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

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 (D_core /D_skin) > 1, the sorption curve was further analyzed using a coaxial cylinder model with dual components that varied (D_core/D_skin) from 1.5 to 7 but fixed the outer and inner radius ratio ( R ₂ / R ₁) 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 D_core values depended much more on vapor pressure than the D_skin values. In contrast, the observed results for Kevlar 149 could be accounted for if we applied the coaxial cylinder model with dual components, assuming ( D_core/ D_skin ) < 1. The significance of the skin-core morphology as well as the interstitial and interfibrillar microvoids upon the sorption rate was discussed.

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