Structure and Thermal Behavior of Pressure-Crystallized Polypropylene

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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 DSCmeasurements 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.

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