SynopsisThe tensile strength degradation of nylon 66 and Kevlar 29 yarns was studied. at elevated temperatures and over a broad range of relative humidities. The degradation rates for nylon are initially slow but increase rapidly, suggesting the depletion of an inhibitor. Kevlar showed no evidence for similar inhibitor action. The effect of relative humidity on degradation can be very large, especially at values in the 9C-100% range and at elevated temperatures. Degradation is governed by thermal-oxidative and moisture-induced mechanisms. At the very high humidities and elevated temperatures, the moisture-induced degradation predominates. Rate relationships were developed in which contributions from each mechanism were considered. Calculated degradation from these relationships agrees well with observed degradation over a broad range of temperatures and humidities.

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Accelerated aging of Nylon 66 and Kevlar 29 yarns was carried out in humid air, smog, and ozone to establish aging trends in these environments. Tensile strengths of nylon yarn aged in a humid environment (50% RH at ambient temperature) for six months degraded from 40% at 1 10 O C to 85% at 150 OC. Kevlar yarns in the same test lost from 10% (130 "C) to 32% (170 "C) of their strength. Based on these data, the estimated energy of activation for the nylon degradation is 18 kcal/mol and for the Kevlar degradation is 13 kcal/mol. Nylon exposed to smog for six months lost -63% of its tensUe strength compared to -7% loss in ozone. Kevlar strength loss was -27% for smog exposure and insignificant for ozone exposure. No trend between oxygen depletion or carbon dioxide generation and strength loss was apparent in the gas analyses. Infrared spectrophotometry did not show changes in materials degraded at significant levels (40 % ). Preliminary nondestructive chemiluminescence studies indicate a linear relationship between thermally induced chemiluminescence and tensile strength.

SYNOPSISThe sorption of water by nylon 66 and Kevlar 29 fabrics was studied over a range of 20-98% relative humidities a t 27°C. Equilibrium and rate relationships were developed from the data in this study and from nylon 66 and nylon 6 data of other studies involving fiber and film. The ratio of sorbed moisture to amide concentration is, on average and at high relative humidities, one water molecule per amide unit. The nylon equilibrium data show that there exist threshold relative humidities above and below which sorption properties differ. One threshold is correlated with the glass transition. Another is identified in the glassy state of nylon 6 in which moisture is tightly bound. The equilibrium constant values are highest and the moisture is most tightly bound a t very low humidities, and the values are lowest in the plastic region at high humidities. NMR, DSC, and mechanical property data from others are correlated with the sorption discontinuities observed in this study. Rate data indicate that sorption is a diffusion-controlled process.

Study of the diffusion in polymers of molecules comparable in size and structure to the segments of the chain molecules is suggested as a means of securing information on structure and molecular friction forces in polymers. A convenient method for measuring the diffusivity in polymers of such compounds tagged with C‐14 was devised and used to study the diffusivity of octadecane, octadecanol, stearic acid, and octadecyl stearate for a range of temperatures in polymers including natural rubber, GR‐S, polybutadiene, polyisoprene, and polyisobutylene. Diffusivities were independent of concentration for the range used. The diffusivities of straight chain hydrocarbons in natural rubber appear to vary inversely as the molecular weight of the diffusing compound. Evidence was found for association effects in the diffusion of stearic acid in natural rubber. Activation energies for diffusion of the compounds were quite similar in any one polymer and of the same order as reported for low molecular weight hydrocarbons and for viscous flow. Larger differences in activation energies were observed for diffusion in the different polymers. Correlations were observed for the diffusivities of octadecane in a series of polymers with their densities and thermal expansivities, especially the latter. Some study was given to the effect on the diffusivity of octadecane of such parameters as polymer unsaturation, solution viscosity, molecular weight, and crosslink density. The diffusivity of octadecane in natural rubber had a pronounced maximum as a function of crosslink density, coinciding with the maximum in tensile strength. Possible implications of this are discussed. The diffusivities of octadecane in a series of elastomers were studied in relation to the internal frictional losses observed for 60 cps vibrations over a range of temperatures. It was concluded that frictional forces deduced from the diffusivity measurements have some correspondence to high frequency viscoelastic phenomena which involve small units of structure comparable in size to the segments of the polymer molecules.