Analysis of the Thermal Behavior of Polypropylene–Camphor Mixtures for Understanding the Pathways to Polymeric Membranes via Thermally Induced Phase Separation

An experimental phase diagram of the isotactic polypropylene - camphor system is constructed using an original optical method. It considerably deviates from the dynamic diagram, which can be obtained using conventional differential scanning calorimetry (DSC), and contains an additional boundary line that describes camphor solubility in the polymer. An accurate phase diagram makes it possible to perform a detailed and consistent thermodynamic analysis of the DSC, optical and scanning electron microscopy data on the cooling of pre-homogenized mixtures of different compositions, which leads to the formation of capillary-porous bodies via thermally induced phase separation. Removal of camphor results in the formation of polypropylene membranes, the morphology and functional properties of which, such as the total pore volume, mean pore size, permeability coefficient, and breaking stress, appear to be highly dependent on the composition of the initial binary system. It is shown that thermally induced phase separation induces formation of microscopic cracks in the studied membranes. The crack density decreases with the polymer content in the initial system but at 53 wt% of polypropylene the membrane becomes completely impermeable to isopropanol despite the presence of large ~4 m pores, thus questioning the perspectives of its practical use. In general, the study makes it possible to achieve a deeper understanding of the membrane formation process via thermally induced phase separation in the mixtures of semicrystalline polymers with low molar mass substances.