Solid-state nuclear magnetic resonance study of linear low-density polyethylenes: 1. Ethylene-1-butene copolymers

Abstract

The effect of crystallization conditions on the partitioning of comonomer units between crystalline and non-crystalline regions has been investigated in a copolymer of ethylene-1-butene, a linear low-density polyethylene, by using solid-state 13C nuclear magnetic resonance (n.m.r.) techniques. It is shown that the cross-polarization ability of the methyl carbons in the ethyl branches is considerably lower than in the case of a hydrogenated polybutadiene taken as a model, indicating a very high degree of mobility of those branches, and resulting in the necessity for longer contact times (about 5 ms) to get a maximum in their signal. Two sample preparations (quenched and slowly crystallized from the melt, respectively) have been analysed, showing very small differences in both the n.m.r. crystallinities and the partitioning of the ethyl branches of the comonomer units. This stands in contrast with the very distinct melting patterns obtained in differential scanning calorimetry, which indicate a rather different distribution of crystallites between the two preparations. The partitioning revealed that the concentration of ethyl branches in the non-crystalline region is about five times higher than in the crystalline region for both preparations. Taking into account the crystallinity, about 9% of total branches were found to be accommodated in the crystal lattice, a number slightly higher than in the case of the model hydrogenated polybutadiene sample, but far away from the much easier inclusion of methyl branches and chain ends.