Fluctuation/Correlation Effects in Symmetric Diblock Copolymers: On the Disordered Phase

While the polymer self-consistent field theory has gained great success in describing various inhomogeneous polymeric systems, particularly the self- assembled morphologies of block copolymers, for spatially homogeneous systems it reduces to the Flory-Huggins theory and gives the simplest, yet often qualitatively incorrect, predictions. We compare, without any parameter-fitting, the thermodynamic and structural properties of the disordered phase of symmetric diblock copolymers obtained from fast off lattice Monte Carlo (FOMC) simulations [Q.Wang and Y.Yin, J.Chem.Phys.130,104903(2009)], reference interaction site model (RISM) and polymer reference interaction site model (PRISM) theories, and Gaussian-fluctuation theory for the same model system of discrete Gaussian chains interacting with soft, finite-range repulsion used in the dissipative particle dynamic simulations. We compared the internal energy, entropy, Helmholtz free energy, pressure, constant-volume heat capacity, chain/block dimensions, and various structure factors in the systems, which unambiguously and quantitatively reveal the consequences of various theoretical approximations and validity of these theories in describing the fluctuations/correlations in disordered diblock copolymers.