Overall crystallization kinetics of polymorphic propylene-ethylene random copolymers: A two-stage parallel model of Avrami kinetics

The isothermal crystallization of propylene ethylene random copolymers evolves with a simultaneous formation of two polymorphic forms, monoclinic crystals (α form) and orthorhombic crystals (γ form). The relative content of each polymorph changes during crystallization and impacts the kinetics and mechanisms of growth. The content of γ crystals developed at high levels of transformation increases with the concentration of ethylene and with the crystallization temperature. In this work, the overall crystallization kinetics of copolymers with an ethylene content ranging from 0.8 to 7.5 mol% were followed by DSC and analyzed according to classical Avrami kinetics. For most copolymers, fits to single-stage nucleation and growth models were poor. Following structural models for lamellar growth that account for epitaxial γ branching from α surfaces, the experimental data were modeled with a parallel two-stage kinetic model with excellent fits up to ∼70% transformations. The Avrami exponents obtained from the fits are consistent with a linear spherulitic growth from pre-existing nuclei for the α stage and homogeneous nucleation (linear with time) for the γ stage, and strongly support the postulated structural growth model. The rate constants of each stage follow the expected temperature dependence. Attempts made to extract the interfacial free energies for α and γ crystals from the values of the rate constants are discussed.