Origin of hysteresis observed in association and dissociation of polymer chains in water

By choosing poly(N,N-diethylacrylamide) which lacks the possibility to form intra- or inter-chain hydrogen bonds, we studied its chain association and dissociation in water by using laser light scattering (LLS), ultrasensitive differential scanning calorimetry (US-DSC) and Fourier transform infrared spectroscopy (FTIR). As the solution temperature increases, the average hydrodynamic radius (〈R_h〉) and average radius of gyration (〈R_g〉) decrease, indicating the intrachain shrinking. When the temperature is higher than its lower critical solution temperature (LCST, ∼30 °C), the apparent weight-average molar mass (M_w,app) increases, reflecting the interchain association. At the same time, FTIR study reveals that as the temperature increases, the area ratio of two absorption peaks, respectively, associated to one hydrogen bonded carbonyl >CO⋯H-O-H and free carbonyl >CO groups increases, while that related to two hydrated hydrogen bonded carbonyl groups decreases, indicating heating-induced dehydration. In the reversible cooling process, 〈R _h〉, 〈R_g〉, M_w,app and area ratios of the absorption peak are similar to those in the heating process for each given temperature, indicating that there is no hysteresis in the interchain association and dissociation. This present study confirms that the hysteresis previously observed for a sister polymer, poly(N-isopropylacrylamide), is due to the formation of some additional hydrogen bonds in its collapsed state at temperatures higher than the LCST.