This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

/pdf/cellulose-nanomaterials-review-structure-properties-and-2z8x922p3v.pdf

Cellulose nanomaterials review: structure, properties and nanocomposites

Cellulose fibrils with widths in the nanometer range are nature-based materials with unique and potentially useful features. Most importantly, these novel nanocelluloses open up the strongly expanding fields of sustainable materials and nanocomposites, as well as medical and life-science devices, to the natural polymer cellulose. The nanodimensions of the structural elements result in a high surface area and hence the powerful interaction of these celluloses with surrounding species, such as water, organic and polymeric compounds, nanoparticles, and living cells. This Review assembles the current knowledge on the isolation of microfibrillated cellulose from wood and its application in nanocomposites; the preparation of nanocrystalline cellulose and its use as a reinforcing agent; and the biofabrication of bacterial nanocellulose, as well as its evaluation as a biomaterial for medical implants.

Nanocelluloses: A New Family of Nature-Based Materials

Due to their abundance, high strength and stiffness, low weight and biodegradability, nano-scale cellulose fiber materials (e.g., microfibrillated cellulose and bacterial cellulose) serve as promising candidates for bio-nanocomposite production. Such new high-value materials are the subject of continuing research and are commercially interesting in terms of new products from the pulp and paper industry and the agricultural sector. Cellulose nanofibers can be extracted from various plant sources and, although the mechanical separation of plant fibers into smaller elementary constituents has typically required high energy input, chemical and/or enzymatic fiber pre-treatments have been developed to overcome this problem. A challenge associated with using nanocellulose in composites is the lack of compatibility with hydrophobic polymers and various chemical modification methods have been explored in order to address this hurdle. This review summarizes progress in nanocellulose preparation with a particular focus on microfibrillated cellulose and also discusses recent developments in bio-nanocomposite fabrication based on nanocellulose.

Microfibrillated cellulose and new nanocomposite materials: a review

Native wood celluloses can be converted to individual nanofibers 3–4 nm wide that are at least several microns in length, i.e. with aspect ratios >100, by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and successive mild disintegration in water. Preparation methods and fundamental characteristics of TEMPO-oxidized cellulose nanofibers (TOCN) are reviewed in this paper. Significant amounts of C6 carboxylate groups are selectively formed on each cellulose microfibril surface by TEMPO-mediated oxidation without any changes to the original crystallinity (∼74%) or crystal width of wood celluloses. Electrostatic repulsion and/or osmotic effects working between anionically-charged cellulose microfibrils, the ζ-potentials of which are approximately −75 mV in water, cause the formation of completely individualized TOCN dispersed in water by gentle mechanical disintegration treatment of TEMPO-oxidized wood cellulose fibers. Self-standing TOCN films are transparent and flexible, with high tensile strengths of 200–300 MPa and elastic moduli of 6–7 GPa. Moreover, TOCN-coated poly(lactic acid) films have extremely low oxygen permeability. The new cellulose-based nanofibers formed by size reduction process of native cellulose fibers by TEMPO-mediated oxidation have potential application as environmentally friendly and new bio-based nanomaterials in high-tech fields.

TEMPO-oxidized cellulose nanofibers

This paper provides an overview of recent progress made in the area of cellulose nanofibre-based nanocomposites. An introduction into the methods used to isolate cellulose nanofibres (nanowhiskers, nanofibrils) is given, with details of their structure. Following this, the article is split into sections dealing with processing and characterisation of cellulose nanocomposites and new developments in the area, with particular emphasis on applications. The types of cellulose nanofibres covered are those extracted from plants by acid hydrolysis (nanowhiskers), mechanical treatment and those that occur naturally (tunicate nanowhiskers) or under culturing conditions (bacterial cellulose nanofibrils). Research highlighted in the article are the use of cellulose nanowhiskers for shape memory nanocomposites, analysis of the interfacial properties of cellulose nanowhisker and nanofibril-based composites using Raman spectroscopy, switchable interfaces that mimic sea cucumbers, polymerisation from the surface of cellulose nanowhiskers by atom transfer radical polymerisation and ring opening polymerisation, and methods to analyse the dispersion of nanowhiskers. The applications and new advances covered in this review are the use of cellulose nanofibres to reinforce adhesives, to make optically transparent paper for electronic displays, to create DNA-hybrid materials, to generate hierarchical composites and for use in foams, aerogels and starch nanocomposites and the use of all-cellulose nanocomposites for enhanced coupling between matrix and fibre. A comprehensive coverage of the literature is given and some suggestions on where the field is likely to advance in the future are discussed.

