Conformational transitions in ultrahigh-molecular-mass polymers and their manifestation in chromatography on monolithic columns

For a monolithic capillary column based on polydivinylbenzene, a change in the elution profile of polystyrene standards with variation in the eluent-flow rate is studied. It is shown that, for polymers with molecular masses up to 3 × 10⁶, the elution profile does not depend on the flow rate. For higher molecular mass polymers at low flow rates, there is a single almost Gaussian peak that splits into two peaks that move to different sides from the initial peak with an increase in the flow rate. A peak with a smaller retention time (peak I) rapidly attains the limiting elution time, and later on, its retention is independent of the eluent-flow rate. In contrast, the retention time of the other peak (peak II) continuously increases with an increase in the flow rate of the mobile phase, so that, at high flow rates, this peak is retained for a longer time than the low-molecular-mass marker. The intensity of peak I decreases, while the intensity of peak II increases with an increase in the eluent-flow rate; the ratio of their intensities tends toward a certain limiting value (≤1). The observed profile of elution of ultrahigh-molecular-mass polymers assumes the presence of a dynamic equilibrium similar to that existing in the case of first-order reversible reactions. For the assumed equilibrium, the rate constants of direct and back reactions are determined. It is found that these constants are close to the inverse maximum relaxation time of a polymer molecule. The character of the transformations of the polymer molecule in the chromatographic column is discussed.