Peptide Domains as Reinforcement in Protein-Based Elastomers

Proteins are a widely available biomass source for synthesizing strong and tough engineering polymers because of their propensity to hydrogen bond, chemically stable amide backbone, and demonstrated efficacy at forming relevant material structures in nature. Because the properties of polypeptides in many ways mimic urethane bonds and hard domains, herein proteins are explored as the reinforcing component in a polyurethane-inspired elastomer. Materials are synthesized using a two-step process: first, protein is methacrylated, and then copolymerized with (meth)acrylate comonomers to link protein domains with rubbery polymer chains. This is demonstrated with water-soluble proteins, whey protein and β-lactoglobulin, and a comonomer, hydroxypropyl acrylate (HPA). The resulting elastomers are amorphous and disordered but have microphase-separated morphologies. Materials with a wide range of stiffnesses have been prepared by varying the fraction of protein macro-cross-linkers in the materials. The protein aggregates function like fillers that strengthen the materials, which are shown to be tougher than both unreinforced homopolymers and unmodified proteins. Materials with low cross-link densities prepared using proteins modified at low methacrylation levels are also stiffer than protein–polymer blends. Above an optimal protein methacrylation level, increasing chemical cross-link densities led to lower extents of protein aggregation and decreased moduli.