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 quantities derived from calorimetric measurements of heat of moisture sorption. The maximum 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 noncrystalline 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 reference. 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.Poly (para-phenylene terephthalamide ) ( PPTA ) is a liquid crystalline polymer developed by E.I. du Pont de Nemours & Company in 1971 to produce a series of aromatic polyamide fibers using a wet spinning process from an anisotropic solution in sulfuric acid [ 1 ] . The fibers, commercially named Kevlarf), exhibit high tensile strength and elastic modulus in bulk [ 7 ] . The tensile modulus in bulk has reaches as high as 125 GPa for Kevlar 49 and 180 GPa for Kevlar 149 [ 8 ] , values that are fairly close to the crystal lattice modulus in the c-axis direction observed by x-ray diffraction as 156 GPa [ 28 ] and to that calculated theoretically from chain conformation as 182 GPa [ 35 ] . The fact that the tensile modulus in bulk is fairly close to the lattice modulus must be interpreted in terms of a high degree of crystal orientation toward the fiber axis as well as a high degree of crystallization, possibly in extended chain fashion, both arising from flow-induced orientation and crystallization of PPTA during the wet spinning process of the liquid crystalline solution.PPTA fibers such as Kevlar 29 and 49 have hygroscopicity as high as nylon 6 and 66 fibers, attaining moisture regains of around 4% (g/g) and 7% (g/g) at 65 and 100% relative humidities, respectively, at 30°C [23]. These moisture sorption behaviors are consistent with the fact that PPTA fibers have almost the same molar concentration of peptide linkages ( which must act as moisture adsorption sites) as nylon 6 and 66, but are inconsistent with t...

Characterization of KEVLAR fibers using selected probes

“…Notice that the hydration enthalpy of -60 kJ mol-' found here is in agreement with the values obtained at low water content by Fukuda et al 38 (one must take into account that in this later case liquid water was taken as the reference state). It is also interesting to mention that Fukuda et al found a decreasing relationship between the enthalpy and the relative humidity.…”

Differential water sorption studies on KevlarTM 49 and as-polymerised poly (p-phenylene terephthalamide): adsorption and desorption isotherms

“…4. For comparison purposes, the results of Fukuda et al (1991) for Kevlar TM 29, 49 and 149 at 30 • C are shown in Fig. 5.…”

“…In the case of Na 2 SO 4 , it has been shown that it is present in Kevlar TM 49 ÿbres at concentrations of up to 1:5 wt% (Morgan and Pruneda, 1987), located predominantly between 60 nm microÿbrils within the ÿbre. While Penn and Table 2 Equilibrium moisture sorption characteristics of Kevlar TM aramid ÿbres (Fukuda et al, 1991) Larsen (1979) showed a direct correlation between sample 'ash' (inorganic component) content and equilibrium moisture content, more recent work conducted by Fukuda et al (1991) has shown no such correlation. Indeed, samples boiled for 30 days (in the process losing up to 50% of the sodium salt) showed no change in equilibrium moisture content compared to those of untreated samples.…”

“…Despite its very high crystallinity, the equilibrium moisture content is unusually high for this polymer: Kevlar TM ÿbres have a moisture regain almost identical to that of Nylon 6, which, while having the same amide group concentration, has a much lower crystallinity (Fukuda et al, 1991). This is illustrated in Table 2.…”

“…Here it is possible that, in addition to monolayer adsorption, above certain concentrations water clusters appear ÿrst as multi-layer sorption and then, if su cient space exists (in the form of microvoids and larger micro-cracks) to allow bridging, as larger water clusters. Fukuda et al (1991) conducted BET, calorimetric and X-ray di raction studies on Kevlar TM 29, 49 and 149, and concluded that, while Kevlar TM ÿbres exhibit a much greater crystallinity than Nylon ÿbres, the bulky phenyl ring associated with Kevlar TM facilitates greater water sorption in the amorphous regions by opening more space for sorption to take place. In addition, adsorption isotherms for different Kevlar TM ÿbres, normalised by the amorphous fraction, were found to be much closer, especially below 30% relative humidity (RH).…”