Ultrahigh-strength cellulose nanofiber foams via the synergy of freeze-casting and solvent exchange.

Ultrahigh-strength and lightweight materials have found wide use. However, it is difficult for artificial materials to maintain high strength while being lightweight, as the mechanical properties of most materials are strongly dependent on density. In this study, we combined the methods of freeze casting and solvent exchange to prepare cellulose nanofiber foams with lightweight and ultrahigh strength. Freeze casting created the continuous 3D network at the microscale while solvent exchange promoted the reconstruction of cellulose nanofibers via the alkali-induced mercerization effect. The foams thus possessed an ordered hierarchical structure that contained lamellar layers at the microscale and a high density of pores at the nanoscale. A quadratic exponential positive correlation between relative modulus and relative density was identified for the foams. The maximum compressive and bending moduli of the foams were 26.09 and 42.10 MPa, respectively, while its density was 210.16 mg/cm. The study provides a robust method for the preparation of lightweight and high-strength cellulose foams.