Preparation and properties of cellulose nanocrystals: Rods, spheres, and network
Introduction
Cellulose, the most abundant biomass in the world, is a linear syndiotactic homopolymer of β-(1 → 4)-glycosidic bonds linked d-anhydroglucopyranose (Kim, Yun, & Ounaies, 2006). Native cellulose is generally known to be fibrillar and crystalline (Saxena & Brown, 2005) and the cellulose fibrils play a significant role in contributing to the high strength of plant cell walls (Zuluaga et al., 2009). Crystalline nanofibers with crystallinities from 65% to 95% have been extracted from a broad range of natural sources including cotton (Favier, Chanzy, & Cavaille, 1995), tunicate (Terech, Chazeau, & Cavaille, 1999), algae (Hanley, Giasson, Revol, & Gray, 1992), bacteria (Grunert & Winter, 2002) and wood (Beck-Candanedo, Roman, & Gray, 2005). These cellulose fibrils were reported to be 2–20 nm wide. Their aspect ratios varied from 40 (∼200 nm long and 5 nm wide) for cotton to around 66 (∼1 μm long and 15 nm wide) for tunicin whiskers (Samir, Alloin, & Dufresne, 2005).
The bending strength and modulus of the cellulose nanofibrils estimated (Helbert et al., 1996, Sakurada et al., 1962, Sturcova et al., 2005) and measured by Raman spectroscopy (Sturcova et al., 2005) were impressively high at ∼10 and ∼150 GPa, respectively. Cellulose nanofibers thus have a bending strength that is nearly one-sixth of the 63 GPa for the carbon nanotubes whose tensile strength is predicted to be as high as ∼300 GPa at E of ∼1 TPa (Wong et al., 1997, Yu et al., 2000), but can be prepared far more economically from readily available renewable resources. Various cellulose nanofibrils, nanocrystals and whiskers have been incorporated into polymer matrices to produce reinforced composites with several tens to hundreds folds higher mechanical strength (Beecher, 2007, Lu and Hsieh, 2009, Svagan et al., 2008) as well as enhanced optical transparency (Ifuku et al., 2007). Cellulose nanofibrils have been used as substrates to determine cellulase activity (Helbert, Chanzy, Husum, Schuelein, & Ernst, 2003) and as carriers for targeted delivery of therapeutics (Dong & Roman, 2007). With the layer-by-layer (LbL) technique, cellulose nanowires have been assembled into antireflective films (Podsiadlo et al., 2007) and high performance nanocomposites (Podsiadlo et al., 2005).
The major challenge of developing the cellulose nanofibers as advanced materials and for further applications is their tendency to form bundles or aggregates. During drying, the abundant hydrogen bonds of cellulose draw the cellulose nanocrystals together to pose significant problems in their re-dispersion for effective processing (Tingaut, Zimmermann, & Lopez-Suevos, 2010). To enable better utilization, it is crucial to develop methods to isolate the nanofibrils after the solvent evaporation in their preparation.
This study was to investigate the hydrolysis and drying processes with the intent to minimize hydrogen bonding, thus reduce and even eliminate aggregation of the cellulose nanocrystals. Homogenous and stable cellulose nanocrystals suspensions were generated by hydrolyzing native cellulose with sulfuric acid to introduce negative charges to the nanocrystal surfaces. Esterification of surface hydroxyl groups of cellulose nanocrystals has shown to introduce sulfate groups to form stable suspensions (Beck-Candanedo et al., 2005). The focus was then to prevent hydrogen bond formation by sustaining repulsion among the cellulose nanocrystals with fast freezing of water among the well-dispersed cellulose nanocrystals with liquid nitrogen to keep them separated and fixed in the solidified ice. The high vacuum in freeze-drying then sublimates the ice in-between the cellulose nanocrystals to substantially reduce or prevent hydrogen bonding, the driving force for the cellulose nanocrystals to aggregate. The induced morphologies and properties of the cellulose nanocrystals were studied to relate to the hydrolysis and drying processes.
