Integral equation theory of block copolymer liquids. II. Numerical results for finite hard-core diameter chains

The spatially local and long wavelength properties of diblock copolymer melts are studied using the polymer reference interaction site model theory. Two new molecular based closure approximations, the "reference-molecular mean spherical approximation" (R-MMSA) and the "reference-molecular Percus-Yevick" (R-MPY) approximation are investigated numerically for structurally symmetric, flexible, and semiflexible copolymers with finite hard-core diameters. For these models both closures lead to a destruction of all spinodal instabilities for finite degrees of polymerization. Results using the R-MMSA closure for the larger chain lengths studied approach the analytic predictions of the Gaussian thread model. On the other hand, numerical results for the R-MPY closure show a temperature regime in which there is an apparent chain length independent fluctuation stabilization for moderate degrees of polymerization in qualitative agreement with recent Monte Carlo simulations. However, we believe this apparent scaling arises from a very slow approach towards the asymptotic, finite size fluctuation behavior analytically derived in the previous paper for the Gaussian thread model. In accord with recent simulations, the peak scattering wave vector exhibits temperature dependence due to collective, many chain fluctuation effects. Predictions of the local structure and composition in the disordered phase are made which show that considerable length scale dependent deviations from homopolymer melt packing emerge at low temperatures.