Derek G. Gray

Nanocelluloses: A New Family of Nature‐Based Materials

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.

Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose

Stable colloidal suspensions of cellulose microcrystallites may be prepared from filter paper by sulfuric acid hydrolysis. Above a critical concentration, the suspensions form a chiral nematic ordered phase, or ‘colloid crystal’. The preparation conditions govern the properties of the individual cellulose microcrystallites, and hence the liquid crystalline phase separation of the cellulose suspensions. The particle properties and the phase separation of the suspensions were strongly dependent on the hydrolysis temperature and time, and on the intensity of the ultrasonic irradiation used to disperse the particles. The particle size of the microcrystallites was characterized with transmission electron microscopy and photon correlation spectroscopy. The surface charge was determined by conductometric titration. It was possible to fractionate the microcrystallites by size using the partitioning between isotropic and liquid crystalline phases; the longer microcrystallites migrate to the liquid crystalline phase

Helicoidal self-ordering of cellulose microfibrils in aqueous suspension

In many skeletal support systems of plants and animals, cellulose, chitin, and collagen occur in the form of microfibrils ordered in a chiral nematic fashion (helicoids). However, these structures remain poorly understood due to the many constituents present in biological tissues. Here we report an in vitro system that attracts by its simplicity. Only one chemical component, cellulose, is present in the form of fibrillar fragments dispersed in water. Above a critical concentration the colloidal dispersion separates spontaneously into a chiral nematic liquid crystalline phase. On drying this phase solidifies into regularly twisted fibrillar layers that mimic the structural organization of helicoids in nature.

Adsorption of n-alkanes at zero surface coverage on cellulose paper and wood fibers

Abstract The Henrys law region of the adsorption isotherms for a series of n-alkanes on cotton cellulose and on wood fibers has been studied by gas chromatography. On wood fibers, the three thermodynamic functions at zero surface coverage were determined. A linear relationship between the standard free energy, the enthalpy and the entropy, and the number of carbon atoms was found. Attempts were made to estimate the London contribution to the surface free energy of cellulose and wood fibers from the results at infinite dilution. A method based on the increment per CH2 group in the free energy of desorption, normalized per unit area of a methylene group, is in reasonable agreement with other estimates.

Cationic surface functionalization of cellulose nanocrystals

The surface of cellulose nanocrystals, prepared by sulfuric acid hydrolysis of cotton, was rendered cationic through a reaction with epoxypropyltrimethylammonium chloride. The resultant nanocrystal suspensions were characterized by ζ-potential, conductometric titration and polarized light microscopy. Atomic force microscopy (AFM) showed no change in the size or shape of the nanocrystals, but the functionalization process reversed the surface charge and led to a reduction of the total surface charge density. These modifications led to stable aqueous suspensions of nanocrystalline cellulose with unexpected gelling and rheological properties. Shear birefringence was observed, but no liquid crystalline chiral nematic phase separation was detected.

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

Abstract 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.

Liquid Crystalline Structure In Aqueous Hydroxypropyl Cellulose Solutions

Abstract Concentrated aqueous solutions of hydroxypropyl cellulose form lyotropic mesophases displaying a range of iridescent colors. Evidence from light microscopy and optical rotatory dispersion suggests that the mesophase has helicoidal structure, resembling in some respects that of cholesteric liquid crystals.

Bactericidal paper impregnated with silver nanoparticles for point-of-use water treatment.

There is an urgent need for cheap point-of-use methods to purify drinking water. We describe a method to deactivate pathogenic bacteria by percolation through a paper sheet containing silver nanoparticles. The silver nanoparticles are deposited by the in situ reduction of silver nitrate on the cellulose fibers of an absorbent blotting paper sheet. The aim is to achieve inactivation of bacteria during percolation through the sheet, rather than removal of bacteria from the effluent by filtration. The silver-nanoparticle containing (AgNP) papers were tested for performance in the laboratory with respect to bacteria inactivation and silver leaching as suspensions of bacteria percolated through the paper. The AgNP sheets exhibited antibacterial properties toward suspensions of Escherichia coli and Enterococcus faecalis, with log reduction values in the effluent of over log 6 and log 3, respectively. The silver loss from the AgNP sheets was minimal, with values under 0.1 ppm (the current US EPA and WHO limit for silver in drinking water). These results show promise that percolation of bacterially contaminated water through paper embedded with silver nanoparticles could be an effective emergency water treatment.

Smooth model cellulose I surfaces from nanocrystal suspensions

Removal of lignin, hemicelluloses and other minor components during pulping results in a porous fibrillar structure. Interactions of cellulose fibre surfaces with wet-end additives and other materials depend both on the interfacial properties of the cellulose and on the morphology of the surface. It would be useful to be able to separate the interactions with the cellulose from those that depend on surface roughness and porosity by preparing flat cellulose surfaces. Current methods give surfaces of amorphous cellulose or of cellulose II, differing in density and crystallinity from the original cellulose I surface. We propose a new route to prepare smooth model surfaces of cellulose I, starting from colloidal dispersions of cellulose I nanocrystals. The nanometer-sized width of these rod-like colloidal particles allows a relatively flat surface to be prepared from the suspension by casting an aqueous suspension on an appropriate surface and allowing the water to evaporate. Oriented films can be prepared by spin-coating or shearing. The surface composition and morphology of the films were examined by X-ray photoelectron spectroscopy and atomic force microscopy.

Surface Grafting of Cellulose Nanocrystals with Poly(ethylene oxide) in Aqueous Media

Aqueous suspensions of poly(ethylene oxide)-grafted nanocrystalline cellulose (PEO-grafted NCC) were prepared in order to achieve steric instead of electrostatic stabilization. A two-step process was employed: in the first step NCC suspensions prepared by sulfuric acid hydrolysis were desulfated with sodium hydroxide, and in the second step the surfaces of the crystals were functionalized with epoxy-terminated poly(ethylene oxide) (PEO epoxide) under alkaline conditions. The PEO-grafted samples were analyzed by conductometric titration, ATR-IR, solid-state NMR, MALDI-TOF MS, SEC MALLS, and AFM. The covalent nature of the linkage was confirmed by weight increase and MALDI-TOF analysis. The PEO-grafted cellulose nanocrystals (CNCs) formed a stable colloidal suspension that remained well dispersed, while the desulfated nanoparticles aggregated and precipitated. Upon concentration of the PEO-grafted aqueous NCC suspension, a chiral nematic phase was observed.