Cellulose is a linear homopolymer consisting of D-glucopyranose units linked by b-(1-4) glucoside bonds. Cellulose is practically insoluble in water unlike starch, which consists of the same glucose units but in which the glucose units of amylose and branched amylopectin are linked by a-(1-4) glucoside bonds, and at branching points also by a-(1-6) bonds. Cellulose, starch, the various hemicelluloses and pectin belong to a group of compounds called polysaccharides which are composed of various monosaccharides such as glucose (cellulose and starch), galactose, xylan, mannose and galacturonic acid.
Native cellulose in plants possesses a high degree of polymerisation (DP), approximately 10,000 for wood cellulose and 15,000 for cotton. Therefore, the molecular weight of cellulose can be more than one million (g/mol). In addition, cellulose is available in grades with a considerably lower molecular weight, where the DP can be as low as 300 to 500.
In nature, cellulose molecules organise themselves into nanofibrils and into larger fibrous structures. In order to swell in water, cellulose requires e.g. a strong alkaline environment, and in order to dissolve it requires additives such as urea (NaOH/urea) or zinc oxide (NaOH/ZnO). Cellulose can also be dissolved in ‘cellulose solvents’ such as N-methylmorpholine N-oxide (NMMO), a mixture of N,N-dimethylacetamide and lithium chloride (DMAc/LiCl) or ionic liquids (IL). These homogeneous solvents systems, in which cellulose completely dissolves, are also used in order to chemically modify cellulose. There are excellent review articles on cellulose dissolution and modifications in various reaction conditions.1,2
Cellulose fibre materials and improving their properties
Regenerated cellulose fibres from cotton and linen are used in the textile industry. Other applications include using the bleached dissolving pulp produced from wood fibres after removal of lignin and hemicelluloses for production of various cellulose derivatives and converted man-made fibre products.
In the regenerated fibre production process, either unmodified of chemically premodified cellulose is first dissolved in an appropriate solvent and then regenerated into fibres. In the Lyocell process, the cellulose is first dissolved in an NMMO system and then regenerated. In the Biocelsol method, the cellulose is first treated with cellulose-degrading enzymes and then dissolved in a NaOH/ZnO system by exploiting a special low-temperature treatment at –40...–30 °C. The resulting cellulose solution is regenerated into fibres in a sulphuric acid bath.
The Biocelsol method has been of particular interest in research in Finland in recent years, for instance in the Future Biorefinery cellulose programme at VTT in cooperation with universities and industry.3 For example, various cellulose ether derivatives have been developed in co-operation with the Tampere University of Technology which are highly suitable for the Biocelsol process: they dissolve more readily into the spin dope and have properties that improve the end product. The textile fibres manufactured using this method have been shown to have more than three times the water retention capacity of conventional viscose fibres. This feature improves the usability of the textile product, especially breathability. Additionally, the reactive groups added to the fibre may be used in the post-processing of textile fibres and in improving their properties even in finished textile materials.4
The viscose process is based on the modification of cellulose with carbon disulfide (CS2) in order to produce cellulose xanthate, which is soluble in alkaline water and can be further regenerated into cellulose fibres in an acid environment. However, due to the toxicity and flammability of carbon disulphide, there has been increasing interest to replace the viscose process in recent years. 5
New materials from nanofibrillated and microfibrillated cellulose
In the past decade, research on microfibrillated and nanofibrillated cellulose has significantly increased, especially in the Nordic countries, Japan and North America. VTT plays a very active role in this context and has participated in several national and international research projects with industrial partners.
In the recently finalised Nanoselect project funded by the European Union, the production of nanocellulose based membrane materials for water treatment and purifications was investigated. The research at VTT involved the development of the membrane material6 using polyvinyl alcohol (PVA) and TEMPO-oxidised nanocellulose (TONC), 7 which can be further functionalised with stimuli-responsive property8. The temperature responsive polymer was attached to the TONC membrane surface. The polymer is able to change its shape and size at a specific temperature (Lower Critical Solution Temperature), simultaneously opening and closing the pores and thus changing permeability through a membrane as a function of temperature. This feature facilitates the efficient cleaning of the membrane using only temperature changes, without any detergents (see Figure 1). Besides, the hydrophilicity, surface charge and nanoscaled pore size of the TONC membrane enable its utilisation for example in organic solvent filtration9 and capturing of valuable metals10.
