New horizons for cellulose nanotechnology
2017; Royal Society; Volume: 376; Issue: 2112 Linguagem: Inglês
10.1098/rsta.2017.0200
ISSN1471-2962
AutoresStephen J. Eichhorn, Sameer S. Rahatekar, Silvia Vignolini, A. H. Windle,
Tópico(s)Polysaccharides and Plant Cell Walls
ResumoYou have accessMoreSectionsView PDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmail Cite this article Eichhorn S. J., Rahatekar S. S., Vignolini S. and Windle A. H. 2018New horizons for cellulose nanotechnologyPhil. Trans. R. Soc. A.3762017020020170200http://doi.org/10.1098/rsta.2017.0200SectionYou have accessIntroductionNew horizons for cellulose nanotechnology S. J. Eichhorn S. J. Eichhorn Bristol Composites Institute (ACCIS), University of Bristol, Queen's Building, University Walk, Bristol BS8 1TR, UK [email protected] Google Scholar Find this author on PubMed Search for more papers by this author , S. S. Rahatekar S. S. Rahatekar Enhanced Composites and Structures Centre, Cranfield University, College Road, Cranfield MK43 0AL, Bedfordshire, UK Google Scholar Find this author on PubMed Search for more papers by this author , S. Vignolini S. Vignolini Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK Google Scholar Find this author on PubMed Search for more papers by this author and A. H. Windle A. H. Windle Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK Google Scholar Find this author on PubMed Search for more papers by this author S. J. Eichhorn S. J. Eichhorn Bristol Composites Institute (ACCIS), University of Bristol, Queen's Building, University Walk, Bristol BS8 1TR, UK [email protected] Google Scholar Find this author on PubMed , S. S. Rahatekar S. S. Rahatekar Enhanced Composites and Structures Centre, Cranfield University, College Road, Cranfield MK43 0AL, Bedfordshire, UK Google Scholar Find this author on PubMed , S. Vignolini S. Vignolini Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK Google Scholar Find this author on PubMed and A. H. Windle A. H. Windle Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK Google Scholar Find this author on PubMed Published:25 December 2017https://doi.org/10.1098/rsta.2017.0200Cellulose is one of the world's primary natural resources, and is the most used material in the world. More than 20 times the volume of steel is used on an annual basis, and since ancient times cellulose, in the form of timber, paper and clothing, has underpinned the development of society. Despite its common use throughout human history, only in recent years has the true potential of cellulose as a high-end functional and sustainable material been realized, especially in the form of nanofibrous materials [1,2]. Natural cellulose is found in every plant as a hierarchical material. Within the trunk of the tree and its branches and roots, there are fibre vessel elements, which in the living tissues transport nutrients and water. In processes such as papermaking, these vessel elements are extracted and we typically then call them 'fibres'. In fact, such fibres possess a layered structure of fibrils, embedded in other plant polysaccharides, and binding materials such as hemicelluloses, lignin, pectins and waxes. These fibrils, or cellulose nanofibrils (CNFs), can be extracted from the cell walls of the fibres through mechanical and/or chemical/enzymatic approaches. They represent not only a truly sustainable resource but also a functional material.Like many other biopolymers, celluloses possess a semicrystalline structure. This semicrystalline structure is susceptible to some degree to hydrolysis using strong acids, the so-called 'amorphous' regions being more susceptible than the crystalline ones [3,4]. This hydrolysis process liberates so-called 'cellulose nanocrystals' (CNCs), a colloidal form of cellulose stabilized by negative surface charge due to the presence of sulfate half-ester groups (if sulfuric acid is used) [4,5]. Such colloidal CNCs are interesting from an industrial perspective because they can be dispersed in water. As cellulose is known to be recalcitrant to a large number of common solvents, its dispersion in water provides a distinct advantage. Moreover, as we have built up a large amount of knowledge on the dispersion of cellulose in water through the papermaking process, CNCs render themselves to an already mature industry.In contrast to CNCs, CNFs are long fibrils that partially retain the amorphous regions of natural fibres, but they can have similar lateral dimensions to CNCs. Knowledge of their existence perhaps pre-dates CNCs. Turbak and co-workers [6] are generally attributed with the discovery of this material, which they called 'microfibrillated cellulose'. This material is typically produced by mechanical means, through homogenization