Cellulose
Table 1 Importance of H-bonding for various physical and chemical properties of cellulose and cellulose based materials, at normal conditions
Property
Importance Comment
Solubility
None
Dominated by solvent effects
Molecular conformation Depends
Negligible in solution but important in the gas-phase
Crystal structure
Some
Defines H-bonding pattern, but structure is to a large extent the result of efficient packing, i.e. steric/dispersion interactions
Elastic properties
Some
Axial modulus is dominated by covalent interactions
Adsorption of polymers/molecules
None
Dominated by solvent effects
Fibril aggregation
Indirect
Mediated by water
Nanopaper prop.
Some
Contributes to dry strength, responsible for high moisture sorption which affects ductility
Strength of paper
Minor
One among many contributions
including H-bonds. Once formed, these interactions will play different roles during the loading of the joints and also the combination of different interactions will contribute. H-bonding naturally influences the strength of these joints, as they will inevitably form wherever a favorable molecular geometry arises, but to what extent they contribute at the fiber scale in comparison to other relevant molecular forces is not possible to determine using any technique available today. However, considering their short range and directional sensitivity, contributions from other inter- actions are most likely dominating.
H-bonding completely in the analysis of cellulose is not a solution; they play an important role for defining the crystal structure of cellulose and give a surpris- ingly large contribution to its elastic modulus. H-bonds can also be expected to contribute to molecular adhesion in dry conditions although com- pletely dry cellulose hardly exist except in theory, and the role of water for interactions at both fibril- and fiber level under normal conditions cannot be stressed enough. This observation also applies to drinking straws. Molecular modeling has indeed been instrumental for our understanding of the molecular-scale proper- ties of cellulose. But at the same time the atomistic picture that permits us to actually count H-bonds has almost been a curse. Furthermore, it is important to realize that when we discuss experimental observa- tions, we almost exclusively limit ourselves to a material that has been taken out of its native environ- ment and has been mechanically and/or chemically modified. This undoubtedly affects its physico-chem- ical properties, particularly so when comparing to computer modeling studies, which are restricted to highly idealized models. The emerging conclusion is that H-bonding is and should be viewed as one interaction among several, and its relative contribution is highly dependent on the specific conditions and cannot easily be determined by intuition alone or, indeed, in some cases not even by careful analysis. Based on our combined experience from working with cellulose and from the review of the scientific literature in this field presented in this
Perspective and outlook
In this paper we have reviewed the role of H-bonding in cellulose-based materials at different scales, from H-bonds within small molecular fragments up to macroscopic fiber-fiber bonds. It is clear that the role of H-bonds has in many cases been exaggerated, at all levels in the structural hierarchy of cellulose. Consid- ering that cellulose H-bonds in most relevant systems can exchange with H-bonds to water molecules it becomes evident that their net effect on the overall energetics is relatively small compared to other factors such as dispersion interactions and hydrophobic effects. A misplaced focus on H-bonding is an oversimplification that has indeed slowed down the development of our fundamental understanding of cellulose-rich materials and impeded their use in more advanced applications. On the other hand, to ignore
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