Three-dimensional (3D) microstructured composite beams are fabricated through the directed and localized infiltration of nanocomposites into 3D porous microfluidic networks. The flexibility of this manufacturing method enables the utilization of different thermosetting materials and nanofillers in order to achieve a variety of functional 3D reinforced nanocomposite macroscopic products.
Microstructured composite beams reinforced with complex three-dimensionally (3D) patterned nanocomposite microfilaments are fabricated via nanocomposite infiltration of 3D interconnected microfluidic networks. The manufacturing of the reinforced beams begins with the fabrication of microfluidic networks, which involves layer-by-layer deposition of fugitive ink filaments using a dispensing robot, filling the empty space between filaments using a low viscosity resin, curing the resin and finally removing the ink. Self-supported 3D structures with other geometries and many layers (e.g. a few hundreds layers) could be built using this method. The resulting tubular microfluidic networks are then infiltrated with thermosetting nanocomposite suspensions containing nanofillers (e.g. single-walled carbon nanotubes), and subsequently cured. The infiltration is done by applying a pressure gradient between two ends of the empty network (either by applying a vacuum or vacuum-assisted microinjection). Prior to the infiltration, the nanocomposite suspensions are prepared by dispersing nanofillers into polymer matrices using ultrasonication and three-roll mixing methods. The nanocomposites (i.e. materials infiltrated) are then solidified under UV exposure/heat cure, resulting in a 3D-reinforced composite structure. The technique presented here enables the design of functional nanocomposite macroscopic products for microengineering applications such as actuators and sensors.
Polymer nanocomposites using nanomaterials, especially carbon nanotubes (CNTs) incorporated into polymer matrices feature multifunctional properties1 for potential applications such as structural composites2, microelectromechanical systems3 (e.g. microsensors), and smart polymers4. Several processing steps including CNT treatment and nanocomposite mixing methods may be required to desirably disperse CNTs into the matrix. Since the CNTs' aspect ratio, their dispersion state and surface treatment mainly influence the electrical and mechanical performance, the nanocomposite processing procedure may vary depending on desired properties for a targeted application5. Moreover, for specific loading conditions, aligning CNTs along a desired direction and also positioning the reinforcements at desired locations enable further improvement of the mechanical and/or electrical properties of these nanocomposites.
A few techniques such as shear flow6-7and electromagnetic fields8 have been used to align the CNTs along a desired direction in a polymer matrix. Moreover, CNT orientation induced by dimensional constraining, specifically in one-dimension (1D) and two-dimension (2D), has been observed during the processing/forming of nanocomposite materials9-11. However, new advances on the manufacturing processes are still needed to allow sufficient control of the three-dimensional (3D) orientation and/or positioning of the nanotube reinforcement during the manufacturing of a product for optimal conditions.
In this paper, we present a protocol for manufacturing 3D-reinforced composite beams via directed and localized infiltration of a 3D microfluidic network with polymer nanocomposite suspensions (Figure 1). First, the fabrication of a 3D interconnected microfluidic network is demonstrated, which involves the direct-write fabrication of the fugitive ink filaments12-13 on epoxy substrates (Figures 2a and 2b), followed by epoxy encapsulation (Figure 2c) and the sacrificial ink removal (Figure 2d). The direct-write method consists of a computer-controlled robot that moves a fluid dispenser along the x, y, and z axes (Figure 3). This technique provides a fast and flexible way to fabricate 3D microdevices for photonic, MEMS and biotechnology applications (Figure 4). Then, the nanocomposite preparation is demonstrated, along with its infiltration (or injection) into the porous network under different controlled and constant pressures to manufacture 3D-reinforced multiscale composites (Figures 2e and 2f). Finally, some representative results along with their potential applications are shown.
1. Fabrication of 3D Microfluidic Networks
2. Nanocomposite Preparation
Note: The nanocomposites are prepared by blending a dual cure (ultraviolet/heat curable) thermosetting resin, either an epoxy resin or a urethane-based resin and nanofillers (here, single-walled carbon nanotubes) at different loadings.
