Phase Diagrams Semicrystalline Polymer–Liquid Revisited: Isotactic Polypropylene-Dibutyl Phthalate and Other Systems

The full phase diagram of an isotactic polypropylene (i-PP)–dibutyl phthalate (DBP) mixture is for the first time constructed by an optical method and discussed within the concept of semicrystalline polymers as microheterogeneous liquids with a three-dimensional network structure. It is demonstrated that the liquidus in this and other polymer–solvent systems is not thermodynamically equivalent to the liquidus in low molecular weight (MW) mixtures. Qualitatively different thermal behavior of those two types of binary systems in the liquidus vicinity is corroborated by differential scanning calorimetry (DSC) experiments. In the former case, a liquid-solid transition resulting in the formation of polymer crystallites does not lead to separating the mixture into crystalline and amorphous phases. On cooling, the system remains macroscopically single phase until the low MW liquid can be fully dissolved in the amorphous regions of the polymer. The correct location of the corresponding borderline is crucially important for the microporous membrane formation via thermally induced phase separation (TIPS). It is also argued that the topology of a phase diagram polymer–low MW liquid does not depend on whether the polymer is amorphous or crystalline.