Hydration characterization of N-vinylcaprolactam polymers by absorption millimeter-wave measurements

Water molecules in the polymer hydration shell known as bound water lose their mobility in comparison with unperturbed water. This effect was quantified by absorption measurements in the millimeter-wave range of microwaves (1–10 mm, 30–300 GHz). Hydration measurements were performed for poly(N-vinylcaprolactam) (PNVCL) and copolymers of N-vinylcaprolactam (NVCL) and N-vinylimidazol (NVIA). The association of hydrophobic groups in PNVCL upon coil-to-globule transition was found to cause a decrease in the relative hydration number, which is the relative amount of bound water per solute molecule as measured by microwave method at 31 GHz. Millimeter-wave hydration measurements were confirmed by the determinations of specific heat capacity (c _p) with differential scanning calorimetry. Hydration determinations of NVCL/NVIA copolymers revealed that they associate via hydrophobic clustering with a decrease in hydration of hydrophobic groups.