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.

中文翻译：

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纳米纤维素材料中水分依赖性振动动力学和声子传输


超绝缘纳米纤维纤维素泡沫有可能取代化石基绝缘材料，但由于水分依赖性的热传输和缺乏对声子传输的直接测量，这一发展受到了阻碍。在这里，非弹性中子散射与广角 X 射线散射 （WAXS） 和小角中子散射一起使用，将水分依赖性结构修饰与来自木材和被毛石的纤维素纳米原纤维以及木材纤维素纳米晶体 （W-CNC） 的振动动力学和声子传输和散射联系起来。水分主要与纳米纤维素中的无序区域相互作用，WAXS 表明结晶度和相干长度随着水分含量的增加而增加。来自方向依赖性声子密度状态 （GDOS） 的声子群在 W-CNC 中沿纤维素链增加 5 至 8 wt% D2O，而垂直于链的声子群相对不受影响，这表明结晶度和相干长度的增加对声子传输的影响被水分诱导的泡沫壁膨胀所补偿。低能 GDOS 中的频率缩放表明，基于吸湿性和半结晶纳米纤维素的材料介于固体和液体的预测行为之间。吸湿性生物聚合物基绝缘材料的声子工程通过对水分依赖性声子传输的见解得到促进。