Thermodynamic properties of a coarse-grained model of hydrocarbon polymers. Computer simulations on articulated chain structures

Abstract

We consider a coarse-grained model for hydrocarbon chains which retains only the branching information inherent in the structure, and ignores any bond and torsional-angle potentials. Additionally, we ignore the hydrogen atoms and the fused nature of the monomers on a real chain. We have utilized molecular dynamics and Monte Carlo simulations to enumerate not only the structural properties of these systems, but also thermodynamic quantities such as pressures and chain chemical potentials. This extremely simple model is, surprisingly, able to reproduce the variation of saturated liquid density and solubility parameters with branching. We find that the presence of branches reduces the efficiency of intermolecular packing as characterized by the pair distribution function. This results in a decrease in the intermolecular excluded-volume contribution to the chain chemical potentials, yielding directly the observed variation in thermodynamic properties. The connection between packing and thermodynamics which has been speculated upon is thus verified quantitatively. We also discuss the effects of chain branching on polymer–solvent phase equilibria. Our primary conclusion is that coarse-grained models which retain only the branching inherent in hydrocarbon chain systems are sufficient to capture qualitatively their thermodynamic behaviour.