Maximum rate of crystallization and morphology of random propylene ethylene copolymers as a function of comonomer content up to 21 mol %

We report a study of the primary chain microstructure, crystallization kinetics, and crystalline morphologies of propylene ethylene copolymers (PE) synthesized with metallocene catalysts in a range of ethylene up to 21 mol %. A C-13 NMR triad distribution analysis lends support for a random distribution of the comonomer in the complete series. Melting (T-m) and crystallization (T-c) temperatures decrease with increasing ethylene more rapidly than the decrease of the glass transition temperature (7). Consequently, for ethylene > 10 mol % the crystallization is observed at T(c)s relatively close to T-g allowing to observe experimentally the maximum of the crystallization rate. In the low T-c range, segmental dynamics play a prominent role in developing the semicrystalline structure, and the crystallization rate decreases dramatically with increasing molar mass. For a fixed molar mass and undercooling, the strong decrease of the crystallization rate with increasing ethylene, especially for copolymers with > 10 mol %, correlates with analogous depletion of crystallizable sequences. A similar trend is predicted for the stereo defect. Analysis of the temperature dependence of the rates following nucleation and growth theory led to values of T-m(o) comparable to those obtained from equilibrium relations. PEs develop gamma phase in the complete range of ethylene studied. However, the gamma phase is not found at T-c < T-max (rate maximum), indicating that in addition to short crystallizable sequences, considerable molecular diffusion is required to form this polymorph. As T. approaches Tg, the effects of nucleation and segmental transport on crystallization affect the lamellar morphologies. Numerous short rodlike lamellae are formed at T-c close to T-g, while sheaflike lamellar structures similar to those found in propylene 1-hexenes (PH) of matched comonomer content (> 10 mol %) develop at the highest T(c)s, yet in PEs a different crystalline unit cell is not found. The ability to propagate long crystalline lamellae by PEs and PHs copolymers as a consequence of the accommodation of a high content of comonomer in the crystalline lattice is discussed.