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 m 0 ) 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 m 0 = 186.2 °C were obtained for a fraction of isotactic polypropylene (iPP) with high tacticity ([ mmmm ] = 99.6%, M n = 64 × 10 3 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.