or grinding, although it is worth mentioning that CNFs can also be produced via the papermaking beating process. When dried, these fibrils can form sheets of material that the research community calls 'nanopaper'. The structure of 'nanopaper' is stochastically similar to paper. Its mechanical properties, however, far exceed those of a typical sheet of conventional paper. These enhanced mechanical properties are thought to derive from the high levels of bonding between fibrils, and the intrinsic strength and stiffness of individual fibrillar elements.This themed issue of the journal contains papers derived from a meeting at the Royal Society (2–3 May 2017) which brought together leading cellulose scientists from different countries to discuss the science of cellulose nanostructures and its application to the development of different forms of cellulose-based materials. We have sequenced the first papers to represent the discussion around the formation of the material cellulose itself within the cell wall (Jarvis, Glasgow [7], and Turner, Manchester [8]). The generation of the cell wall structure remains somewhat of a mystery, particularly how the cellulose synthase (rosettes) operate to deposit the material with such regularity. The intimate contact of cellulose with other materials is an important area of research, because our ability to extract nanofibres requires understanding, at a fundamental level, of the forces present at the nanoscale. So, we turn to this subject with a paper on the interaction forces within the plant cell walls (Nishiyama, CERMAV [9]). Such forces appear to be dominated by hydrogen bonding, but these alone cannot be solely responsible for the recalcitrance of cellulose to dissolve in common solvents.As already noted, CNCs can be dispersed in aqueous solvents, and some issues with the control of these dispersions are discussed by Gray (McGill) [10]. With the control of surface charge being most important to maintaining aqueous dispersions of material, but also to aiding their self-assembly, Cranston (McMaster) [11] presents a paper on how phosphoric acid can be used in this respect. Indeed, many acids can be used for the hydrolysis process, but phosphoric acid has the added benefit of maintaining surface charge, while not compromising on thermal stability.Perhaps one of the most remarkable discoveries around aqueous suspensions of CNCs is their ability to form lyotropic liquid crystals (LCs). It is noted that the formation of these LC phases, while interesting in its own right, has further enabled us to better understand the dynamics of LCs in general. The paper by MacLachlan (University of British Columbia) [12] on tactoid formation, growth and deposition provides a beautiful illustration of the translative power of this research area. The cholesteric phase of CNC suspensions enables us to form 'frozen-in' helicoidal structures in the dry state which display structural colour. These frozen-in structures give us some insight into the formation of LC phases, but also have great potential for the development of new industrial products.CNCs also possess an interesting property in that they are able to reside at the oil–water interface, stabilizing these emulsions (called 'Pickering emulsions'). This ability to stabilize emulsions is discussed in a paper by Hamad (FP Innovations) [13]. The formation of Pickering emulsions opens up opportunities to use CNCs in paints and inks—indeed, one application of CNCs has been in pen inks, where they also act as a rheological modifier. Given the high mechanical stiffness of CNCs, and their mechanically extracted counterpart CNFs, we then turn to applications in composite materials. Dufresne (University of Grenoble) [14] has been a pioneer in this area. Perhaps, one of the 'Holy Grails' of composite manufacture is to incorporate seemingly hydrophilic cellulose fibres into hydrophobic resin materials (thermoplastic resins). This challenge forms the subject of Dufresne's paper, although CNCs/CNFs can also be incorporated rather more readily into many resin materials other than thermoplastics. It ought to be possible to form composite materials with the 'frozen-in' helicoidal structures (displaying Bouligand curves when sectioned). The paper by Gilman (National Institute of Standards and Technology) [15] reports such an achievement, and these materials were shown to display remarkable fracture properties as well as structural colour. Cellulose nanofibres are of course optically interesting in that they can possess diameters less than the wavelength of light if they are suitably processed. While paper, even without fillers, is still white due to air scattering, reducing the diameter of the fibrils and densifying the structure