Characterization and modeling the hygroscopic behavior of cellulose acetate membranes

Exploiting materials with the ability to respond to the environmental stimuli is experiencing an enormous research interest. In particular, polymers that are sensitive to the changes of humidity levels attract great attention as self-actuators. The sensitivity of these materials to the level of moisture is expressed by their hygroscopic properties, namely, the coefficient of hygroscopic expansion. In this context, this study details the effect of moisture absorption on cellulose acetate membranes, as potential material for humidity-responsive self-actuators. The aim is two-fold. The first deals with the evaluation of the coefficient of hygroscopic expansion (alpha) through the determination of the absorbed moisture concentration at saturation (C-sat) and the relevant moisture absorption induced strain (epsilon(hygro)). The second assesses the accuracy of a finite element modeling in describing the coupling of moisture absorption in cellulose acetate membranes and the corresponding dimensional variation, using the material properties experimentally measured. The experimentally measured C-sat and epsilon(hygro) resulted a non-linear dependency on relative humidity. Also the coefficient of hygroscopic expansion (alpha = C-sat /epsilon(hygro)) resulted to have a non-linear dependency on the relative humidity, as well. By this input, numerical simulations were performed for different relative humidity levels, showing accurate description of experimental data.