Moisture Sorption Mechanism of Aromatic Polyamide Fibers. (Part 4). Effect of Water on the Crystal Structure of Regular Kevlar.

Abstract

: Wide-angle X-ray diffraction studieswere performed for regular Kevlar under dried and wet conditions. The ratio of the diffracted intensity from the 110 crystal plane to that from the 200 crystal plane was reversibly changed from 0.92 at dryness to 0.74 at wetness. The 001 diffraction was detectable both in the dried and wet specimens. The longitudinal size of the paracrystallite, Door, also reversibly changed to about 1.3 times larger at dryness than at wetness, but the lattice distortion factor, go. evaluated by Hosemann's paracrystalline equation was kept nearly constant. The difference in the second order moments of the uniaxial orientation was only slight between the dried and wet specimens, but more complete orientation was detected in the dried specimen. These results suggested that the sorbed water penetrated the interstitial noncrystalline regions between the crystallites and may interfere with the hydrogen bonds of the outermost molecular chains which resulted in the distortion of the molecular packing both in the hydrogen bond direction and in the fiber axis.

Differential Water Sorption Studies on Kevlar 49 and As-Polymerized Poly(p-phenylene terephthalamide): Determination of Water Transport Properties

“…In parallel with the diffusive process there is, as has been mentioned, an intercalation process whereby the diffusing penetrant can be incorporated into the surface layer of the crystallites as a result of the partial relaxation of the crystallite surface by the water, and which is facilitated further at higher penetrant activities/concentration. This is consistent with the observations of Jackson et al, and to a lesser extent the observations of Fukuda and Kawai, , where evidence for water penetration into the surface layer of the crystallites of PPTA is suggested. This process of intercalation can be modeled as a “pseudo” reaction of the form where A represents the penetrant in the amorphous component, S the surface of the crystallite, and B represents the penetrant−surface complex.…”