Moisture Sorption Mechanism of Aromatic Polyamide Fibers: Stoichiometry of the Water Sorbed in Poly(para-phenylene Terephthalamide ) Fibers1

Abstract

The water sorbed in a series of poly( para-phenylene terephthalamide) (PPTA) fibers (regular Kevlar, Kevlar 49, Kevlar 149, and a heat treated PPTA fiber) was characterized by analyzing the moisture sorption isotherm and thermodynamic quan tities derived from calorimetric measurements of heat of moisture sorption. The max imum volume of adsorbed water in Langmuir's monolayer fashion per unit mass of dry material v_m normalized by the degree of noncrystallinity was much larger for the series of PPTA fibers than that for nylon fibers, such as 6, 66, and 46, possessing almost the same concentration of peptide groups as the PPTA fibers. The number of water molecules adsorbed in monolayer fashion per each peptide site in the noncrys talline region was around 0.4 moles/mole for the PPTA fibers and about 2.2 times larger than that for the nylon fibers, all at 30°C. The hydration enthalpy ΔH_L at dryness and the hydration entropy TΔS_L near dryness were -25 ∼-35 and -10 ∼ -12 kJ /mol, respectively, both at 30°C, when liquid water was taken as the ref erence. These values are about 1.5 times larger than those of nylon 6 fibers, large enough to assign the initially sorbed water in the PPTA fibers as doubly hydrogen bonded and in a highly ordered state like the crystal lattice of ice. These results indicate that the bulky and rigid phenyl ring in the aromatic polyamide chain opens the space for each peptide site to adsorb moisture more efficiently than the site in the aliphatic polyamide chain.