The Enhanced Mechanical Properties of a Covalently Bound Chitosan-Multiwalled Carbon Nanotube Nanocomposite Xiaodong Cao, 1,2 Hua Dong, 2 Chang Ming Li, 2 Lucian A. Lucia 1 1 Department of Wood and Paper Science, North Carolina State University, Raleigh, North Carolina 27695-8005 2 Division of Bioengineering, School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639798 Received 16 July 2008; accepted 2 January 2009 DOI 10.1002/app.29984 Published online 19 March 2009 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: In the present work, chitosan (CS)-grafted multiwalled carbon nanotube (MWCNT) nanocomposites were prepared via covalently bonded CS onto MWCNTs that had weight fractions of MWCNTs ranging from 0.1 to 3.0 wt % by a simple method of solution casting. The structure, morphology, and mechanical properties of the films were investigated by Fourier transform infrared spectroscopy, field emission scanning electron microscopy, optical microscopy, wide-angle X-ray diffraction, contact angle, and tensile testing. The results indicated that the CS chains were attached onto the MWCNTs successfully via covalent linkages. More interestingly, the MWCNTs pro- vided a matrix that facilitated the crystallization of CS. Compared with the pure CS, the tensile strength and Young’s modulus of the nanocomposites were enhanced significantly from 39.6 to 105.6 MPa and from 2.01 to 4.22 GPa with an increase in the MWCNT loading level from 0 to 3.0 wt %, respectively. The improvement in the tensile strength and modulus were ascribed to the uniform dis- persion of MWCNTs covalently linked to the CS matrix. V V C 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 466–472, 2009 Key words: chitosan; multiwalled carbon nanotubes; nanocomposite; tensile strength INTRODUCTION Carbon nanotubes (CNTs) have been considered ideal reinforcing fillers for polymer matrices since their dis- covery in 1991 because of their nanometer size, high as- pect ratios, and, more importantly, their excellent mechanical properties and high electrical and thermal conductivity. 1–3 These nanocomposites are usually developed to provide enhanced mechanical perform- ance such as improved load transfer and tear resistance and to achieve certain levels of electric conductivity by providing a percolation network for charge migration and electromagnetic shielding. 4,5 It is well known that the physical properties of the nanocomposites mainly depend on the dispersion of fillers in the polymer mat- rices. However, CNTs are generally insoluble and severely aggregate because of intrinsic van der Walls attractions among the nanotubes, and their homogene- ous dispersion in the desired polymer therefore presents a great hurdle. 6 To overcome this hurdle, vari- ous physical and chemical approaches have been previ- ously considered, including the direct suspension of carbon nanotubes in the polymer solution via sonica- tion, 7–12 in situ polymerization in the presence of CNTs, 13–16 and the chemical modification of CNTs to enhance solubilization. 17–21 Overall, the solubilization approach of CNTs via chemical functionalization is con- sidered one of the most effective ways to achieve a ho- mogeneous dispersion of CNTs in polymer matrices for high-quality nanocomposites. For example, it has been shown that by simply refluxing CNTs with nitric acid or mixed acids, the existing defects on the surface and ends of the CNTs that contain pendant hydroxyl groups can be preferentially oxidized by the acid to carboxylic groups. This thus greatly enhances the formation of nanocomposites with a great variety of polymer via covalent linkages. For example, it has already been shown that the carboxylic groups bound on CNTs can be used to attach aminopolymers via the formation of amide linkages. 22 Another important approach to fabricate high- quality nanocomposites is through the proper selec- tion of functionalities for chemical modification of the multiwalled carbon nanotube (MWCNTs). The most ideal and obvious way is to use a matrix poly- mer to provide functionality because it ensures com- patibility of the dispersed CNTs with the matrix and avoids ‘‘impurities’’ associated with the dispersion agents and any potential microscopic phase separa- tion in the nanocomposites. Sun and coworkers 15 Journal of Applied Polymer Science, Vol. 113, 466–472 (2009) V V C 2009 Wiley Periodicals, Inc. Correspondence to: X. Cao (xcao@ncsu.edu).