In situ monitoring the change of mechanical response induced by the diffusion of saline water in glassy cellulose acetate

Dynamic mechanical analysis (DMA) in submersion mode has been used as an in situ and new technique to study the diffusion behavior of water and different salt solutions that are usually used as draw solutions in forward osmosis (FO) process, in glassy cellulose acetate (CA). The dramatic decay of elastic modulus (E′) as a mechanical response upon water uptake against time at constant temperature (27 °C) and constant frequency (1 Hz) has been employed to calculate the diffusion coefficient of water and different salt solutions in CA near its glass transition temperature (Tg) based on the diffusion-controlled stress-relaxation theory. Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy has been used as an in situ conventional method to evaluate diffusion coefficients based on the simple Fickian model. Diffusion coefficients that have been obtained from both methods show appropriate order though no correlation has been found between two methods. From the results of two methods it may be concluded that DMA submersion mode measurements provide a robust and useful in situ method not only to elucidate at least qualitatively the type of sorption behavior of penetrant in a polymer solid but also to evaluate the diffusion coefficient if it combined properly with an adequate theory.