Elsevier

Carbohydrate Polymers

Volume 82, Issue 2, 5 September 2010, Pages 329-336
Carbohydrate Polymers

Preparation and properties of cellulose nanocrystals: Rods, spheres, and network

https://doi.org/10.1016/j.carbpol.2010.04.073Get rights and content

Abstract

Cellulose nanocrystals with rod, sphere, and network morphologies were prepared by acid hydrolysis of cotton cellulose, followed by freeze-drying. Hydrolysis with sulfuric acid introduced sulfate groups to these nanocrystal surfaces permitting their dispersion in aqueous as well as organic media, including ethanol and N,N-dimethylformamide, in a matter of seconds. Freeze-drying, on the other hand, induced mesoporosity (91.99 ± 2.57 Å average pore width) and significantly improved specific surface (13.362 m2/g) that is about 9 times of the original cellulose (1.547 m2/g). Moreover, the nanocrystals exhibited improved thermal conductivity and considerably higher (nearly 30%) carbonaceous residue, possibly due to direct solid-to-gas decomposition. These results demonstrated that a combination of surface charge introduction and fixation of mesoporosity in cellulose nanocrystals is an efficient route to prepare large quantity of high quality cellulose nanocrystals with quick re-dispersion capability for practical applications.

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.

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