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Biomacromolecules 2009, 10, 712–716
Communications Polymer Nanocomposites with Nanowhiskers Isolated from Microcrystalline Cellulose Jeffrey R. Capadona,†,‡,§,| Kadhiravan Shanmuganathan,†,‡ Stephanie Trittschuh,‡ Scott Seidel,‡ Stuart J. Rowan,‡,⊥ and Christoph Weder*,‡,⊥ Departments of Macromolecular Science and Engineering, Biomedical Engineering, and Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, and Rehabilitation Research and Development, Louis Stokes Cleveland DVA Medical Center, 10701 East Blvd. Cleveland, Ohio 44106 Received September 25, 2008; Revised Manuscript Received February 5, 2009
The ability to produce polymer nanocomposites, which comprise a percolating, three-dimensional network of well-individualized nanofibers, is important to maximize the reinforcing effect of the nanofibers. While microcrystalline cellulose (MCC) has been previously shown to improve the mechanical properties of polymer composites, the formation of fibrous percolating networks within the nanocomposites has been stifled. Through the utilization of a template approach, nanocomposites based on an ethylene oxide/epichlorohydrin copolymer and nanowhiskers isolated from MCC were produced that display the maximum mechanical reinforcement predicted by the percolation model.
Introduction The incorporation of small amounts of high-stiffness, highaspect-ratio nanometer-sized fillers into polymers is a design approach that has rapidly emerged to a broadly exploited framework for the creation of new materials with tailored mechanical properties.1-3 Crystalline cellulose nanofibers are attracting significant interest in this context, mainly due to their intriguing mechanical properties and the abundance of cellulose in the biomass.4 These fiber-like crystals, often referred to as nanowhiskers, display an elastic modulus of 120-150 GPa5 and are readily obtained from renewable biosources such as bacteria, wood, cotton, and sessile sea creatures called tunicates.4 Owing to their strongly interacting surface hydroxyl groups,6 cellulose nanowhiskers have a significant tendency for self-association, which is advantageous for the formation of load-bearing percolating architectures within the host polymer matrix: the spectacular reinforcement of polymers observed for this class of materials is attributed to the formation of rigid nanowhisker networks in which stress transfer is facilitated by hydrogen-bonding between the nanowhiskers;7 van der Waals interactions also have been shown to play a significant role.8 However, these same nanowhisker-nanowhisker interactions can also lead to aggregation during the nanocomposite fabrication,9 which significantly reduces the mechanical properties of the resulting materials compared to predicted values.7 The traditional approach to solve this problem is surface functionalization, * To whom correspondence should be addressed. Tel.: (216) 368 6374. E-mail:
[email protected]. † These authors contributed equally. ‡ Department of Macromolecular Science and Engineering, Case Western Reserve University. § Department of Biomedical Engineering, Case Western Reserve University. | Louis Stokes Cleveland DVA Medical Center. ⊥ Department of Chemistry, Case Western Reserve University.
which mediates particle-particle and particle-polymer interactions and significantly influences nanoparticle dispersion.10-13 However, the advantages of surface groups are often negated because they suppress desirable nanoparticle interactions and limit their reinforcing impact. To solve this dilemma, we recently explored a template approach to nanocomposite fabrication.14 The process is based on the formation of a three-dimensional scaffold of well-individualized nanowhisker, which is subsequently filled with a polymer of choice. We demonstrated that this technique, which is somewhat different from other recently developed “impregnation schemes”,15 is applicable to cellulose nanowhiskers isolated from tunicates16 and cotton4 (both exhibit average diameters of 20-26 nm, and lengths of 2.2 µm or ∼210 nm, respectively). Both nanowhisker types were incorporated into a range of polymers, and the resulting nanocomposites display significantly improved mechanical properties. We here report that the template approach is also suitable for the fabrication of polymer nanocomposites with commercially available microcrystalline cellulose (MCC). MCC is produced by partially hydrolyzing cellulose pulp, for example, from wood, with mineral acids. The properties of the resulting particles are influenced by the conditions of the hydrolysis process.17 Typical MCC features particles with dimensions on the order of 10-100 µm.18 However, the material can be dispersed by ultrasonication so that mainly nanowhiskers with dimensions on the order of 20 × 200 nm are obtained.18,19 The nanowhiskers produced here from MCC were of similar dimensions (23 × 260 nm, Figure 1a). The main applications of MCC are as an excipient in the formulation of tablets, emulsifier, stabilizer, and anticaking agent. The commercial availability of MCC has also triggered significant interest in its use as reinforcing filler in polymer composites.20-22 Unfortunately, the dramatic reinforcement effects displayed by cellulose nanowhiskers from other sources have hitherto
10.1021/bm8010903 CCC: $40.75 2009 American Chemical Society Published on Web 03/03/2009
Communications
Biomacromolecules, Vol. 10, No. 4, 2009
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Figure 1. Transmission electron microscopy (TEM) images of cellulose nanowhiskers prepared by (a) dispersion of microcrystalline cellulose (MCC) in water and subsequent fractionation; (b) dispersion of microcrystalline cellulose (MCC) in DMF; (c) hydrolysis of cotton and dispersion in water; scale bars (a,c) ) 500 nm; (b) ) 2 µm.
not been observed in the case of MCC-based composites. Wu et al. attributed this shortcoming to the inability of MCC to form percolating fibrous networks within the nanocomposites.21 The present study supports this finding for materials that comprise small volume fractions of ultrasonicated, but not fractionated MCC dispersions. We further demonstrate that MCC nanocomposites with properties that appear to reach the theoretical limit can be fabricated by fractionation of the MCC and using the template approach.
Experimental Section Materials. All reagents, except acetone, were used as received. Dimethyl formamide (DMF, water
