EQUILIBRIUM MELTING TEMPERATURE AND UNDERCOOLING DEPENDENCE OF THE SPHERULITIC GROWTH-RATE OF ISOTACTIC POLYPROPYLENE

Studies of the temperature dependence of spherulitic growth rates, in the context of the Lauritzen-Hoffman secondary nucleation theory, and investigations of the crystallization time and temperature dependence of the melting behavior, in the context of the nonlinear Hoffman-Weeks (HW) extrapolation, are reported for isotactic polypropylene prepared with Ziegler-Natta catalysts. The quantitative agreement between esti mations of the equilibrium melting temperature, T-m, through an analys is of spherulitic growth rate data and use of the nonlinear HW approac h provides strong support for the latter method. The results of these studies indicate specifically that T-m (it-PP, alpha-phase) is in the vicinity of 212-215 degrees C, some 30 degrees C above the most common ly quoted value. This new value of the equilibrium melting temperature of it-PP leads to predictions of the crystallization temperature depe ndence of the initial lamellar thickness and of the correlation betwee n the lamellar thickness and observed melting temperature, which are i n good agreement with the limited morphological information available. The present studies confirm the existence of a regime II/III transiti on at T-x = 139.5 degrees C. The results of this study furthermore cas t some doubts as to the current value of the small-angle X-ray scatter ing technique for the analysis of morphological parameters in quiescen tly crystallized isotactic polypropylene.