3. Nanocomposite Infiltration (Injection)
Figures 8a and 8b show a representative image of manufactured beams and an optical image of its cross-section, consisting of nine layers of the nanocomposite filaments.
Figures 8c and 8d show typical SEM images of a manufactured beams fracture surface and a higher magnification image of filled channels (i.e. embedded nanocomposite microfibers), respectively. Since no debonding is seen at the channels wall, it is fair to say that the surrounding epoxy and the infiltrated materials are well adhered as a result of proper cleaning of the channels with hexane after the ink removal.
Figure 9 shows a representative optical image of a beam broken during the mechanical testing in which hexane is not used during the ink removal. Fiber debonding, as a result of poor mechanical interface is observed which might be due to fugitive ink traces remained after network cleaning.
Figure 10 shows the storage modulus, E', of the molded bulk epoxy samples (as benchmarks) and the 3D-reinforced beams. The results show unique trends for the manufactured beams which are the combination of the embedded and surrounding epoxy materials with superior properties with the presence of only ~0.18 wt. % CNTs.
Figure 11 shows the three-point bending test results of the manufactured composite beams using a DMA. As a result of CNTs positioning, the flexural modulus of the 3D reinforced beams showed an increase of 34% compared to the pure epoxy-infiltrated (whole epoxy) beams.
Figure 1. Schematic representation of a 3D-reinforced nanocomposite manufactured by the microinfiltration approach. Click here to view larger image.
Figure 2. Schematic representation of the manufacturing of 3D-reinforced beams. (a) Ink filament direct deposition using a dispensing robot, (b) Deposition of several layers on top of each other by incrementing the dispensing nozzle in z-direction, (c) Filling the pore space between filaments using a low viscosity resin, (d) Taking the ink out of the network by its liquefaction, resulting in the fabrication of microfluidic channels. (e) Filling the empty network with the nanocomposite suspension followed by curing, and (f) Cutting the excess epoxy parts. Click here to view larger image.
Figure 3. A photo of the robotic deposition stage consisting of a computer-controlled robot, a dispensing apparatus, and a live camera. Click here to view larger image.
Figure 4. A few images of microstructures manufactured by the direct-write assembly. Click here to view larger image.
Figure 5. An isometric view and a SEM image the 3D-connected microfluidic empty network. Click here to view larger image.
Figure 6. Nanocomposite mixing strategies including nanotube noncovalent functionalization, ultrasonication, and/or three-roll mill mixing which lead to nanotube dispersions with different qualities (optical images of nanocomposite films). Click here to view larger image.
Figure 7. Nanocomposite curing under UV illumination of a UV-lamp followed by post-curing in the oven. Click here to view larger image.
Figure 8. (a) Isometric image of a 3D-reinforced beam, (b) Typical cross-section of a nanocomposite-injected beam, (c) A beam surface fracture SEM image, and (d) A close-up view of (c). Click here to view larger image.
Figure 9. Fracture surface image of a polyurethane nanocomposite-infiltrated beam. Click here to view larger image.
Figure 10. Temperature-dependent mechanical properties (storage modulus) of the bulk epoxies and the manufactured beams using a dynamic mechanical analyzer. Click here to view larger image.
Figure 11. Quasi-static mechanical properties (flexural) of the bulk epoxies and the manufactured beams (three-point bending test). Click here to view larger image.