renders it transparent. This was, and is still, the process by which transparent papers called 'glassine' are manufactured—the transparent window in old envelopes. Greaseproof paper is also combined with waxy material, providing a refractive index match, thus making the material transparent. If the scattering regions of wood (the pores) are eradicated through resin infusion, and the scattering elements of the cell walls, the fibrils, are used appropriately, the production of transparent wood is possible. A paper by Berglund (KTH Royal Institute of Technology) [16] covers this subject, and we were delighted during his talk with the vision of producing a wood-based laser! Other applications of cellulose nanofibres, such as thermoelectric materials, are covered in the paper by Tammelin (VTT Technical Research Centre of Finland) [17]. The subject of composite materials is further explored by Gindl (BOKU University of Natural Resources and Life Sciences) [18], where it is shown that retaining some of the lignin component of the plant fibre structure assists in the interfacial interactions with other polymers. This, and the fact that hierarchical and architectural diversity are important (Bismarck, University of Vienna/Imperial College) [19] for composite materials, is not surprising because we know they are critical in the function of the original plant material. We would do well to remember that these are originally biological materials, and our exploitation of them may be more successful if we learn from Nature's approach to their use.So, returning to the natural world, and the place and context of cellulose nanofibres and their industrial exploitation, we should consider these in a mounting concern over global warming. It will become more critical for humans to use more sustainable sources of materials, and to move away from oil-based products. Cellulose production as plant life plays a major part in a balanced global ecosystem, and it is harvested as bulk or fibre materials. While forests are responsible for huge uptakes of CO2 from the atmosphere, which are more or less balanced by a return flow, there is a net uptake estimated at 9.3 Gt p.a. (gigatonnes per annum),1 about half of which, though, is cancelled out by deforestation, especially if the timber is burnt on site [20]. Some 2.2 Gt of wood is harvested each year to be used as a material (other than for burning) [21], while natural fibres and wood pulp for paper manufacture add a further 0.3 Gt [21,22], giving a total retained CO2 equivalent of 4 Gt of CO2. Most cellulose products at the end of their service life will either decay or be burnt, either way returning the carbon to the atmosphere, so they represent at best a medium-term sequestration of the carbon that the ecosystem extracts from the atmosphere. However, 4 Gt of CO2 p.a. is but a fraction of the man-made burden put on the atmosphere, of the order of approximately 30 Gt p.a. emitted CO2 [20], derived mainly from fossil fuels and cement production. However, any carbon sequestration, albeit temporary, will buy time in the campaign to limit CO2 and, by implication, global warming. The on-going pursuit of cellulose science and technology leading to the development of new applications and markets can therefore make a small but useful contribution to the quest to control CO2 levels. A deeper understanding of cellulose itself, which involves structures at the nanoscale, has the ability to open up the new potential for exploitation, whether at a small scale for device applications or on a larger scale for structural materials. The development of new high value-added applications for cellulose may also encourage the forestry industry at least to maintain the status quo in terms of production and provide some bastion against the reduction in wood pulp manufacture, itself triggered by a trend towards the 'paperless society'.Data accessibilityThis article has no additional data.Competing interestsWe declare we have no competing interests.FundingWe received no funding for this study.Footnotes1 Especially where they are the difference between two large estimates, these values are at best approximate. The industry-derived utilization tonnages are likely to be more accurate, although not necessarily above scrutiny.One contribution of 14 to a discussion meeting issue 'New horizons for cellulose nanotechnology'.© 2017 The Author(s)Published by the Royal Society. All rights reserved.References1Habibi Y, Lucia LA, Rojas OJ. 2010Cellulose nanocrystals: chemistry, self-assembly, and applications. Chem. 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All rights reserved. Citations and impact Subjectsmaterials sciencenanotechnology
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