Comparison of partitioning of a bimodal polymer mixture into micropores in good and Θ solvents.: A Monte Carlo study

We performed lattice Monte Carlo simulations to study partitioning of a mixture of short and long chains with a slitlike pore in good and Theta solvents, allowing unrestricted exchange of chains between the pore and the exterior solution. At high concentrations in the good solvent, the concentration of the short chains became higher in the pore than it was in the exterior. This partitioning inversion was not observed in the Theta solvent. The partition coefficients of the two components were nearly unchanged in the mixture as well as in solutions of pure polymer until the solution exterior to the pore reached the overlap concentration. The concentration that maximizes the resolution in size exclusion chromatography (SEC) shifted to a higher concentration in the Theta solvent. The purity of the short chains in the pure remained high over a wide range of concentrations. In contrast, the good solvent exhibited a drastic decrease in the purity as soon as the concentration increased slightly from zero. The comparison paves the way for potential operation of SEC at high concentrations in Theta solvent. The concentration profiles of the two components in the slit always exhibited a deep depletion layer at the walls in the Theta solvent, different from the profiles in the good solvent that showed thinning of the depletion layer and segregation of the short chains near the walls. These differences are ascribed to A(2) = 0 or clustering of macromolecules in the Theta solvent.