Fundamental studies on the interaction between moisture and textiles. Part IV Moisture sorption isotherm of polypeptide and polyamide fibers.

Abstract

The interaction of water molecules with textiles was investigated by analyzing the moisture sorption isotherm in terms of thermodynamic parameters as well as the parameters of Brunauer, Emmett and Teller (B. E. T.)'s multi-layer adsorption model or Hill's multi-molecular adsorption model of water cluster formation for a series of natural polypeptide and synthetic polyamide fibers. The investigation was further proceeded by a broadline proton NMR spectroscopy to discuss the interaction more directly by molecular parameters than by the parameters basing on the physico-chemical and physical models.The following conclusions were obtained to characterize the interaction: (i) From temperature dependence of the moisture sorption isotherm, the differential heat of moisture sorption and, subsequently, the excess energy (entropy decrease relative to bulk water) were estimated for scoured wool and nylon 6 fibers as a function of relative humidity at a given temperature of 30°C, validating the concept of rather regular structure of sorbed water in the B. E. T. multi-layer adsorption model or the Hill's multi-molecular adsorption model of cluster formation. (ii) From the analysis of sorption isotherm of the polypeptide and polyamide fibers in terms of the B. E. T. parameters, nmax, the maximum number of layers below which the calculated sorption isotherm is closest but never exceeds the observed isotherm, is found to be 6 for almost every specimen tested. The molar concentration of monolayer-adsorbed water (Langmuir's adsorption) normalized by the degree of noncrystallinity, [Vm/(l-Xx)], is found to increase gradually at first and then rapidly with increase in the molar concentration of [CONH] groups in the back bone chain for a series of aliphatic nylons including a scoured silk, but excluding the scoured wool and an aromatic polyamide fiber, Kevlar 29; i.e., the denser the distribution of [CONH] groups along the back-bone chain, the greater is the water adsorptive capacity per [CONH] group. (iii) The narrow linewidth ΔHn in the broadline proton NMR spectrum exhibits a transition at a certain moisture regain at which the sorbed water changes in its nature from layered-adsorbed water with the number of layers up to nmax to less interacted water with the number of layers of n>nmax, but not to an extent of far less interaction as comparable to the water in liquid state; i.e., the so-called bulk water.