Shear-Induced Nematic Alignment in Polysulfone Melts

Polymer melt processing often requires conditions of temperature and shear that create flow-induced structures that impact the properties of the resulting material. Such an effect is connected to chain stretching, often leading to shear-induced nematic alignment of at least the longest chains, reported for rod-like polymers but also polyolefins. A polysulfone melt is shown here to undergo such nematic alignment, as can be predicted from its chain stiffness. Its convenient chain linearity, verified by very good agreement of the linear viscoelastic data with a tube model (BoB) for entangled polymer melts, and its inability to crystallize make it suitable for exploring the temperature dependence of the nematic alignment, monitored by both rheology and birefringence. The critical shear rate for nematic alignment at various temperatures is determined and contrasted with that expected from the Rouse time of the longest chains, often considered as the control parameter. Although the onset shear rate for nematic alignment is shown to follow the same temperature dependence as the chain relaxation times, suggesting that chain stretching is the underlying mechanism, the critical shear rate is much smaller than expected. This anomalous behavior of polysulfone is discussed in relation with possible π-stacking interactions stabilizing the nematic domains.