Cellulose
Fig. 7 Drying-induced deformation of nanofibrils driven by capillary effects. The ‘‘dry’’ fibril-fibril joint contains structured and immobile water, which can contribute to both total adhesion and deformation mechanisms
at 50% relative humidity. The emerging picture is that the extent to which H-bonding contributes to the adhesion between fibrils, even in ‘‘dry’’ conditions, is limited and an indirect effect of the high surface tension of liquid water. Interfacial water also affects the friction between fibrils. In completely dry conditions, which can only be realized in simulations, the traction is dominated by a stick-slip behavior, in part related to breaking and reformation of interfacial H-bonds (Zhang et al. 2021). In that respect, the small size and relatively high mobility of water molecules make them into an effective lubricant, which can significantly decrease the resistance to interfacial shear stress (Sinko and Keten 2014), and possibly also serve as a toughening mechanism since a weaker interface reduces brittle- ness (Hou et al. 2021).
substantial at the nanoscale and are strong enough to deform fibrils plastically (Ogawa et al. 2020). More- over, MD simulations have shown that the capillary force can act over large distances (several nanometers) through liquid capillary bridges (Sinko and Keten 2014; Zhang et al. 2021) and thus fulfils both criteria. When two cellulose surfaces finally are in contact there will of course be H-bonds. However, with respect to breaking , since the H-bond energy is smaller than the dispersion energy even for the best possible interface with respect to H-bond formation (i.e., the crystal structure), H-bonding likely plays a minor role for the stability also here. Thus, if the surfaces are close enough to be in molecular contact (i.e., van der Waal’s and/or H-bonding) the London dispersion will dominate over the H-bond contribution to the interac- tion energy. However, there is undoubtedly water present even in cellulose material that is perceived as dry and this will complicate the picture further. Even if the porosity of cellulose nanomaterials can be surprisingly low, unless fibrils are perfectly aligned, they are prohibited from fusing completely into one continuous phase. Thus, as mentioned above, sub- nanometer sized cavities are formed, for instance around fibril-fibril joints, where surface hydroxyl groups are available for H-bonding. This space can be effectively filled by water molecules which will saturate the H-bonds, as long as the entropic penalty of confinement is not too high (Fig. 7). Thus, it is not only difficult to remove the water fraction that is most tightly attached to cellulose, but it is even more difficult to prepare and maintain samples (either as a specimen to be studied or a starting point for controlled hydration) that are devoid of all water (Lindh et al. 2017), because the initial water uptake of dry cellulose from ambient air is extremely rapid and amounts to about 8 wt% (based on cellulose dry mass)
Cellulose nanopaper films
A unique feature of cellulose nanofibrils is that ‘‘non- porous’’ nanopaper films can be formed by drying from colloidal dispersions of the nanofibers in water. Filtration and drying results in nanofibrils which are oriented random-in-plane, with slightly swirled con- formation (Henriksson et al. 2008). The formation process can be compared to the formation of photonic crystals where capillary force through liquid bridges (Fig. 7) is the main adhesion mechanism, not hydro- gen bonding (Zhou et al. 2006). Such forces can become very large, enough to deform the nanofibers plastically (Ogawa et al. 2020), which explains the low porosity of cellulose nanopaper dried from water. The Young’s modulus of dry wood cellulose nanofibril films was recently reported to be 24.9 GPa by careful strain field measurements (Yang et al. 2021). This result is much higher than for any polymer
123
Made with FlippingBook Online document maker