Thiol-branched graphene oxide and polydopamine-induced nanofibrillated cellulose to strengthen protein-based nanocomposite films

Soy protein isolate (SPI)—based composite materials possess a variety of excellent properties including biodegradability, biocompatibility, low cost, and easy availability, which have great potential for replacing petroleum-based materials. However, the inferior mechanical properties and high moisture sensitivity severely restrict their practical applications. In this study, novel integrated ternary hybrid nanocomposite films with multiple network structures were fabricated through introducing functionalized building blocks [graphene oxide (GO) sheets and nanofibrillated cellulose (NFC)] in a SPI matrix, in which, the interface interactions and building blocks worked synergistically to result in excellent integrated mechanical properties. First, the functionalized GO (SGO) sheets were successfully prepared via the thiol-ene click reaction with four-armed HS-terminated compound pentaerythritol tetra (3-mercaptopropionate) (PETMP) and were thus covalently cross-linked with catechol moieties of polydopamine layers on NFC (PNFC) by the Michael addition reaction. Then, the catechol moieties of PNFC covalently cross-linked to form the other crosslinked network with amino-containing SPI chains through Michael addition and/or Schiff base reactions. Moreover, H-bonding and π–π interactions also formed in such cross-linked networks. The resultant ternary SPI/SGO/PNFC films exhibited an optimal tensile strength of 11.10 MPa, which was nearly 2.81-fold higher than that of the neat SPI film. These films had the lowest water vapor permeability value of 3.91, which was 64.70% less than that of the neat SPI film. This work provides an effective and scalable design strategy for combining the synergistic effects of multiple building blocks and interfacial interactions and may show promise for various biomass applications.