High pressure miscibility predictions of polyethylene in hexane solutions based on molecular dynamics

Abstract

Models that describe miscibility of polyethylene in hydrocarbon solvents at high pressures are of on-going interest in fundamental polymer physics and applied polymer processing. The present study aims to characterize the pressure dependence of the solubility of polyethylene (PE) in hexane using an atomistic-level simulation technique to avoid the need for expensive and difficult to obtain experimental data. To achieve this, isobaric–isothermal molecular dynamics (NPT-MD) simulations based upon a well-established force field (OPLS-AA) are utilized to predict the pressure dependence of Hildebrand’s solubility parameter of high-density polyethylene (HDPE) and hexane at high pressure. The NPT simulations also capture molar volume data computed at high temperature (425K) and high pressures ranging from 100 to 3000bar. Further, internal pressures are estimated at high pressures and it is shown that for PE internal pressure is not identical to cohesive energy density (CED). However for PE the ratio of these two quantities tends to unity with increasing pressure. Subsequently, the Flory–Huggins binary interaction parameter is predicted from the knowledge of pressure dependence of solubility parameters and molar volumes. It is demonstrated that the computed binary interaction parameter decreases upon increasing the pressure indicating that the miscibility of the PE/hexane system improves by raising the pressure. This conclusion is in agreement with the solution polymerization process for producing PE where pressure-induced phase separation (PIPS) is applied to separate the polymer product from the polyolefin solution. Exclusion of electrostatic potentials in the molecular mechanics model results in larger interaction parameters while the monotonically decreasing trend remains intact in both molecular mechanics models with and without electrostatic forces. In addition, it has been found that there is a pressure limit beyond which the binary interaction parameter demonstrates less sensitivity to pressure indicating that PE miscibility is not further affected by pressure changes. Based upon the pressure dependence of the interaction parameter the negative contribution of volume change on mixing is predicted where the change in volume upon mixing decays with increasing pressure. Moreover, it is shown that the increase in system pressure increases the chemical potential factor of the phase stability condition indicating that at higher pressures this term tends to stabilize the polymer–solvent system. It has also been revealed that the chemical potential factor estimated by the molecular mechanics model, incorporating the atomic partial charges, is qualitatively more consistent with the miscibility predictions from phase diagrams. The presented results contribute to the fundamental understanding of PIPS, an important demixing process poorly understood when compared to thermally-induced phase separation.