Linear growth rates of random propylene ethylene copolymers. The changeover from γ dominated growth to mixed (α+γ) polymorphic growth

The spherulitic linear growth rates of a homo-poly(propylene) and a series of propylene-ethylene copolymers, all synthesized with the same type of metallocene catalyst were analyzed. The inter-chain distribution of comonomer content is uniform in these copolymers and the intra-chain distribution adheres to the random behavior. Furthermore, the concentration of stereo and regio defects is constant for all copolymers. Thus, with these polymers it was possible to investigate the influence of ethylene content on the crystallization kinetics, as extracted from their linear growth rates. All iPPs investigated display mixed polymorphic behavior during isothermal crystallization and major emphasis was given to integrate the simultaneous development of the α and γ polymorphs, and their intimate structural relations during growth, in the analysis of the crystallization kinetics. A sharp break in spherulitic growth is found between times domains of mixed α+γ growth and growth of pure γ crystals reflecting a drastic change in growth mechanisms at the point where α development ceases. The rates corresponding to α growth are significantly higher than those characteristics of γ growth. In addition, growth data in the domain of mixed α+γ growth, which, following the structural models for α and γ branching, reflect the growth pattern of α crystals, display a discontinuity in the temperature gradient at the changeover from growth of mixed α+γ to γ dominated growth. This behavior correlates with that found in other systems that undergo a similar extended chain to folded change in crystallization mechanisms. The temperature coefficient of the linear growth rates is analyzed according to regime theory for both domains of growth. The results allow a quantitative framework for discussion of the interfacial free energies of α and γ crystals obtained from this analysis.