Heterogeneous Dynamics and Polymer Plasticity

We extend a model that has been proposed for describing mechanical properties of glassy polymers and that takes into account the heterogeneous nature of the dynamics. We describe their nonlinear mechanical properties, up to a few 10% deformation. We propose that the elastic energy stored in the volume ∼ ξ3 of dynamical heterogeneities effectively reduces the free energy barriers present for internal relaxation. It allows to calculate yield stresses of order a few 10 MPa, which are consistent as compared to experimental data without additional adjustable parameters than the scale ξ. The model is solved in 3D by numerical simulations with spatial resolution the scale of dynamical heterogeneities. We show that the prediction of our model as regards to yield behavior is consistent with the value of 3–5 nm for the scale for dynamical heterogeneities. Yield appears as the result of an acceleration of the dynamics of subunits with intermediate relaxation times which relax under stress before subunits with very long relaxation. Our simulations describe the onset of plastic behavior and the reorganization at the scale of dynamical heterogeneities. We predict the appearance of shear bands on a scale of a few tens of nanometers at yield and beyond.