We report on the thermal, electrical, and mechanical properties of alginate biopolymer nanocomposites prepared by solution casting with various amounts of graphene oxide (GO) or reduced GO (rGO). Our data shows that the thermal stability of alginate nanocomposites can be improved by the introduction of cross-linking through divalent metal cations, albeit that under these conditions little influence by the amount of rGO remains. On the other hand, the electrical conductivity of divalent metal ion cross-linked-rGO improves approximately 10 orders of magnitude with increasing weight fraction of rGO, whereas it declines for sodium alginate-GO composites. In addition, storage moduli and glass to rubber transition temperatures show strong composition dependence as a consequence of complex interactions of the ions with both polymer and filler. We propose a mechanical model that allows for the accurate prediction of reinforcement by GO sheets in sodium alginate-GO composites taking into account the orientational order of the sheets. Creep tests reveal the complex nature of multiple stress relaxation mechanisms in the nanocomposites although the stretched exponential Burgers' model accurately describes short time creep compliance.

The water sorption and diffusion in (reduced) graphene oxide-alginate composites of various compositions is analyzed. Water sorption of sodium alginate can be significantly reduced by the inclusion of graphene oxide sheets due to the formation of an extensive hydrogen bonding network between oxygenated groups. Crosslinking alginate with divalent metal ions and the presence of reduced graphene oxide can further improve the swelling resistance due to the strong interactions between metal ions, alginate, and filler sheets. Depending on conditions and composition, the overall water barrier properties of alginate composites improve upon (reduced) graphene oxide filling, making them attractive for moisture barrier coating applications. Water sorption kinetics in all alginate composites indicate a non-Fickian diffusion process that can be accurately described by the Variable Surface Concentration model. In addition, the water barrier properties of sodium alginate-graphene oxide composites can be adequately predicted using a simple model that takes the orientational order of filler sheets and their effective aspect ratio into account. (Figure presented.).