A molecular picture of motion in polyolefins.

We examined three united atom models in light of their description of polyolefin dynamics and investigated the relative influence of various potentials on the resulting dynamics. Results were compared with a collection of experimental data on polyethylene, poly(ethylene-alt-propylene), polypropylene, and head-to-head polypropylene, including quasielastic neutron scattering measurements that we report for two of these materials. For materials with branching, differences between force fields are apparent at low temperature, with the NERD force field most accurate. Differences between NERD and the others are the strength of nonbonded interactions and the height of torsional barriers. We artificially raised each, both of which leads to a slow down in dynamics similar to that observed when lowering temperature. Increasing nonbonded interaction strength slows dynamics with the Vogel dependence of the alpha-relaxation, while raising torsional barriers, slows dynamics with the Arrhenius dependence of the beta-relaxation.