(149c) Reducing Moisture Sensitivity of Protein-Based Thermosets through Protein Charge Modification and Melt Polymerization with Hydrophobic Monomers

Proteins are promising renewable feedstock for materials manufacture due to their large volumes produced in agricultural waste streams. The extensive intermolecular interactions and highly hydrogen bonded nature of proteins can be leveraged to provide mechanical reinforcement in engineering plastics. Proteins have a wide range of reactive groups amenable to chemical modification, enabling strong and tough protein copolymers to be prepared via various graft polymerization strategies. However, as proteins have high softening temperatures and limited solubility in organic solvents, many strategies were limited to aqueous-based chemistries and water soluble copolymers. Previously, this challenge was addressed by using a surfactant as both compatibilizer and plasticizer, allowing protein macromonomers to be blended with hydrophobic acrylate comonomers and copolymerized solvent-free using thermoplastic processing techniques. However, proteins are hydrophilic and are easily plasticized by water, leading to materials with low extensibility in dry conditions and low mechanical strength at high humidity levels. This critically limits the application of protein-based materials to a limited humidity range where mechanical properties are optimal.

Here, we investigate the influence of charged protein functional groups on humidity sensitivity, and demonstrate the reduction of water uptake through the copolymerization of modified whey proteins with the hydrophobic n-butyl acrylate. A combination of esterification and acetylation reactions superneutralize the protein by installing uncharged ethyl esters and amide groups onto carboxylic acids and amines respectively. Superneutralization reduced water solubility of the protein, and resulted in a 46% reduction in water absorption at 90% relative humidity, compared to unmodified protein. Incorporation of the charge modified proteins into a copolymer is enabled by the flexible nature of the employed surfactant compatibilization strategy, where a range of ionic surfactants were shown to be effective at compatibilizing the otherwise immiscible protein-monomer mixtures. Subsequent methacrylation allows the protein to be incorporated into protein-polyacrylate network, where the hydrophobic polyacrylate further reduced moisture absorption. The lowered water uptake of superneutralized proteins is reflected in the copolymer mechanical properties, which remain stable across a wider humidity range compared to non-charge modified and supercharged protein copolymers. Structure-property relationships were further studied using Fourier-transform infrared spectroscopy and atomic force microscopy. Overall, this work provides a strategy to address the performance variability of protein-based materials due to changes in humidity.