Abstract
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.
Highlights
Hygroscopic expansion, which is defined as dimensional variation due to moisture absorption, can be a double-edged sword
For the considered cellulose acetate, the concentration at saturation at a relative humidity of 74–76% is about 1 9 10–5 g mm-3 higher than the value extrapolated from the linear fit of the experimental data below 53% of relative humidity
The concentration at saturation (Csat) at room temperature and different relative humidity levels (RH = 21 7 76%) has been determined via gravimetric measurements performed during moisture absorption
Summary
Hygroscopic expansion, which is defined as dimensional variation due to moisture absorption, can be a double-edged sword. To overcome some of the mentioned drawbacks, in this study, the concentration at saturation of a cellulose acetate has been evaluated by direct gravimetric measurements of membranes during the absorption process Despite this technique is time consuming, it is more suitable for highly hygroscopic materials having a different absorption and desorption kinetics (Crank and Park 1951; Crank 1953; Alfrey Jr et al 1966; Roussis 1981; De Wilde and Shopov 1994; Mensitieri and Scherillo 2012). Specimens for gravimetric measurements, with dimensions of 30 9 30 mm, and for Thermomechanical Analysis, with dimensions of 20 9 4 mm, were respectively punched and cut from the obtained films (Fig. 1c)
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