Many natural organisms use “protein rubbers” to store and release an imposed strain energy with high efficiency to make motion easier. Protein rubbers exist in a complicated environment surrounded by water and other molecules such as sugars, implying that amino acid composition and its environment are important in protein rubber behavior. Gelatin, the hydrolysis product of animal collagen, is hydrated or “plasticized” with water, ethylene glycol, glycerol, corn syrup, and aqueous solutions of sorbitol, glucose, and fructose. The rubber formed is “dry”, that is, is not fully immersed in liquid, and has the appearance and feel of a soft rubber band. The mechanical and thermodynamic behavior of each rubber is characterized with low strain dynamic and high strain tensile experiments with good agreement between the two. Plasticized gelatin rubbers are incompressible and follow the neo-Hookean model for rubber elasticity up to moderate extension ratios. Higher molecular weight polyols with more hydrogen bond donors and acceptors create gelatin networks with lower crosslink density. Ethylene glycol–, glycerol-, sorbitol syrup–, and fructose syrup–plasticized gelatin rubbers have similar molecular relaxation mechanisms and are the most efficient rubbers when probed in the rubbery plateau region prior to approaching the glass transition. The other plasticizers have different molecular relaxation mechanisms that detract from the efficiency of energy storage and return that is not related to network formation but perhaps the individual solvation ability of each plasticizer.

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