Shear-induced nematic alignment in polysulfone melts

Polymer melt processing often requires conditions of temperature and shear that create flow-induced structures which deeply affect the properties of the resulting material. Such 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. The chain stiffness of a polysulfone melt is numerically determined from the tangeant correlation decay along a freely rotating chain model. Its reasonable chain stiffness and linearity, verified by very good agreement of the linear viscoelastic data with a branch-on-branch (BoB) tube model, as well as its inability to crystallize make it a suitable material to explore the temperature dependence of the nematic alignment. Such behavior is monitored by both rheology, exhibiting a failure of the Cox-Merz rule, and birefringence by means of an in situ reflection polariscope. The critical shear rate for nematic alignment at various temperatures is determined and contrasted with that expected from the Rouse relaxation time of the longest chains, often considered as the control parameter. Although the onset shear rate for nematic alignment share the same temperature dependence as the chain relaxation times, suggesting chain stretching is the underlying mechanism, the critical shear rate is much smaller than expected. The anomalous behavior of polysulfone is discussed in relation with possible π-stacking interactions stabilizing the nematic domains.