Humidity and Multiscale Structure Govern Mechanical Properties and Deformation Modes in Films of Native Cellulose Nanofibrils

Abstract

Nanopapers formed by stiff and strong native cellulose nanofibrils are emerging as mechanically robust and sustainable materials to replace high-performance plastics or as flexible, transparent and "green" substrates for organic electronics. The mechanical properties endowed by nanofibrils crucially depend on mastering structure formation processes and on understanding interfibrillar interactions as well as deformation mechanisms in bulk. Herein, we show how different dispersion states of cellulose nanofibrils, that is, unlike tendencies to interfibrillar aggregation, and different relative humidities influence the mechanical properties of nanopapers. The materials undergo a humidity-induced transition from a predominantly linear elastic behavior in dry state to films displaying plastic deformation due to disengagement of the hydrogen-bonded network and lower nanofibrillar friction at high humidity. A concurrent loss of stiffness and tensile strength of 1 order of magnitude is observed, while maximum elongation stays near constant. Scanning electron microscopy imaging in plastic failure demonstrates pull-out of individual nanofibrils and bundles of nanofibrils, as well as larger mesoscopic layers, stemming from structures organized on different length scales. Moreover, multiple yielding phenomena and substantially increased elongation in strongly disengaged networks, swollen in water, show that strain at break in such nanofibril-based materials is coupled to relaxation of structural entities, such as cooperative entanglements and aggregates, which depend on the pathway of material preparation. The results demonstrate the importance of controlling the state of dispersion and aggregation of nanofibrils by mediating their interactions, and highlight the complexity associated with understanding hierarchically structured nanofibrillar networks under deformation.

Water-Resistant, Transparent Hybrid Nanopaper by Physical Cross-Linking with Chitosan

“…In the presence of water these physical bonds compete with water interactions leading to reduced direct interfibrillar hydrogen bonding. 12 It is known that water forms strong hydrogen bonds with polar solutes, such as carbohydrates. 50,51 Overall, the combined effects manifest in a loss of macroscopic mechanical properties under wet conditions as the network is only poorly connected.…”

Toughening mechanisms in cellulose nanopaper: the contribution of amorphous regions

“…The dependence of yield stress on the degree of polymerization of the cellulose molecules and length of the nanofibres was also investigated but no clear trend was observed (Fukuzumi et al 2013;Henriksson et al 2008). The inelastic behaviour can be changed by changing humidity (Benitez et al 2013). It was reported that inelasticity is facilitated by high humidity, and it was proposed that here inelasticity is caused by inter-fibrillar debonding and possible sliding.…”

“…It was reported that inelasticity is facilitated by high humidity, and it was proposed that here inelasticity is caused by inter-fibrillar debonding and possible sliding. Inter-fibrillar hydrogen bonds can be weakened and broken by water molecules, leading to breakage of hydrogen bonds and reduced inter-fibrillar friction (Benitez et al 2013;Quero et al 2011). Robust cellulose nanopapers have been produced by hot pressing, with inelastic behaviour being more pronounced for less well pressed nanopapers (Ö sterberg et al 2013).…”

Nanocomposite films based on cellulose nanofibrils and water-soluble polysaccharides

“…Nevertheless, the ionic interaction between anionic fibrils and the cationic polymer removed water from the gel, and systems relying on ionic interactions could be susceptible to aggregation. It is important to control the state of dispersion and aggregation of CNF by controlling the interactions between the fibrils in order to achieve high strength and stiffness in cellulose nanopapers [17]. In natural composites like wood, silk, mollusk shells or bone the interactions are nonionic, mainly van der Waals interactions and multiple hydrogen bonds.…”