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).

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Cellulose nanomaterials review: structure, properties and nanocomposites

Cellulose constitutes the most abundant renewable polymer resource available today. As a chemical raw material, it is generally well-known that it has been used in the form of fibers or derivatives for nearly 150 years for a wide spectrum of products and materials in daily life. What has not been known until relatively recently is that when cellulose fibers are subjected to acid hydrolysis, the fibers yield defect-free, rod-like crystalline residues. Cellulose nanocrystals (CNs) have garnered in the materials community a tremendous level of attention that does not appear to be relenting. These biopolymeric assemblies warrant such attention not only because of their unsurpassed quintessential physical and chemical properties (as will become evident in the review) but also because of their inherent renewability and sustainability in addition to their abundance. They have been the subject of a wide array of research efforts as reinforcing agents in nanocomposites due to their low cost, availability, renewability, light weight, nanoscale dimension, and unique morphology. Indeed, CNs are the fundamental constitutive polymeric motifs of macroscopic cellulosic-based fibers whose sheer volume dwarfs any known natural or synthetic biomaterial. Biopolymers such as cellulose and lignin and † North Carolina State University. ‡ Helsinki University of Technology. Dr. Youssef Habibi is a research assistant professor at the Department of Forest Biomaterials at North Carolina State University. He received his Ph.D. in 2004 in organic chemistry from Joseph Fourier University (Grenoble, France) jointly with CERMAV (Centre de Recherche sur les Macromolecules Vegetales) and Cadi Ayyad University (Marrakesh, Morocco). During his Ph.D., he worked on the structural characterization of cell wall polysaccharides and also performed surface chemical modification, mainly TEMPO-mediated oxidation, of crystalline polysaccharides, as well as their nanocrystals. Prior to joining NCSU, he worked as assistant professor at the French Engineering School of Paper, Printing and Biomaterials (PAGORA, Grenoble Institute of Technology, France) on the development of biodegradable nanocomposites based on nanocrystalline polysaccharides. He also spent two years as postdoctoral fellow at the French Institute for Agricultural Research, INRA, where he developed new nanostructured thin films based on cellulose nanowiskers. Dr. Habibi’s research interests include the sustainable production of materials from biomass, development of high performance nanocomposites from lignocellulosic materials, biomass conversion technologies, and the application of novel analytical tools in biomass research. Chem. Rev. 2010, 110, 3479–3500 3479

Cellulose nanocrystals: chemistry, self-assembly, and applications.

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

Review of Recent Research into Cellulosic Whiskers, Their Properties and Their Application in Nanocomposite Field

Enzymatic hydrolysis combined with mechanical shearing and high-pressure homogenization for nanoscale cellulose fibrils and strong gels.

Suspensions of rod-like cellulose crystallites of axial ratio ≈ 20–40, prepared by acid hydrolysis of natural cellulose fibres with sulphuric acid, give stable ordered fluids that display well-formed textures and disclinations characteristic of chiral nematic liquid crystalline phases. The critical volume fraction for phase separation of salt-free suspensions is typically 0.03, with a relatively narrow biphasic region. Because of the negative diamagnetic susceptibility of cellulose, the ordered phase becomes oriented in a magnetic field with its chiral nematic axis parallel to the applied field.

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

Small angle neutron scattering, SANS, was used to characterize the enhanced ordering induced by magnetic and shear alignment of chiral nematic liquid crystals of cellulose microfibrils in aqueous suspension. In a ∼2 T magnetic field the chiral nematic phase exhibits a uniform orientation over an entire 10 mL sample. SANS data confirmed that the cholesteric axis of this phase aligns along the magnetic field with implications that the distance between microfibrils is shorter along the cholesteric axis than perpendicular to it. This is consistent with the hypothesis that cellulose microfibrils are helically twisted rods. Under shear flow, the alignment of microfibrils changes from chiral nematic to nematic with relative order increasing with increasing shear rate. The axial ratio (length/width) is the key parameter in determining the relative order achieved and in determining the relaxation behavior after shear ceases.

Enhanced Ordering of Liquid Crystalline Suspensions of Cellulose Microfibrils: A Small Angle Neutron Scattering Study

De nouveaux materiaux solides iridescents peuvent etre produits dans l'eau par simple evaporation a partir de suspensions cristallines liquides nematiques, chirales et lyotropiques faites de microcrystallites cellulosiques. Les microcrystallites sont des fragments de microfibrilles colloidalement stables produits par l'hydrolyse controlee de la pâte kraft a faible rendement, du papier filtre ou d'autres sources de cellulose. Quand on laisse evaporer dans l'eau des suspensions de ces microcrystallites sur une surface plane, un film de cellulose se forme dans lequel les microcrystallites retiennent leur orientation nematique chirale. En modifiant la concentration de l'electrolyte dans les suspensions, le pas nematique chiral du film ainsi produit peut etre controle. Pour ce qui est des films dont les longueurs de pas sont dans la plage des longueurs d'onde de la lumiere visible, ils presentent les proprietes optiques des cristaux liquides nematiques chiraux et, plus particulierement, ils reflechissent circulairement la lumiere polarisee qui change de couleur selon l'angle de vision. Ces films peuvent ainsi offrir un certain interet comme composants des papiers de securite et comme films et pigments decoratifs.

Solid self-assembled films of cellulose with chiral nematic order and optically variable properties

Atomic force microscopy (AFM), tapping mode atomic force microscopy (TM-AFM) and transmission electron microscopy (TEM) have been used to image the cell wall, ultrathin sections of whole cells and cellulose microfibrils prepared from the green alga Micrasterias denticulata. Measurements of the microfibril dimensions are in agreement with earlier observations carried out by electron microscopy. Images at the molecular level of the surface of the microfibrils were obtained with AFM and show regular periodicities along the microfibril axis that correspond to the fibre and glucose repeat distances of cellulose. Twisted regions visible at intervals along the microfibrils dried down onto substrates were noted to be right-handed in over 100 observations by TEM, AFM and TM-AFM.

Atomic force microscopy and transmission electron microscopy of cellulose from Micrasterias denticulata; evidence for a chiral helical microfibril twist

Louis Godbout

Papers

Chiral nematic suspensions of cellulose crystallites; phase separation and magnetic field orientation

Enhanced Ordering of Liquid Crystalline Suspensions of Cellulose Microfibrils: A Small Angle Neutron Scattering Study

Solid self-assembled films of cellulose with chiral nematic order and optically variable properties

Atomic force microscopy and transmission electron microscopy of cellulose from Micrasterias denticulata; evidence for a chiral helical microfibril twist

Origin of the twist of cellulosic materials.