Nonlinear rheological response of phase separating polymer blends: Poly(styrene-co-maleic anhydride)/poly(methyl methacrylate)

We investigate the thermorheological properties of a partially miscible pol ymer blend exhibiting lower critical solution temperature phase diagram and having high glass transition and molecular weight constituents, namely pol y(styrene-co-maleic anhydride)/poly(methyl methacrylate), SMA/ PMMA, in the nonlinear regime. By using parallel plate, sliding plate, and capillary rh eometry, with the knowledge of the blend's phase behavior, we address the f ollowing aspects of its dynamic response: (i) the sensitivity of nonlinear viscoelastic properties to the phase behavior, which is reflected in the ob served thermorheological complexity and a dual-peak stress response in larg e amplitude oscillatory shear; (ii) the effects of shear history on the mor phological and rheological properties, which corroborate the picture of she ar-induced fibrilar-type growth of the SMA-rich inclusions in the phase sep arated regime; and (iii) the evolution of shear and normal stresses and the relevant morphological changes in the phase separated blend during startup of simple shear flow. These results suggest that the main rheological feat ures of complex partially miscible polymer blends can be understood using i deas from the behavior of immiscible blends, provided that their phase stat e is precisely known. (C) 2000 The Society of Rheology. [S0148-6055(00)0020 1-7].