Steady and oscillatory shear flow alignment dynamics in a lamellar diblock copolymer solution

Shear flow alignment in a symmetric polystyrene-polyisoprene (PS-PI) diblock copolymer solution in dioctyl phthalate has been studied using flow birefringence and theology. 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 low frequencies even close to the ODT. This suggests that flow alignment in steady shear is more complex than that in oscillatory shear flow, perhaps owing to an increased likelihood for defect formation. Finally, no flow-induced changes were observed in the order-disorder transition in either steady or large amplitude oscillatory shear flow.