Evolution of a dispersed morphology from a co-continuous morphology immiscible polymer blends

We observed, via transmission electron microscopy, the evolution of a dispersed morphology from a modulated co-continuous morphology in immiscible blends of two amorphous polymers, poly(methyl methacrylate) (PMMA) and polystyrene (PS). Upon rapid precipitation of a homogeneous solution, which can be regarded as being equivalent to spinodal decomposition via temperature quenching, we observed a modulated co-continuous morphology for all three blend compositions, 70/30, 50/50, and 30/70 PMMA/PS blends. This observation is interpreted with the Cahn's Linearized theory, which has been found to be accurate in describing phase separation in the early stage of spinodal decomposition. When a rapidly precipitated PMMA/PS blend specimen having asymmetric (70/30, 60/40, 55/45 or 30/70) blend composition was annealed under isothermal conditions at 170 degrees C for varying periods, the modulated co-continuous morphology evolved into a dispersed morphology, in which the major component formed the continuous phase and the minor component formed the discrete phase, and into a 'dual mode' of dispersed morphology in the symmetric (50/50) PMMA/PS blend. This observation is interpreted in terms of the percolation-to-cluster transition mechanism proposed by Hashimoto and co-workers. The morphology evolution, during isothermal annealing, of a rapidly precipitated blend may be regarded as being equivalent to late stages of spinodal decomposition, which is controlled by diffusion and coalescence. The rate of morphology development in PMMA/PS blend during isothermal annealing was found to depend on the zero-shear viscosity ratio of the constituent components. This observation is interpreted in terms of Siggia's theory for late stages of spinodal decomposition. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.