Moisture sorption and diffusion in polyacrylonitrile fiber: Interaction between nitrile groups and sorbed water

Moisture sorption properties of polyacrylonitrile (PAN) fiber has been investigated. Sorption isotherm at 25 degrees C and heat of wetting for PAN fiber were compared with those for acrylic fibers with known chemical compositions. The moisture regain at 65% r.h. was 1.37% (w/w) and heat of wetting for the dried PAN fiber was 0.6 cal/g, which were comparable values to those for the acrylic fibers. Diffusion of moisture into PAN fiber was observed using a gravimetric method at 25 degrees C. The sorption/desorption curve below 70% r.h. was nearly identical, which indicated that interaction between water molecules and nitrile group in polymer chain was not significant. The results obtained in this study suggest that the averaged interaction between water and nitrile group is very weak. On the other hand, the exothermic property for heat of wetting and IR spectrum pattern for OH stretching indicated the possibility of the strong interaction between the sorbed water molecules and a very few nitrile groups.