Superinsulating nanofibrillar cellulose foams have the potential to replace fossil-based insulating materials, but the development is hampered by the moisture-dependent heat transport and the lack of direct measurements of phonon transport. Here, inelastic neutron scattering is used together with wide angle X-ray scattering (WAXS) and small angle neutron scattering to relate the moisture-dependent structural modifications to the vibrational dynamics and phonon transport and scattering of cellulose nanofibrils from wood and tunicate, and wood cellulose nanocrystals (W-CNC). The moisture interacted primarily with the disordered regions in nanocellulose, and WAXS showed that the crystallinity and coherence length increased as the moisture content increased. The phonon population derived from directional-dependent phonon density of states (GDOS) increased along the cellulose chains in W-CNC between 5 and 8 wt% D₂O, while the phonon population perpendicular to the chains remained relatively unaffected, suggesting that the effect of increased crystallinity and coherence length on phonon transport is compensated by the moisture-induced swelling of the foam walls. Frequency scaling in the low-energy GDOS showed that materials based on hygroscopic and semicrystalline nanocellulose falls in between the predicted behavior for solids and liquids. Phonon-engineering of hygroscopic biopolymer-based insulation materials is promoted by the insights on the moisture-dependent phonon transport.

Nanocellulose is a promising alternative to fossil-derived materials, but its development is hindered by a limited understanding of cellulose–water interactions. Herein, quasielastic neutron scattering (QENS) is used to investigate how hydration and temperature affect the localized rotations in cellulose nanocrystals (CNC) and the diffusion of mobile water. QENS reveals that the C6 hydrogens and the C6 OH in the surface regions of CNC exhibit an isotropic rotation. The extracted mean square displacement shows that hydration enhances the overall hydrogen mobility in the cellulose chains. The mobile water diffusion at 270 K is unaffected by cellulose and consistent with diffusion of supercooled bulk water. At 310 K, the diffusion slows compared to bulk water, consistent with water diffusing on the CNC surface. Decoupling of the translational and rotational motion provides insight into the local cellulose–water motions. The localized motions of the nondiffusing water are found to be coupled with cellulose at 310 K, indicating a more complex dynamics at higher temperature. These findings provide new insights into how hydration modulates hydrogen mobility in cellulose, highlighting the interplay between water diffusion and molecular motion.

We have produced foams from cellulose nanofibrils from upcycled cotton (upCNF) and wood (wCNF) through unidirectional (UIT) and multidirectional ice-templating (MIT) and investigated the structural humidity response through in-situ WAXS, SAXS, and micro tomography (μCT) between 10 and 95 % relative humidity (RH). The upCNF and wCNF WAXS patterns displayed a shape- and position shift as the RH was increased, with a compression in the (200) direction and an elongation in the (004) direction. The average separation distance extracted from the 1D SAXS patterns revealed no significant change for the upCNF foams regardless of RH and processing route, while a significant increase was observed for the wCNF foams. The μCT measurements of the upCNF foams showed a slight shift in macropore distribution towards larger pores between 50 and 80 % RH which can be attributed to the weakening and partial disintegration of the pore wall as more moisture is introduced. The humidity-induced structural alterations of the upCNF foam were significantly lower compared to the wCNF foams, confirming our claim of upCNF being more moisture resistant than wCNF foams.