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

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 was 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 exp 5 , the crystallite thickness is reduced to approx 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 measurements give significantly higher values in the low-size range. The conventional extrapolative method of plotting the observed melting temperature against the crystallization temperature, to obtain the equilibrium melting temperature, failed for the random copolymers at low levels of crystallinity. A straight line resulted that paralleled the 45 deg 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 led to unreasonable values for the equilibrium melting temperature.