SYMMETRICAL DIBLOCK COPOLYMERS UNDER LARGE-AMPLITUDE OSCILLATORY SHEAR-FLOW - ENTANGLEMENT EFFECT

The effect of large amplitude oscillatory shear flow on the morphology of two nearly symmetric polystyrene-polyisoprene diblock copolymers of intermediate molecular weights has been studied. Using two dimensional SAXS experiments and electron microscopy micrographs the orientation of the lamellae after shear flow has been determined. It has been shown that under the appropriate experimental conditions an orientation component can be observed with the unit normal of the lamellae parallel to the flow which on the timescale of hours is not observed in low molecular weight samples. It has been proposed that this orientation is the result of topological constraints, i.e., entanglement effects, on the diblock copolymer dynamics, as evidenced by dynamic mechanical experiments on the samples before flow. With the help of mechanical data collected during the flow, the orientation dynamics has been studied. Using concepts from the tube model of entangled polymer melts and characteristic features of the chain conformational statistics of diblock copolymers, a mechanism for the formation of this orientation component is suggested. (C) 1995 American Institute of Physics.