Solidification and melting of isotactic polypropylene at elevated pressure

Solidification and melting of isotactic polypropylene, iPP, has been studied at elevated pressure. Previous studies showed that the α and γ forms of iPP coexist in samples crystallized at elevated pressures up to 200 MPa (2 kbar) where only the γ-form exists. Current research using a wider range of supercoolings proves that the amount of each form varies with crystallization temperature at a specific pressure. It appears from experiment that the lower the supercooling the higher the amount of γ-form produced. The results of melting at high pressures show that the equilibrium melting point of iPP is linearly proportional to the pressure, and when the data is fitted to the Clapeyron equation the heat of fusion, ΔH_f, is determined from the slope of the line to be 144.8 J/g. The equilibrium melting point of the γ-form at atmospheric pressure is the intercept of the line and has a value of 187.6°C.