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Microscopic origins of the mechanical response of nanostructured elastomeric materials Parker, Amanda J.


We use a molecular dynamics (MD) framework to study the mechanical properties of triblock copolymer materials which form thermoplastic elastomers (TPEs). These materials form physical, rather than chemical, cross-links as a result of their phaseseparated nano-structure. It is difficult, or impossible, to measure the details of network chains and monomers experimentally. However, it is these microscopic features that give rise to the material’s elastomeric properties. We use a coarsegrained bead-spring model which retains the vital details of the chain network and the nano-structured regions while removing unnecessary atomistic detail. We first present a simulation strategy for the equilibration of nano-structured copolymer melt morphologies. MD simulations with a soft pair potential that allows for chain crossing result in efficient modelling of phase segregation. We successfully reintroduce excluded volume pair interactions with only a short re-equilibration of the local structure, allowing configurations generated (with this method) to be used for studies of structural and mechanical properties. We then study the plastic deformation of triblock TPEs, probing the microscopic mechanisms operative during deformation and how they connect to the macroscopic stress response. We compare two deformation modes, uniaxial stress and strain, which emulate experimental tests and conditions around material failure. We find that triblocks’ stress response exhibits a significant increase in strain hardening compared to homopolymeric chains. We analyse several microscopic properties, including: the chain deformation, monomer displacement, deformation and division of glassy domains, and void formation. We introduce an entropic network model for the stress response utilising microscopic information about chain configurations and their topological constrains. The model assumes additive contributions from chain stretch and the stretch between chain entanglement points and results in quantitative prediction of the stress response. Only one parameter fit is required to describe both triblock and homopolymers systems. We compare our model to recent entropic models developed for vulcanised rubbers and probe its limitations and more general applicability. Extensions to more complicated architectures are possible (e.g. stars).

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