Mechanical and Thermo-Mechanical Studies of …

1 Mechanical and thermo - Mechanical Studies of Double Networks Basedon thermoplastic ElastomersNAVEEN K. SINGH, ALAN J. LESSERU niversity of Massachusetts at Amherst, Silvio O. Conte National Center for Polymer Research, Amherst,Massachusetts 01003-45303 Received 20 October 2009; revised 18 December 2009; accepted 19 December 2009 DOI: online in Wiley InterScience ( ).ABSTRACT:A new approach to prepare and characterize doublenetwork elastomeric systems was investigated. A styrene-ethyl-ene-co-butylene-styrene (SEBS) triblock copolymer system con-taining physical crosslinks was used to achieve a doublenetwork by additional crosslinking using ultra-violet (UV) ethylene propylene diene monomer (EPDM) terpolymersystem containing chemical crosslinks was used to achieve aconventional double network using UV crosslinking.

2 Propertiesfrom conventional monotonic tensile tests, dynamic mechani-cal analysis, and thermomechanical properties were double network elastomers show a transition betweencompetitive and collaborative behavior in their Mechanical proper-ties and lower coefficients of thermal expansion arising from acompetition of the Wiley Periodicals, Polym Sci Part B: Polym Phys 48: 778 789, 2010 KEYWORDS:crosslinking; elastomers ; Mechanical properties;networks; thermal propertiesINTRODUCTIONT raditional curing of a rubber or elastomerresults in isotropic networks and the ki-netic theory, the elasticity of rubber has been attributed tothe changes in the conformations and configurational en-tropy of a system of long-chain molecules.

3 When the chainsundergo deformation, the internal energy is considered toremain , when loading and unloading thenetwork, the heat exchange with the surroundings is due pri-marily to entropic general schematic of an elastomer s heat exchange with itssurroundings, during deformation is shown in Figure 1. Heatis given out from the elastomer to the surroundings whileloading, and is absorbed while heat exchange governs the Mechanical and thermal prop-erties of these networks. If a partially cured elastomer is firstdeformed and then subjected to additional crosslinking, a sec-ond crosslinked network can be formed within the initial net-work, as depicted in Figure 2.

4 This results in materials withunusual and enhanced properties which have been termed double network elastomers . 1,3 6 These properties arise froma competition between these two networks during smalldeformations, where heat released by one network is beingabsorbed by the other network . Thus, a lower modulus isexpected in this low strain, competitive regime relative to asingle network system. At higher elongations these networkswork in parallel to each other to provide a collaborativebehavior, and consequently a higher modulus than in a singlenetwork system is of entropic dependence of these elastomers , thestretched rubber is expected to have a lower coefficient ofthermal expansion in the stretched direction.

5 Therefore,these systems can be very useful in applications where posi-tive thermal expansion must be reduced, without using acomposite material exhibiting a negative coefficient of ther-mal concept of a double network was introduced more than50 years ago by Tobolsky and studiesexplained the physical ageing of rubbers, which involvedfree-radical scission and crosslinking under deformation, andthe independent network hypothesis they proposed has beenrigorously 14 This hypothesis suggested that theconstitutive relation for postcured networks can be calcu-lated as the sum of the stress contributions from the inde-pendent networks, each described by the classical stress strain expression.

6 Each independent network has its ownstate of a very general theoretical treatment of simi-lar systems and his expression for the elastic free energy ofthe double network system, assuming Gaussian chains, isgiven in eq t12 k2x k2y k2z 3 t22 k2x:2 k2y:2 k2z:2 3 12 t1 t2 lnVV0 1 Correspondence to: A. J. Lesser (E-mail: of Polymer Science: Part B: Polymer Physics, Vol. 48, 778 789 (2010)VC2010 Wiley Periodicals, ,ky,andkz extension ratios relative to the initial iso-tropic state;kx:2,ky:2,andkz:2 extension ratios relative to the state inwhich the second set of crosslinks is introduced;V the actual volume;Vo the reference volume;t1 number of elastically effective strands between cross-links in the isotropic unstretched state;t2 number of additional elastically effective strandsbetween crosslink in the cured sample at a stretched state;In this scheme, there are two opposing forces acting on thissystem.)

7 One is due to the network introduced in the iso-tropic state, and the other is due to the network introducedin the strained stat; thus there is a stress transfer betweenthese also proposed a theory to explain double network sys-tems using protein and Edwards16carriedout theoretical treatment to explain deformation-dependentproperties of polymer networks constructed by addition ofcrosslinks under strain. Toughness improvements were theo-retically proven by Okumura,17and Meissner and Matjka18provided a Langevin-elasticity-theory- based description ofthe tensile properties of double network rubbers. Moleculardynamics simulation Studies were also carried out on thesedouble network elastomers to understand and explain per-manent 23 Until recently, the concept of double network elastomers wasconfined to Studies of the physical ageing of elastomers .

8 Inthe last 2 decades, many Studies probed the physical proper-ties of double networks, with the intent of applying them topractical applications. Properties studied included stressrelaxation behavior,24orientation and crystallizationeffects,1,25electrical conductivity,26tensile properties,27fatigue,28birefringence,29c reep-recovery,30,31swelling,32,33failure properties,34dynamic properties, and filler , little attention has been given to the thermal andthermomechanical behavior, despite their significance inapplications such aso-rings, actuators, encapsulants, anddampers. Moreover, there has been almost very little work inthe field of thermoplastic elastomers (TPEs) have recently been studiedas a possible replacement for traditionally cured follow the thermodynamic behavior similar to tradi-tional elastomers , and their deformational behavior has beenstudied in the ,37 TPEs are generally block copoly-mers, having hard and soft segments.

9 In contrast to the tra-ditionally cured elastomers , which once cured cannot bereshaped, TPEs provide ease of processibility to give rubberymaterials which can be reshaped again, like other thermo -plastics. However, TPEs lack thermal stability and their rigid-ity is limited by the stiffness of hard ,the thermoelastic properties of TPEs make them an interest-ing competitor to traditional this work, initial results are presented from an ongoinginvestigation39,40aimed to elucidate the mechanisms behindthe Mechanical and thermomechanical behavior of competi-tive double network thermoplastic elastomers . Herein, a dou-ble-networked styrene-ethylene-co-butylene-styrene (SEBS)triblock copolymer is prepared by using physical crosslinksfrom the hard styrenic phase as the first network and curingit in a deformed state to achieve a second chemicallyFIGURE 1 Heat exchange with the surrounding for traditional isotropic 2 Formation of double network ELASTOMERIC SYSTEMS, SINGH AND LESSER779crosslinked network .

10 To compare the double networksformed from physical crosslinks and from chemical cross-links, a double-networked ethylene-co-propylene-diene-monomer (EPDM) terpolymer was also prepared, by utilizingonly the chemical crosslinks from the unsaturation. Theproperties of these materials are compared at similar totalcrosslink times, but at different extension ratios beforeimposing the second triblock copolymer was obtained from structure is shown in Figure 3, containing 20% poly-styrene in the endblocks and 20% polystyrene in terpolymer was obtained from Exxon MobilVRand itsgeneral structure is shown in Figure 4. The diene monomerpresent in the EPDM was ethylene norbornene (ENB), at aconcentration of by SEBS and EPDM were mixed with 5% benzophenone(Alfa Aesar), a UV crosslinking initiator, and melt blendedusing a Brabender batch mixer at 250 and 150 C, respec-tively.