NMR and calorimetric investigation of water in a superabsorbing crosslinked network based on cellulose derivatives

In this study we have investigated the state of water in a superabsorbing network based on hydroxyethylcellulose (HEC) and carboxymethylcellulose sodium salt (CMCNa) crosslinked with divinylsulfone (DVS). This type of network, at low degree of crosslinking, when exposed to distilled water is able to form a stable hydrogel containing an amount of water as high as 1000 times its own weight. De-hydrated/re-hydrated networks, containing different amounts of absorbed water, have been studied using differential scanning calorimetry (DSC) and NMR relaxometric methods. DSC analysis allowed the evaluation of freezable and non-freezable fractions of absorbed water showing also the presence of two types of freezable water. On the other hand, NMR relaxometry evidenced the presence of two hydration shells, characterized by a different mobility, which in both cases is lower than that of bulk water. An excellent quantitative agreement was found in the determination of the amount of freezable water using the two techniques. A comparison of the state of water in the crosslinked network and in the corresponding uncrosslinked mechanical mixture shows that in the last case micro-heterogeneity arises. (C) 2003 Elsevier Ltd. All rights reserved.