Hiromichi Kawai

FTi.r. study on the nature of water sorbed in poly(ethylene terephthalate) film

Abstract The interaction between moisture and poly(ethylene terephthalate) (PET) was investigated by FTi.r. spectroscopy over a near infrared range of wavenumber 4500–8000 cm−1 for an amorphous-isotropic film specimen as a function of relative humidity from dryness up to saturation at 30°C. From two water sensitive bands appearing at 5200 and 7000 cm−1, the latter band (a combination of vasOH and vsOH) was minaly adopted to characterize the water sorbed in the specimen. To separate the water sensitive bands from the entire spectrum, two types of differential procedure were proposed: subtracting the spectrum at dryness from that at x% relative humidity (procedure A) and subtracting the spectrum at a given x% from that at (x + Δx)% relative humidity (procedure B), both as a function of x%. The moisture sorption isotherm determined from the area of the water sensitive bands thus separated by procedure A or B agreed well with that determined from a gravimetric method. The isotherm was found to follow a modified Henrys solution law. The 7000 cm−1 band was resolved into three sub-bands of Lorentzian type function centred at 6810, 7010 and 7090 cm−1, which were compared with those of bulk water centred at 6715, 6887 and 7039 cm−1 in terms of three kinds of water species differing in the degree of hydrogen bonding. It is obvious that the three sub-bands of water sorbed in PET film shift to higher wavenumber than those of bulk water, but do not approach the spectrum of water vapour at ≈7200–7400 cm−1. The relative humidity dependence of this blue shift was hardly found, with the exception of a slight increase of the shift at low relative humidities near dryness, leading to the conclusion that the water sorbed in PET is less interacted than in the liquid state and that few water species are strongly interacted with PET from dryness up to saturation.

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’.

Moisture sorption mechanism of aromatic polyamide fibers. V: Growth of crystallites in as‐spun wet poly(p‐phenylene terephthalamide) fiber during dehydration

Wide-angle x-ray diffraction studies were performed for as-spun wet poly(p -phenylene terephthalamide) fiber. The effects of sorbed water on the equatorial diffractions from the (110) and (200) crystal planes and on the meridional diffractions from the (002),(004), and (006) crystal planes were analyzed during desorption and absorption. There was no significant change in the d-spacing from the respective crystal plane irrespective of the moisture (water) regain. The ratio of the diffracted intensity from the (110) diffraction to that from the ( 200 ) diffraction remarkably increased by removing the sorbed water. The crystallite size estimated from the (110) diffraction, L 110 , also increased as the moisture regain decreased, while the L 200 did not increase. The longitudinal size of paracrystallite, D 001 , also remarkably increased with the decrease in moisture regain with the lattice distortion factor, g II , kept unchanged. These results strongly suggested the growth of the crystallite via hydrogen bonds in the lateral (b-axis) direction. The growth of the lateral size of crystallite also accompanied the longitudinal growth of crystallite during desorption.

Moisture Sorption Mechanism of Kevlar Fibers

The moisture sorption characteristics of an aromatic polyamide fiber, Regular Kevlar, were investigated in terms of the moisture sorption isotherm and the heat of moisture sorption, both at 30 °C, and compared with those of an aliphatic polyamide fiber, nylon 6 fiber, having nearly the same molar concentration of moisture adsoptive sites of peptide group. Despite of much higher degree of crystallinity in Regular Kevlar than in nylon 6 fiber, both fibers sorbed moisture in almost the same extent as high as around 7 to 8% regain at saturation. The sorption characteristics of Regular Kevlar were intended to explain in terms of particular emphases of either paracrystalline nature of the material associated with an unusually large value of crystal imperfection factor attaing as large as 8 or existence of micro-voids or -cleavages whose internal surfaces acting as adsorptive surface but not being detected as noncrystalline region by X-ray diffraction.