Ultraviolet-Assisted Direct-Write Fabrication of Carbon nanotube polymer nanocomposite microcoils

Ultraviolet-Assisted Direct-Write Fabrication of Carbon nanotube polymer...

Ultraviolet-Assisted Direct-Write Fabrication of Carbon Nanotube/Polymer Nanocomposite Microcoils

By Louis Laberge Lebel, Brahim Aissa, My Ali El Khakani, and Daniel Therriault*

There is currently a worldwide effort to achieve advances in micro- and nanotechnologies because of their high potential for technologic applications in fields such as microelectromechanical systems (MEMS) and organic electronics. In these applications, carbon nanotube/polymer nanocomposites represent interesting material options compared to conventional resins for their enhanced mechanical[1] and electrical[2] properties. However, several significant scientific and technologic challenges must first be overcome in order to rapidly and cost-effectively fabricate nanocomposite-based microdevices. For the microfabrication of 3D microdevices using photopolymers, stereolithographic techniques based on photopolymerization and two-photon absorption are readily available.[3] Two-photon absorption technique stands out as a powerful procedure to manufacture 3D products with a spatial resolution down to 120nm.[4] However, the low viscosity and transparency of the photopolymer used limit the application of this technique to functional nanocomposite devices.

Fabrication techniques such as the direct-write of ink filaments[5] of various material types have emerged for fabricating 3D miniature devices. These techniques consist of the robotized deposition of continuous ink filaments that allow the construction of 3D devices through a layer-by-layer building sequence. The techniques rely on specific ink viscoelastic properties. For example, low to moderate viscosity is required for the extrusion through capillary micronozzles, and an increase in rigidity after extrusion is necessary for shape retention of the deposited filament. There are various materials that possess these properties. Organic fugitive inks with superior stiffness are ideal for use as self-supporting spanning filaments and exhibit moderate shear yield stress to facilitate extrusion through a deposition nozzle.[6] This type of ink can be used to create a sacrificial scaffold enveloped in an epoxy matrix, in which the ink can be heated to liquid state and then drained to manufacture 3D microfluidic devices.[7] Low-viscosity polyelectrolyte inks have been developed for direct-write, using the coagulation mechanism occurring when filaments are deposited in an alcohol/water reservoir.[8] The fivefold increase in rigidity after coagulation allows the deposition of microscopic woodpile structures for photonic crystal fabrication.[9] Flocculated colloidal suspensions have also been employed for direct-write. Appropriate pH calibration suppresses the repulsive forces between the particles, which allows the long-range van der Waals forces to drive the agglomeration.[10] Flow could also occur when the shear stress reaches a specific yield point due to the destruction of the agglomerated network, since the van der Waals forces are weak by nature. This preparation technique has been employed to prepare silver nanoparticle inks for spanning electronic contacts in microelectronic applications.[1] Another approach is to use the rigidity increase after the temperature phase transition from liquid to solid that a hydrogel[12] or a thermoplastic polymer[13] undergoes in specific temperatures ranges, which is often applicable for the fabrication of tissue engineering scaffolds.

These various direct-write techniques have been limited mainly to supported structures in a layer-by-layer building sequence and straight spanning filaments between support points. This limitation comes from the fact that for the ink systems previously mentioned, the ink rigidity increases at the same moment when the ink exits the extrusion nozzle, i.e., when the shear strain applied to the material returns to a near zero value. Therefore, the extruded filament has the same rigidity from its extrusion point to the previous support point. Given these conditions, the application of a side force due to the extrusion nozzle changing direction results in a bending moment that reaches a maximum at the support point; this results in a deflection of the whole deposited filament. In order to create a curved shape by changing the moving path of the extrusion nozzle, another increase in rigidity has to occur slightly away from the extrusion point. As a result, the filament bending would occur at the transition zone due to the low bending rigidity of the newly deposited material compared to the higher bending rigidity of the spanning filament previously deposited. A high increase in rigidity is naturally observed in cross-linking reactions of thermosetting polymers, in which the rigidity can increase several orders of magnitude from its liquid uncured state to its final polymerized state. Using this rigidity increase for the direct-write technique is dependent on the capability of triggering the cross-linking reaction at the desired distance from the extrusion point and also of achieving the high rate of conversion from liquid to solid state that is required for the fast w.MaterialsViews.com w.advmat.de

