Aggregation of dispersed rod-like particles like nanocellulose can improve the strength and rigidity of percolated networks but may also have a detrimental effect on the foamability. However, it should be possible to improve the strength of nanocellulose foams by multivalent ion-induced aggregation if the aggregation occurs after the foam has been formed. Lightweight and highly porous foams based on TEMPO-mediated oxidized cellulose nanofibrils (CNF) were formulated with the addition of a nonionic surfactant, pluronic P123, and CaCO3 nanoparticles. Foam volume measurements show that addition of the non-ionic surfactant generates wet CNF/P123 foams with a high foamability. Foam bubble size studies show that delayed Ca-induced aggregation of CNF by gluconic acid-triggered dissolution of the CaCO3 nanoparticles significantly improves the long-term stability of the wet composite foams. Drying the Ca-reinforced foam at 60 degrees C results in a moderate shrinkage and electron microscopy and X-ray tomography studies show that the pores became slightly oblate after drying but the overall microstructure and pore/foam bubble size distribution is preserved after drying. The elastic modulus (0.9-1.4 MPa) of Ca-reinforced composite foams with a density of 9-15 kg/m(3) is significantly higher than commercially available polyurethane foams used for thermal insulation.

Organosilica foams are commonly formed by a multistep process involving hydrolysis and condensationof organosilanes followed by solvent exchange and e.g. supercritical CO2 drying. Here, we propose astraightforward route to synthesize lightweight hybrid foams from aqueous dispersions of a surfaceactiveaminosilane (AS) and TEMPO-oxidized cellulose nanofibrils (TCNFs). Air bubbles were introducedin the TCNF/AS dispersion by mechanical blending, and the foam was solidified by oven-drying.Evaporative drying at mild temperature (60 1C) resulted in dry foams with low densities (25–50 kg m3),high porosities (96–99%) and macropores of 150–300 mm in diameter. The foaming and foam stabilizationwere successful for a pH range of 10.4–10.8 for foams containing 55–65 wt% of organosilica inthe dry state. The protonation of AS increased the ionic strength of the dispersion and enhanced theinterparticle interactions with TCNFs and, in turn, the foam viscosity and foam stability upon drying. Theevaporation of water catalyzed the condensation of the AS to form low-molecular linear polymers,which resulted in an increased stiffness and strength of the foam lamella. The crosslinking of the ASpolymeric network with the TCNF matrix allowed lightweight and homogeneous macroporous foams tobe obtained with controlled densities and high amine content (amine content 44.5 mmol g1) using anenvironmentally friendly technique.

We show that unidirectional freezing of nanocellulose dispersions produces cellular foams with high alignment of the rod-like nanoparticles in the freezing direction. Quantification of the alignment in the long direction of the tubular pores with X-ray diffraction shows high orientation of cellulose nanofibrils (CNF) and cellulose nanocrystals (CNC) at particle concentrations above 0.2 wt % (CNC) and 0.08 wt % (CNF). Aggregation of CNF by pH decrease or addition of salt significantly reduces the particle orientation; in contrast, exceeding the concentration where particles gel by mobility constraints had a relatively small effect on the orientation. The dense nanocellulose network formed by directional freezing was sufficiently strong to resist melting. The formed hydrogels were birefringent and displayed anisotropic laser diffraction patterns, suggesting preserved nanocellulose alignment and cellular structure. Nondirectional freezing of the hydrogels followed by sublimation generates foams with a pore structure and nanocellulose alignment resembling the structure of the initial directional freezing.