Polymer International Polym Int 55:1209–1214 (2006)

Novel AB crosslinked polymernetworks based on 1-vinylimidazole-terminated polyurethane and poly(methylmethacrylate)Sriram Venkataramani,1 Tharanikkarasu Kannan,2 and Ganga Radhakrishnan3∗1Aortech Biomaterials Pty Ltd, Melbourne, Australia2Department of Chemistry, North Campus, University of Delhi, Delhi-110007, India3Advanced Centre in Polymers, Central Leather Research Institute, Adyar, Chennai-600020, India

Abstract: 1-Vinylimidazole-terminated telechelic polyurethanes were prepared from 1-vinylimidazole andbromine-terminated polyurethane. This vinyl-terminated telechelic polyurethane (VTPU) and methyl methacry-late were polymerized in the presence of benzoyl peroxide to prepare novel AB crosslinked polymer networks(ABCPs). These were characterized by spectral, thermal and mechanical studies. The absence of the characteristicpeak of vinyl group in infrared spectra of ABCP films confirms the occurrence of crosslinking. Static mechanicaltesting showed that the tensile strength of ABCP increases with increasing poly(methyl methacrylate) content.Dynamic mechanical studies revealed that ABCPs, at equal compositions of VTPU and methyl methacrylate, showgood damping properties. 2006 Society of Chemical Industry

Keywords: 1-vinylimidazole; AB crosslinked polymers; polyurethane; damping properties; quaternization;mechanical properties

INTRODUCTIONThe term AB crosslinked polymer (ABCP) refers toa polymer network where polymer A is bonded topolymer B at both ends and various points alongthe chains.1–9 Here, polymer A is bonded primarilyto polymer B and is not crosslinked to itself. ABCPshave wide commercial application and in particular areemployed in melt processing also. ABCPs are a newclass of materials, and exhibit good physical propertiessuch as improved damping properties. They are usedas adhesives, transfer-coating materials in leather andare very well suited for controlling vibration andnoise.10–12

Bamford and Eastmond described a general prepa-ration method for ABCP,13,14 where polymer A, whichcarries a reactive halogen group in its side-chains,is reacted with a metal carbonyl in the presence ofa vinyl monomer. The metal carbonyl specificallyremoves atoms from polymer A to produce radi-cal sites. These macro radicals can then initiate thepolymerization of the vinyl monomer. If the graftedpropagating chains from two polymer A chains termi-nate by combination, crosslinks are formed betweenpolymer A and polymer B, whereas termination bydisproportionation generates B branches on polymerA chains.

Ionomers have attracted technological and scientificinterest due to the low-polarity backbone combinedwith high ionic content. The ionic interactions andthe properties of the ionomer depend on the type ofpolymer backbone, ionic functionality, type of ionicmoiety and degree of neutralization/quaternization.Polyurethane ionomers are an important class ofblock copolymers that can be synthesized predom-inantly by using an ionic diisocyanate or an ionicdiol at the chain extension stage, and by post-modification of the pre-formed polyurethane.15 Moststudies on polyurethane ionomers are concentratedon random ionomers.15 Preparation of AB crosslinkedpolymers with specific arrangement of ionic groupshas seldom been reported. To this effect telechelic,i.e. end-functional polyurethane ionomers, wouldbe of great interest. In continuation of our effortson the synthesis of novel ABCPs, end-functional4-vinylpyridine telechelic polyurethane16 was suc-cessfully synthesized and used to prepare ABCPs.7

In this paper, for the first time, the synthesis of1-vinylimidazole telechelic polyurethane containingquaternized nitrogen atoms is reported. Using thesevinyl-terminated telechelic polyurethane, synthesisof novel ABCPs from methyl methacrylate is alsoreported.

