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

• Published in: Journal of applied polymer science Vol. 83; no. 5; pp. 1140 - 1156
• Main Authors: Starck, P., Malmberg, A., Löfgren, B.
• Format: Journal Article
• Language: English
• Published: New York Wiley Periodicals, Inc 31.01.2002; Wiley
• Subjects: Applied sciences; Exact sciences and technology; long-chain branching; metallocene catalysts; Organic polymers; Physicochemistry of polymers; polyethylene (PE); Properties and characterization; rheology; Rheology and viscoelasticity; short-chain branching; Ziegler-Natta; Melting; Molten state; Branched polymer; Butylene derivative copolymer; High density ethylene polymer; Hexene copolymer; Experimental study; Low density ethylene polymer; Dynamic mechanical properties; Ethylene copolymer; Very low density ethylene polymer; Linear low density ethylene polymer; Comparative study; Rheological properties
• ISSN: 0021-8995; 1097-4628
• DOI: 10.1002/app.10152

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. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1140–1156, 2002