Thermally induced phase separation in semicrystalline polymer solutions: How does the porous structure actually arise?

Abstract Thermally-induced phase separation in semicrystalline polymer – solvent systems is revisited to analyze the formation of membrane precursors beyond the existing concepts. It is argued that the phase diagrams of such systems should include a commonly overlooked curve describing the solubility of a low-molar-mass liquid in the amorphous regions of a semicrystalline polymer. To illustrate our approach, the binary mixtures of isotactic polypropylene with dioctyl phthalate (good solvent), dibutyl phthalate (poor solvent), and their ternary mixtures are considered. Quasi-equilibrium phase diagrams are constructed using an original optical method and analyzed in terms of phenomenological thermodynamics, including the previously poorly studied region below the polymer crystallization temperature. The proposed scenario of morphology development during the cooling of pre-homogenized mixtures is illustrated with the data of optical and scanning electron microscopy, small-angle X-ray scattering, differential scanning calorimetry, and cloud point methods. It is found that cooling down and subsequent solvent extraction generally lead to the formation of capillary-porous bodies with large (micron-sized) pores formed via the liquid-liquid phase separation and small (submicron-sized) pores appeared due to the microphase separation of amorphous polymer regions under strain imposed by the presence of crystallites. The composition range most suitable for the formation of filtration membrane precursors extends between the polymer mass fraction values corresponding to the upper critical solution point and “monotectic” point, in which the solubility curve meets the curve of complete polymer amorphization.