Elasticity and Fracture of Polymer Networks with Entanglements and Weak Crosslinkers

Abstract

Topological entanglements and crosslinkers are two essential molecular structures that govern the mechanical properties of polymer networks. This dissertation combines theoretical models and molecular dynamics simulations to investigate the mechanical properties of polymer network materials, such as slide-ring gels, entangled polymer networks, and polymers with sacrificial bonds and weak crosslinkers. These materials are notable for their potential for superior mechanical performance, including better extensibility and toughness.The first part of this dissertation discusses topological entanglements, which are classified into collective and pairwise types. We first explore the elasticity of slide-ring gels, demonstrating how the sliding of pairwise entanglements modifies the elasticity and improves the extensibility of polymer networks. Then, we analyze the impact of collective entanglements by measuring them through bond-vector correlation functions and linking them to the strain-softening behavior of entangled polymer networks under uniaxial or triaxial deformations. Finally, we investigate the fracture mechanism of polymer melts under fast extensional flow, where chains form a network by these topological entanglements.The second part of the dissertation discusses using sacrificial bonds and weak crosslinkers as strategies to enhance the energy dissipation capabilities of polymers, thus improving their toughness and fracture resistance. We present theoretical models for the force-extension curves of single polymer chains with sacrificial bonds and stored length, which are compared with simulation results. Then, we explore the effect of weak crosslinkers in polymer networks and demonstrate how the extension of network strands can be achieved by preferentially breaking weak crosslinkers. Finally, polymer networks with a mixture of weak and strong crosslinkers are simulated to determine the optimal composition of weak and strong crosslinkers for the optimal material properties.