(708b) Crystallization-Induced Stress Generation in Crosslinked Elastomers

Semi-crystalline elastomers undergo stress relaxation upon crystallization, and this enables efficient stress-memory and shape-memory behavior. Furthermore, freestanding, thermo-reversible self-stretching of semi-crystalline elastomers has been demonstrated by incorporating permanent anisotropy into the network. Anisotropy is achieved through a two-step curing procedure where mechanical strain is introduced and fixed to generate a bias in chain orientation. Dual-stage curing yields an unconventional type of semi-crystalline elastomer that resists typical crystallization-induced stress relaxation. On the contrary, crystallization-driven stress generation can be observed, depending on the annealing conditions.

Differential scanning calorimetry and wide angle X-ray scattering (WAXS) suggest that configurational anisotropy is successfully embedded in the network. In situ thermo-mechanical and WAXS studies allow the direct observation of stress evolution as the elastomer crystallizes. In situ WAXS data can be used to calculate instantaneous crystallization rates for the chains parallel and perpendicular to the chain-bias direction. A comparison of crystallization kinetics with real time stress transients allows the interplay between crystallization and stress relaxation or stress accumulation to be evaluated. Data are interpreted with the goal of developing a physical model to predict how mechanical stress is influenced by crystallization and annealing conditions. Stress prediction models are valuable when designing materials for specialized applications, for example, biomedical devices and smart textiles.