Contribution to elasticity from trapped entanglements in polymer networks

CAMPISE F; ACOSTA RH; MONTI GA; AGUDELO, D.C.; VILLAR MA; EM VALLES; DA VEGA

Fox do Iguazu

Encuentro; Encuentro de otoño 2018 de la Brazilian Physical Society Meeting; 2018

Sociedad Brasilera de Física

Trapped entanglements, cross-linker functionality, and elastically e®ective chains are the sources of elasticityof polymer networks and gels. However, despite more than 80 years of theoretical and experimental researchin this ¯eld, still little is known about their relative contribution to network elasticity.The simplest models that capture the elastic behavior of polymer networks in terms of the average size of thepolymer chains that make up the system (network strands) and the functionality of cross-linker points arethe a±ne and phantom model.1;2. In the phantom model, the cross-link junctions are not ¯xed in space butcan °uctuate around their average positions. 3 As a consequence, the elasticity of the network is reduced bya factor (f ¡ 2)=f, with f being the functionality of the cross-linker. It has been observed that these modelscan underestimate the elasticity of real networks due to the contribution of trapped entanglements. 4 Defects,like entangled loops5 and free and dangling molecules, 6 reduce the elasticity and dictate the nonrecoverabledissipative response.In this work, double quantum (DQ) NMR experiments on model end-linked networks with cross-linkers ofmixed functionality and well-known structural parameters were addressed to study the correlations betweenelasticity and network architecture. We study the relationship between the residual dipolar coupling constantDres obtained by DQ experiments and the network topology on poly- (dimethylsiloxane) (PDMS) networkswith di®erent average functionalities and accurately controlled contents of defects.An order parameter that condensates the elastic response within the theoretical framework of the entangledphantom theory for rubber elasticity was identi¯ed7. Here we show that the contribution of trapped entangle-ments may equal the contribution coming from elastically active material and that it is independent of network topology.