Memory and recovery effects in the strain hardening regime of glassy polymers: theory and simulations

Complex memory effects under applied strain are a defining feature of glassy polymers in the strain hardening regime. We proposed recently a theory for plastic flow and strain hardening as controlled by two contributions to the free energy barriers in glassy polymers submitted to an applied deformation. Free energy barriers decrease under the effect of the stress, which leads to yielding and the onset of plastic flow. Conversely, monomer orientation increases the barriers. These two contributions have very different kinetics. The contribution related to the stress relaxes quickly as a function of the applied stress whereas the orientation contribution relaxes by rotational diffusion, which is very slow at depth in the glassy state. This description could account for the main feature of the Bauschinger effect when considering a stop of the deformation followed by a resuming of the deformation after some waiting time, or for describing deformation cycles, e.g. a tensile test followed by a compression, or the reverse. We show here that the same model allows for interpreting and explaining memory effects in complex deformation histories as regard the distribution of relaxation times, the evolution of tangent delta measured by dielectric spectroscopy and the kinetics of recovery of the reference curve as a function of the waiting time and allows for the interpretation of recent experimental results obtained by small probes reorientation dynamics and dielectric spectroscopy.