Giant Strain-Induced Crystallization in Ideal-Network Elastomers

Aligned crystalline domains can form in the amorphous network of elastomers when subject to large elongations. The increased crystallinity can effectively pin and blunt cracks, giving enhanced mechanical strength and fracture toughness. Strain-induced crystallization is reversible since ordered chains can disassemble during bulk retraction. However, the effect of strain-induced crystallization is limited in conventional elastomers by polydispersity, random network architecture, and dense chain entanglements. This work demonstrates that strain-induced crystallization in ideal-network elastomers can reach up to 50%, more than twice the effect of strain-induced crystallization reported in existing elastomers (e.g., 20% for natural rubber). Our experiments show that an ideal-network elastomer with a low polydispersity, high degree of homogeneity, and low chain entanglement density attains an amplified extent to which reversible hydrogen bonds form during stretching. This giant strain-induced crystallization simultaneously gives the ideal-network elastomer high fracture toughness and low mechanical hysteresis. We also show that the limiting stretch of ideal-network elastomers follows an abnormal scaling since giant strain-induced crystallization enables the material to stretch near its elastic limit. Polymer chains in the network display shorter initial end-to-end lengths, producing a higher elongation ratio when they straighten and break. We found that the ultimate stretch of an ideal-network elastomer follows the abnormal scaling, exceeding the established theoretical stretch limit, which is the square root of the number of monomers per chain. Ideal-network elastomers provide a foundation for the design of the next frontier of enhanced rubbery materials whose properties approach fundamental mechanical limits.