A Molecular-Based Equation of State for Simple and Chainlike Fluids

Abstract

A perturbed Lennard-Jones chain (PLJC) equation of state is developed for simple chainlike fluids and polymers. The PLJC equation is formulated on the basis of a well-defined molecular model - the Lennard-Jones chains. Analytical expressions for the Helmholtz energy and compressibility factor were derived on the basis of a first-order variational perturbation theory using the hard-sphere chain as reference. The PLJC equation of state is characterized by three segment-based parameters, namely, the number of segments per molecule m, segment size a, and segment energy e/k. Unlike other perturbation theory-based models, the diameter of the hard-sphere chain segments depends not only on temperature kT/e but also on chain length m. Model parameters for nonassociating molecular fluids were obtained by regressing against vapor pressure and saturated liquid density data. The PLJC model was also applied to high molecular weight polymers; model parameters were obtained by fitting measured specific volumes.