Crystallization of Random Propylene-Ethylene Copolymers at Atmospheric and High Pressures

One of the main goals of the polymer research is to modify the properties of polymers in order to increase the range of their end−use applications. This is especially true in the case of isotactic polypropylene (

i−PP). By simple modification of the molecular structure of i−PP with small amount of comonomer a variety of polymer grades with different characteristics can be produced. Random propylene copolymers with low ethylene content have lower crystallinity and melting temperatures than the homopolymer, as well as ability to crystallize into γ−crystal form of i−PP at atmospheric pressure. Isothermal crystallization of i−PP under high pressure can significantly affect the resulting crystallographic structures. While at atmospheric pressure it crystallizes exclusively in α−crystal from, when crystallized isothermally under high pressure a mixture of α− and γ−crystals, as well as pure γ−crystals form.

Random propylene copolymers with low ethylene content synthesized by Ziegler−Natta catalysts were used is this study to investigate their isothermal crystallization at atmospheric and high pressure. Copolymers were fractionated and their microstructure analyzed in detail by ^{13C NMR to determine the concentration and distribution of defects since they have crucial role in the crystallization behavior and polymorphism of these copolymers.}

Isothermal crystallization and melting studies showed that these random propylene-ethylene copolymers crystallize in a mixture of α− and γ−crystals. Their observed linear growth rates at atmospheric pressure were found to be dependent on the copolymer composition. Crystallization kinetic data were analyzed using the Lauritzen−Hoffman secondary nucleation theory. Copolymers exhibited two or three crystallization regimes depending on their defect content and molecular weight.

Combined DSC, WAXD and SAXS experiments were used to evaluate the copolymer crystallization models on these random copolymers. It was found that even though the exclusion model fairly well describes the behavior of copolymers with lower defect content some defect inclusion has to occur to account for the lowering of their equilibrium melting temperatures. Defect inclusion increased considerably with the increase of the total defect content in the case of copolymers.

It was shown that γ−phase content in the copolymer crystals increases with increasing defect content, crystallization temperature and pressure. Temperature-pressure-composition α−γ phase diagram of i−PP was constructed based on the Gibbs free energy approach. This diagram enabled the extrapolation of the equilibrium melting temperatures of both phases for defect free i−PP.