Figure 1. Principle of the stimuli-responsive membrane material that reacts to temperature changes (left); photo of the membrane (centre); and an atomic force microscope (AFM) image of the membrane surface (right). Chemical modification pathway used to produce the membrane: TEMPO-oxidation of cellulose fibre; ethyl esterification of carboxylic acid groups of the TEMPO-oxidised fibre; and amidation using the thermo-responsive polymer PNIPAM-NH2. (Reprinted with permission from Hakalahti, M. et al. ACS Appl. Mater. Interfaces, 2016, 8, 2923−2927. Copyright 2017 American Chemical Society.)
Cellulose nanofibrils (CNF of NFC) have been used to produce films which are suitable for packaging and barrier materials. CNF films exhibit low oxygen permeability; and by using interfacial surface modification methods for example by attaching aminosilane groups directly to the CNF film surface, the moisture sensitivity can be considerably reduced.11
Nanofibrillated cellulose can be used as a cell growth matrix or for immobilising various biomolecules such as enzymes on the surface of the cellulose fibre. Reactive substituents attached to cellulose fibrils, such as acrylate or allyl groups, can be used for further chemical modifications such as cross-linking, grafting or immobilisation of biomolecules on the fibre surfaces. For instance, the double bond in an allyl group attached to a nanofibril may be converted into a reactive epoxy group; this method has been used for attaching enzyme proteins to microfibrillated cellulose.12 An allyl group may also be used to attach biomolecules to fibrous material through a very quick and effective thiol-ene ‘click’ reaction.13
Chemical modification of cellulose fibres and examples of applications
Cellulose can thus either be modified in its fibrous state in a heterogeneous reaction system or be dissolved completely in a homogeneous reaction environment. The industrial process for producing carboxymethyl cellulose (CMC) is a good example of a heterogeneous method where cellulose fibres react in an alkaline mixture of water and isopropanol. Depending on what substituents and how much of them are attached to the cellulose, the result is a cellulose derivative that either retains its fibrous structure through the process or can be regenerated into fibres after being dissolved. On the other hand, in both reaction environments the cellulose can be modified so extensively that it is completely converted into water-soluble products (examples include cellulose ethers such as hydroxyethyl cellulose (HEC) or hydroxypropyl cellulose (HPC)) or into products soluble in organic solvents (examples include the thermoplastic materials cellulose acetate (CA) and cellulose acetate butyrate (CAB)).
VTT has an extensive history of researching and developing a variety of cellulose derivatives and thermoplastic cellulose-based materials that are water-soluble and/or better suited to fibre production processes and adding new properties to them. In cellulose derivatives, the degree of substitution and/or the quality of the substituent is regulated according to the requirements of the application. The selection of the cellulose raw material is of crucial importance for the properties of the end product. Often the molecular weight of the cellulose has to be adjusted to achieve the desired properties in an optimum way.
Of the most recent cellulose-based thermoplastic materials developed at VTT, we may mention hydroxypropyl cellulose acetates and cellulose palmitate esters. As an example, adjusting the molecular weight of the cellulose used to produce thermoplastic cellulose palmitate and the degree of substitution of the palmitate allowed a significant improvement in the properties of the end product, such as its melting point, temperature resistance and various barrier properties such as water vapour transmission rate (WVTR) and oxygen transmission rate (OTR).14
Harri Setälä PhD is a Senior Scientist at VTT, mainly working on chemical modification of cellulose and hemicelluloses, and also on lignin and lignin-based materials. His dissertation (2008) was on the chemistry of wood and lignin. Before joining VTT in 1999, Setälä was employed for several years at the research centre of Suomen Sokeri (Cultor) in Kantvik. In addition to conference papers and posters, he has published 25 peer-reviewed articles. In 2012, he received the gold medal of the Filtration Society for his work in developing thermal stimuli controlled functional filter cloths.
