Cellulose nanofibril foams: Links between ice-templating conditions, microstructures and mechanical properties

This study aimed at investigating how ice templating conditions affected the shrinkage, the microstructure, and the mechanical properties of cellulose nanofibril (NFC) foams. Enzymatic and TEMPO-oxidized NFC foams were fabricated using two solidification techniques, i.e., quenching NFC suspensions in temperature-controlled baths, and mechanical stirring of NFC suspensions during solidification followed by quenching, prior to freeze-drying. Foams prepared by direct quenching using stabilized TEMPO-oxidized NFC suspensions exhibited higher specific mechanical properties and more regular anisotropic cells with unimodal size than enzymatic NFC foams. In addition, NFC concentration and NFC morphology severely affected the foam shrinkage and the geometry of foam cells. Controlling the solidification had also a drastic effect on the foam microstructure, e.g. foams prepared by mechanical stirring and quenching exhibited bimodal cell size and enhanced mechanical properties. The compressive behavior of foams showed successive elastic, strain-hardening and densification regimes with auxetic effects and strain localization. Both the elastic modulus and the yield stress were power-law functions of the foam relative density whose exponents reached unusual high values for enzymatic NFC foams, potentially because of their chaotic microstructures. The evolution of a typical TEMPO-oxidized NFC foam was studied under compression using X-ray microtomography, unveiling deformation mechanisms at the cell scale. (C) 2016 Elsevier Ltd. All rights reserved.