Stress Aging in Anhydrous Nylon 6–10

A delay time t_d to re‐establish steady‐state neck motion is observed if a partially drawn nylon 6–10 monofilament is aged for a time t_a at a stress σ_a below the stress used to propagate the neck in a tensilecreep experiment. The delay time increases as kσ_at_a^m for long t_a, where 0<m<1, as long as the neck is immobile. The factor k increases steadily with the aging temperature T_a below 40°C, the glass transition temperature T_g of amorphous nylon 6–10, but much more rapidly with T_a above T_g. Aging at σ_a>0 is necessary to observe a delay time and, in fact, the effects of aging at σ_a can be erased by subsequently aging a short time at zero stress. At aging stresses just below the propagation stress σ_p, the neck still moves, but at a lower velocity than at σ_p. In this regime the delay time increases as c(σ_p−σ_a), 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 ų. 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.