Growth transition and morphology change in polypropylene

The growth rate of isotactic polypropylene is deduced from microscopic observations during isothermal crystallizations. A change in the growth regime is observed at 138 C and interpreted as a Regime III Regime II transition, according to Hoffman's kinetic theory of polymer crystallization. A Regime II Regime I transition is also theoretically predicted at 155 C, i. e. at a temperature outside the investigated temperature range. The Regime III Regime II transition is related to the positive to negative change in the spherulite birefringence, which is generally attributed to a change in the organization of crystalline lamellae: quadritic arrays of intercrossing lamellae atT_c < 138 C (Regime III) and preferentially radiating lamellae atT_c > 138 C (Regime II). It is suggested that such a morphological change could be interpreted using the concept of non-adjacent re-entry introduced in Hoffman's kinetic theory. This interpretation could also explain the interspherulitic ruptures observed in negative spherulites.