Steady and Oscillatory Shear Flow Alignment Dynamics in a Lamellar Diblock Copolymer Solution

Abstract

Shear flow alignment in a symmetric polystyrene−polyisoprene (PS−PI) diblock copolymer solution in dioctyl phthalate has been studied using flow birefringence and rheology. In large amplitude oscillatory shear, we find behavior similar to that previously reported in lamellar PS−PI melts. At high reduced frequency, parallel alignment of the layers is preferred, confirmed with birefringence measurements using oblique light paths. At low reduced frequency, perpendicular alignment is favored. In steady shear flow experiments well below the ODT, a similar behavior pattern is found: perpendicular alignment results at low shear rates, and parallel alignment is found at high shear rates. These are the first results demonstrating flipping by a change in steady shear flow conditions. As temperature is increased, the degree of perpendicular alignment obtained at low reduced shear rates decreases, unlike the behavior in large amplitude oscillatory flow, where high degrees of perpendicular alignment are achieved at l...