Section snippets
Materials
Cotton cellulose from filter paper (Q2, Whatman) was purchased from Fisher Scientific (Pittsburgh, PA). Sulfuric acid (95–98%) for hydrolysis was provided by EMD (Gibbstown, NJ). Water used in all experiments was purified by a Millipore Milli-Q Plus (Billerica, MA) water purification system.
Sulfuric acid hydrolysis
The cellulose filter was milled (Thomas-Wiley Laboratory Mill model 4, Thomas Scientific, USA) to pass through a 60-mesh screen. Hydrolysis was performed using 64–65% (w/w) sulfuric acid (10 mL/g cellulose)
Preparation of cellulose nanocrystals and homogenous suspension
Acid hydrolysis of cellulose in sulfuric acid involves rapid protonation of glucosidic oxygen (path 1) or cyclic oxygen (path 2) by protons from the acid, followed by a slow splitting of glucosidic bonds induced by the addition of water (Fig. 1a). This hydrolysis process yields two fragments with shorter chains while preserving the basic backbone structure. In native cellulose, the amorphous regions are more accessible to acid molecules and susceptible to the hydrolytic actions than the
Conclusions
Cellulose nanocrystals with rod, sphere, and network-structured morphologies were prepared by acid hydrolysis and freeze-drying of cotton cellulose. Hydrolysis with sulfuric acid removed amorphous cellulose to produce isolated cellulose nanocrystals with newly introduced sulfate groups on the nanocrystal surfaces. Repulsion among the negatively charged cellulose nanocrystals and quick freezing with liquid nitrogen were very effective in preventing aggregate formation driven by the strong
Acknowledgement
This research was made possible by funding from the National Textile Center (project M02-CD05), the Jastro-Shields Graduate Research Award, and Summer Graduate Researcher Award from the University of California, Davis.
References (33)
- et al.
Atomic force microscopy of cellulose microfibrils: Comparison with transmission electron microscopy
Polymer
(1992) - et al.
Hydrodynamic properties of neutral suspensions of cellulose crystallites as related to size and shape
Journal of Colloid Science
(1961) - et al.
Heterogeneous preparation of cellulose-polyaniline conductive composites with cellulose activated by acids and its electrical properties
Carbohydrate Polymers
(2009) - et al.
Facile synthesis of spherical cellulose nanoparticles
Carbohydrate Polymers
(2007) - et al.
Cellulose microfibrils from banana rachis: Effect of alkaline treatments on structural and morphological features
Carbohydrate Polymers
(2009) - et al.
Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions
Biomacromolecules
(2005) Organic materials: Wood, trees and nanotechnology
Nature Nanotechnology
(2007)- et al.
Fluorescently labeled cellulose nanocrystals for bioimaging applications
Journal of the American Chemical Society
(2007) - et al.
Polymer nanocomposites reinforced by cellulose whiskers
Macromolecules
(1995) - et al.
Nanocomposites of cellulose acetate butyrate reinforced with cellulose nanocrystals
Journal of Polymers and the Environment
(2002)
Thermoplastic nanocomposites filled with wheat straw cellulose whiskers. Part I: Processing and mechanical behavior
Polymer Composites
Fluorescent cellulose microfibrils as substrate for the detection of cellulase activity
Biomacromolecules
Wood and cellulosic chemistry
Surface modification of bacterial cellulose nanofibers for property enhancement of optically transparent composites: Dependence on acetyl-group DS
Biomacromolecules
Discovery of cellulose as a smart material
Macromolecules
Cellulose nanocrystal-filled poly(acrylic acid) nanocomposite fibrous membranes
Nanotechnology
Cited by (818)
Dynamically bonded cellulose nanocrystal hydrogels: Structure, rheology and fire prevention performance
2024, Carbohydrate PolymersNanocellulose-based functional materials for physical, chemical, and biological sensing: A review of materials, properties, and perspectives
2024, Industrial Crops and ProductsHybridized cellulose nanocrystals enhanced the temperature resistance and viscoelasticity of the fracturing fluid network
2024, Journal of Molecular LiquidsSelf-assembled sodium alginate polymannuronate nanoparticles for synergistic treatment of ophthalmic infection and inflammation: Preparation optimization and in vitro/vivo evaluation
2024, International Journal of Biological Macromolecules