Binary miscible blends of poly(methyl methacrylate)/poly(α-methyl styrene-co-acrylonitrile). III. Investigation of the phase behavior and morphology during shear flow

The phase behavior and morphology of poly(methyl methacrylate) (PMMA) and poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) blends have been investigated under different shear rates using a simple shear apparatus and transmission electron microscopy (TEM), respectively. The lower critical solution temperature-type (LCST-type) phase diagram detected under quiescent condition was shifted a few degrees to higher temperatures with increasing shear flow, i.e., the shear could suppress the phase separation and enlarge the homogeneous region. There was no sign detected for shear-induced demixing over a wide range of shear rates and temperatures for all the measuring compositions. The normalized shift in the cloud point ΔT(γ̇)/T(0) vs. shear rate (γ̇) for this blend was also investigated and compared with previous studies for high molecular weight polymer blends, oligomer mixtures, and polymer solutions as well as simple liquid mixtures. It was found that the characteristic relaxation time for concentration fluctuations is the most sensitive parameter, which controls the behaviour of the mixtures under shear flow i.e., the larger the characteristic time for concentration fluctuations are, the larger the change in the cloud points under shear will be. The morphology of the critical blend composition (PαMSAN/PMMA = 25/75) was investigated as a function of shear rates at two different quenching temperatures (different quenching depth). At 189°C (17°C above the quiescent cloud point), the morphology of the blend was strongly elongated and oriented parallel to the flow direction and, shear-induced mixing was detected at a critical shear rate value (10 sec^-1), at which no morphology was detected. At 193°C (21°C above the quiescent cloud point), the blend morphology showed a slightly elongated co-continuous two-phase structure with a significant decrease in the periodic distance; however, no shear-induced mixing could be detected at this temperature at shear rates up to 40 sec^-1.