∗ Correspondence to: Ganga Radhakrishnan, Advanced Centre in Polymers, Central Leather Research Institute, Adyar, Chennai-600020, IndiaE-mail: clrieco@md3.vsnl.net.inContract/grant sponsor: Council of Scientific and Industrial Research (CSIR), New DelhiContract/grant sponsor: Central Leather Research Institute, Chennai(Received 25 March 2005; revised version received 18 December 2005; accepted 6 February 2006)Published online 15 August 2006; DOI: 10.1002/pi.2042

2006 Society of Chemical Industry. Polym Int 0959–8103/2006/$30.00

S Venkataramani, T Kannan, G Radhakrishnan

EXPERIMENTALMaterialsPoly(tetramethylene oxide) glycol of number averagemolecular weight (Mn) 1000 g mol−1 (PTMG1000),polycaprolactone diol with Mn 1250 g mol−1

(PCL1250) and poly(propylene oxide)glycol of Mn

1000 g mol−1 (PPG1000) (Aldrich, USA) were driedunder vacuum at 105 ◦C before use. Tolylene diiso-cyanate (TDI; a mixture of 80% 2,4- and 20%2,6- isomers; Aldrich, USA) and dibutyltin dilau-rate (DBTDL; Aldrich, USA) were used as received.2-Bromoethanol (Aldrich, USA), methyl methacrylate(MMA; SD Fine Chem, India) and 1-vinylimidazole(Aldrich, USA) were purified using conventionalpurification methods and the middle portions fromdistillation were stored at 0–4 ◦C until use. Benzoylperoxide (BPO; SD Fine-Chem., India) was recrystal-lized from ethyl alcohol and stored at 0–4 ◦C until use.

CharacterizationFourier transform nuclear magnetic resonance(FT-NMR) spectra were recorded on a Bruker MSLp300 MHz instrument using deutrated dimethyl sul-foxide as the solvent and tetramethylsilane as aninternal standard. The FTIR spectra were recordedwith a Nicolet Avatar 360 FTIR instrument. Ther-mogravimetric analysis (TGA) was carried out usinga DuPont 951 thermogravimetric analyzer at a heat-ing rate of 10 ◦C min−1 under nitrogen atmosphere.Differential scanning calorimetry (DSC) analysis wascarried out using a DuPont 910 DSC instrumentat a heating rate of 10 ◦C min−1 under nitrogenatmosphere. Number-average (Mn) molecular weightswere determined by gel permeation chromatography(GPC) using a Waters liquid chromatograph equippedwith a 410 differential refractometer and 4 µ-Styragelcolumns (106, 105, 104 and 103 A) in series. Dimethyl-formamide (DMF; 0.01% LiBr added) was used as aneluent at a flow rate of 1.0 mL min−1 and the molec-ular weight calibration was done using polystyrenestandards. Dynamic mechanical studies were carriedout using a DMA 2980 dynamic mechanical analyzer(TA instruments, USA). For DMA studies, ABCPfilms having dimensions of 20 mm × 10 mm × 2 mmwere used. The tensile mode was studied over thetemperature range −100 ◦C to +100 ◦C at a heat-ing rate of 5 ◦C min−1. A strain amplitude of 20 µmand a frequency of 1 Hz were used as characteriza-tion conditions in DMA. The ABCP samples (at leastfive specimens each) for stress–strain analysis werecut into dimensions 40 mm × 10 mm and all the spec-imens were kept for conditioning at a temperatureof 20 ± 2 ◦C and relative humidity of 65 ± 2% for24 h before testing. Tensile testing was done using anInstron Universal Testing machine (Model 4501) atan elongation rate of 100 mm min−1. The specimensconformed to an ASTM D-142.

Synthesis of VTPUsDried polyol (10 g, 0.01 mol) was placed in a100 mL three-necked round-bottom flask, fitted with

a mechanical stirrer, nitrogen inlet, a dropping funnel,and heated in an oil bath. When the temperaturereached 65 ◦C, TDI (3.48 g, 0.02 mol) was addedduring stirring. The temperature was then increased to70 ◦C and the reaction proceeded until the isocyanatecontent reached half the initial value (as determinedby dibutylamine titration). Then 2-bromoethanol(2.48 g, 0.02 mol) was added dropwise followed bythe addition of DBTDL (0.01 g) catalyst. After 3 h,the content was cooled to room temperature, 1-vinylimidazole (1.88 g, 0.02 mol) was added, and themixture stirred for another 24 h at room temperature.The resulting vinyl-terminated polyurethane (VTPU)was stored at 0–4 ◦C until use.

