Equilibrium melting temperature of isotactic polypropylene with high tacticity: 1. Determination by differential scanning calorimetry

A new method to determine the correct Gibbs-Thomson plot and equilibrium melting temperature (T_m⁰) of polymers was proposed. The Gibbs-Thomson plot method is reliable, because the Gibbs-Thomson equation is directly derived from thermodynamical relations. In this method, the heating rate dependence of melting temperature (T_m) was omitted by applying the theory of the "melting kinetics", and the effect of lamellar thickening on T_m was also omitted by observing thick lamellae. A differential scanning calorimeter (DSC) was used for observation of T_m as a conventional method. Transmission electron microscope (TEM) was used to observe a distribution of lamellar thickness (l). It was shown theoretically that peak temperature of melting endotherm (T_m(DSC)) corresponded to averaged reciprocal l (〈l^-1〉) for the case of sharp distributions of T_m and l^-1. The Gibbs-Thomson plot, T_m (DSC) vs 〈l^-1〉, was carried out. A reliable Gibbs-Thomson plot and T_m⁰ = 186.2 °C were obtained for a fraction of isotactic polypropylene (iPP) with high tacticity ([mmmm] = 99.6%, M_n = 64 × 10³ and M_w/M_n = 2.4). It was shown that DSC double melting endotherm corresponded to the number-distribution of l^-1, when lamellar thickening did not occur.