Chain packing in ethylene-butene copolymers

We investigate intermolecular packing in a series of five ethylene/butene random copolymers and various binary mixtures of the pure components using molecular dynamics simulation. The simulation model provides chain dimensions and solubility parameters as a function of butene content that are consistent with experiment. In pure materials, intermolecular packing varies directly with butene content, with low butene content materials packing more efficiently. If butene placement is made alternating instead of random, the material behaves as if its butene content has been lowered. In mixtures, the dominant trend is for the more efficient packer to increase its self-packing, while the less efficient packer decreases its self-packing-in other words, the two distributions move further apart on mixing. In these cases, the cross-correlation is described by an average of the two components. In some instances, large preferences for cross-correlations are observed at the expense of self-correlations. In addition, an efficient packer is more resistant to changes of self-packing on mixing than a poor packer, and self-packing changes more if the material is placed in a dissimilar environment.