Shear-induced structural change and phase behavior of polymer blends

Date of Completion

January 1998

Keywords

Chemistry, Physical|Chemistry, Polymer|Engineering, Chemical|Plastics Technology

Degree

Ph.D.

Abstract

The effect of shear on the miscibility and phase structure change of polymer mixtures has been extensively studied for decades. One on-going debate concerns whether experimental observations of flow-induced miscibility of polymer blends actually result from a “true” shift of the critical temperature. To elucidate the origin of the phenomenon more clearly, the time-resolved structural changes of phase domains under the influence of shear flow have been investigated for two polymer blends. One is a poly(styrene-co-acrylonitrile)/poly(methyl methacrylate) near-critical-composition blend; both components have high glass transition temperatures. The other is a poly(styrene-co-acrylonitrile)/poly(ϵ-caprolactone) off-critical-composition blend, which contains one high-Tg and one low-Tg component. Both blends exhibit LCST type of phase behavior. ^ Rheo-SALS (small angle light scattering) was used to probe the time-dependent structure evolution during shear flow. The scattering patterns for phase-separated blends were found to evolve from a symmetrical, spinodal ring to a “two-wing” type of pattern with a divergent “dark-streak” in between, and to an “H-shaped” pattern, and eventually to a “bright-streak” pattern. Quenched samples were examined with TEM and phase-contrast light microscopy, and the Fourier transforms of digitized micrographs were compared with two-dimensional light scattering measurements of the same samples. The morphology of the phase domains developed from bicontinuous, to chevron, to a deformed spinodal structure, and finally reached a fibrillar structure at steady state. The blends were also subjected to pressure-driven flows and drag flows at very high stresses, and their morphology under various shear rates were similarly studied. The results showed no shear-induced phase transition occurred under all stresses applied. The structure evolution due to flow could be explained by hydrodynamic effects consistent with droplet breakup theory. ^

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