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

Abstract

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 threedimensional 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.

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

“…The boundary curve A 1 B 1 C 1 by DSC is noticeably lower than the curve ABC by the optical method because of the several reasons: (i) endotherm maximum is always below the temperature of full polymer amorphization in the presence of an LMM component, (ii) the samples for DSC and optical experiments have different thermal history, (iii) our optical experiments take a long time during which the polymer crystallites may become more perfect and, therefore, increase their thermal stability …”

“…Phase diagrams published in the literature for high-density polyethylene–camphor and poly­(vinylidene fluoride)–camphor systems , are incomplete since all of them miss one boundary curve reflecting the temperature dependence of the low molar mass (LMM) component solubility in the amorphous regions of a semicrystalline polymer. As shown in our previous publications, − the presence of this curve is essential for a self-consistent interpretation of the phase diagrams of semicrystalline polymers with LMM substances.…”

“…For constructing the phase diagram of PP with camphor, we implemented a well-established − original optical method described in detail in ref and DSC.…”

“…The coordinates of point B are shifted down from 150 °C (optical) to 136 °C (DSC heating) and 98 °C (DSC cooling) along the temperature axis and from 0.42 (optical) to 0.59 (DSC heating) and 0.70 (DSC cooling) along the composition axis. (2) The boundary curve A 1 B 1 C 1 by DSC is noticeably lower than the curve ABC by the optical method because of the several reasons: (i) endotherm maximum is always below the temperature of full polymer amorphization in the presence of an LMM component, 26 (ii) the samples for DSC and optical experiments have different thermal history, (iii) our optical experiments take a long time during which the polymer crystallites may become more perfect and, therefore, increase their thermal stability. 46 (3) Since DSC cannot distinguish the thermal effects of camphor dissolution in the amorphous PP regions from those of polymer amorphization in the presence of camphor, it cannot detect the temperature at which the camphor dissolution is over.…”