/pdf/review-current-international-research-into-cellulose-vduhai1vrx.pdf

Review: current international research into cellulose nanofibres and nanocomposites

M Optically Transparent Nanofiber Paper M U N IC By Masaya Nogi,* Shinichiro Iwamoto, Antonio Norio Nakagaito, and Hiroyuki Yano A T IO N Glass has well-suited low thermal expansion for use in electronic devices, but it is fragile, and the search for a stronger, more flexible optically clear medium has gone on for many years. Plastics have been widely studied; however, most of them have large coefficients of thermal expansion (CTE, approx. 50 ppm K ), and foldable plastics in particular exhibit extremely large CTEs, in excess of 200 ppm K . Further, the functional materials deposited on plastic substrates are prone to be damaged by the temperatures involved in the assembly and mounting processes due to themismatch between CTEs from differentmaterials. This article reports on what might be best described as optically transparent paper. It is a foldable nanofiber material with low thermal expansion (CTE <8.5 ppm K ) prepared using 15 nm cellulose nanofibers with the same chemical constituents as conventional paper and a production process also similar to that of conventional paper. The only difference is in the fiber width and the size of the interstitial cavities (Fig. 1). The foldable, low-CTE, and optically transparent nanofiber paper is the perfect candidate for substrates for continuous roll-to-roll processing in the future production of electronic devices, such as flexible displays, solar cells, e-papers, and a myriad of new flexible circuit technologies, and could replace the costly conventional batch processes based on glass substrates currently used. We project that it will also replace conventional paper as an advanced information medium, which can still be produced using traditional paper-making equipment that is used in production today. Cellulose nanofibers are the main component of plant and wood pulp fibers. These tiny elements with diameters of 15–20 nm are composed of bundles of cellulose microfibrils smaller than 4 nm in width, which, in turn, are composed of long cellulose molecules laterally stabilized by hydrogen bonds forming highly crystalline domains. As such, cellulose nanofibers have a CTE of 0.1 ppm K , which is as low as that of quartz glass, and an estimated strength of 2–3 GPa, rendering it five times stronger than mild steel. The nanofibers also exhibit good heat-transfer properties comparable to glass. Another significant property of the nanofibers is that light scattering can be suppressed. If the cellulose nanofibers are densely packed, and the interstices between the fibers are small enough to avoid

Optically Transparent Nanofiber Paper

Obtaining cellulose nanofibers with a uniform width of 15 nm from wood.

Elastic Modulus of Single Cellulose Microfibrils from Tunicate Measured by Atomic Force Microscopy

Pulp fibers were fibrillated uniformly into nano-sized fibers using a grinder with a specially designed set of grinding disks. To investigate the effect of the fibrillation through the grinder on the physical properties of the composites, dissolved pulp fibers were subjected to various passes through the grinder, and the resulting fibrillated pulp fibers were used to make fibrillated pulp fibers/acrylic resin composites. Scanning electron microscopy observations showed that at above five passes, the structure of the fibrillated pulp fibers did not change significantly. The light transmittances of the composites were increased to 80% up to five passes through the grinder, and did not change after further passes. However, the tensile test and thermal expansion analysis indicated that a degradation of the fibrillated pulp occurred during the grinding treatment. To evaluate the fiber degradation, the degree of crystallinity and degree of polymerization of cellulose were measured. Both decreased as the number of passes through the grinder increased. In addition, to reduce the thermal expansion of composites, the fibrillated pulp fibers were additionally treated by sulfuric acid. The thermal expansion of composites was decreased, because the amorphous region of cellulose was removed.

Shinichiro Iwamoto

Papers

Optically Transparent Nanofiber Paper

Obtaining cellulose nanofibers with a uniform width of 15 nm from wood.

Elastic Modulus of Single Cellulose Microfibrils from Tunicate Measured by Atomic Force Microscopy

Nano-fibrillation of pulp fibers for the processing of transparent nanocomposites

The effect of hemicelluloses on wood pulp nanofibrillation and nanofiber network characteristics.