Analytic integral equation theory for the critical properties of homopolymer fluids

We apply the analytic version of the polymer reference interaction site model theory to determine the critical properties of homopolymer fluids. The Gaussian thread is used tnroughout, together with a Yukawa form for the attractive interaction between chain segments. Atomiclike as well as molecular closures are employed, and results are presented using both the compressibility and free-energy route approaches to the thermodynamics. Predictions derived based on different closure approximations for the chain length (N) dependence of the theta and critical temperatures, and of the critical density, are compared with the results of simulations of the liquid-vapor equilibrium in homopolymer systems, as well as with experimental results for the demixing transition in polymer solutions. The large N asymptotic scaling laws, and finite size corrections, for the critical properties depend strongly on the closure employed for treating attractive interactions, and for all cases studied significant deviations from the mean-field Flory-Huggins lattice theory are found. The importance of simultaneously including fluctuation effects associated with both the repulsive and attractive interactions is demonstrated. Model calculations are also presented for the liquid-vapor spinodal and coexistence curves.