Relating Tensile Properties To Microstructural Heterogeneities In Elastomeric Materials

Abstract

Through both molecular modeling and experimental measurements, this investigation seeks a deeper understanding of the underlying relationships between nanostructure and tensile properties in elastomeric networks. We study how the elasti c response strongly depends upon microstructural heterogeneities in two different classes of elastomers: first, networks made of chains with bimodal molar mass distributions where heterogeneity arises from purely entropic effects; and second, model liquid crystal elastomers with idealized regular connectivity where microstructural heterogeneities arise from orientational entropic contributions in combination with enthalpic interactions. First, we examine why certain bimodal polymer networks exhibit pronounced improvements of their tensile properties over that of unimodal networks with similar elastic modulus. We explore the impact of composition on the network topology, chain-segment orientation, and tensile enhancement through molecular simulations in addition to 2H-NMR and SANS measurements on PDMS samples. We also derive a novel method, based on the maximum entropy principle, to extract segment orientation distributions from 2H-NMR spectra. After validating our simulation model by comparison with experimental data, we find that optimal tensile properties occur in systems where the concentration of shorter chains slightly exceeds their percolation transition and hence spatial cross-linking heterogeneity is small. Through the use of network topological footprints, we attribute the higher extensibility of certain bimodal networks (than that of unimodal networks with similar modulus) to their greater spread in the distribution of shortest path lengths between pairs of monomers. Lastly, we explore the tensile response of idealized regular liquidcrystalline elastomers via molecular simulations. Such networks exhibit a distinct sawtooth-shaped elastic response similar to that of super-tough natural materials (e.g., nacre, titin, spider silk). This unique tensile response arises from a deformation mechanism that entails the successive creation and distortion of ordered (smectic C A) chain domains in concert with crosslink segregation and layering. We also investigate the impact of chain stiffness, network architecture, and the use of end-linked tri-block copolymer chains on the elastic response. This chemical bidispersity further stabilizes the smectic chain domains, via microphase separation of the different blocks, and consequently enhances the tensile properties (e.g., toughness and modulus) of these idealized networks with mesogenic tri-block copolymer chains.