TPT2 and SAFTD equations of state for mixtures of hard chain copolymers

We present the second-order thermodynamic perturbation theory (TPT2) and th e dimer statistical associating fluid theory (SAFTD) equations of state for mixtures consisting of heteronuclear hard chain molecules based on extensi ons of Wertheim's theory for associating fluids. The second-order perturbat ion theory, TPT2, is based on the hard sphere mixture reference fluid. SAFT D is an extension of TPT1 (= SAFT) and is based on the non-spherical (hard disphere mixture) reference fluid. The TPT2 equation of state requires only the contact values of the hard sphere mixture site-site correlation functi ons, while the SAFTD equation of state requires the contact values of site- site correlation functions of both hard sphere and hard disphere mixtures. We test several approximations for site-site correlation functions of hard disphere mixtures and use these in the SAFTD equation of state to predict t he compressibility factor of copolymers. Since simulation data are availabl e only for a few pure copolymer systems, theoretical predictions are compar ed with molecular simulation results for the compressibility factor of pure hard chain copolymer systems. Our comparisons show a very good performance of TPT2, which is found to be more accurate than TPT1 (= SAFT). Using a mo dified Percus-Yevick site-site correlation function SAFTD is found to repre sent a significant improvement over SAFT and is slightly more accurate than TPT2. Comparison of SAFTD with generalized Flory dimer (GFD) theory shows that both are equivalent at intermediate to high densities for the compress ibility factor of copolymer systems investigated here.