Moisture‐Dependent Vibrational Dynamics and Phonon Transport in Nanocellulose Materials

Abstract 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% D 2 O, 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.