Intra- and intermolecular packing in polyolefin blends

The response of chain packing for four saturated hydrocarbon polymers, poly(ethylene-propylene), poly(ethylene-butene), isotactic polypropylene, and head-to-head polypropylene, to changes in environment are considered. The changes arise from mixing each material with the three others in the series. Intra- and intermolecular packing are considered separately by using molecular dynamics simulation, where unambiguous separation of the two is possible. We assess changes from the pure state in chain dimensions [radii of gyration and end-to-end distances], the intramolecular pair distribution functions, and the intermolecular pair distribution functions of each component [the A/A and B/B distributions]. Although it cannot be compared to the melt, the cross [A/B] intermolecular pair distribution function is also considered. We find that chain dimensions and intramolecular packing are insensitive to mixing. Regardless of the nature of the second component, these quantities are invariant within our ability to measure them. In contrast, intermolecular packing changes considerably on mixing. The changes are not energetically driven; i.e., the packing differences do not cause a favorable energetic response in the system. Of the six possible mixtures considered, two appear athermal, one has a preference for A/B contacts, two have decreased A/B contacts, and one changes only slightly from the pure component distributions.