Synthesis of ABCPDifferent weight ratios (8/2, 6/4, 5/5, 4/6, 2/8) ofPTMG1000-based VTPU and MMA were prepared.As the total weight of VTPU/MMA taken was fixedat 10 g, varying weight ratios of VTPU (2–8 g;0.00224–0.00897 mol based on vinyl group) weremixed homogeneously with calculated amounts offree-radical initiator (BPO, 0.0012 mol) at roomtemperature. This mixture was injected into aglass mould (toughened glasses of thickness 4 mm)in such a way that 2 mm thick sheets wereproduced after polymerization. The glass mould,which consists of VTPU, MMA and BPO mixture,was placed in a thermostated water bath at 75 ◦Cfor 48 h. The resulting 2 mm thick sheets were thenplaced in a vacuum oven at 60 ◦C for 48 h toremove unreacted monomer. The same procedurewas adopted to prepare ABCPs of 5/5 weightwise(VTPU/MMA) composition with different polyols.When the specimens were immersed in DMF for aspecific period of time (48 h), they swelled but did notdissolve, confirming complete crosslinking. After 48 h,the films were kept in a vacuum desiccator to removeexcess DMF.

RESULTS AND DISCUSSIONThe preparation of 1-vinylimidazole-terminatedpolyurethane is reported in this paper for the first timeas shown in Scheme 1, which also gives the outline ofthe synthetic route of ABCP from 1-vinylimidazole-terminated polyurethane and MMA.

Synthesis and purification of ABCPsAs given in Scheme 1, ABCPs were prepared from1-vinylimidazole-terminated polyurethane and MMA.To study the effect of polyurethane (PU) andpoly(methyl methacrylate) (PMMA) content on theproperties of the resulting ABCPs, different weightratios (2/8, 4/6, 5/5, 6/4, 8/2) of 1-vinylimidazoleterminated-polyurethane and MMA were used toprepare ABCPs. Then a Soxhlet extraction was carriedout to eliminate any unreacted MMA or PU. Forthis, ABCP strips (1 g; 2 mm thick) were extractedusing benzene as a solvent. After 24 h, the extracted

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Novel AB crosslinked polymer networks

Scheme 1. Synthesis of ABCP from VTPU and MMA.

strips were dried under vacuum. The weight loss wasfound to be very low (1–4%) in all cases and thisis evidence for complete crosslinking. The observedweight loss may be due to the presence of tracesof trapped solvent (DMF). Moreover, there is nopeak corresponding to the vinyl double bond in theFTIR spectrum of the extracted material and hencethe weight loss cannot be due to any unreactedmonomer.

Spectral studiesThe telechelic polyurethanes and ABCPs werecharacterized using FTIR and NMR spectroscopyto determine their structure. 1H-NMR data ofbromo-terminated polyurethane and VTPU are givenin Table 1. In 1H-FT-NMR spectra of bromo-terminated polyurethane, the CH2 attached tobromine resonates at 3.45 ppm, and Br–CH2 –CH2

attached to urethane groups resonates at 4.37 ppm,whereas in the 1H-FT-NMR spectrum of VTPU,the protons of Br–CH2 at 3.45 ppm completelydisappeared. The complete disappearance of the peakat 3.45 ppm in the case of cationomers shows that thenucleophilic substitution of the bromine by the tertiarynitrogen of the 1-vinylimidazole proceeds to completeconversion. The conversion of the cationomers isaccompanied by the appearance of new peaks between4.6 and 4.8 ppm which correspond to new CH2 –N+

bonds. The protons of the double bond presentin 1-vinylimidazole moieties resonate between 5.7and 6.7 ppm. The other protons present in bromo-terminated polyurethane and VTPU are also given inTable 1. Figure 1 shows the FTIR spectrum of VTPUprepared from PTMG1000, TDI, 2-bromoethanol and1-vinylimidazole. The peak at 1680 cm−1 confirmsthat vinyl double bonds are present. The characteristicpeaks due to the NH and C=O of urethane groupsappear at 3300 and 1725 cm−1, respectively. Thepolyurethane cationomers show a strong peak at1640 cm−1 and this is specifically due to quaternizationof heterocyclic nitrogen.6 This peak is due to C=Nstretching, which is influenced by the electromericeffect and undergoes a shift to lower frequency.The peak at 1540 cm−1 is due to C–N stretchingand N–H deformation. The C–O–C stretching

