Monte Carlo simulation of head-to-head, tail-to-tail polypropylene and its mixing with polyethylene in the melt

Simulations have been performed at 473 K for one-component melts of polyethylene (PE) and head-to-head, tail-to-tail polypropylene (hhPP) as well as a mixture of the two species. The densities are 0.760, 0.753, and 0.756 g/cm(3) for these three NVT simulations, respectively. The Monte Carlo simulation uses coarse-grained representations of the chains on a sparsely occupied high coordination lattice. The short-range intramolecular interactions are controlled by rotational isomeric state models for the two types of chains, and the intermolecular interactions are represented by a discretized version of Lennard-Jones potential energy functions. Equilibrated coarse-grained replicas are reverse-mapped to atomistically detailed models in continuous space. The pair correlation functions clearly demonstrate the onset of demixing for the two-component melt, which is qualitatively consistent with the conclusion from small-angle neutron scattering reported by Jeon et al. [Macromolecules 1998, 31, 3340]. Analysis of the components of the energy in the simulations shows that the positive energy change on mixing is completely dominated by the intermolecular Lennard-Jones contributions, with negligible contributions from the short-range intramolecular interactions in the rotational isomeric state models. Quantitative comparison with experiment shows that the chi deduced from the simulations is larger than the chi deduced from the experiments. Several factors in the experiments and in the simulations may contribute to the quantitative difference.