Modification of carbon nanotube-polystyrene matrix composites through polyatomic-ion beam deposition: predictions from molecular dynamics simulations
Introduction
There has been considerable effort spent to incorporate carbon nanotubes into polymer-matrix composite materials [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] to make use of their high Young's modulus and resistance to brittle failure [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]. The nanotubes are found to stiffen [2], [3], [36], [37], [38] the composite and retard the thermal degradation of the polymer [36]. However, when as-synthesized nanotubes are used either individually or in bundles, the composites fail through nanotube pullout [2], [9], [14]. This occurs because of the shearing of the nanotube bundles in response to the applied stress and the weak van der Waals interactions between the nanotubes and polymer chains [14]. Nanotubes that are chemically functionalized however, stick more strongly to the polymer matrix [39]. Molecular dynamics simulations illustrate why this is the case [40], [41]. When van der Waals bonds are what hold together the nanotube-polymer matrix composite, there is no permanent load-transfer between the polymer and the nanotubes. In contrast, when the same system is considered with covalent chemical bonds between the nanotubes and the polymer, the material as a whole is found to have an improved shear yield strength.
In previous computational studies [42], [43] polyatomic ion (CH3+) deposition was predicted to be an effective method of chemical functionalization of single-walled and multi-walled carbon nanotubes arranged in a bundle at low and medium incident energies of 10–80 eV. These results were confirmed by experiments where beams of CF3+ were deposited on multi-walled nanotubes [43]. The simulations also predicted that at 80 eV in the case of single-walled nanotubes, and 45 and 80 eV in the case of multi-walled nanotubes, covalent cross-linking would occur between neighboring nanotubes or between shells within multi-walled nanotubes as a result of the ion deposition.
In this paper we use molecular dynamics simulations to explore the modification of a composite of (10,10) single-walled nanotubes and polystyrene (PS) through the deposition of a beam of polyatomic ions of C3F5+. One objective is to determine if polyatomic ion beam deposition is a suitable approach for inducing covalent cross-links between otherwise unfunctionalized nanotube and a PS matrix that would toughen the composite. Another objective of the proposed work is to determine how the presence of the nanotube in the PS affects the outcome of the polyatomic ion beam deposition relative to deposition on a pristine PS surface.
Section snippets
Computational methods
The computational approach used is classical molecular dynamics [44] with a time step of 0.20 fs. The interatomic potential used is the analytic reactive empirical bond-order (REBO) potential of Brenner for hydrocarbons [45] that has been extended to include carbon–fluorine [46] and hydrogen–fluorine interactions [47]. The REBO potential is coupled to Lennard–Jones potentials to model the long-range van der Waals interactions within the PS and between the PS and the nanotube [48], [49].
The REBO
Results and discussion
Prior to the deposition of the ion beam, the substrates are equilibrated at 300 K. The structures shown in Fig. 1 are after this equilibration but prior to the ion beam deposition. The ion beam consists of 20 C3F5+ (CF2–CF+–CF2) ions that are deposited on each substrate at random positions. The incident energy is 50 eV/ion and there is an approximately 2 ps time interval between the impacts of any two consecutive depositing ions. Therefore, the substrate is fully equilibrated after the
Conclusions
This computational study has demonstrated that polyatomic ion beam deposition can induce covalent cross-links between otherwise unfunctionalized nanotubes and the backbones of a PS matrix. Such covalent tethers have been shown by others to lead to a tougher composite [39], [40], [41]. It has also shown that the response of a nanotube-PS composite to ion beam deposition is significantly different from the response of a pristine PS surface.
Acknowledgements
The authors gratefully acknowledge the support of the National Science Foundation (CHE-0200838) and the Petroleum Research Fund, administered by the American Chemical Society.
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