Gel permeation chromatography with crosslinked polystyrene gels and poor and theta solvents for polystyrene, 2. Separation mechanism

Abstract

AbstractFor separations performed with crosslinked polystyrene gels, an interpretation is proposed for the displacement to high retention volume of plots of log hydrodynamic volume versus retention volume for polystyrene (PS) in poor and theta solvents, e.g. cyclohexane, trans‐decalin, and N,N‐dimethylformamide, with respect to plots for other polymers for which these same eluents are good solvents. PS separates by steric exclusion and by solute‐gel interaction effects giving rise to partition and adsorption mechanisms. The retention volume VR is given by where V0 is the interstitial volume, Vi is the solvent volume within the gel, KD is the distribution coefficient for steric exclusion, and Kp is the distribution coefficient for solute‐gel interaction effects. For polymers in good solvent media separating solely by steric exclusion, Kp is unity. For PS in poor and theta solvents, network‐limited partition and network‐limited adsorption mechanisms are proposed in which Kp is greater than unity. The hydrodynamic volume universal calibration method is applicable when Kp is greater than unity, provided the retention parameter is changed from VR to (VR–V0)/Kp. From the network‐limited adsorption mechanism, a plot of log[(VR–V0)/Kp] versus polymer hydrodynamic radius yields the average pore size of the crosslinked polystyrene gel. Experiments are reported showing the dependence of Kp on gel porosity and on solvent power by varying the temperature of the eluent.