Embedded carbon-nanotube-stiffened polymer surfaces.

Conducting surface coatings are useful for antistatic applications, whereas surface hardening of materials is useful for improving the wear and abrasion resistance. 3] For polymer materials, surface conductivity and stiffness may be improved by applying coatings or adding fillers to the polymer matrix. For example, polymers can be made scratch resistant by the addition of hard fillers. However, for the case of polymers, achieving excellent mechanical and electrical properties only at the surface is a challenge. Conventional hard fillers, such as alumina or silica, improve the scratch resistance of the polymer, but do not help improve the conductivity. On the other hand, conducting fillers such as micrometer-scale graphite particles do not considerably improve the mechanical properties of the polymer. Thus, there is a need to develop a surface engineering approach to alter the mechanical and electrical properties of polymer coatings. Multiwalled carbon nanotubes (MWNTs) are stiff macromolecular structures having outer diameters of 30 nm, and lengths on the order of a few tens of micrometers. The MWNTs also have a very high conductivity ( 10 Scm ), high modulus ( 1 TPa) along their length direction, as well as a high bending modulus (0.9 to 1.24 TPa). Possibilities of improving bulk mechanical and electrical properties of composites by nanotube-reinforcement have been discussed in the literature. 8–12] For example, the polymer-intercalated nanotube sheets have shown significant improvement in the modulus of the film. Our approach is to incorporate the excellent properties of nanotubes at a polymer surface in a well-ordered and distributed fashion resulting in the improvement in the electrical as well as mechanical properties of the polymer. This would enable multifunctional surface characteristics for polymer coatings. This paper describes the first report of the generation of such surface-engineered polymer coatings with nanotubes. In the present work, a thickness-aligned MWNT/polymer disc was prepared, where the MWNTs were reinforced into one of the surfaces of the disc, and were aligned in the thickness direction. The discs were made from two different polymers: polymethyl methacrylate (PMMA) and polydimethyl siloxane (PDMS). 14] Both PMMA and PDMS are insulating. However, PMMA is a glassy, rigid polymer at room temperature, whereas PDMS is a soft elastomer at room temperature. The synthesis of the composite disc was performed as follows: First, the aligned arrays of MWNTs ( 30 mm in length) were grown on a quartz substrate by chemical vapor deposition. 15] Subsequently, the quartz substrate with aligned nanotube arrays was gently immersed, with the nanotube side facing the top, into the excess monomer (or uncured resin) solution in a vial. By using the excess quantity, the resulting polymer not only occupied the inter-nanotube gaps in the MWNT arrays, but also formed a thick layer above the surface of the MWNT arrays. A portion of the same monomer solution was taken in a separate vial to make pure polymer as a control sample. After the in situ polymerization was complete, polymer discs were taken out of the quartz substrate. In order to make MWNT/PMMA discs, the monomer (methyl methacrylate (MMA)), the initiator (2,2’-azobisisobutyronitrile (AIBN)), and the chain-transfer agent (1-decanethiol) were mixed together in a given proportion (60 mL MMA: 0.17 g AIBN: 30 mL 1-decanethiol) in a quartz vial. The polymerization was carried out in a water bath at 55 8C, for 24 h. The weight fraction of MWNTs in the MWNT/PMMA composite films was estimated to be approximately 4%. Schematics of the synthesis process and the cross-sectional SEM micrographs of the thickness-aligned MWNT/PMMA discs are shown in Figure 1. Similarly, PDMS as well as MWNT/ PDMS films were prepared by infiltration of a mixture of silane resin and a curing agent (in a proportion of 10:1 by weight) into aligned MWNT arrays, followed by typical thermal cure cycles. The surface resistivity of the MWNT side of the polymer disc was compared with that of the pure PMMA side by measurement with a four-probe setup with a probe spacing of 500 mm. A dc current (I), on the order of a few hundred microamps, was applied through the sample, and the voltage (V) was measured in millivolts. The MWNT-reinforced side of the PMMA disc showed a dc conductivity of 0.60 0.07 Scm , whereas the pure PMMA side was not conducting. The reported conductivity of PMMA is 5 10 11 Scm . Thus, the addition of MWNTs increases the surface conductivity of PMMA significantly. The MWNTs used in the present analysis are macroscopically aligned but show less overall alignment owing to the waviness of the nanotubes. Thus, the percolation threshold is expected to be drastically lower than that which is expected for perfectly aligned fibers in a matrix. Therefore, the nanotube loading ( 4% by weight or 2% by volume) in the present composites is expected to be above percolation threshold, as indicated by the conductivity of 0.60 0.07 Scm . This value is higher than that reported in the literature for similar loadings of pure, non-aligned MWNTs (10 3 to 10 2 Scm ), but lower than for similar loadings of Fe-containing MWNTs in PMMA (>1 Scm ). The conductivity of the aligned MWNT/PMMA surfaces is large enough for poten[*] Dr. N. R. Raravikar, A. S. Vijayaraghavan, Prof. P. Keblinski, Prof. L. S. Schadler, Prof. P. M. Ajayan Departments of Materials Science and Engineering Rensselaer Polytechnic Institute, Troy, NY 12180 (USA) Fax: (+1)518-276-8554 E-mail: [email protected] [email protected] [] These authors contributed equally to the work