Volume Dependence of Hydrogen Sorption and Desorption Diffusion in Cylindrical Polymers

Abstract

We have investigated the volume effects on hydrogen diffusion properties in both sorption and desorption processes by employing a volumetric analysis technique. The total uptake (𝐶∞), total desorbed content (𝐶0), sorption diffusion coefficient (Ds), desorption diffusion coefficient (Dd), sorption and desorption equilibrium time of hydrogen in two rubbery polymers are determined relative to the diameter and thickness of the cylindrical sample in the two processes. 𝐶∞ and 𝐶0 do not demonstrate the appreciable volume dependence for all rubbers. The identical values in 𝐶∞ and 𝐶0 indicate the reversibility between sorption and desorption, which is interpreted by the occurrence of physisorption rather than chemisorption by introducing hydrogen molecules. The larger Dd values in the desorption process than Ds may be attributed to increased amorphous phase and volume swelling caused by increased hydrogen voids and polymer chain scission after decompression. The time to reach equilibrium in both sorption and desorption processes was found to be linearly proportional to the square of thickness above an aspect ratio of 3.7, which is consistent with the numerical simulations based on the solution of Fick’s law. This finding could be used to predict the equilibrium adsorption time depending on the sample size in the polymer.