Tekla Tammelin D.Sc.(Tech.) is a Principal Scientist and a Principal Investigator on the ‘New fibre products’ - team. Her research focuses on the potential of plant cell wall components in new material applications. Tammelin gained her doctorate on surface interaction between fibre components in 2006. Tammelin holds the title of Docent in Bioproduct Technology at Aalto University. She has published more than 50 peer-reviewed papers and more than 90 scientific meeting proceedings. She received the Akzo Nobel Nordic Research Prize in 2010, and in 2016 she received the VTT Award for scientific excellence.
Photos: Sirpa Levonperä
 Gericke M., Fardim P., Heinze T. (2012) Ionic Liquids – Promising but challenging solvents for homogeneous derivatization of cellulose. Molecules 17: 7458-7502.
 El Soud O. A., Nawaz H., Areas E. P. G. (2013) Chemistry and applications of polysaccharide solutions in strong electrolytes/dipolar aprotic solvents: An overview. Molecules 18: 1270-1313.
 Vehviläinen M., Kamppuri T., Grönqvist S., Rissanen M., Maloney T., Honkanen M., Nousiainen P. (2015) Dissolution of enzyme-treated cellulose using freezing-thawing method and the properties of fibres regenerated from the solution. Cellulose 22: 1653-1674.
 Vehviläinen M., Kamppuri T., Setälä H., Grönqvist S., Rissanen M., Honkanen M., Nousiainen P. (2015) Regeneration of fibres from alkaline solution containing enzyme-treated 3-allyloxy-2-hydroxypropyl substituted cellulose. Cellulose 22: 2271-2282.
 Wang S., Lu A., Zhang L. (2016) Recent advances in regenerated cellulose materials.
Prog. Polym. Sci. 53: 169-206.
 Hakalahti M., Salminen A., Seppälä J., Tammelin T. Hänninen, T. (2015) Effect of interfibrillar PVA bridging on water stability and mechanical properties of TEMPO/NaClO2 oxidized cellulosic nanofibril films. Carbohydr. Polym.
 Saito T., Nishiyama Y., Putaux J., Vignon M., Isogai A. (2006) Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose. Biomacromolecules 7: 1687–1691.
 Hakalahti M., Mautner A., Johansson L.-S., Hänninen T., Setälä H., Kontturi E., Bismarck A., Tammelin T. (2016) Direct Interfacial Modification of Nanocellulose Films for Thermoresponsive Membrane Template. ACS Appl. Mater. Interfaces 8: 2923−2927.
 Mautner A., Lee K.Y., Lahtinen P., Tammelin T., Li K., Bismarck A. (2014) Nanopapers for organic solvent nanofiltration. Chem. Commun. 50: 5778-5781.
 Karim Z., Hakalahti M., Tammelin T., Mathew A. (2017) In situ TEMPO surface functionalization of nanocellulose membranes for enhanced adsorption of metal ions from aqueous medium. RSC Adv. 7: 5232-5241.
 Peresin M. P., Kammiovirta K., Heikkinen H., Johansson L.-S., Vartiainen J., Setälä H., Österberg M., Tammelin T. (2017) Surface functionalized nanocellulose film with controlled interactions with oxygen and water. Under revision in Carbohydr. Polym.
 Arola S., Tammelin T., Setälä H., Tullila A., Linder M. B. (2012) Immobilization-stabilization of proteins on nanofibrillated cellulose derivatives and their bioactive film formation. Biomacromolecules 13: 594-603.
 Nurmi L., Salminen R., Setälä H. (2015) Modular modification of xylan with UV-initiated
thiol-ene reaction. Carbohydr. Res. 404: 63–69.
 Willberg-Keyriläinen P., Talja R., Asikainen S., Harlin A., Ropponen J. (2016) The effect of cellulose molar mass on the properties of palmitate esters. Carbohydr. Polym. 151: 988-995.