SOLVENT-MEDIATED INTERACTIONS AND CHAIN CONFORMATIONS IN BLOCK-COPOLYMER LIQUIDS

Approximate medium-induced interactions acting on a tagged chain immersed in its melt are studied for AB copolymers using liquid-state polymer reference interaction site model (PRISM) theory. The contributions from both density and concentration fluctuations are examined. In analogy to the homopolymer case, we identify the alloy density-fluctuation-mediated term as a local, or 'intrinsic', contribution which survives in the long-chain limit. On the other hand, the concentration-fluctuation-like term is finite size in nature, and is viewed as the liquid-state theory analogue to the 'screened' potentials employed in previous field theoretic studies. Here, the screened form can be written in terms of fluctuation renormalized collective structure factors and corresponding effective chi-parameters, which replace the phenomenological Flory chi-parameter in the mean field treatment. The conformational properties of idealized structurally and interaction-symmetric diblock copolymers under the screened conditions are then investigated. As the diblock melt is cooled, the mean squared end-to-end distance is predicted to grow, while the individual blocks may either shrink or expand depending on composition. The roughly sigmoidal shape of the end-to-end distance versus inverse temperature is in good agreement with recent lattice Monte Carlo simulations. The degree to which the overall chain dimensions deviate from the unperturbed value is predicted to be <10% over the relevant temperature range, also in accord with simulation results. The predicted saturation of conformational changes at low temperatures is a direct consequence of the fluctuation-corrected liquid-state theory solvation potential, and physically arises from the emergence of a 'ground-state' fluid structure on the microdomain length. We also discuss issues relating to the screening of the intrachain excluded volume, and the consequences of non-universal coupled density and concentration fluctuations on the solvent-induced potential, for more general models of structurally asymmetric block copolymers.