Structure and Thermal Behavior of Pressure-Crystallized Polypropylene

Abstract

Samples of isotactic polypropylene have been crystallized at a pressure of 3.86 kbar and under a variety of crystallization conditions. These include isothermal crystallization at five different degrees of supercooling ranging from ΔT=70°C to ΔT=34°C and crystallization by slow cooling from 248°C. Each sample was studied by means of x-ray diffraction at both room and elevated temperature, by differential scanning calorimetry, and by electron microscope observation of replicas of the fracture surface. The x-ray diffraction results show that all the pressure-crystallized samples have formed in the γ, or triclinic, modification. Depending upon crystallization conditions, this phase may be stable or metastable. For high values of ΔT, the triclinic phase shows a partial conversion to the α, or monoclinic, phase under elevated temperature annealing. For low values of ΔT, the γ phase does not convert to α even at elevated temperatures. The DSC measurements indicate that the melting temperatures of the pressure-crystallized samples are dependent upon the degree of supercooling. For a heating rate of 40°C/min, the melting temperature is at 157°C for ΔT=70°C; it rises with decrease of the degree of supercooling and occurs in two stages at 175° and at 182°C for ΔT=34°C. Fracture-surface replicas of the slowly cooled sample show that the bulk polymer contains well-delineated, lath-shaped crystals, similar in morphology to those obtained from dilute-solution crystallization. For the samples isothermally crystallized, the fracture-surface replicas show the presence of long thin fibril-like entities. Consideration is given to the possibility that these fibrillar elements are single-crystal lamellas of low-molecular-weight polymer that have crystallized during cooling of the whole polymer from the crystallization temperature.