The effect of branch distribution on morphology, chain dynamics and rheological behavior of metallocene and Ziegler -Natta linear low density polyethylenes.

摘要

The effects of the branching distribution in ethylene-alpha-olefin copolymers (LLDPEs) on morphology and molecular mobility in the solid state, and rheological properties in the melt and during crystallization were studied. Two types of ethylene-alpha-olefin-copolymers were examined: (I) Ziegler-Natta LLDPE containing significant amounts of highly and low-branched chains, and (II) metallocene LLDPE where branched alpha-olefin units are statistically distributed among the molecules.;Solid-state NMR techniques, WAXS, DSC, TEM and Raman spectroscopy were utilized to characterize LLDPEs in the solid state. A novel solid-state NMR technique for the determination of the lamellar thickness distribution was developed and tested during this investigation. The crystallization kinetics was measured by DSC. Rheological properties in the melt and during crystallization were characterized using oscillatory and steady shear techniques.;In Ziegler-Natta copolymers the crystalline domains are predominantly formed by almost linear chains while highly branched molecules are excluded into the amorphous region. In the metallocene system, crystalline and amorphous domains are formed by segments belonging to the same chains. So numerous covalent links exist between crystalline and amorphous regions. As a result, the metallocene system has thinner crystalline and amorphous domains. It shows lower molecular mobility in the amorphous phase than its more branched Ziegler-Natta analog.;The existence of a crystalline-amorphous interface formed by all-trans yet partially mobile chains was proven for LLDPEs by NMR. The morphological partitioning of branched units and local chain conformation near the crystalline defects in LLDPEs was characterized by solid-state NMR techniques.;It was shown that for both metallocene and Ziegler-Natta LLDPEs the critical crystallinity &phis;cr, at the gel point, i.e. the melt-solid transition, does not exceed 5% [w/w]. A higher crystallization rate and a narrower solidification interval is observed for a Ziegler-Natta copolymer and can be related to the significant content of almost linear molecules. Step-crystallization and partial melting temperature programs for the preparation of stable near-critical physical gels, whose crystallinity remains in the vicinity of &phis;cr on the timescale of hours, were developed. The overall crystallinity and local chain conformation in stable near-critical gels prepared from metallocene LLDPE were studied using Raman spectroscopy.