Effects of temperature and water on the γ → α crystalline transition of nylon 6 caused by stretching in the chain direction

Abstract The effects of temperature and water on the crystal transition of nylon 6 from the γ phase to the α phase, which is caused by stretching along the chain direction, were investigated. The γ‐phase fibers with high crystallite orientation were stretched at constant load under various conditions. An inversion of the effect of water on the transition occurs at about −60°C. Stretching in the wet state is more effective for the transition at higher temperature. In contrast, at low temperatures water in the crystalline regions actsasa cohesive agent for the chains and increases the activation energy for the transition. Thus, dry stretching is more effective than wet stretching at very low temperature. The fraction of transformed α‐phase crystallites increases abruptly over a narrow range of stress. Thus the critical stress can be determined for the transition. The critical stress changes appreciably with temperature; the higher the temperature, the lower the critical stress. The relation between stretching temperature and critical stress was analyzed by Flory's equation for the shift of transition temperature by stress. About 220°C. was estimated as the zero‐stress transition temperature. The heat content of the γ‐phase crystal was estimated to be smaller by 500 cal./mole than that of the α‐phase crystal. This result suggests that the free energy of the γ‐phase crystal is lower than that of the α‐phase crystal at temperatures lower than the transition point. The irreversible strain of a sample in which the crystal transition has taken place is very small at low temperature. This small extension of the sample is evidence that the γ → α transition produced by stretching along the chain axis is a crystal‐crystal transition.