Rigid Amorphous Fraction in Isotactic Polypropylene

Abstract

The rigid amorphous fraction in isotactic polypropylene is comprehensively analyzed as a function of the history of crystallization. The crystallinity, crystal morphology, and superstructure were independently and precisely controlled by the rate of cooling on melt crystallization and by the temperature/time of subsequent annealing. Formation of lamellae and spherulites on slow cooling and formation of nodular domains on rapid cooling are observed. Annealing allowed an increase of the crystallinity and perfection/growth of crystals, without affecting their habit and higher order organization. Analysis of the glass transition of the amorphous phase in preparations with a crystallinity between 40% and 75% proves coexistence of differently mobile amorphous structures. The rigid amorphous fraction and the ratio between rigid and mobile amorphous fractions decrease with increasing crystallinity, which is due to reduced covalent coupling of crystals and amorphous phase in preparations of high crystallinity. Analysis of the effect of the crystal habit on the rigid amorphous fraction allows identification of the structure at the basal planes of crystals as a major source of the immobilization of the amorphous phase. The mobile amorphous fraction cannot primarily be related to interspherulitic or interlamellar stack regions of the semicrystalline superstructure. The presence of crystals in semicrystalline isotactic polypropylene also affects the properties of the mobile amorphous fraction, which is detected by a distinct decrease of its glass transition temperature in samples of high crystallinity, which contain crystals of high stability. This confirms increasing decoupling of the crystalline and amorphous phases in preparations of high crystallinity, as was independently derived by the analysis of the rigid amorphous fraction.