Influence of molecular weight on the melting and phase structure of random copolymers of ethylene

Abstract

The melting temperatures and phase structures of a series of hydrogenated polybutadienes with fixed co-unit content (2.3 mol % branch points) and varying molecular weights have been studied. These copolymers represent molecular weight and composition fractions of randomly ethyl-branched ethylene copolymers. Also studied were a set of random ethylene-hexene copolymers with a lower co-unit content. The observed melting temperatures, after a variety of crystallization procedures, were found to decrease with increasing molecular weight for both copolymer types. This unusual result could be attributed to the decreasing crystallite thickness in the chain direction with molecular weight. At the higher molecular weights, M = 4.6 X 10®, the crystallite thickness is reduced to about 30 A. Associated with the crystallite is a relatively large disordered overlayer. Although small-angle X-ray measurements and thin-section transmission electron microscopy give results that are in quantitative agreement for the crystallite thickness, the Raman LAM measurement give significantly higher values in the low-size range. The conventional extrapolative method of plotting the observed melting temperature against the crystallization temperature, in order to obtain the equilibrium melting temperature, failed for the random copolymers at low levels of crystallinity. A straight line resulted that paralleled the 45°line. Therefore, extrapolation to the equilibrium melting temperature could not be accomplished. Although the extrapolation could be made for higher levels of crystallinity, this procedure was arbitrary and lead to unreasonable values for the equilibrium melting temperature.

Investigation of the Melting and Crystallization Behavior of Random Propene/α-Olefin Copolymers by DSC and CRYSTAF

“…The influence of the molecular weight for ethene/a-olefin copolymers is most pronounced in copolymers with a low branch content and in the range of molecular weights between 5 000 and 100 000. [6,22] The propene/higher aolefin copolymers in this study have a notably higher molecular weight ( It is well known that the crystallization of a polymer occurs only at temperatures below the melting temperature and generally the crystallization rate reaches a maximum at a temperature between the melting point and the glass point. Commonly, in kinetic studies of crystallization processes, a temperature below the melting point is Table 1).…”

Influence of ionic groups on the crystallization and melting behavior of segmented polyurethane ionomers

“…The extrapolation of the observed melting temperatures to the line T m ϭ T c has been widely employed to calculate the T e of different copolymers and homopolymers. 19 However, in the case of the study on segmented poly(ester-urethanes) based on poly(-caprolactone) by Bogdanow et al, 12 linear polyethylene and random copolymers at low level of crystallinity by Alamo et al, 32,33 this extrapolation method fails to describe the relations between the T m and T c . Bogdanow et al have used different approaches to understand this scenario.…”

Nonisothermal melting and crystallization studies of homogeneous ethylene/?-olefin random copolymers

“…There has been extensive reporting of melting and structure studies, [1][2][3] including a review, 4,5 of ethylene/␣-olefin random copolymers with a variey of different ␣-olefins obtained from different sources. The studies used fractions of ethylene copolymers with narrow molecular weight distribution and demonstrated the importance of the comonomer distribution, or sequence length distribution, of the copolymer chains.…”