Molecular thermodynamic theory for polymer systems part II. Equation of state for chain fluids

Abstract

An equation of state for fluids containing chain-like molecules has been established by combining a reference equation for hard-sphere chain fluids (HSCF) with Alder et al.'s square-well attractive potential served as a perturbation term. The equation for HSCFs has been derived through the r-particle cavity-correlation function (CCF) based on a sticky-point model for chemical association, where r is the number of segments of a chain molecule. The r-particle CCF is approximated by a product of effective two-particle CCFs accounting for correlations in nearest-neighbour and next-to-nearest-neighbour segment pairs. Their density dependence is determined by using the Tildesley-Streett equation and computer-simulation results for hard-sphere trimers. The obtained HSCF equation can excellently predict compressibility factors and second virial coefficients for homonuclear HSCFs covering a wide range of chain length as well as for homonuclear HSCF mixtures. The final equation of state has been satisfactory tested by fitting vapor pressures and volumetric properties for various pure fluids and polymers.