STEADY AND OSCILLATORY SHEAR-FLOW ALIGNMENT DYNAMICS IN A LAMELLAR DIBLOCK COPOLYMER SOLUTION

Shear flow alignment in a symmetric polystyrene-polyisoprene (PS-PI) d iblock 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-P I melts. At high reduced frequency, parallel alignment of the layers i s preferred, confirmed with birefringence measurements using oblique l ight paths. At low reduced frequency, perpendicular alignment is favor ed. In steady shear flow experiments well below the ODT, a similar beh avior pattern is found: perpendicular alignment results at low shear r ates, and parallel alignment is found at high shear rates. These are t he first results demonstrating flipping by a change in steady shear fl ow conditions. As temperature is increased, the degree of perpendicula r alignment obtained at low reduced shear rates decreases, unlike the behavior in large amplitude oscillatory flow, where high degrees of pe rpendicular alignment are achieved at low frequencies even close to th e ODT. This suggests that flow alignment in steady shear is more compl ex 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 a mplitude oscillatory shear flow.