Mechanical and thermo-mechanical studies of double networks ...

ARTICLEFIGURE 5 Free radical crosslinking mechanism in styrenic and olefinic phase.benzophenone was used. 44 Using benzophenone can resultin an increase in crosslinking in both styrenic as well as olefinicphases, and reduce the scission reactions.The photo crosslinking of EPDM has also been studied, 35,45–48and the mechanism is shown in Figure 6. A photoinitiatorsuch as benzophenone is used to crosslink the ethylene norborneneunsaturation.Infra-Red SpectroscopyFourier transform infrared (FTIR) spectroscopy was utilizedto qualitatively determine the extent of crosslinking. Infraredspectra were obtained on a Perkin–Elmer Spectrum OneFTIR spectrometer.Mechanical PropertiesMonotonic uniaxial tensile properties were determined usingan Instron 4411 tensile test machine, at a constant crossheadspeed of 20 mm/min at 23 C. ASTM D1708 test geometrywas used for all the samples used for mechanical testingas shown in Figure 7. 49 Samples were tested in the directionparallel to the direction of stretch during cure.The engineering stress versus extension ratio curves wereplotted for these double networks, where extension ratios(k t ) were calculated based on the final equilibrium length ofthese double networks. These stress versus extension ratiocurves were used to calculate the effective crosslink density,which contains a contribution attributed to chain entanglementsand loose chain ends acting as crosslinks in rubber.The effective crosslink density was calculated according tothe Mooney–Rivlin equation, shown in eq 2.t ¼r2RTðk t 1k t2 Þwheret ¼ the effective crosslink density;r ¼ the engineering stress;R ¼ the universal gas constant;T ¼ the absolute temperature;k t ¼ the extension ratio of the sample.Dynamic Mechanical AnalysisDynamic mechanical properties were determined using aDynamic Mechanical Analyzer (TA Instruments-DMA 2980)at a constant frequency of 1 Hz and 0.1% strain. Thin filmsamples were oscillated in the direction parallel to thestretch direction during cure, over a temperature range of80 to 250 C, ramped at 5 C/min.Thermo-Mechanical PropertiesThermo-mechanical properties, including the thermal stabilityunder load and coefficient of thermal expansion, were(2)FIGURE 6 Free radical crosslinking mechanism via norborneneunsaturation of EPDM.FIGURE 7 Specimen geometry for the ASTM D 1708 standard.DOUBLE NETWORK ELASTOMERIC SYSTEMS, SINGH AND LESSER 781

JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS DOI 10.1002/POLBFIGURE 8 Infrared spectrograph (a) SEBS double network and (b) EPDM double network, each compared with samples with k c ¼ 1.studied using the film/fiber probe in TA instrument Thermo-Mechanical Analyzer (TMA) 2940CE, at constant load of0.05 N and heating rate of 3 C/min.RESULTS AND DISCUSSIONExtent of CrosslinkingThe extent of crosslinking in both SEBS and EPDM wasdetermined qualitatively using infrared spectroscopy. Thepeak chosen to compare the different samples of SEBS wasat 3400 cm 1 , which corresponds to the hydroxyl-groupbased side products which are formed in competition withthe crosslinking reaction. Hence, higher peak intensity correspondsto lower extent of crosslinking. The peak chosen tocompare the different samples of EPDM was at 808 cm 1 ,which corresponds to the norbornene unsaturation. Hence,higher peak intensity corresponds to higher extent of crosslinking.In both systems, the chosen peak intensity decreaseswith an increase in k c , as shown in Figure 8.The increase in extent of curing can be attributed to decreasein thickness and increase in surface area available for UVcrosslinking when the samples are stretched to higher k c .AsUV crosslinking is governed by the penetration depth, a lowerthickness