Investigation of the state and dynamics of water in hydrogels of cellulose ethers by 1H NMR spectroscopy

Abstract

The aim of this work was to study the effect of the type of substituent of the cellulose ethers and the molecular mass on the state and dynamics of water in the respective hydrogels to specify the quantity of adsorbed water on the polymers or, more explicitly, to calculate the average number of water molecules bound to a polymer repeating unit (PRU). 1H NMR relaxation experiments were performed on equilibrated systems of cellulose ether polymers (HEC, HPC, HPMC K4M, and HPMC K100M) with water. In particular, the water proton spin-lattice (T1) and spin-spin (T2) relaxation times were measured in these systems at room temperature. The observed proton NMR T1 and T2 of water in hydrogels at different cellulose ether concentrations at room temperature were shown to decrease with increasing polymer concentration. The relaxation rate 1/T1 is sensitive to the type of polymer substituent but insensitive to the polymer molecular mass. The rate 1/T2 appears much less influenced by the polymer substitution. The procedure developed for calculating the amount of water bound per PRU, based on the analysis of the T1 and T2 data, shows that this amount is the largest for HPC followed by HEC, HPMC K4M, and HPMC K100M. The results correlate well with the degree of hydrophilic substitution of the polymer chains. This NMR analysis deals with a single molecular layer of adsorbed water for the investigated cellulose ether polymers at all concentrations, while the rest of the water in the hydrogel is bulk-like. Therefore, the mesh size of polymer network in the view of a single molecular layer is not effectively changed.