Bond Orientation-Assisted Enthalpic Stress in Polymer Glasses: A Simulation Study on Elastic Yielding

After cold drawing and storage below glass temperature T-g, the polymer glasses are shown to display sizable retractive stress when annealed above the storage temperature but still well below T-g. The emerging so-called "elastic yielding stress" is around one order of magnitude higher than the stress from entropic elasticity. With molecular dynamics simulation, current results show that two elements are necessary for the emergence of elastic yielding, namely, (i) particles with mobility to rearrange their positions and (ii) structural ordering at the level of bonds. We propose a theoretical model to correlate elastic yielding stress with the bond orientation parameter and dissipated energy, which corresponds to the extent of the local rearrangement of particles. The theoretical model is in good agreement with the simulation results. Such bond orientation-assisted enthalpic stress is also expected to play a crucial role in mechanical stress during deformation. The magnitude of elastic yielding stress approaches that of the weak hardening region of polymer glasses during deformation. The existence of such bond orientation-assisted enthalpic stress may be the dominant origin of the stress in weak hardening, while the strong hardening region may revolve around other influence factors.