Research on nanoparticles extracted from renewable and highly available sources is motivated by both the development of functional nanomaterials and the drive to replace widely used materials based on fossil resources. In particular, cellulose, in the form of cellulose nanomaterials (CNM), has attracted increased attention for the development of sustainable and high performance products, thanks to properties that include high specific mechanical strength, chemical versatility and anisotropic thermal conductivity. Ice-templated CNM foams display super-insulating properties across the direction of the aligned particles (radially) and could potentially compete with fossil-based insulation materials. This thesis investigates the alignment and co-assembly of widely available inorganic nanomaterials with CNM in aqueous dispersions, and the relative importance of phonon scattering in anisotropic thermally insulating composite foams.

Time resolved small-angle X-ray scattering (SAXS) experiments have been conducted to study assembly and alignment in composite aqueous dispersions containing cellulose nanocrystals (CNC) and montmorillonite (MNT) clay nanoplatelets. The co-assembly of CNC and MNT in slowly evaporating levitating droplets was dominated by the interactions between the dispersed CNC particles but MNT promoted gelation and assembly at lower total volume fractions than in CNC-only droplets. Combining SAXS with rotational rheology showed that shear induced a high degree of orientation of CNC in both the CNC-only and mixed CNC:MNT dispersions. The shear-induced CNC orientation relaxed quickly in the CNC-only dispersion but relaxation was strongly retarded and partially inhibited in the mixed CNC:MNT dispersions.

Analysis of previous works suggests that anisotropic and multiscale CNM-based foams with a high number of interfaces can favour heat dissipation by phonon scattering within the foam walls. Measurements and theoretical estimates of the thermal conductivities of CNC-only ice-templated foams over a wide range of densities confirmed the importance of phonon scattering to achieve super-insulating radial thermal conductivity values. 

Ice-templated CNC:MNT composite foams displayed a lower radial thermal conductivity compared to CNC-only foams, which suggests that the introduction of heterogeneous interfaces between the biopolymer and the clay enhanced the dissipation of heat through phonon scattering. Composite ice-templated foams of colloidal silica and TEMPO-oxidised cellulose nanofibrils (TCNF) were significantly stronger under mechanical compression and less sensitive to moisture uptake than TCNF-only foams, and maintained radial thermal conductivities that are comparable with widely used thermally insulating materials. These examples could pave the way towards the development of super-insulating, strong and moisture-resilient CNM-based composite foams.