REVERSED-PHASE GRADIENT ELUTION BEHAVIOR OF POLYSTYRENES IN A DICHLOROMETHANE METHANOL SOLVENT SYSTEM

A study was made of the gradient elution behaviour of the high-molecular-weight polystyrenes on C-18 reversed-phase columns in methanol-dichloromethane solvent systems. The polymers were injected into mobile phase compositions which were expected to precipitate the polymer on the column and separation of the polymers was then expected as solvents able to dissolve the various molecular weights entered the column in the solvent gradient. The effects of column pore size, sample load and flow-rate were examined. Results were determined mainly by polymer molecular weight and its relation to column pore size. When columns were chosen in which polymer had access to the pores, elution occurred after the expected solvent solubility composition indicating that normal adsorption processes were occurring. Polymers which were excluded from the pores underwent unusual elution processes which resulted in their elution at the solvent solubility composition, or before. The extent of this unexpected elution in a solvent of poorer solvating ability was more predominant on columns of small pore size. The effects of these elution processes was to contribute to band broadening and to reduce selectivity between higher-molecular-weight polymers on small pore size adsorbents. An explanation as to the causes of such elution processes is presented, in which non-equilibrium solvation and precipitation of eluting polymer molecules occurs in the changing solvent gradient mixture.