Water Dynamics in Hydrated Carboxylated Cellulose Nanofibril Membranes

Abstract

Cellulose nanofibrils (CNF) are a class of materials with good mechanical properties, surface functionality and bio-/environmental friendliness. They have been used in many applications as loading material or function materials, where water-cellulose interaction determines the materials performance.Especially, CNF with carboxylated groups can be used as the separation membrane in polymer electrolyte membrane fuel cell. The water dynamics is closely related to the proton conductivity. The Non-destructive quasi-elastic neutron scattering (QENS) is used to characterized water movement in hydrated membrane made of CNF prepared by TEMPO-oxidation with different surface charges.However, neither surface charge nor the nanoconfinement due to membrane swelling has large impact on water dynamics mechanism. A slow diffusive motion is found with the diffusion coefficient close to bulk water and that in hydrated Nafion membrane regardless the surface charge, while a fast motion is rather localized with a correlation time increasing as temperature increase, which might related to the hydrogen bond network formation between water and CNF. † Present address: RISE Introduction:The water -cellulose interaction is one of the most important scientific topics in the field of woodbased material science. It does not only conceal the secret behind the plant growth, but also determines processing parameters and performance of the final wood-based materials. In nature, cellulose polymer chains self-assemble into nanofibrils which consist of alternative crystalline and non-crystalline region along the fibril long axis. These so-called cellulose nanofibrils (CNF) serve as basic building block and are further organized with other biopolymer in water media to form the plant cell wall. 1 Compared to many wood-based materials, CNF to the largest extent keep the strong nature of cellulose materials, i.e. preserving excellent mechanical properties 2,3 . Meanwhile, the high surface-to-volume ratio enables a large surface area per unit weight and the extensive hydroxyl group exposed on the CNF surface could be easily modified and functionalized 4,5 . Especially, the carboxylation of CNF by the TEMPO-oxidation process has now become a common step for most of application as it delivers fine cross-section and activated carboxylated surface ligands 6 . Additionally, CNF are readily extractable from the pulp fibres and biodegradable. Thus, they have become a popular material platform for developing novel composites 4,5,7-9 . Among many applications, the good mechanical property of CNF has attracted most of the attention for engineering materials. Moisture content in CNF based materials influence both stiffness and strength 10 . On top of the robustness, the CNF could be used alone and/or as a carrier to load other functional materials for versatile purposes in optics 11 , electronics 12 and energy device 13 , where moisture content and electrolyte are closely relavent. Recently, CNF membranes with surface functional group have been successfully tested a...