Dynamic viscoelastic properties of carboxymethylcellulose during isothermal water sorption

Abstract

The dynamic viscoelastic properties of carboxymethylcellulose sodium salt (NaCMC) were measured isothermally as a function of relative humidity (r.h.). The dynamic modulus decreased with increasing r.h. and dropped markedly at about 70% r.h. In the tan δ curve, a small peak appeared at about 20–30% r.h. and a large peak was observed at 80% r.h. To study the mechanism of these relaxations, the dynamic modulus was measured isothermally as a function of time at constant r.h. and the data were analysed in terms of the kinetics. Below about 70% r.h. bound water may be formed, having an activation energy of 15.0–15.6 kJ mol−1. On the other hand, above 70% r.h., two types of mechanism may occur simultaneously: formation of bound water around hydrophilic groups with activation energy of 15.6 kJ mol−1, and formation of free water with activation energy of 13.0 kJ mol−1. Consequently, the tan δ peak occurring at about 20–30% r.h. may be caused by the onset of local molecular motion of the hydrophilic groups, and the peak at about 80% r.h. by the onset of micro-Brownian motion of chain molecules plasticized with free water.