Stress Aging in Anhydrous Nylon 6–10

Abstract

A delay time td to re-establish steady-state neck motion is observed if a partially drawn nylon 6–10 monofilament is aged for a time ta at a stress σa below the stress used to propagate the neck in a tensilecreep experiment. The delay time increases as kσatam for long ta, where 0<m<1, as long as the neck is immobile. The factor k increases steadily with the aging temperature Ta below 40°C, the glass transition temperature Tg of amorphous nylon 6–10, but much more rapidly with Ta above Tg. Aging at σa>0 is necessary to observe a delay time and, in fact, the effects of aging at σa can be erased by subsequently aging a short time at zero stress. At aging stresses just below the propagation stress σp, the neck still moves, but at a lower velocity than at σp. In this regime the delay time increases as c(σp−σa), where c is markedly increased by decreases in temperature. It is proposed that stress aging is caused by the stress-induced formation of small regions of better interchain packing (microcrystals) in nominally amorphous portions of the nylon and that the delay time is the time necessary to break up the microcrystalline structure induced by stress aging into the structure characteristic of steady-state flow. The breakup process is thermally activated with an activation enthalpy of 3.85 eV and stress activated with a shear activation volume of approximately 104 Å3. A modified Eyring model of flow is developed which assumes that the microcrystals are primary flow units which use up free volume in the amorphous regions as they are formed. This model accounts qualitatively for all, and quantitatively for some, of the features of the dependence of delay time on propagation and aging variables. Other theories, in particular free-volume collapse and dislocation dynamics theories, cannot be reconciled with all the observed characteristics of stress aging.