Multiscale modeling and simulation of crosslinked polymers

Abstract

The combination of physics-based modeling and coarse-grained molecular dynamics simulations is a powerful tool to understand how molecular properties and processes affect the mechanical properties of crosslinked polymers. In this dissertation polymer network structure and chain behavior are analyzed in order to connect these microscopic characteristics and deformation mechanisms to the macroscopic material properties of nonlinear elasticity and the shape memory effect. A new physics-based model of rubber elasticity is constructed which can capture the strain softening, strain hardening, and deformation-state dependent response of rubber materials undergoing finite deformations. This model is unique in its ability to capture large-stretch mechanical behavior with only three parameters that are each connected to the polymer chemistry and the important characteristics of the macroscopic stress-stretch response. Coarse-grained molecular dynamics simulations are used to analyze chain behavior during deformation. This work is the first to track primitive path length changes in a deformed polymer network. The primitive path of a polymer chain is defined as the shortest path from one end of the chain to the other which preserves the topological state of the network (i.e. retaining all inter-chain entanglements). Through a comparison of simulated networks with different structures, it is demonstrated that changes in average primitive path length are always nonaffine, even for long, entangled chain networks. A visualization of time-dependent chain conformations and the restraining “tube” in deformed networks demonstrates the viability of using primitive path analysis to quantify micro-macro deformation in crosslinked polymers. The shape memory effect in crosslinked polymers is the ability of a material to hold a deformed shape, then subsequently recover the initial shape when heated above the glass transition temperature. This work is the first to construct a suitable coarse-grained model for examining shape memory polymer behavior via molecular dynamics simulation. It is found that simply including monomer-monomer attraction in the simulation model is sufficient to reproduce the nonlinear thermomechanical trends seen experimentally. Because of the simplicity of the simulation model, these results give important information as to how to model and understand these systems.