Volume Dependence of Hydrogen Diffusion for Sorption and Desorption Processes in Cylindrical-Shaped Polymers

In the actual application of gas transport properties under high pressure, the important factors are sample size dependence and permeation efficiency, related to gas sorption. With a modified volumetric analysis technique, we firstly measured the overall diffusion properties and equilibrium times for reaching the saturation of hydrogen content in both hydrogen sorption and desorption processes. The measured parameters of total uptake (C(∞)), total desorbed content (C(0)), diffusion coefficient in sorption (D(s)), diffusion coefficient in desorption (D(d)), sorption equilibrium time (t(s)) and desorption equilibrium time (t(d)) of hydrogen in two polymers were determined relative to the diameter and thickness of the cylindrical-shaped polymers in the two processes. C(∞) and C(0) did not demonstrate an appreciable volume dependence for all polymers. The identical values of C(∞) and C(0) indicate the reversibility between sorption and desorption, which is interpreted by the occurrence of physisorption by sorbed hydrogen molecules. However, the measured diffusivity of the polymers was found to be increased with increasing thickness above 5 mm. Moreover, the larger D(d) values measured in the desorption process compared to D(s) may be attributed to an increased amorphous phase and volume swelling caused by increased hydrogen voids and polymer chain scission after decompression. The t(s) and t(d) were found to be linearly proportional to the square of the thickness above an aspect ratio of 3.7, which was consistent with the numerical simulations based on the solution of Fick’s law. This finding could be used to predict the t(s) in a polymer without any measurement, depending on the sample size.