Cooling History Redefines DSC-Derived Intermediate-Water Descriptors in Hydrated Poly(2-methoxyethyl acrylate)

Hydration at polymer-water interfaces is often discussed using experimentally defined descriptors of sorbed water. In poly(2-methoxyethyl acrylate) (PMEA), differential scanning calorimetry (DSC)-derived intermediate-water-related fractions have been widely used to characterize hydration states associated with polymer-water interactions. However, these fractions are defined through freezing, cold crystallization, and melting events during a prescribed thermal scan, and their quantitative values may therefore depend on the imposed thermal history. Here, we examined hydrated PMEA by DSC while systematically varying cooling and heating rates at fixed water content. Decreasing the cooling rate below 1.0 K·min-1 markedly reduced the cold-crystallizable-water and low-temperature-melting-water fractions, whereas the free-water fraction increased. Although no distinct crystallization exotherm was resolved during cooling, the mismatch between crystallization- and melting-based water balances is consistent with gradual crystallization or reorganization of sorbed water during cooling that is calorimetrically indistinct under the present conditions. In contrast, changing the heating rate mainly shifted apparent transition temperatures and had little effect on the calculated water fractions. These results demonstrate that DSC-derived hydration descriptors in PMEA are not material-intrinsic quantities determined solely by polymer chemistry and water content. Rather, they are operational descriptors defined jointly by polymer-water interactions and the imposed cooling-heating protocol. This protocol dependence should be explicitly considered when DSC-derived water fractions are used to compare hydrated polymer interfaces.