will lead to a higher extent of crosslinking. Polymerchain orientation causes crowding of chains and there is ahigher probability of two chains combining together to form acrosslink than leading to other side scission reactions.Tensile ResponseSEBS samples were stretched to various k c andthencured.Whereas EPDM samples were partially cured and thenextended to k c before curing them further. The percent permanentset and the residual extension ratio of the variousdouble-networked rubbers, along with their mechanicalproperties, are given in Tables 2 and 3. The residual strainand permanent set were calculated based on eqs 3 and 4,respectively. 4wheree R ¼ 1 s1 u(3)e P ¼ l s l ul x l u(4)e R ¼ Residual Straine P ¼ Permanent Setl s ¼ Set lengthl u ¼ Unstretched Lengthl x ¼ Stretched LengthTABLE 2 Summary of Mechanical Properties of SEBS Samples Cured at Different k ck cPermanentSet (e P )ResidualStrain (e R )InitialModulus (MPa)FinalModulus (MPa)Elongation atBreak (%)Toughness(MPa)1 – 1.00 10.63 2.6 130 3.231.5 0.72 1.36 9.35 3.7 150 5.462 0.57 1.57 7.55 4.6 133 4.253 0.56 2.12 7.03 7.1 103 3.334 0.49 2.48 6.88 8.8 118 4.58782 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLB

ARTICLETABLE 3 Summary of Mechanical Properties of EPDM Samples Cured at Different k ck cPermanentSet (e P )ResidualStrain (e R )InitialModulus (MPa)FinalModulus (MPa)Elongationat Break (%)Toughness(MPa)1 – 1.00 3.65 1.68 318 3.581.5 0.74 1.37 3.04 1.83 183 1.872 0.67 1.67 2.81 2.02 136 1.413 0.57 2.14 3.56 2.28 100 1.094 0.52 2.56 3.35 2.36 121 1.96The aforementioned parameters are shown in Figure 9.From Tables 2 and 3, it is evident that there is a significantimprovement in overall mechanical properties of double networksformed in a stretched state as opposed to anunstretched state (k c ¼ 1). Figure 10 shows uniaxial stressversus k t curves for the SEBS double network systems. Samplestested were single network (uncured) and double networksprepared at k c ¼ 1, 1.5, 2, 3, and 4. It is evident fromFigure 10(a) that, even having the same crosslink time in allthe cured samples, the double network samples exhibithigher ultimate modulus compared with both the uncuredand the k c ¼ 1 double network. This agrees with independentnetwork hypothesis. Analyzing the data carefully, it isevident from Figure 10(b) that there is a transition in modulusdetectable in the double network elastomer which isabsent in both the uncured sample and the sample cured atk c ¼ 1. This is due to the fact that, below the transitionpoint, the two networks are in competition with each other,while above it, they become collaborative and act in parallel,leading to an increase in modulus. The position of the transitionpoint is dependent on k c .Compared with the uncured polymer, the samples cured atk c ¼ 1.5 and 2 show a lower modulus at lower k t and ahigher modulus at higher k t , but do not exhibit strain hardeningat higher k t . Samples cured at k c ¼ 3 and 4 show aninitial drop, as compared to the one cured at k c ¼ 1, followedby an increase in the modulus, along with strain hardening.The decrease in initial modulus, while carrying outmonotonic uniaxial tensile test in all the samples is a resultof the competitive behavior of the two networks and due toheat exchange between these networks. The samples curedat higher k c shows strain hardening behavior at a lower k tvalue as compared to the one cured at k c ¼ 1, hence indicatingthe permanent anisotropy embedded in the double networkelastomer. This results in increase in the modulus ofthe double networks even at moderate k t .FIGURE 9 Diagram representing various parameters used tocalculate permanent set and residual strain.Similar behavior was observed in the case of EPDM asshown in Figure 11(a). There is