Biobased super-insulating materials could mitigate climate change by minimizing the use of petroleum-based materials, creating artificial carbon sinks and minimizing the energy needed to maintain pleasant interior conditions. Cellulose nanomaterials (CNM) produced from abundantly available cellulose sources constitute versatile, highly anisotropic raw materials with tunable surface chemistry and high strength. This thesis includes the evaluation of the thermal conductivity of isotropic and anisotropic CNM-based foams and aerogels and analysis of the dominant heat transfer mechanisms. 

We have developed a customized measurement cell for hygroscopic materials in which the humidity and temperature are carefully controlled while the thermal conductivity is measured. Anisotropic cellulose nanofibrils (CNF) foams with varying diameters showed a super-insulating behavior perpendicular (radial) to the nanofibril direction, that depended non-linearly on the relative humidity (RH) and foam density. Molecular simulations revealed that the very low thermal conductivity is related to phonon scattering due to the increase of the inter-fibrillar gap with increasing RH that resulted in a 6-fold decrease of the thermal boundary conductance. The moisture-induced swelling exceeds the thermal conductivity increase due to water uptake at low and intermediate RH and resulted in a minimum thermal conductivity of 14 mW m^-1 K^-1 at 35% RH and 295 K for the foams based on the thinnest CNF.

The density-dependency of the thermal conductivity of cellulose nanocrystal (CNC) foams with densities of 25 to 129 kg m^-3 was investigated and a volume-weighted modelling of the solid and gas thermal conductivity contributions suggested that phonon scattering was essential to explain the low radial thermal conductivity, whereas the replacement of air with water and the Knudsen effect related to the nanoporosity in the foam walls had a small effect. Intermediate-density CNC foams (34 kg m^-3) exhibited a radial thermal conductivity of 24 mW m^-1 K^-1 at 295 K and 20% RH, which is below the value for air.

The moisture uptake of foams based on CNMs with different degree of crystallinity and surface modifications decreased significantly with increasing crystallinity and temperature. Molecular simulations showed that the narrow pore size distribution of the amorphous cellulose film, and the relatively low water adsorption in the hydration cell around the oxygen of the carboxyl group play an important role for the moisture uptake of amorphous and crystalline CNM-based materials.

Isotropic CNF- and polyoxamer based foams as well as CNF-AL-MIL-53 (an aluminum‑based metal-organic framework) foams were both moderately insulating (>40 mW m^-1 K^-1) and comparable with commercial expanded polystyrene. The thermal conductivity of CNF and polyoxamer foams displayed a very strong RH dependency that was modelled with a modified Künzel’s model. The presence of hydrophobic AL-MIL-53 decreased the moisture uptake of CNF-AL-MIL-53 aerogels by 42% compared to CNF-polyoxamer foams.

Solid and gas conduction are the main heat transfer mechanisms in hygroscopic nanofibrillar foams and aerogels that depend on the interfacial phonon scattering, Knudsen effect and water uptake. It is essential that the thermal conductivity measurements of hygroscopic CNM-based foams and aerogels are determined at controlled RH and that parameters such as the temperature, density, nanoporosity, fibril dimensions and alignment are characterized and controlled for systematic development and upscaling of biobased foams for applications in building insulation and packaging.

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.