Dynamic macroscopic heterogeneities in a flexible linear polymer melt

This article describes shear-experiments on flexible linear polymer melts (polystyrene) with molecular weights (4,000 and 17,500 g/mol) lower than that of critical entanglement. The technique used is a shear piezoelectric rheometer, enabling the complex shear-modulus to be measured in a broad frequency domain (ranging from a few hundredths Hz to some kHz), for weak imposed strains ( 10−4), and for thicknesses between 15 and 100 μm. The results obtained show that the behavior of the shear modulus progressively shifts from a liquid-type behavior to a solid-type as sample-thickness decreases from 100 to 15 μm. This unexpected change in behavior, which is only observed for strong anchoring conditions of the polymer on the substrate, indicates the presence of macroscopic heterogeneities (elastic clusters). We show that these clusters are associated with the glass transition and suggest that they are due to long-range density-fluctuations which are frozen as a result of their ultra-slow relaxation times and thus display an elastic response.