Thermal and rheological studies on the molecular composition and structure of metallocene-and Ziegler-Natta-catalyzed ethylene-α-olefin copolymers

The relationship between the molecular structure and the thermal and rheological behaviors of metallocene- and Ziegler-Natta (ZN)-catalyzed ethylene copolymers and high-density polyethylenes was studied. Of special interest in this work were the differences and similarities of the metallocene-catalyzed (homogeneous) polymers with conventional coordination-catalyzed (heterogeneous) polyethylenes and low-density polyethylenes. The short-chain branching distribution was analyzed with stepwise crystallization by differential scanning calorimetry and by dynamic mechanical analysis. The metallocene copolymers exhibited much more effective comonomer incorporation in the chain than the ZN copolymers; they also exhibited narrower lamellar thickness distributions. Homogeneous, vanadium-catalyzed ZN copolymers displayed a very similar comonomer incorporation to metallocene copolymers at the same density level. The small amplitude rheological measurements revealed the expected trend of increasing viscosity with weight-average molecular weight and shear-thinning tendency with polydispersity for the heterogeneous linear low-density polyethylene and very-low-density polyethylene resins. The high activation energy values (34-53 kJ/mol) and elevated elasticity found for some of our experimental metallocene polymers suggest the presence of long-chain branching in these polymers. This was also supported by the comparison of the relationship between low shear rate viscosity and molecular weight. (C) 2002 John Wiley & Sons, Inc.