STUDY ON THE RETENTION BEHAVIOR OF LOW-MOLAR-MASS POLYSTYRENES AND POLYESTERS IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY BY EVALUATION OF THERMODYNAMIC PARAMETERS

Polymers can be characterized under sorption conditions, to obtain inf ormation on molar mass and chemical composition. Tn order to get a bet ter understanding of their retention behaviour under such conditions, the evaluation of thermodynamic parameters obtained from van't Hoff an alyses on low-molar-mass polystyrenes (PS) and polyesters (PE) in vari ous THF-water mixtures on a C-18 column is described in this study. Li near van't Hoff behaviour was observed in almost all cases. Negative V alues for both Delta H and Delta S were found for both PS and PE oligo mers, which increase with increasing %THF. For Delta S this is explain ed from multi-site attachment effects. For PS, the non-linear relation s between Delta H and Delta S, and degree of polymerization (p) could be properly described by the Stockmayer-Fixman equation. Although less clear, similar trends were found for PE. For PS, evidence for penetra tion effects of oligomer chains into the bonded chains was obtained. M artin plots for both PS and PE were shown to be non-linear in all inve stigated eluent compositions. The extent of non-linearity is suggested to depend on the conformation of a polymer in solution. No distinct e nthalpy-entropy-compensation temperature (EECT) independent of p was f ound for PS, thus confirming the findings of an earlier study in which no exact molar mass independence was found under critical conditions. Further evaluation of EECT for PS oligomers revealed a retention mech anism independent of the binary eluent composition. This indicates tha t conclusions from this study can also be used far a qualitative under standing of sorption mechanisms in the gradient elution mode. Finally, for PS it was shown that Delta G equals zero under critical condition s, thus confirming theoretical predictions. (C) 1997 Elsevier Science B.V.