Ordering in Block Copolymer Thin Films Induced by Compressible Solvents
Manas R. Shah and Venkat Ganesan. Department of Chemical Engineering, The University of Texas at Austin, CPE 3.402, 1 University Station, Code C0400, Austin, TX 78712
Recently there has been an surge of interest in understanding the effect of compressible solvents such as CO2 on the phase behavior of block copolymers. In bulk systems, the effect of slightly selective solvent is to screen out unfavorable interactions between the different segments of block copolymer, thus decreasing the ODT (dilution approximation). However, recent experimental results have indicated that diblock copolymer thin films can become ordered in compressible solvents at temperatures where its bulk analog is disordered. We carry out a theoretical study of such systems in both bulk and thin film systems. We combine a simple lattice-gas model for compressible fluids along with block polymer self consistent mean-field theory to study the formation of lamellar morphologies of symmetric diblock copolymers in confined and bulk systems. Using a free energy analysis, the stable and unstable thicknesses and ordering transitions for a copolymer thin film for given solvent conditions are obtained. It is observed that the interplay between confinement, the compressibility of the solvent and its selectivity to polymer component can lead to non -trivial effects in the ordering phenomena of diblock copolymer thin films.