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/cm3 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 χ deduced from the simulations is larger than the χ deduced from the experiments. Several factors in the experiments and in the simulations may contribute to the quantitative difference.