Rapid Cooling of Random Isotactic Propylene-1-Alkene Copolymers: Structure and Properties

The possibility to join unlike macromolecule units together in random sequences, known as copolymerization, allowed the development of a new class of polymer materials with unique properties. Incorporation of branches of different length and concentration along the main polymer chain influences significantly the crystallization kinetics, crystal structure, resultant morphology and final properties of the homopolymer. Knowledge about the physical structure and its crystal modifications in semicrystalline polymers is of particular importance. Among the numerous parameters which determine the final order and shape of crystals in semicrystalline polymers, the most prominent are the chemical composition and conditions of material processing.The present work attempts to provide a detailed analysis of the effect of random insertion of low amount of 1-alkenes into the isotactic polypropylene chain on the structure formation at non-isothermal conditions. The crystal structure formation in propylene copolymers with ethylene, 1-butene, 1-hexene or 1-octene was analyzed in the range of cooling rates from 10 -1 to 10 3 K s -1 . The conditions of lamellar crystallization and mesophase formation were evaluated on increasing cooling rate as a function of comonomer content and type. The data allowed establishment of the critical cooling rates for replacement of monoclinic lamellar crystals by mesomorphic nodular domains. The thermodynamic stability of the ordered structures formed at different cooling rates was analyzed, as well. Finally selected mechanical and optical properties were related to the structure and morphology formed via different pathways of crystallization.Summarizing, the present work provides a quantitative characterization of the kinetics of the melt-crystallization/mesophase formation and of the conversion of the mesophase into crystals in propylene random copolymers on heating/annealing. Furthermore, the impact of the comonomer inclusion in the propylene chain with combination of different pathways of crystallization on engineering properties is highlighted.