Supramolecular double networks of cellulose nanofibrils and algal polysaccharides with excellent wet mechanical properties

Abstract

Supramolecular double network films, consisting of cellulose nanofibrils (CNF) entangled with the algal polysaccharides alginate or carrageenan, were prepared using a rapid vacuum filtration process to achieve water-resistant CNF nanopapers with excellent mechanical properties in both the wet and dry states following the locking of the structures using Ca²⁺. The rigid network of calcium alginate was more efficient than the more flexible network of calcium carrageenan and 10% by weight of alginate was sufficient to form a network that suppressed the swelling of the CNF film by over 95%. The resulting material could be compared to a stiff rubber with a Young's modulus of 135 MPa, a tensile strength of 17 MPa, a strain-at-break above 55%, and a work of fracture close to 5 MJ m⁻³ in the wet state, which was both significantly stronger and more ductile than the calcium-treated CNF reference nanopaper. It was shown that the state in which Ca²⁺ was introduced is crucial, and it is also hypothesized that the alginate works as a sacrificial network that prevents the CNF from aligning during loading and that this leads to the increased toughness. The material maintained its barrier properties at elevated relative humidities and the extensibility and ductility made possible hygroplastic forming into three-dimensional shapes. It is suggested that the attractive force in the CNF part of the double network in the presence of multivalent ions is due to the ion–ion correlation forces generated by the fluctuating counter-ion cloud, since no significant ion coordination was observed using FTIR.