Tensile Stress Generation on Crystallization of Polymer Networks

When crystallized under constant strain, conventional semicrystalline networks exhibit a loss in tension due to directionally preferred crystallization of chains along the strain axis. This study shows that a dual-cured, semicrystalline polymer network with programmed chain bias can generate similar to 400 kPa of tensile stress upon crystallization. Constant strain experiments were conducted to monitor stress in configurationally biased networks as they were cooled and crystallized. Time resolved wide-angle X-ray scattering was used to study crystallization kinetics and the evolution of crystallite orientation. In configurationally biased networks without external load, initial crystallization along the internal chain bias direction is rapidly followed by crystal growth in the orthogonal direction. Under external loads, crystallization first occurs along the external load direction followed by the configurational chain bias direction. Calorimetry measurements indicate that two distinct crystal populations form upon cooling with different crystallization temperatures. In summary, embedding chain bias can act as a source of tension. The magnitude of tensile stress can be controlled though different crystallization temperatures. These findings open up new opportunities for engineering of thermoresponsive networks in applications where stress must be retained or generated upon crystallization.