an increase in modulus withincrease in k c , at values of k t beyond the transition points asshown in Figure 11(b). As the curing conditions of theEPDM double network samples have not been optimized, thetransition points are not easily separated and are very lowdue to incomplete curing.This nonlinear behavior is also evident from the Mooney–Rivlincurveshown in Figure 12, where the reduced stressrðk is plotted against the inverse of k t .Itisobservedthatt 1 Þ k t2the SEBS double network elastomers are highly nonlinear systems,whereas EPDM samples are more linear. SEBS samplesshow nonlinear behavior at smaller k t as compared to theuncured samples and the samples cured at k c ¼ 1, and thisnonlinear behavior increases with an increase in k c . EPDMdouble network samples show slight nonlinearity withincrease in k c .On the basis of the Mooney–Rivlin equation, the concentrationsof physically effective crosslinks were calculated andare shown in Figure 13. The best fit was calculated based onthe linear region of the Mooney–Rivlin plot. Even with thesame curing dosages, the crosslink density increases withincrease in k c . This is in agreement with the FTIR resultsand can be attributed to the increase in surface area anddecrease in thickness with increase in k c . This also concurswith an increase in the polymer chain orientation, leading tohigher probability of crosslinking reactions as comparedwith chain scission reactions.Dynamic Mechanical AnalysisThe introduction of double networks changes the modulus ofthe material. The storage modulus values for SEBS andEPDM samples are plotted against temperature for samplescured at different k c , as shown in Figures 14 and 15. Thegraphs can be divided into three distinct regions: the glassyregion, the transition (leathery) region, and the rubberyregion. The plateau modulus in the rubbery region is lowerfor double-networked samples cured at higher k c . Hence, thestorage modulus curves follow the thermodynamic behaviorand are governed by the entropy of the system. The drop instorage modulus plateau with an increase in k c may be associatedwith the competitive nature of the two networksformed in the sample, which was strained at a very smallstrain and at low frequency. This causes a decrease in overallDOUBLE NETWORK ELASTOMERIC SYSTEMS, SINGH AND LESSER 783

JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS DOI 10.1002/POLBFIGURE 10 Tensile results of SEBS samples cured at different k c (a) Full stress-extension results, (b) Closeup of low extension regime.FIGURE 11 Tensile results of EPDM samples cured at different k c (a) Full stress-extension results, (b) Closeup of low extension regime.FIGURE 12 Mooney–Rivlin plot (a) SEBS, (b) EPDM; corresponding to Figures 10 and 11.784 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLB

ARTICLEFIGURE 13 Physical effective crosslink densities of double networkSEBS and EPDM calculated from the Mooney–Rivlin plotshown in Figure 12.modulus of the double network system in the rubbery regionof the elastomer.The loss modulus values versus temperature for SEBS andEPDM are shown in Figure 16. In the rubbery regions ofboth the SEBS and EPDM double network, the loss modulusof the networks decreases with increase in k c . Even in theglassy regime there is slight decrease in the loss modulus,which suggests that there is an increase in crosslinking withincrease in k c . This also suggests that there is less viscousbehavior of the samples with higher degree of crosslinking.Crosslinking increases the glass transition temperature (T g )of a polymer by introducing restrictions in the molecularmotions of the chains. The plot of tan d as a function of temperaturefor SEBS and EPDM is shown in Figure 17. Thepeak of the curve is taken