Sorption kinetics of water vapor in wool fibers

Using a modification of the vibroscope technique, studies have been made of the sorption kinetics of water vapor in single wool fibers. Small changes in water content, described as interval sorption and desorption, occur in two stages, the first occupying a few minutes and the second many hours (at room temperature). This behavior is similar to that recently reported for interval sorption of organic vapors in “glassy” polymers (e.g., acetone in cellulose acetate). The first stage of sorption is considered to be due to diffusion in accordance with Fick's law; the slow increase in concentration during the second stage is thought to result from partial relaxation of the swelling stress. Large changes in water content are described as integral sorption and desorption. In integral sorption the slow second stage of sorption is not observed; the explanation is thought to be that relaxation of swelling stress, and hence the second stage, are greatly accelerated by the transient stress associated with the steep concentration gradient present during integral sorption. The slow second stage is present in integral desorption, the probable reason being that in this case there is no steep concentration gradient. Integral sorption to a given value of relative humidity (R.H.) results in a higher equilibrium water content than if the same R.H. is reached in a series of interval steps. This effect is thought to be related to the hysteresis which is observed in the isotherm in a sorption‐desorption cycle; the transient stress present in integral sorption leaves the material in the same condition as if it had been brought down the desorption curve of the isotherm. The sorption hysteresis is thought to result from metastability of the interchain bonds which undergo relaxation in the second stage of interval sorption and desorption. All the effects observed are likely to be qualitatively characteristic of vapor sorption in any polymer at temperatures below the second‐order transition.