Liquid structure, thermodynamics, and mixing behavior of saturated hydrocarbon polymers. 1. Cohesive energy density and internal pressure

Thermodynamic properties related to the miscibility of saturated hydrocarbon polymers were investigated by simulation methods. Cohesive energy density, Pi(CED), widely used to estimate the mutual solubilities of ordinary liquids, cannot be measured for polymers, but values of Pi(CED) have been inferred from data on internal pressure, Pi(IP). Both Pi(CED) and Pi(IP) were obtained in this work by molecular dynamics simulations with a united atom model. The effects of chain microstructure and chain length were examined. The simulation model was tested with data for various heptane isomers (the C7 series), for which Pi(CED) and Pi(IP) are known. It was then applied to oligomers of various polymer species (the C30 series) with known flip. Simulation values of Pi(CED) and Pi(IP) were also extrapolated to their long-chain limits in selected cases. The values and trends with structure were generally consistent with the experimental data available for the C7 and polymeric liquids. The ratio a = Pi(CED)/Pi(IP) was found to decrease from near unity for the C7 series to polymeric values of approximately 0.75. This result agrees remarkably well with a = 0.72 +/- 0.11, a range of values that had been inferred from the analysis of interactions in blends of saturated hydrocarbon polymers and PVT data on the pure components. The implication of these results and their relationship to various mixing: theories are discussed.