Departmental Oral Examination (Thesis Title: “Microscopic origins of the mechanical response of nano-structured elastomeric materials")

Speaker: 
AMANDA PARKER
Event Date and Time: 
Fri, 2017-10-13 10:00 - 12:00
Location: 
Room 288, Brimacombe extension
Local Contact: 
Physics and Astronomy, UBC
Intended Audience: 
Public

Abstract:
We study triblock copolymer materials which form thermoplastic elastomers (TPEs). These materials form physical, rather than chemical, cross-links as a result of their phase-separated nano-structure. It is difficult, or impossible, to measure the details of network chains and monomers experimentally. However, it is these microscopic features give rise to the material's elastomeric properties. So, we use a coarse-grained bead-spring model within a molecular dynamics framework to study triblock copolymer TPE materials. This modelling approach retains the vital details of the chain network structure and the nano-structured regions while losing 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 results in efficient modelling of phase segregation. We successfully reintroduce harder pair interactions with only a small 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 the 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 an experimental uniaxial strain test 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 simulation details which results in quantitative prediction of the stress response. The stress response is made up of additive contributions from chain stretch and stretch between chain entanglement points. The model requires only one parameter fit 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.

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