Moisture Sorption Mechanism of Aromatic Polyamide Fibers: Diffusion of Moisture in Poly (p-phenylene Terephthalamide) Fibers 1

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Abstract
Diffusion of moisture into three kinds of Kevlar fibers—regular Kevlar, Kevlar 49, and Kevlar 149—was observed using a gravimetric method that varied vapor pressure at 25.0°C. The apparent diffusion coefficients for sorption and desorption, which were small, on the order of 10-12 cm2 / s, were found by fitting the calculated curves to the observed curves at the initial stage of the process using a Fickian model of a homogeneous cylinder. At intermediate and later stages of the sorption process, however, the observed results deviated from the calculated ones in two contrasting ways; i.e., the observed results were larger than the calculated ones for regular Kevlar and Kevlar 49 and vice versa for Kevlar 149. These results suggest a difference in the sorption mechanism, possibly due to the skin-core morphology, which is distinct for regular Kevlar and Kevlar 49 but indistinct for Kevlar 149. Referring to the skin-core mor phology with ( Dcore/ Dskin) > 1, the sorption curve was further analyzed using a coaxial cylinder model with dual components that varied (Dcore/Dskin) from 1.5 to 7 but fixed the outer and inner radius ratio ( R 2 / R1) at 1.2. The calculated results agree, in geheral, with the observed results for regular Kevlar and Kevlar 49 over the whole range of the sorption process. The Dcore values depended much more on vapor pressure than the Dskin values. In contrast, the observed results for Kevlar 149 could be accounted for if we applied the coaxial cylinder model with dual components, assuming ( Dcore/ Dskin ) < 1. The significance of the skin-core morphology as well as the interstitial and interfibrillar microvoids upon the sorption rate was discussed.