- PublicationNew Kinetics Equation for Stress Relaxation of Semi-crystalline Polymers below Glass Transition Temperature(2023)The stress relaxation of semi-crystalline nylon 1010 cannot be fitted by the Kohlrausch-Williams-Watts formula when the experiments were performed at pre-yielding regime below the glass transition temperature. We study this problem and identify the two-step mechanism of stress relaxation. At short time scale, relaxation is fast, dominated by stress biased thermal fluctuation with a fixed short-range length scale (activation volume). At long time scale, relaxation is slow due to the emergence of a cooperative long-range length scale determined by the stress fluctuation. The cooperative length scale is proportional to the reciprocal of stress and the amplitude of stress fluctuation is the product of stress and activation volume. Based on this two-step mechanism, we propose a new kinetics equation to capture the stress relaxation effectively, where the short time relaxation is described by an Eyring-like local activation and the long-time relaxation is captured by a cooperative excitation process resorting to an extension from the random first order transition theory. Our equation fits the experimental data well and can serve as a model to guide the related experiments of relaxation processes in crystalline solids.