Impact of Chain Orientation on the Melting of the α-Form of Isotactic Polypropylene

Isotactic polypropylene (iPP) exhibits polymorphism in the crystalline state that has long been leveraged to tune the material properties. It has been established that even the most prevailing α-iPP can exhibit significant variability with two distinct melting points Tm often attributed to α1 and α2 forms. Several structural models with specific ordering in chain orientations and correlations along the crystals have been proposed. Precise structural investigation relies on inverse modeling of diffraction patterns, but the impact of thermal history, a potential transition between the two forms, and any existence of multiple solutions challenge characterization. Recent detailed atomistic simulations offered a direct route to link postulated structural arrangements to melting transitions with studies focusing on the α1 and β forms of iPP. In this study, we employ atomistic molecular dynamics to examine models of α1 and α2 with variable levels of ordering in terms of chain axis orientation. At rapid heating rates, we find high melting points that can differ by up to 30 K depending on the fraction of the material that follows the P21/c structural arrangement. To contrast systems in the absence of superheating, we assembled configurations with free surfaces as well as models of semicrystalline iPP. We report melting kinetics above Tm for the P21/c and Cc crystals and provide Tm estimates in agreement with literature data. In addition, our study provides valuable microscopic information about the dynamics of melting of α-iPP along specific directions.