On the metastability of β phase in isotactic polypropylene: Experiments and numerical simulation

Abstract

Phase transitions in isotactic polypropylene were investigated during isothermal crystallization and heating after isothermal crystallization using various experimental techniques. The results obtained by wide angle x-ray scattering (WAXS), light depolarization technique (LDT), differential scanning calorimetry (DSC) and optical microscopy show that crystallization of isotactic polypropylene can result in simultaneous formation of two crystal modifications, α and β. There is clear experimental evidence that β phase tends to convert into α modification during crystallization as well as during subsequent heating. Experimental results are compared with numerical simulation performed according to the model of nucleation-controlled phase transitions in multiphase systems. The results of simulation show that β phase is not thermodynamically stable in any temperature range. The reason for the appearance of β phase is related to low interfacial tension of melt vs. β. It has been also shown that maximum crystallinity reached in experiments does not exceed 40-50% in agreement with the concept of constrained amorphous phase.