Table 1. 1H-NMR data of bromo-terminated polyurethane and VTPU

Bromo-terminated polyurethane VTPU

1HChemical

Shift (ppm) 1HChemicalshift (ppm)

Br–CH2 – 3.45 +N–CH2 – 4.6–4.8Br–CH2 –CH2 – 4.37 +N–CH2 –CH2 – 4.37N–H 8.5–9.5 N–CH=CH2 – 5.7–6.7Aromatic 1H 7.0–7.4 N–H 8.5–9.5CH3 of 2,4-TDI 2.19 Aromatic 1H 7.0–7.4CH3 of 2,6-DI 2.11 CH3 of 2,4-TDI 2.19–CH2 of PTMG 1.6 CH3 of 2,6-DI 2.11–OCH2 of PTMG 3.3 –CH2 of PTMG 1.6–OCH2 in urethane PTMG 4.05 –OCH2 of PTMG 3.3–OCH2 of PPG 3.3 –OCH2 of urethane of PTMG 4.05CH3 of PPG 1.1 –OCH2 of PPG 3.3–OCH2 in urethane PPG 4.1 CH3 of PPG 1.1–OCH of PPG 3.3–3.6 –OCH2 of urethane of PPG 4.1–OCH in urethane PPG 4.5 –OCH of PPG 3.3–3.6–OCH2 of PCL 3.3 –OCH of urethane of PPG 4.5–CH2 –CH2 –CH2 – of PCL 1.2–1.6 –OCH2 of PCL 3.3–CH2 –COO– of PCL 3.6 –CH2 –CH2 –CH2 – of PCL 1.2–1.6–OCH2 in urethane PCL 4.0–4.5 –CH2 –COO– of PCL 3.6–OCO–CH2 – of PCL 4.0–4.5 –CH2 –COOCH2 of PCL and –O–CH2 – of urethane PCL 4.0–4.5

Polym Int 55:1209–1214 (2006) 1211DOI: 10.1002/pi

S Venkataramani, T Kannan, G Radhakrishnan

Figure 1. FTIR spectra of: (a) VTPU; (b) ABCP from PTMG1000-basedVTPU and MMA.

of the polyurethane is seen as a broad peak at1100 cm−1 showing that the ether group is alsoinvolved in hydrogen bonding. The FTIR spectrumof PTMG1000-based ABCP of 5/5 (VTPU/MMA)composition is also given in Fig 1. The absence ofabsorption at 1680 cm−1 suggests the absence of vinylgroups and complete crosslinking.

Dynamic mechanical studiesThe results of dynamic mechanical studies aretabulated in Table 2 for composition variation andin Table 3 for polyol variation. For comparison, theglass transition temperature (Tg) of VTPU obtainedfrom DSC and the literature Tg of atactic PMMAare also given in Table 2. Similarly Tg of VTPUbased on PPG1000 and PCL1250 are given in Table 3.The 8/2 and 2/8 compositions exhibit two maxima,which suggests that these ABCPs show two phasetransitions. The 8/2 composition corresponds to theelastomeric phase, while 2/8 composition correspondsto the rigid phase. But the mid-range compositionexhibits a single maximum at an intermediate Tg ofthe individual components and hence exhibits gooddamping. The Tg of pure PMMA is around 104 ◦C,17

whereas in ABCP there is a shift in the Tg. TheTg of PU increases at the same time as the Tg ofPMMA decreases. This kind of shift in Tg is calledinward shift.7 This is in good agreement with dataavailable in the literature12 and it indicates strong

Table 2. Static and dynamic mechanical properties of ABCPs from

PTMG1000 based VTPU and MMA

Composition(weightwise)VTPU/MMA

Max. tensilestrength(MPa)

Elongationat break

(%)tan

δmax 1

Tg1(◦C)

tanδmax 2

Tg2(◦C)

10/0a – – – −51 – –8/2 5.2 142 0.23 −47 0.47 606/4 10.0 244 0.20 −25 – –5/5 16.0 327 0.27 −22 – –4/6 24.0 30 0.13 −18 – –2/8 35.0 14 0.10 −30 0.49 890/10b – – – – – 104

a Mechanical and dynamic mechanical analyses could not be done forpure VTPU as it is a viscous liquid and does not form a film. The Tg

values given here were obtained by DSC.b Pure PMMA is brittle and hence mechanical analysis could not bedone. The Tg given here is from ref. 17.

