Spectroscopic Analysis of Hydration Structures in PEG‐ and PMPC‐Containing Hydrogels by Pressure‐Controlled ATR‐IR Combined With Multivariate Curve Resolution

ABSTRACT Hydration structures govern the properties and functions of hydrogels, yet selectively measuring hydration water in the presence of abundant bulk water remains analytically challenging. Here we applied pressure‐controlled attenuated total reflection infrared spectroscopy (ATR‐IR) combined with multivariate curve resolution (MCR) analysis, a method established in our previous work, to selectively characterize the hydration water of poly(2‐hydroxyethyl methacrylate) (PHEMA)‐based hydrogels containing poly(ethylene glycol) (PEG) or poly(2‐methacryloyloxyethyl phosphorylcholine) (PMPC). Controlled pressure applied to the gels on the ATR prism yielded spectral datasets with systematically varying bulk‐to‐hydration water ratios, which were decomposed by MCR. The resolved spectra showed that increasing PEG content decreased the intensity near 3200 cm −1 , indicating enhanced polymer–water interactions that disrupt the hydrogen‐bond network of neighboring water. In contrast, PMPC increased the population of tetrahedrally coordinated water, indicating that it preserves the bulk‐like hydrogen‐bond network. The ν(O–H)/ν(C–H) area ratio further showed that PMPC‐containing hydrogels retain more hydration water than their PEG counterparts at equivalent compositions. The resolved hydration water is discussed in relation to the nonfreezing, intermediate and free water populations defined by calorimetry and NMR, and pressure‐induced dehydration is interpreted as an osmotic and poroelastic response of the network. These findings provide molecular‐level insight into the distinct hydration behaviors of PEG and PMPC, offering a mechanistic rationale for the superior lubricity of PMPC‐based soft contact lens materials.