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Reinforcing composite materials with carbon nanotubes (CNTs) has the potential to improve mechanical and/or multifunctional properties due to their nano-size. Research has been done on using CNTs to reinforce the interlaminar strength of carbon fiber reinforced composites (CFRPs), but most of the previous work is about randomly oriented carbon nanotubes. Currently, one of the main challenges regarding CNT integration into polymers is mitigating their agglomeration and controlling their dispersion in the polymer matrix. By aligning CNTs with an external field, more tailored structure control can be achieved, and a better understanding of how CNT agglomeration and dispersion relate to external field application and CNT concentration is needed. In this work, we studied the effects of magnetic field magnitude, CNT concentration, and matrix viscosity on CNT agglomeration and morphology. We measured the fracture toughness reinforcement of epoxy-CNT nanocomposites at various CNT concentrations (0.1 vol.% and 0.5 vol.%), magnetic field magnitudes (no field, 180 G, and 300 G), and matrix viscosities (older epoxy-hardener system with higher viscosity and newer epoxy-hardener system with lower viscosity). Our results demonstrated that aligning CNTs with a magnetic field can lead to extra reinforcement when compared to using randomly oriented CNTs if the field magnitude, CNT concentration, and matrix viscosity are selected accordingly. The maximum fracture toughness reinforcement achieved with the higher viscosity epoxy-hardener system (~72%) was with 0.5 vol.% of CNTs with a 180 G field, whereas the maximum reinforcement with the lower viscosity epoxy-hardener system (~62%) was observed for the samples fabricated with 0.1 vol.% of randomly oriented CNTs. COMSOL simulations were also conducted to understand the behavior of CNT agglomeration and alignment at different field magnitudes and CNT concentrations, and were compared with the experimental results. To maximize CNT reinforcement, more work needs to be conducted to address the challenge of CNT agglomeration and dispersion control in a polymer matrix, such as a more in-depth study of how different field magnitudes affect fracture toughness improvement and new methods to improve CNT dispersion. 

Organization of carbon nanotubes (CNTs) within epoxy matrices can be effectively achieved using magnetic field application. In our previous experimental work, multi-walled CNTs were magnetized, diazotized, and magnetically aligned to form aligned CNT-epoxy composites. While effective toughness improvement was experimentally observed with small CNT addition, more understanding about magnetic assembly of CNTs is desired, to effectively complete CNT assembly before the epoxy cures and also to avoid re-agglomeration of CNTs. In this work, assembly behaviors of ellipsoid particles, that simulate CNT bundles, in a fluid domain, that simulates the epoxy matrix, under the static magnetic field are being studied. Higher ellipsoid aspect ratio was observed to be effective to decrease the magnetic assembly time, while some ellipsoid lower aspect ratio and larger original ellipsoid separate distance combination prevented their magnetic assembly. When assembly is achieved, the assembly time was observed to be much smaller (<0.1 second) than the currently dedicated assembly time in our experiments (~10s of minutes). Further studies with more ellipsoids, varying ellipsoid positions, and increasing magnetic field strength are planned in future. 

Smart structures with actuation function are desired for aerospace applications, including morphing airfoils, deployable structures and more. While shape memory alloys and piezoelectric ceramics and polymers are currently a popular smart material options for such applications, magnetoelastomers (MEs) can be uniquely actuated with application of non-contact magnetic field. Magnetoelastomers (MEs), composite materials made of magnetic particles and soft, non-magnetic matrix, can potentially contribute to such smart structures as a light-weight, smart material option with large strain change, fast response time (milliseconds) and anisotropic actuation properties. Other than aerospace applications, MEs, as soft actuators, have been investigated for flexible electronics, soft robotics, and biomedical applications. Anisotropic actuation properties of MEs can be controlled with particle organization within the elastomer. To provide this control, parametric studies on fabrication of MEs need to be performed. This study presents experimental work on nanoparticle organization within MEs using uniaxial, biaxial and triaxial magnetic fields and on the structure-property relationships of MEs. Iron oxide nanoparticles were used as a model nanofillers, and their surfaces were treated with silane coupling agent to improve dispersion and suspension within a polydimethylsiloxane (PDMS) elastomer. The fabricated MEs were inspected using microCT, and their anisotropic susceptibilities are being measured. 

Polymer nanocomposites have been sought after for their light weight, high performance (strength-to-mass ratio, renewability, etc.), and multi-functionality (actuation, sensing, protection against lightning strikes, etc.). Nano-/micro-engineering has achieved such advanced properties by controlling crystallinity, phases, and interfaces/interphases; hierarchical structuring, often bio-inspired, has been also implemented. While driven by the advanced properties of nanofillers, properties of polymer nanocomposites are critically affected by their structuring and interfaces/interphases due to their small size (< ~50 nm) and large surface area per volume. Measures of their property improvement by nanofiller addition are often smaller than theoretically predicted. Currently, application of these novel engineered materials is limited because these materials cannot often be made in large sizes without compromising nano-scale organization, and because their multi-scale structure-property relationships are not well understood. In this work, we study precise and fast nanofiller structuring with non-contact and energy-efficient application of oscillating magnetic fields. Magnetic assembly is a promising, scalable method to deliver bulk amount of nanocomposites while maintaining organized nanofiller structure throughout the composite volume. In the past, we have demonstrated controlled alignment of nanofillers with tunable inter-assembly distances with application of oscillating one-dimentional magnetic fields (~100s of G), by taking advantage of both magnetic attraction and repulsion. The low oscillation frequency (< 1 Hz) was tuned to achieve maghemite nanofiller alignment patterns, in an epoxy matrix, with different amount of inter-nanofiller contacts with the same nanofiller volume fraction (see Figure 1a). This work was recently expanded to three-dimensional assembly using a triaxial Helmholtz coil system (see Figure 1b); the system can apply the triaxial magnetic fields of varying magnitude (max. ±300G, ±250G, ±180G (x-y-z)) and frequency (0 to 1 Hz, ~0.1 Hz resolution) with controlled phase delay to the sample size of 1.5” x 2.5” x 3.5”(x-y-z). Two model systems are currently studied: maghemite nanofillers in an elastomer for magnetoactuation, and nickel-coated CNTs in an thermoset for mehcniacl and transport property reinforcement. The assembled nanofiller structures are currently characterized by microCT; microCT scan data (see Figure 1b) are segmented through a machine learning algorithm, and will be modeled for their transport properties using a Monte Carlo method. These estimated properties will be compared with the experimentally characterized mechanical, transport, and actuation properties, providing the structure-interphase-property relationships. In future, we plan to integrate these nanocomposites to CFRPs for interlaminar property reinforcement, possibly with an out-of-autoclave composite processing. 

Polymer composites with small amount of CNTs (< 5 wt%) have been studied as a light-weight wear-resistant material with low friction, among other applications, but their modulus improvement often plateaus or diminishes with increasing CNT fraction due to agglomeration. Here, polymer nanocomposites were fabricated with randomly oriented or aligned CNTs across their volume (up to 5 mm length) by CNT surface diazotization and by static magnetic field application (400 G for 40 min). With the improved CNT dispersion and thus less agglomeration, the reduced moduli of PNCs stayed improved with addition of up to 1 vol% (or 1.3 wt%) of CNTs. In this work, the PNCs with randomly oriented CNTs exhibited higher stiffness than the PNCs with magnetically aligned and assembled CNTs, indicating again the negative effect of CNT agglomeration on stiffness. In future, other CNT structuring methods with controlled inter-CNT contacts will be conducted to dissociate alignment from local agglomeration of CNTs and thus to simultaneously improve hardness and modulus of PNCs with small CNT addition.