With a view toward the application of highly hygroscopic polymers as a humidity responsive self-actuator, the evaluation of the real time moisture concentration in the material becomes a priority. In this paper, the moisture diffusion process in a cellulose acetate (53.3% of acetylation) has been studied. Membranes of cellulose acetate (thickness within the range 66–200 µm) have been prepared, and the moisture absorption at room temperature and at a different relative humidity (RH within the range 21–53%) has been monitored. An analytical model has been used to describe the observed non-Fickian sigmoidal behavior of moisture diffusion. A relaxation factor (β) of about 0.026 s−1 and a moisture diffusion coefficient (D) of 3.35 × 10–6 mm2/s have been determined. At constant room temperature, the moisture concentration at saturation (Csat) has shown a linear relation with relative humidity. The identified values β, D and Csat of the analytical model have been used as input for the finite element simulation of the non-Fickian diffusion. The reliability of the finite element simulations has been confirmed with a second set of experiments.

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 (α) through the determination of the absorbed moisture concentration at saturation (Csat) and the relevant moisture absorption induced strain (εhygro). 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 Csat and εhygro resulted a non-linear dependency on relative humidity. Also the coefficient of hygroscopic expansion (α = Csat /εhygro) 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.

The use of stimuli-responsive polymers to produce environmentally responsive self-actuators continues to rise. Highly hygroscopic materials are attracting great interest for the design of humidity-responsive self-actuators. In this context, bilayer composites, formed by the coupling of a hygroscopic layer with a non-hygroscopic one, are relevant as they allow for the response to be tuned through the design of the composite layers. Therefore, the meticulous material characterization and the definition of descriptive models of their hygroscopic behavior are the primary steps towards the development of humidity-responsive self-actuators. This study is aimed at measuring and predicting the response of a bilayer composite made of a hygroscopic material layer and a layer of a non-hygroscopic material when subjected to changes in environmental humidity levels, to be used as a humidity-responsive self-actuator. A cellulose acetate was used as the hygroscopic material. Predictions for the induced hygroscopic deformation in the bilayer composite, based on two-physics finite element simulations, are compared to experimental measurements.