Heterogeneity in semidilute solutions of a random copolymer and its relevance to phase fluctuation chromatography for separation by chemical composition

Phase fluctuation chromatography (PFC) involves injection of a semidilute solution of a copolymer for separation by the chemical composition. It is considered that PFC utilizes naturally occurring spatial fluctuations of composition in the semidilute solution of the multi-component polymer to enhance segregation by a selective surface. We conducted PFC for semidilute solutions of poly(styrene-co-acrylonitrile) in various solvents that have a different solubility for the constituent monomeric units. Heterogeneity in the semidilute solution was studied by using dynamic light scattering (DLS). In selective solvents, DLS autocorrelation functions had a slow mode ascribed to composition fluctuations in addition to a fast cooperative diffusion mode PFC of these solutions produced fractions with a broad span in the styrene content. The early eluent was enriched with components preferred by the solvent regardless of the surface. The resolution was better when the surface moieties repelled components preferred by the solvent. As the solvent selectivity decreased, the solution became more uniform, losing the heterogeneity mode in the autocorrelation functions. Then the separation by PFC became worse and more dependent on the surface preference. The surface controlled partitioning was effective only when the solvent selectivity was low. We thus find that when the solvent is selective, the heterogeneous solution is segregated according to the preference of the mobile phase rather tan to the preference of the pore surface.