as the T g . The T g values of bothFIGURE 15 Storage modulus of the double network EPDMsamples.the polyolefin and the polystyrene phases show an increasewith increase in crosslink density for SEBS. EPDM alsoexhibits an increase in T g . This increase in T g can beexplained on the basis of increase in crosslink density withincrease in k c and negligible effect of orientation of chains asstudied by Munch et al. 50 The peak area is a measure of mechanicaldamping ability 27 and can be related to the serviceperformance of the products. A slight increase in damping isobserved in both SEBS and EPDM as shown in Figure 18,with an increase in k c . This may be attributed to the competitivenature of the networks in the low strain regime, andmay lead to better fatigue life as shown by Santangelo andRoland. 28The tan d and loss modulus curves appear to correlate withtrue crosslink densities, showing an increase in crosslinkingwith increase in k c . However, the storage modulus followsthe competitive behavior of the double network and hencedecreases in the low k t regime with an increase in k c .Thermo-Mechanical AnalysisThe dimensional change measured at constant load as afunction of temperature is shown in Figure 19. The coefficientof thermal expansion (CTE) was calculated using thefollowing equation:a ¼ DL KDT L(5)wherea ¼ coefficient of expansion (lm/mm C);L ¼ sample length (mm);DL ¼ change in sample length (lm);DT ¼ change in temperature ( C);K ¼ cell constant.FIGURE 14 Storage modulus of the double network SEBSsamples.It is evident that the curing at k c ¼ 1 leads to a decrease inthe coefficient of thermal expansion (CTE) of both SEBS andDOUBLE NETWORK ELASTOMERIC SYSTEMS, SINGH AND LESSER 785

JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS DOI 10.1002/POLBFIGURE 16 Loss modulus of the double network (a) SEBS and (b) EPDM samples as compared to samples with k c ¼ 1.EPDM samples and an improved thermal stability under loadas compared to the uncured sample. Hence, it may be suitablefor high temperature applications.It is also evident that the samples cured at various k c showsinteresting behavior along with increase in temperature. Allthe SEBS samples with k c ¼ 1–4 show a transition at 55–60 C, at which only the physically crosslinked uncured samplesfailed. The CTE values of all the samples, before and afterthe transition point, are given in Table 4. The CTE valuesbefore the transition point are positive for the sample curedat k c ¼ 1. The samples cured at higher k c show a decrease inCTE and in the case of k c ¼ 4, it almost reaches zero. Thiscan be attributed to that fact that the second network isleading to increase in anisotropy in the direction of stretchdue to an increase in residual strain. They also show a transitionat 55–60 C, beyond which a negative CTE isobtained. This may be due to the expansion of the double networkelastomers before the transition point, with both the networkscontributing in the expansion. However, beyond the transitionpoint, where one of the networks fails, the secondnetwork tries to attain equilibrium, and following entropicbehavior, contracts, which is seen as a negative CTE value. Thenegative CTE values also increase with increasing k c .Similar behavior is observed in the case of EPDM. The doublenetwork elastomers show a negative CTE and there is atransition point at 45 C, beyond which the slope decreasesfurther. The CTE values of all samples, before and after thetransition point, are given in Table 5. The samples do notshow a contraction after failure of the first network becauseboth networks are formed from chemical crosslinks.The first explanation of this kind of behavior of elastomerswas given by Govi to explain the Joule effect, whereFIGURE 17 Tan d of the double network (a) SEBS and (b) EPDM samples as compared to samples with k c ¼ 1.786 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLB

