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 alpha-phase, whereas for crystallization pressures above 0.9 kbar the values are typical of the gamma-phase. The principal technique used, to be reported in this paper, was the Hoffman Weeks plot of melting point versus crystallization temperature. Unlike the alpha-phase, the gamma-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 gamma-phase, when extrapolated back to atmospheric pressure, is similar to that of or-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. (C) 1998 Elsevier Science Ltd. All rights reserved.