Weak nonlinearities in viscoelastic mechanical properties of polymers near their glass transition: Local versus macroscopic laws for stress-induced acceleration of the mechanical response

We focus on the role of dynamical heterogeneities on the weak mechanical nonlinearities of amorphous polymers near and above the glass transition temperature T g by combining experiments and numerical coarse-grained simulations. The acceleration of the macroscopic nonlinear modulus relaxation resulting from the applied stress is measured below yielding. As a result of dynamic disorder, the macroscopic acceleration differs from the local acceleration. We obtain a good agreement of experimental measurements with simulations computed by using an exponential function of the square stress for the local acceleration. Further, the length scale of dynamical heterogeneities is deduced.