Mechanics and chemistry of highly stretchable materials

Rapid advances are taking place to develop highly stretchable materials (elastomers and gels). Such a material is a hybrid of strong and weak bonds. The strong intramolecular bonds link monomers into polymer chains. The weak intermolecular interaction aggregates polymer chains in an elastomer, and aggregates polymer chains and solvent in a gel. The hybrid strong and weak bonds give rise to the characteristic mechanical behavior of these materials. Familiar examples include entropic elasticity and giant swelling of a polymer network, and high fracture energy in natural rubber due to stretch-induced crystallization. Several recent findings show that hydrogels can achieve properties and applications well beyond previously imagined. Most existing hydrogels, like Jell-O and tofu, are fragile and dry out in open air. We make hydrogels as tough as natural rubber, and retain water in low-humidity environment. Hydrogels are stretchable, transparent, ionic conductors. We demonstrate their use in artificial muscles, skins, and axons. This talk describes the mechanics and chemistry of these materials, with focus on strength, toughness, adhesion, and fatigue resistance.