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.

By combining small-angle X-ray scattering and differential scanning calorimetry measurements, we analyze the widening of the glass transition measured for miscible mixtures of semicrystalline PEKK chains and fully amorphous PEI chains. The calorimetric analysis reveals that the amorphous interlamellar phase is subject to confinement effects. In addition, we measure wide composition distributions for the amorphous interlamellar and interfibrillar phases. We quantitatively identify, through careful analysis of the signals, the fraction and average composition of each amorphous phase and the composition distribution. We conclude that the final distribution of the polymer chains that do not crystallize in the interlamellar and interfibrillar phases results from the crystallization dynamics of the mixtures.