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

Wide-angle X-ray diffraction studies were 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, D001, also reversibly changed to about 1.3 times larger at dryness than at wetness, but the lattice distortion factor, g_??_. 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.