA, BAand Sty exhibit a single tan dmax value at an intermediate Tg of theindividual components and hence these ABCP networks exhibitgood damping. It is well known that Tg values of polymethylmethacrylate, polybutyl acrylate, poly(4-vinyl pyridine) andpolystyrene are 104, �54, 142, and 1008C respectively.[38] ButABCP networks based on these polymers and TVBCPU1000 show ashift in Tg and this kind of shift in Tg is called inward shift.[9,13] Asin the MMA case, a forced compatibility develops due to chemicalcrosslinking and interlocking in the case of ABCP networks fromdifferent monomers also. This may be a reason for the absence ofa second tan dmax value in all ABCP networks except 4-VP-basedABCP. In the case of 4-VP-based ABCP, two tan dmax values are

observed. The upper transition is more pronounced whichcorresponds to a plastic phase and the lower one appears as ashoulder which corresponds to the elastomeric phase. Thepresence of two maxima indicates the presence of hompolymer-ization of 4-VP apart from ABCP network formation.

Stress–strain properties

Stress–strain curves for PU-PMMA-based ABCP networks areshown in Fig. 4 and the corresponding results are given in Table 2.One can observe from Table 2 that as the molecular weight ofpolyol (PPG) increases from 1000 to 4000, the tensile strengthinitially increases and then decreases as the molecular weightincreases. Similar type of results were observed for ABCPnetworks prepared from vinyl terminated polycaprolactone/PMMA.[29] As discussed in the previous section, interactionbetween PPG and PMMA segments, and free volume of PMMAchains in the ABCP decide the tensile properties of thenetworks.[29] For a given ratio of TVBCPU/MMA, the tensilestrength of PPG2000 based ABCP exhibits the highest value oftensile strength which shows that PPG and PMMA chains havemore interaction in this network. In the other networks, the freevolume of PMMA plays a major role and hence tensile strength isless than that of a PPG2000 based ABCP network. Except in PPG2000

based ABCP network, other ABCP networks show a decrease intensile strength with increasingmolecular weight of the PPG. Thismay be due to the reason that as the molecular weight of PPGincreases the number of double bonds decreases, leading to lesscrosslinked networks which is reflected by the decrease in tensilestrength as we go from PPG1000 to PPG4000.

Figure 3. Dynamic mechanical analysis curves of ABCPs prepared fromTVBCPU1000 and different monomers: (a) PU-PMMA, (b) PU-PBA,

(c) PU-PSty, and (d) PU-PVP.

Figure 4. Stress–strain curves of PU-PMMA ABCPs: (a) PU1PMMA,

(b) PU2PMMA, (c) PU3PMMA, and (d) PU4PMMA.



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Stress–strain curves of ABCPs based on different monomers aregiven in Fig. 5 and the tensile data are given in Table 3. While4-VP-based ABCP behaves as a hard and brittle materialexhibiting no yield point, Sty and MMA-based ABCPs exhibitan elastomeric behavior with a smeared yield point as for soft andtough materials, which may be due to moderate physicalcrosslinking. As expected, the behavior of BA-based ABCP issimilar to that of soft and weak materials because of low Tg. FromTable 3, one can observe that 4-VP-based ABCP exhibits thehighest value of tensile strength while BA-based ABCP shows thelowest value. Depending on the nature of the polyacrylate chains,the materials can behave as tough elastomers or as hard andbrittle polymers. Introduction of a hetero atom on the styrenesystem (4-VP), increases the tensile strength and Tg tremen-dously, accompanied by a significant reduction in the dampingproperties.

Thermogravimetric analysis

Thermal stability of ABCP networks was evaluated by thermo-gravimetric analysis. TGA curves of ABCP network prepared fromTVBCPU and MMA are given in Fig. 6 and TGA curves of ABCPnetworks from TVBCPU and different types of monomers aregiven in Fig. 7. The initial decomposition, which was taken as thepoint of onset, shows no significant weight loss up to 3008Cindicating enhanced thermal stability due to covalent cross-linking. As the molecular weight of PPG increases, thermalstability of ABCP network decreases slightly as shown in Fig. 6.This is due to the decrease in the number of crosslinking siteswith increase in the molecular weight of PPG and hence thermal

stability decreases. In the case of ABCP networks from differentmonomers, MMA and Sty-based ABCP networks are thermallymore stable than the BA and 4-VP-based ABCP networks.

CONCLUSIONS

Novel ABCPs can be prepared by polymerizingmonomers such asstyrene, vinylpyridine, methyl methacrylate and butyl acrylate inthe presence of telechelic 4-vinylbenzyl carbamate terminatedpolyurethane using AIBN as an initiator. Thermal, mechanical,and damping properties can be improved by preparing ABCPnetworks. Dynamicmechanical thermal analysis results show thatMMA, Sty, and BA-based ABCP networks formed good covalentcrosslinking and hence they have good damping behavior. Staticmechanical analysis results show that MMA and Sty-based ABCPnetworks have better properties than BA and 4-VP-based ABCPnetworks. Thermal analyses show enhanced thermal stability forsynthesized ABCP networks.

Figure 5. Stress–strain curves of ABCPs prepared from TVBCPU1000

and different monomers: (a) PU-PMMA, (b) PU-PBA, (c) PU-PSty, and

(d) PU-PVP.

Figure 6. TGA thermograms of PU-PMMA ABCPs: (a) PU1PMMA, (b)

PU2PMMA, (c) PU3PMMA, and (d) PU4PMMA.

Figure 7. TGA thermograms of ABCPs prepared from TVBCPU1000 anddifferent monomers: (a) PU-PMMA, (b) PU-PBA, (c) PU-PSty, and (d)

PU-PVP.



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Acknowledgements

The authors thank the University Grants Commission, New Delhi,India (No. 32-291/2006 (SR) dated 26th February 2007) for finan-cial support. One of the authors (Mr Palsh Jyothi Das) thanks theCouncil of Scientific and Industrial Research, New Delhi, India forthe Junior and Senior Research fellowships.

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Novel AB crosslinked polymer networks from telechelic 4-vinylbenzyl carbamate terminated polyurethanes and different vinyl monomers