Polydispersity of ethylene sequence length in metallocene ethylene/α‐olefin copolymers. I. Characterized by thermal‐fractionation technique

Abstract

AbstractIn this article, the polydispersity of the ethylene sequence length (ESL) in ethylene/α‐olefin copolymers was studied by atomic force microscopy (AFM) and the thermal‐fractionation technique. The crystal morphology observation by AFM showed that morphology changed gradually with decreasing average ESL from complete lamellae over shorter and more curved lamellae to a granular‐like morphology, and the mixed morphology was observed after stepwise crystallization from phase‐separated melt. This result indicated that the ethylene sequence with different lengths crystallized into a crystalline phase with a different size and stability at the copolymer systems. The thermal‐fractionation technique was used to characterize the polydispersity of ESL. Three of the following statistical terms were introduced to describe the distribution of ESL and the lamellar thickness: the arithmetic mean L̄n, the weight mean L̄w, and the broadness index I = L̄w/L̄n. It was concluded that the polydispersity of ESL could be quantitatively characterized by the thermal‐fractionation technique. The effects of temperature range, temperature‐dependent specific heat capacity Cp of copolymer, and the molecular weight on the results of thermal fractionation were discussed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 813–821, 2002