Rheology of Miscible Polymer Blends

AbstractBroadly classified, there are three types of polymer blends, namely, (1) miscible polymer blends, (2) immiscible polymer blends, and (3) partially miscible polymer blends. There are many different experimental methods that can be used to investigate the miscibility of polymer blends, such as differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), dielectric measurement, cloud point measurement, microscopy, light scattering, small-angle X-ray scattering, small-angle neutron scattering, fluorescence technique, and nuclear magnetic resonance (NMR) spectroscopy. Each of these experimental methods can only probe the homogeneity (or heterogeneity) in a polymer blend at a certain scale range. Thus, the determination of the miscibility in polymer blends depends on the resolution limit of the experimental method(s) employed. For instance, DSC and DMTA have frequently been used to determine the miscibility in polymer blends by determining glass transition temperature Tg. When a single Tg value is observed in a polymer blend, the blend can be considered miscible. However, there is a general consensus among researchers that such an experimental criterion, while very useful, cannot guarantee that a polymer blend is miscible on a segmental level. Therefore, a serious question may be raised as to whether a polymer pair can be regarded as being miscible on the segmental level (say, less than approximately 5 nm). It has been reported that DMTA can resolve the size of domains (or separated phases) on the order of 5–10 nm (Molnar and Eisenberg 1992) and DSC is not as sensitive as DMTA for determining the Tg of a polymer blend (Stoelting et al. 1970). In the use of DSC to investigate the miscibility of polymer blends, one often encounters the situation where a very broad (say, 40–60 ◦C) single glass transition appears for certain blend compositions, such as polystyrene/poly(α-methyl styrene) (PS/PαMS) blends (Kim et al. 1998; Lin and Roe 1988; Saeki 1983) and polystyrene/poly(vinyl methyl ether) (PS/PVME) blends (Kim et al. 1998; Schneider and Wirbser 1990; Schneider et al. 1990). Under such circumstances, it is not clear how an unambiguous, single value of Tg can be read off from a DSC thermogram.