Infrared spectroscopic insight into hydration behavior of poly(N-vinylcaprolactam) in water

IR spectroscopy in combination with two-dimensional correlation spectroscopy (2DCOS) and the perturbation correlation moving window (PCMW) technique is employed to illustrate the dynamic hydration behavior of poly(N-vinylcaprolactam) (PVCL) in water, which exhibits a typical type I continuous lower critical solution temperature (LCST) behavior. PCMW easily determined the transition temperature to be ca. 43.5 °C during heating and ca. 42.5 °C during cooling and the transition temperature range to be 39.5-45 °C. On the other hand, 2DCOS was used to discern the sequence order of different species in PVCL and concluded that hydrogen bonding transformation predominates at the first stage below LCST while hydrophobic interaction predominates at the second stage above LCST. In combination with molecular dynamics simulation results, we find that there exists a distribution gradient of water molecules in PVCL mesoglobules ranging from a hydrophobic core to a hydrophilic surface. Due to the absence of self-associated hydrogen bonds and topological constraints, PVCL mesoglobules would form a "sponge-like" structure which can further continuously expel water molecules upon increasing temperature, while poly(N-isopropylacrylamide) (PNIPAM) with self-associated hydrogen bonds forms mesoglobules with a "cotton-ball-like" structure without an apparent distribution gradient of water molecules and does not change much upon increasing temperature.