Infusion of a semidilute bimodal polymer solution into small pores: Partitioning inversion and oscillatory behavior

When a semidilute polymer solution is equilibrated with a porous medium filled with solvent, the high osmotic pressure due mostly to high molar mass components (HMC) drives low molar mass components (LMC) preferentially into the pores. Infusion transients of solvated polystyrene into a confined space of a porous medium from a free external solution were studied for an equal mass mixture of two polystyrene standards at several concentrations higher than the overlap concentration. The total pore volume was almost equal to the volume of the surrounding solution. Initially, both components, especially LMC, filled the pores until their concentrations became excessively high. Thus, the exterior solution was depleted in LMC; i.e., the partitioning was inverted for LMC, but enrichment of LMC in the pore was small. Subsequently, both components decreased the concentrations in the pores. The approach to the equilibria after the rapid infusion was slow and accompanied by an oscillatory pattern, especially evident for HMC. We explain the phenomenon as the one caused by coupling of the chain contraction of HMC due to overcrowded LMC in the pores and a large difference in the diffusivity between the two components.