Fabrication and Nanocompression Testing of Aligned Carbon-Nanotube-Polymer Nanocomposites

The exceptional electronic, thermal, and mechanical properties of carbon nanotubes (CNTs) have motivated extensive research on their manufacturing and applications. At bulk scales, there is particular interest in property enhancements by adding CNTs to polymers to make composite materials. Most work on CNT-based composites presented in the literature to date has focused on dispersion of singleor multiwalled CNTs (SWNTs or MWNTs) in the matrix; however, bulk CNTs embedded in a polymeric matrix tend to form aggregates that are not only poorly adhered to the matrix but also concentrate stresses, compromising the effect of the CNTs as reinforcement. On the other hand, establishing order and alignment among CNTs within a composite matrix offers significant further potential to harness the properties of individual CNTs at bulk scales by realizing the anisotropic properties of CNTs in desired directions, and by enabling packing and dispersion of CNTs at much higher volume fractions than in tangled configurations. In this work, CNT–polymer composites are manufactured by wetting as-grown arrays of vertically aligned CNTs rapidly and effectively (lack of voids) using off-the-shelf polymers. The wetting process not only preserves the alignment of the CNTs, but also allows the controlled manufacturing of nanocomposite test structures. Direct characterization of the mechanical properties of the nanocomposite structures is also presented in this work. The mechanical-reinforcement results support the feasibility of using these CNT arrays in large-scale hybrid advanced composite architectures reinforced with aligned CNTs. Such composites will also benefit from multifunctional property (e.g., electrical conductivity) enhancements owing to the aligned CNTs. The mechanical properties (e.g., Young’s modulus, hardness) of thin films containing randomly oriented CNTs inside a polymer matrix have been assessed using various techniques, that is, analytical models, standard tension and compression tests, and nanoindentation techniques. The composite modulus along the CNT axis (stiff axis) of well-aligned CNT–polymer composites has not been studied to date. In the most relevant related work, Fang et al. used Berkovich nanoindentation to determine the hardness of chemical vapor deposition (CVD)-grown aligned MWNT– parylene nanocomposites, but did not report their modulus. Parylene is a gas-phase deposited polymer typically used for electrical isolation. Modulus characterization in the transverse direction (i.e., CNTs aligned perpendicular to the direction of modulus measurement) indirectly inferred from structural tests was reported, with a significant increase (by a factor of 20–30) over the assumed modulus of parylene. Modulus characterization using direct measurement methods, not dependent on structural models, is preferred when available. The present work is the first to characterize the mechanical properties of well-aligned CNT–epoxy nanocomposites along the CNT axis using direct measurement methods; this allows an assessment of the effectiveness of the CVD-grown CNT reinforcement by direct comparison to results obtained for nonreinforced polymers. Compression tests, obtained by using a diamond flat punch mounted on a nanoindenter, were used to acquire direct measurements of the load-displacement response of the aligned CNT–epoxy nanocomposites fabricated by using a submersion technique, and compared directly to the measured modulus for pure epoxy pillars (identical test structure). Two grades of SU-8 UV-curing thermoset epoxy (Microchem 2000.1 and 2025) were selected to create the nanocomposites. The direct measurement results show that Young’s modulus increased from 3.7 GPa for the non-reinforced epoxy to 11.8 GPa for the vertically aligned CNT-reinforced composite material (220 % increase at 2 % volume loading). The mechanical properties of the pure polymer are the same for both grades; the CNT-reinforced pillars were manufactured with SU-8 2000.1. SU-8 epoxy was selected for four main reasons: i) Because of its UV curing activation, it is possible to selectively cure 2D patterns to create 3D structures. SU-8 has been used in the fabrication of microfeatures such as microchannels, micromolds, and pure epoxy pillars similar to the ones used in this research; allowing the fabrication of test structures for direct comparison with the nanocomposites. ii) The surface of the pure SU-8 films or pillars is sufficiently regular to allow for C O M M U N IC A IO N