ARTICLETABLE 4 Summary of Coefficient of Thermal Expansionof SEBSk ca BeforeTransition(lm/mm C)a AfterTransition(lm/mm C)1 0.60 1.811.5 0.33 1.042 0.58 1.553 0.24 2.454 0.020 2.47stretched state, which lead to contraction upon heating, evenwith higher crosslinking densities.FIGURE 18 Increase in area under the olefenic tan d peak ofSEBS and EPDM with increase in k c .elastomers show a contraction when they are under strain.He assumed rubber as a ‘‘sort of solid foam composed of innumerablevesicles full of gas.’’ On stretching the rubber,according to him, the vesicles stretch in the direction ofextension and contract in the perpendicular direction. Onapplication of heat, the gas in the vesicles will result inexpansion and each vesicle would want to return to its originalminimized free energy state of a sphere. This would leadto linear contraction. 51 The analogy to the gaseous behaviorof rubbers suggests the entropic dependence of these elastomers.These double networks hence have stretched chains,frozen in the system, by introducing the second network inCONCLUSIONSSEBS- and EPDM-based double networks have been formedand investigated. In both systems, a competitive regime anda collaborative regime have been identified, and the transitionpoint is related to elongation of the sample during theformation of the second network. The storage modulusdecreases in the competitive regime, even though crosslinkdensity and T g increase and loss modulus decreases. Thisindicates the competitive regime is purely entropic and independentof the network changes, as observed between thephysical networks for SEBS and chemical networks forEPDM. Constitutive equations governing this behavior arestill to be investigated. These double network systems alsoform stiffer materials with higher crosslinking densities, yetshow a lower or even a negative coefficient of thermalexpansion, without addition of any filler. This is due to theentropic restrictions in this class of material. These materialsFIGURE 19 TMA results of (a) SEBS, (b) EPDM double network samples cured at different k c at constant load of 0.05 N and temperatureramp rate of 3 C/min.DOUBLE NETWORK ELASTOMERIC SYSTEMS, SINGH AND LESSER 787

JOURNAL OF POLYMER SCIENCE: PART B: POLYMER PHYSICS DOI 10.1002/POLBTABLE 5 Summary of Coefficient of Thermal Expansionof EPDMk ccan be very useful in many industrial applications such aslamination, where a balance of stiffness and expansion coefficientis required. They also have the potential to showimprovement in other properties, such as fatigue and toughness,because of a high dependence on the entropic behaviorof chains. These properties, along with the thermodynamicsof these systems are still to be investigated.The authors gratefully acknowledge Kathryn Wright, KratonPolymers LLC for and Sudhin Datta, Exxon Mobil for providingthe materials used in this study.REFERENCES AND NOTESa BeforeTransition(lm/mm C)a AfterTransition(lm/mm C)1 0.112 1.171.5 0.61 1.162 2.442 5.393 2.832 6.834 2.914 9.031 Roland, C. M.; Warzel, M. L. Rubber Chem Technol 1990, 63,285–297.2 Treloar, L. R. G. The Physics of Rubber Elasticity; Oxford UniversityPress: London, England, 1975.3 Santangelo, P. G.; Roland, C. M. Rubber Chem Technol 1994,67, 359–365.4 Andrews, R. D.; Tobolsky, A. V.; Hanson, E. E. J Appl Phys1946, 17, 352–361.5 Baldwin, F. P.; Woodbridge, N. J. U.S. Pat. 2,488,112 (1945).6 Erman, B. Biol Synth Polym Networks 1988, 497, 497–508.7 Miller, W.; Smith, C. W.; Mackenzie, D. S.; Evans, K. E. JMater Sci 2009, 44, 5441–5451.8 Flory, P. J. Trans Faraday Soc 1960, 56, 722–743.9 