The experimental procedure presented here is a new and flexible manufacturing method in order to tailor mechanical performance of polymer-based materials for material design purposes. Using this method, desired properties could be achieved based on the proper choice of components (i.e. infiltrated materials and main matrix) as well as engineering the composite structures. First, the technique enables the manufacture of a single material, composed of different thermosetting polymers, representing a unique temperature-dependent feature which is different than those of the components bulks15. Another advantage of the present technique over other nanocomposite fabrication techniques by which the nanofillers are uniformly distributed through whole matrix is the ability to spatially place the reinforcements at desired locations in these 3D-reinforced composite beams. Due to this positioning capability, a lower amount of possibly expensive nanofillers is needed to obtain a specific mechanical performance13. Since the reinforcement pattern obeys the original direct-writing of the ink scaffold, the filaments' spacing in a given layer is limited to approximately ten times the ink filaments diameter owing to the viscoelastic properties of the fugitive ink. On the other hand, a small spacing may limit flow of liquid epoxy during the epoxy encapsulation step. Moreover, the ink filament's diameter should be large enough (e.g. above 50 µm) for ease of fabrication (e.g. extrusion of high viscous ink) and subsequent manufacturing steps such as nanocomposite infiltration into the microfluidic networks.
Another potential of the present method might be the capability of aligning the individual CNTs or other nanofillers in the flow direction under shear flow16 by nanocomposite infiltration at higher speeds/pressures, if the nanofillers are well-dispersed in during the nanocomposite mixing process. However, a high degree of alignment could only be achieved at very high infiltration pressures (due to small channel diameter), which may cause air entrapment in the network during the infiltration.
Representative optical images in Figure 6 show the nanocomposites prepared by the mixing procedure presented in Protocol 2 (two images at the bottom of the figure). The observed dark spots are thought to be nanotube aggregates. For the ultrasonicated nanocomposite, the micron-size aggregates with a diameter of up to ~7 µm are present while a drastic change of the size of the aggregates (with an average of ~1 µm) is observed for the shear-mixed nanocomposite. Since the nanofiller dispersion affects the mechanical and electrical properties of the manufactured 3D nanocomposite beams, an improved dispersion should be achieved to take the full advantage of 3D positioning of nanofillers using the present manufacturing technique. Therefore, a further study is needed to systematically investigate the dispersion states of nanotubes and the use of other nanofillers, which can be more easily dispersed within the epoxy matrix.
The present manufacturing technique might enable the design of functional 3D nanocomposite products for microengineering application17. The technique is not limited to the materials used in this study. Therefore, the application of this technique could be extended by the utilization of other thermosetting materials and nanofillers. Among several applications, structural health monitoring, vibration absorption products and microelectronics can be mentioned.
The authors have nothing to disclose.
The authors acknowledge financial support from FQRNT (Le Fonds Québécois de la Recherche sur la Nature et les Technologies). The authors would like to thank the consulting support of Prof. Martin Levesque, Prof. My Ali El Khakani and Dr. Brahim Aissa.
Dispensing Robot | I & J Fisnar | I & J2200-4 | – |
Robot software | I & J Fisnar | – | JR-Point Dispensing |
Syringe Barrel | Nordson EFD Inc. | 7012072 | 3cc |
Dispensing Nozzle | Nordson EFD Inc. | 7018225 | Stainless Steel Tip (ID: 0.51 mm) |
Dispensing Nozzle | Nordson EFD Inc. | 7018424 | Stainless Steel Tip (ID: 0.15 mm) |
Fluid Dispenser | Nordson EFD Inc. | HP-7X | – |
Fluid Dispenser | Nordson EFD Inc. | 800 | – |
Live camera | MediaCybernetics | QI, Cool, Color | 12 Bit, Qimaging |
Live Camera Software | Image-pro Plus | – | Version 6 |
Precision Saw | Buehler (IsoMet) | 622-ISF-03604 | Low-Speed Saw |
Flexible plastic Tube | Saint-Gobain PRL Corp. | Tygon 177936 | – |
Stirring hot plate | Barnstead international | SP131825 | – |
Vacuumed-oven | Cole-Parmer | EW-05053-10 | – |
Ultrasonic cleaner | Cole-Parmer | EW-08891-11 | – |
Three-roll mill mixer | Exakt Technologies | Exakt 80E | – |
Dynamic Mechanical Analyzer | TA Instruments | DMA Q800 | – |
UV-lamp | Cole Parmer | RK-97600-00 | Intensity of 21mW/cm² |