Investigation of water dynamic behaviour in poly(HEMA) and poly(HEMA-co-DHPMA) hydrogels by proton T2-relaxation time and self-diffusion coefficient NMR measurements

Proton transverse relaxation time (T2) and molecular self-diffusion coefficient (D) n.m.r. measurements of water in three hydrogels based on poly(2-hydroxyethyl-methacrylate) (pHEMA) have been performed in order to investigate the state of water and its interaction with the polymer network. Measured T2 values are discussed and quantitatively interpreted by assuming the chemical exchange process between water protons and hydroxyl protons of polymer chains as the major relaxation source, with no recourse to the bound water concept. The pseudo first-order kinetic constants and the activation energies for the exchange process in the different hydrogels have been calculated. The Pulsed Field Gradient (PFG) spin-echo technique has been used to measure diffusivity of water internal to the hydrogels; for hydrogels surrounded by external water, PFG multiple spin-echo experiments at different echo times and subsequent Diffusion Analysis by Relaxation-Time-Separated (DARTS) have been performed in order to obtain simultaneous determination of both internal and external water diffusion coefficients.