Moisture sorption mechanism of aromatic polyamide fibres: diffusion of moisture into regular Kevlar as observed by time-resolved small-angle X-ray scattering technique

Abstract

The general trend of equatorial and meridional small-angle X-ray scattering (SAXS) was investigated for four kinds of Kevlar fibres: regular Kevlar, Kevlar 49, Kevlar 149 and a heat-treated PPTA fibre, over a scattering angle 2θ from 0.3 to 6.0°. Reflecting the fibre structure of highly oriented and extended chain conformation, the equatorial scattering is much more intensive and decreases in intensity more gradually with increasing scattering angle than the meridional scattering: i.e. the electron density fluctuations in the radial direction of the fibre due to interfibrillar and interstitial microvoids were expected to have Bragg spacings in the range from several to a few decades of angstroms, while no particular periodic fluctuation could be found in the fibre axis direction. The effects of total reflection upon the equatorial scattering intensity distribution were also discussed. The moisture sorption rate in a bone-dry regular Kevlar was observed at 25°C at saturated vapour pressure by the time-resolved SAXS technique. The experimental results were analysed assuming the moisture to be sorbed only in the microvoids to form various sizes of water cluster. Normalized moisture sorption rate, {1 − [ J(t) J(0) ] 1 2 } {1 − [ J(∞) J(0) ] 1 2 } , determined from the change in integrated scattering intensity over the angular range 0.3–6.0° with time, J( t), was compared with that observed by a gravimetric method, M(t) M(∞) , at a relative vapour pressure of 0.896 at 25°C. Although both rate curves attained equilibrium at almost the same time of about 670 min, the rate curve from the SAXS method showed a sigmoidal shape and the moisture diffusivity was much smaller than that determined from the gravimetric method. The large discrepancy between the two diffusivities was explained using the assumption that the gravimetric method detects the sorbed water in the form of both ‘micro-water clusters’ and ‘macro-water clusters’, but the SAXS method detects only the ‘macro-water clusters’.