Structure evolution and phase behavior of polymer blends under the influence of shear

The effect of shear how on the phase behavior, structure formation and morp hology of a near-critical composition blend of poly(styrene co-acrylonitril e)/poly(methyl methacrylate) (SAN/PMMA) and an off-critical composition ble nd of SAN/poly(epsilon-caprolactone) (SAN/PCL) were investigated. Both blen ds exhibited lower critical solution temperature (LCST) behavior. Rheo-SALS (small angle light scattering) was used to characterize the time-dependent structure evolution during flows with shear rates from 0.01 to 0.05 s(-1) and stresses from 8 to 60 kPa. Two types of transient "dark-streak" scatter ing patterns were observed. One featured a dark streak that diverged with i ncreasing scattering vector, q, which corresponded to a chevron-like struct ure in the morphology of the blend. The other type of dark streak converged with increasing q, and may be due: to a spinodal structure that was stretc hed in the flow direction. A capillary viscometer was used to achieve shear stresses as high as 400 kPa for the SAN/PCL blends, and microscopy analysi s of microtomed extrudates revealed a dispersed phase morphology of cylindr ical PCL-rich domains highly oriented in the flow direction. No shear-induc ed phase transitions were observed for either blend for the range of shear stress studied. (C) 2000 Elsevier Science Ltd. All rights reserved.