The γ-phase of high molecular weight isotactic polypropylene: III. The equilibrium melting point and the phase diagram

The equilibrium melting point of polypropylene has been determined as a function of pressure. For pressures of crystallization below 0.5 kbar the melting point observed is characteristic of the α-phase, whereas for crystallization pressures above 0.9 kbar the values are typical of the γ-phase. The principal technique used, to be reported in this paper, was the Hoffman Weeks plot of melting point versus crystallization temperature. Unlike the α-phase, the γ-phase does not show significant levels of abnormal lamellar thickening and the use of the Hoffman Weeks plot is accurate, correlating well with results from small angle X-ray scattering studies. Results demonstrate that the equilibrium melting point of the γ-phase, when extrapolated back to atmospheric pressure, is similar to that of α-polypropylene. The heat of fusion has been determined using the Clapeyron equation and the phase diagram constructed using the Gibbs equation. Reasons for the relative stability of the phases are proposed.