Cellulose
• Interfibril level (how fibrils aggregate into larger structures). • Fiber and interfiber level (how fibril aggregates are assembled in fibers and how fibers form joints in paper). Based on a critical survey of suitable literature, both old and recent, our aim is to present a more nuanced description of the role of H-bonds in cellulose research. Molecular modeling such as molecular dynamics (MD) simulations has played an important role over the years for the understanding of molecular- scale phenomena in cellulose (Zhou et al. 2020) since they offer both a level of detail that surpasses what can be reached by experimental methods and the ability to quantitatively extract almost any experimental param- eter from the simulated ensemble. However, how simulated microscale properties are related to the macroscale is not always clear. A molecular simula- tion typically represents less than one millionth of a real sample that is observed during less than one microsecond, and great care must naturally be taken when transferring simulation results to larger scales. Nevertheless, simulated molecular interactions are a strong qualitative and quantitative tool to understand
modulus and strength of fibrils, strong fiber-fiber bonding, or forming of paper from fibers are com- monly explained based on ‘‘hydrogen bonding effects’’. Sometimes this explanation is wrong (high modulus and strength of fibrils) or very often incom- plete (fiber-fiber bonding and forming of paper), and in most cases the effect of moisture is neglected. However, simplistic explanations to complex prob- lems are convenient and thereby tend to survive. Thus, there is a need from time to time to re- examine the claims made with respect to H-bonding in cellulose and cellulose-based materials, and this is the purpose of this review. To this end, we discuss H-bonds in the context of the research on cellulose and their role at different length scales (Fig. 1) including: • Molecular level (conformation of a cellulose molecule). • Intermolecular level (how cellulose molecules interact with each other). • Fibril level (how cellulose is arranged into crystals). • Interfacial level (how cellulose fibrils interact with other molecules).
Fig. 1 Structure of cellulose at various length scales and organizational levels. The dihedral angles u and w are defined by the atomic sequences O5’-C1’-O4-C4 and C1’-O4-C4-C3,
respectively. The figure depicts a tentative model for elementary fibrils from wood, although its exact dimensions and shape are still matters of debate
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