Gamma phase in isotactic polypropylene at elevated pressures: Thermodynamics prediction

The γ-form of polypropylene was first noted during 1960s and largely produced by crystallization at elevated pressures. When varying the pressure only the α- and γ- forms are observed. 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. Research reported here using a wider range of supercoolings proves that the amount of each form varies with crystallization temperature at a specific pressure. It appears that the lower the supercooling the higher the amount of the γ-form. The coexistence of these two phases can be predicted from their thermodynamics parameter differences. Calculated values of the Gibbs free energy (ΔG) as a function of temperature show that as the temperature decreases from the equilibrium melting point the ΔG of the γ-form is lower than that of the α-form up to a transition temperature, where ΔG_γ = ΔG_α. This transition temperature, above which pure γ-form is predicted, is in good agreement with the experimental results for several pressures.