Strain Hardening of Polymer Glasses: Entanglements, Energetics, and Plasticity

Abstract

Simulations are used to examine the microscopic origins of strain hardening in polymer glasses. While stress-strain curves for a wide range of temperature can be fit to the functional form predicted by entropic network models, many other results are fundamentally inconsistent with the physical picture underlying these models. Stresses are too large to be entropic and have the wrong trend with temperature. The most dramatic hardening at large strains reflects increases in energy as chains are pulled taut between entanglements rather than a change in entropy. A weak entropic stress is only observed in shape recovery of deformed samples when heated above the glass transition. While short chains do not form an entangled network, they exhibit partial shape recovery, orientation, and strain hardening. Stresses for all chain lengths collapse when plotted against a microscopic measure of chain stretching rather than the macroscopic stretch. The thermal contribution to the stress is directly proportional to the rate of plasticity as measured by breaking and reforming of interchain bonds. These observations suggest that the correct microscopic theory of strain hardening should be based on glassy state physics rather than rubber elasticity.

arXiv comment15 pages, 12 figures: significant revisions

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• Why is Understanding Glassy Polymer Mechanics So Difficult?
• Scaling of the Strain Hardening Modulus of Glassy Polymers with the Flow Stress
• Strain hardening in bidisperse polymer glasses: Separating the roles of chain orientation and interchain entanglement
• Viscoplasticity and large-scale chain relaxation in glassy-polymeric strain hardening
• Modeling the Relaxation of Polymer Glasses under Shear and Elongational Loads
• Evolution of entanglements during the response to a uniaxial deformation of lamellar triblock copolymers and polymer glasses
• Influence of ensemble boundary conditions (thermostat and barostat) on the deformation of amorphous polyethylene by molecular dynamics
• Segmental Dynamics in the Strain-hardening Regime for Poly(methyl methacrylate) Glasses with and without Melt-stretching
• Elasticity and Plasticity in Stiff and Flexible Oligomeric Glasses