interlocking between PU and PMMA chains. A forcedcompatibility develops due to chemical crosslinkingand interlocking of the entangled chains that confinephase separation. This may be the reason for theabsence of a second maximum (Table 2). In the caseof polyol variation, both ether-based polyol systemsexhibit a single maximum, whereas the ester polyol-based system exhibits two maxima. This is due to themelting point of polycaprolactone diol block presentin the ABCP.

Static mechanical studiesStress–strain results are also shown in Table 2. Itcan be seen that when the VTPU content increases,the tensile strength decreases. However, the elonga-tion at break does not show a monotonic increase ordecrease. Instead, it increases initially with increasingMMA content (or decreasing VTPU content), reachesa maximum value of 327% as the weights of the twocomponents are equal and then decreases as the MMAcontent increases (or VTPU content decreases) fur-ther. These results are in good agreement with resultsobtained from polyurethane–polystyrene ABCPs.18,19

The increase in both tensile strength and elongationup to 5/5 composition is due to intimate physicalinterlockings18,19 between VTPU and PMMA chains.With an increase in MMA content going from the sam-ple containing 8 VTPU/2 MMA to 5 VTPU/5 MMA,the resultant ABCP exhibit viscoelastic properties withincreased tensile strength and elongation. But, withfurther increase in MMA content (and simultaneousdecrease in VTPU content), the ABCP changes fromelastic to plastic nature and hence the tensile strengthincreases and elongation decreases. This indicates thatby adjusting the feed ratio ABCPs can be tuned to betough plastics or elastomers.

Table 3 shows the effect of polyol variation in thestatic and dynamical mechanical properties of ABCPsprepared with the same composition of VTPU andMMA (5/5). It is observed that PTMG-based ABCPshows the highest tensile strength. The pendant CH3

groups in PPG-based ABCP lower the strength of

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Table 3. Static and dynamic mechanical properties of ABCPs with VTPU prepared from different polyols and MMA at 5/5 weight ratio

PolyolVariation

Mol. wt ofVTPU

(Mn × 10−3)Tg of VTPU

by DSC

Max. tensilestrength(MPa)

Elongationat break

(%)tan

δmax 1

Tg1

(◦C)tan

δmax 2

Tg2

(◦C)

PPG1000 3.9 −47 7.4 30 0.28 −19 – –PTMG1000 4.6 −51 16 327 0.27 −22 – –PCL1250 5.1 −43 12 290 0.18 −14 0.59 60a

a A close value of Tg2 was published in ref. 20.

van der Waals forces between the polymer moleculessince these groups prevent polymer cohesion andhence PPG-based ABCP has lower tensile strengththan other ABCPs. Despite the higher crystallinityof PCL compared to PTMG, the tensile strengthof PCL-based ABCP is lower. This may be due todisorientation of the polymer chains, where crystallitescan act as stress concentrators and thereby weaken thecrosslinked polymer.7,21

Thermogravimetric analysisTGA curves of ABCPs of all compositions basedon PTMG1000 are given in Fig. 2 and those at 5/5composition for different polyols are given in Fig 3.The initial decomposition temperature is taken asthe onset point. Virgin VTPU starts decomposing at180 ◦C whereas all the ABCPs, as shown in Figs 2and 3 exhibit a thermal stability higher than 300 ◦C.This indicates enhanced thermal stability of ABCPscompared to conventional polyurethane,22 which maybe due to the covalent crosslinking. From Fig. 2one observes that, as the MMA content increases,the thermal stability increases, which is due to theincreasing rigidity of the system. In the case of polyolcontent variation (Fig. 3), it shows that PCL-basedABCP shows higher thermal stability compared to theother two. In the case of PPG-based ABCP, the methylgroups in the side-chains reduce the thermal stability.TGA results in Fig. 3 show that the ester groups inPCL-based ABCP are more stable than ether groupspresent in PTMG based ABCP.

