In-plane elastic moduli of covalently functionalized single-wall carbon nanotubes

Effective utilization of single-wall carbon nanotubes (SWCNTs) as reinforcements in composites necessitates their strong interfacial bonding with the surrounding matrix. The covalent functionalization of SWCNTs is an effective method to enhance this bonding. However, covalent bonds introduced by a functional group may alter the pristine structure of the SWCNT and affect its mechanical properties. Thus it is important to delineate effects of covalent functionalization on elastic properties of a SWCNT. We study here effects of covalent functionalization on Young's modulus, Poisson's ratio and shear modulus of a SWCNT in the graphitic plane. We consider hydrogen (-H), hydroxyl (-OH), carboxyl (-COOH), and amine (-NH2) as model functional groups in this work. We use molecular mechanics (MM) simulations with the MM3 potential and the software TINKER to analyze simple tension and torsional deformations of pristine and functionalized SWCNTs. As is commonly assumed, we hypothesize that the response of a SWCNT to these deformations is the same as that of an energetically and geometrically equivalent continuum cylinder of wall thickness 3.4 Å. From curves of the strain energy density of deformation versus the axial strain and the shear strain for each functionalization group, values of Young's modulus and the shear modulus, respectively, are deduced. From results of the tension tests on a pristine and a SWCNT fully functionalized with hydrogen (-H), Poisson's ratio is computed. It is found that for each functional group studied and 20% functionalization, Young's modulus and the shear modulus decrease by about 34% and 43%, respectively, and Poisson's ratio of the functionalized SWCNT is more than that of the pristine SWCNT. These results should help in determining mechanical properties of SWCNT reinforced nanocomposites by using a micromechanics approach.