Kramer, O.; Ty, V.; Carpente, R. L.; Ferry, J. D. Bull Am PhysSoc 1973, 18, 318–318.10 Kramer, O.; Carpenter, R. L.; Ty, V.; Ferry, J. D. Macromolecules1974, 7, 79–84.11 Carpenter, R. L.; Kramer, O.; Ferry, J. D. Bull Am Phys Soc1977, 22, 429–429.12 Carpenter, R. L.; Kramer, O.; Ferry, J. D. Macromolecules1977, 10, 117–119.13 Carpenter, R. L.; Kramer, O.; Ferry, J. D. J Appl Polym Sci1978, 22, 335–342.14 Berry, J. P.; Scanlan, J.; Watson, W. F. Trans Faraday Soc1956, 52, 1137–1151.15 Flory, P. J. J Am Chem Soc 1956, 78, 5222–5234.16 Baxandall, L. G.; Edwards, S. F. Macromolecules 1988, 21,1763–1772.17 Okumura, K. Eur Lett 2004, 67, 470–476.18 Meissner, B.; Matjka, L. Polymer 2003, 44, 4611–4617.19 Budzien, J.; Rottach, D. R.; Curro, J. G.; Lo, C. S.; Thompson,A. P. Macromolecules 2008, 41, 9896–9903.20 Rottach, D. R.; Curro, J. G.; Budzien, J.; Grest, G. S.; Svaneborg,C.; Everaers, R. Macromolecules 2006, 39, 5521–5530.21 Rottach, D. R.; Curro, J. G.; Budzien, J.; Grest, G. S.; Svaneborg,C.; Everaers, R. Macromolecules 2007, 40, 131–139.22 Rottach, D. R.; Curro, J. G.; Grest, G. S.; Thompson, A. P.Macromolecules 2004, 37, 5468–5473.23 Svaneborg, C.; Everaers, R.; Grest, G. S.; Curro, J. G. Macromolecules2008, 41, 4920–4928.24 Gillen, K. T. Macromolecules 1988, 21, 442–446.25 Roland, C. M.; Santangelo, P. G. World Pat. 95/26,367(1995).26 Roland, C. M.; Peng, K. L. Rubber Chem Technol 1991, 64,790–800.27 Aprem, A. S.; Joseph, K.; Thomas, S. J Appl Polym Sci2004, 91, 1068–1076.28 Santangelo, P. G.; Roland, C. M. Rubber Chem Technol2003, 76, 892–898.29 Mott, P. H.; Roland, C. M. Macromolecules 2000, 33,4132–4137.30 Kaang, S.; Gong, D.; Nah, C. J Appl Polym Sci 1997, 65,917–924.31 Kaang, S.; Nah, C. Polymer 1998, 39, 2209–2214.32 Kaang, S.; Gong, D.; Nah, C.; Kim, S.; Kim, J. M. PolymKorea 1997, 21, 282–288.33 Marykut-Ry, C. V.; Mathew, G.; Thomas, S. Rubber ChemTechnol 2007, 80, 809–819.34 Santangelo, P. G.; Roland, C. M. Rubber Chem Technol1995, 68, 124–131.35 Wang, J.; Hamed, G. R.; Umetsu, K.; Roland, C. M. RubberChem Technol 2005, 78, 76–83.36 Indukuri, K. K.; Lesser, A. J. Polymer 2005, 46, 7218–7229.37 Wright, K. J.; Indukuri, K.; Lesser, A. J. Polym Eng Sci 2003,43, 531–542.38 Holden, G.; Legge, N. R.; Quirk, R.; Schroeder, H. E. ThermoplasticElastomers; Hanser/Gardner Publications, Inc.: Cincinnati,1996.39 Singh, N. K.; Lesser, A. J. Soc Plast Eng Annu Tech Conf2009, 67, 868–873.40 Lesser, A. J.; Singh, N. K.; Mamodia, M. (to University ofMassachusetts, Amherst). U.S. /12/615,717, (2009).41 Allen, N. S.; Edge, M.; Wilkinson, A.; Liauw, C. M.; Mourelatou,D.; Barrio, J.; Martinez-Zaporta, M. A. Polym Degrad Stab2000, 71, 113–122.788 INTERSCIENCE.WILEY.COM/JOURNAL/JPOLB

ARTICLE42 Luengo, C.; Allen, N. S.; Edge, M.; Wilkinson, A.; Parellada, M. D.;Barrio, J. A.; Santa, V. R. Polym Degrad Stab 2006, 91, 947–956.43 Allen, N. S.; Edge, M.; Mourelatou, D.; Wilkinson, A.; Liauw,C. M.; Parellada, M. D.; Barrio, J. A.; Quiteria, V. R. S. PolymDegrad Stab 2003, 79, 297–307.44 Ramaratnam, K.; Malphrus, J.; Luzinov, I. Polym Mater SciEng 2006, 95, 856–857.45 Hilborn, J.; Ranby, B. Rubber Chem Technol 1988, 61,568–576.46 Hilborn, J.; Ranby, B. Rubber Chem Technol 1989, 62,592–608.47 Wang, W. Z. J Appl Polym Sci 2004, 93, 1837–1845.48 Wang, W. Z.; Wu, Q. H.; Qu, B. J. Chem Res Chin Univ2004, 20, 111–113.49 ASTM. Standard test method for tensile properties of plasticsby use of microtensile specimens, ASTM D 1708–02a, WestConshohocken, PA, 2002; p 3.50 Munch, E.; Pelletier, J. M.; Sixou, B.; Vigier, G. Polymer2006, 47, 3477–3485.51 Whitby, G. S. Monographs on Industrial Chemistry—PlantationRubber and the Testing of Rubber; Longmans Green andCo.: Manchester, London, 1920; p 459.DOUBLE NETWORK ELASTOMERIC SYSTEMS, SINGH AND LESSER 789