Figure 2. TGA thermograms of ABCPs prepared fromPTMG1000-based VTPU and MMA with different weightratios: (a) 8/2; (b) 6/4; (c) 5/5; (d) 4/6; (e) 2/8.

Figure 3. TGA thermograms of ABCPs prepared from differentpolyols. VTPU/MMA is 5/5 weight ratio: (a) PCL1250; (b) PPG1000;(c) PTMG1000.

CONCLUSIONSVTPU/MMA-based ABCPs with (1) varying contentsof VTPU and MMA and (2) polyol content weresynthesized, and their static and dynamic mechanicalproperties studied. Completion of crosslinking wasconfirmed by FTIR and Soxhlet extraction. FTIRshowed the absence of vinyl peaks, whereas nosignificant weight loss was observed in Soxhletextraction. The dynamic mechanical study showedthat mid-composition of PTMG1000-based ABCPexhibits highest damping. This could be due to thestrong interlocking taking place between PU andPMMA chains. Polyol variation in the mid-rangeVPTU/MMA composition shows that PTMG-basedABCP has the highest tensile strength. This maybe due to strain-induced crystallization of PTMGunits. TGA analysis revealed that the thermal stabilityincreases with increased MMA incorporation.

ACKNOWLEDGEMENTSOne of the authors (VS) would like to thankthe Council of Scientific and Industrial Research(CSIR), New Delhi, for financial support, and KTwould like to thank the Central Leather ResearchInstitute, Chennai, for awarding a quick-hire scientistfellowship.

REFERENCES1 Li H and Ruckenstein E, Polymer 36:2281 (1995).

Polym Int 55:1209–1214 (2006) 1213DOI: 10.1002/pi

S Venkataramani, T Kannan, G Radhakrishnan

2 Chiang WY and Tsai CD, E Polym J 35:1139 (1999).3 Baek SH and Kim BK, Colloid Surface A 220:191 (2003).4 Sriram V, Subramani S and Radhakrishnan G, Polym Int

50:1124 (2001).5 Sriram V, Aruna P, Naresh MD and Radhakrishnan G,

J Macromol Sci Pure Appl Chem A38:945 (2001).6 Sriram V, Mahesh GN, Jeevan RG and Radhakrishnan G,

Macromol Chem Phys 201:2799 (2000).7 Sriram V, Aruna P, Tharanikkarasu K, Venkateswarlu U and

Radhakrishnan G, J Appl Polym Sci 81:813 (2001).8 Sriram V and Radhakrishnan G, Cationomeric telechelic

polyurethane based AB crosslinked polymer networks, inPPS 2004 Polymer Processing Society Asia/Australia Meeting,pp. 49–49 (2004).

9 Jiang LJ, Hong LW, Rong Z, Rong HC and Shaoru N, Polymer32:1361 (1991).

10 Chern CH, Tseng SM and Hsieh KH, J Appl Polym Sci 74:328(1999).

11 Valea A, Gonzalez ML and Mondragon I, J Appl Polym Sci71:21 (1999).

12 Chen CH, Chen WJ, Chen MH and Li YM, J Appl Polym Sci71:1977 (1999).

13 Bamford CH, Eastmond GC and Whittle D, Polymer 10:771(1969).

14 Bamford CH, Eastmond GC and Whittle D, Polymer 12:247(1971).

15 Ramesh S, Tharanikkarasu K, Mahesh GN and Radhakrish-nan G, J Macromol Sci Rev Macromol Chem Phys 38:481(1998).

16 Mahesh GN, Philip TG and Radhakrishnan G, Polym Bull37:737 (1996).

17 Brandrup J and Immergut EH, Polymer Handbook. Wiley, NewYork (1976).

18 Lee SJ and Kim BK, J Appl Polym Sci 82:1315 (2001).19 Sperling LH, Interpenetrating Polymer Networks and Related

Materials. Plenum, New York, p. 17 (1981).20 Cuiqing T, Kai Y, Ping J and Muhuo Y, J Polym Sci Part A:

Polym Chem 42:5045 (2004).21 Sperling LH, Recent Advances in Polymer Blends, Grafts, Blocks.

Plenum Press, New York (1974).22 Oretel G, Polyurethane Handbook. Hanser, New York (1985).

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