| The application of reversed-phase liquid chromatography in separations of macromolecules has been studied for over a decade. There are many reports on the success of this technique. However, the fundamental retention mechanism remains unclear. While some assert that a special retention model is needed for polymers, others contend that polymers just behave like small molecules and conventional chromatographic theory can well explain the retention behavior. The controversy in retention mechanism hinders the full exploitation of the power of reversed-phase liquid chromatography in polymer separations.; In this study, the retention behavior of three model polymers, poly(methyl methacrylate) (PMMA), poly(ethylene glycol) (PEG) and poly(tryptophan) (PTRP) in reversed-phase liquid chromatography is examined. The key to this research is to obtain isocratic retention data under various conditions and to correlate isocratic retention with gradient retention. With various solvent mixtures as mobile phases, isocratic retention is observed for all three polymers and a linear relationship between the logarithm of the capacity factor, k{dollar}spprime{dollar} and the volume fraction of the organic modifier in aqueous binary or ternary solvent mixtures is found. The influence of mobile phase on retention and the interaction of polymers with the stationary phase are studied using the results from the chromatographic experiments. The success in interrelating isocratic elution with gradient elution using the Linear-Solvent-Strength model (LSS) supports that the polymer retention mechanism fundamentally resembles the one for small molecules, though there are some unique characteristics about polymers. The LSS model is found to provide a way of predicting retention and separation of polymers with moderate accuracy (5% to 15% error). As a gateway to the retention mechanism, the temperature effect on retention is investigated. The thermodynamic parameters such as {dollar}Delta{dollar}H and {dollar}Delta{dollar}S of the polymer retention process are obtained from Van't Hoff plots. These parameters are found useful not only for studying retention mechanisms but for obtaining separation and information about polymer molar mass as well. Preliminary study of factor analytical modeling of retention data indicates that factor analysis is a useful technique for study of retention mechanisms. Factor analysis is performed on the retention data under different temperatures. The molar mass is found to be a real factor for the solute space and unknown molar mass can be calculated via the free-floating method. Some interesting topics for future study of polymer chromatography are suggested as the extension of this study. |