[*] Prof. D. Therriault, L. L. Lebel

Laboratory of Multi-scale Mechanics, Center for Applied Research on Polymers (CREPEC) Ecole Polytechnique of Montreal C.P. 6079, succ. Centre-Ville, Montreal, QC H3C 3A7 (Canada) E-mail: daniel.therriault@polymtl.ca

B. Aissa, Prof. M. A. E. Khakani Institut National de la Recherche Scientifique, INRS-Energie, Materiaux et Telecommunications 1650 Blvd. Lionel-Boulet, Varennes, QC J3X 1S2 (Canada)

DOI: 10.1002/adma.200902192

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manufacturing conditions. Photocurable polymers are suitable candidates due to their localized curing abilities and rapid conversion rates.[14]

The ultraviolet-assisted direct-write (UV-DW) fabrication method presented in this study was developed and used to build continuous 3D-coiled geometries in a free-form fashion (see Scheme 1). The method employs the robotically controlled microextrusion of a UV-curable nanocomposite ink filament combined with a UV exposure that follows the extrusion point.

Upon curing, the increased rigidity of the extruded filament enables the creation of multi-directional shapes along the trajectory of the extrusion point. The UV-DW apparatus consists of a syringe barrel containing the uncured material fitted with an extrusion nozzle having a defined cross-section (e.g., circular, square) and specific characteristic dimensions. The ink is extruded through a capillary nozzle by an applied pressure and forms a filament that is exposed to UV illumination by a set of six optical fibers arranged in a circular pattern by way of an attachment ring above the filament. The height of the ring, and thus the UV zone, is adjusted such that the filament is exposed to the UV radiation slightly after extrusion. This allows the increase in rigidity upon curing to occur away from the extrusion point. However, the UV radiation must nonetheless remain as close as possible to the extrusion point in order to reproduce the specific path of the moving extrusion device.

For successful UV direct writing, the extrusion point moving speed, the pressure applied to the material, and the UV illumination intensity and position have to be adjusted according to the rheologic behavior and the curing rate of the extruded ink. A UV-curable nanocomposite consisting of a blend of singlewalled carbon nanotubes (C-SWNT) in a polyurethane matrix was developed to manufacture the microcoils. The C-SWNTs were functionalized (during their nitric acid-based purification) with carboxylic groups, which are expected to anchor to the polyurethane backbone through hydrogen bonds. The anchoring mechanism of the C-SWNTs to the polymer matrix and related effect on the mechanical properties of the resulting nanocomposite has been previously reported by Sahoo et al.[15] The viscoelastic behavior of the nanocomposite ink was tailored to form a continuous filament once extruded through the micronozzle. The process-related rheologic characterization of the nanocomposite ink developed in this study is presented in Figure 1a. A shear-thinning rheologic behavior was observed as the process-related apparent viscosity decreased with increasing process-related shear rate. This crucial property, resulting from the addition of fumed silica nanoparticles,[16] allows the material to be extruded from the capillary nozzle under high shear rates due to an increase in viscosity. Following extrusion, the shear rate at which the material is exposed drops to a near zero value leading to a viscosity increase. This phenomenon prevents sagging of the extruded filament prior to curing under UV exposure.

Another important property for UV-DW is the curing rate of the extruded material. Upon curing, the rigidity of the material increases and provides structural support for the material being deposited in a continuous manner and along different directions, therefore allowing for free-form writing in open air. The polyurethane matrix used cures by thiol-polyene polymerization. This radical polymerization employs thyil radicals generated by UV radiation.[14] The curing behavior of a UV-curable resin can be characterized by measuring the heat generated by the crosslinking reaction using differential scanning calorimetry (DSC) coupled with a UV-radiation source. Figure 1b shows the exothermic heat Figure 1. UV-DW ink characterization. a) Process-related apparent viscosity and b) exothermic heat flux measured under DSC coupled with a 50mW cm 2 intensity UV lamp for 30s.

Scheme 1. Schematic representation of the UV-assisted fabrication of a nanocomposite coil. The UV-curable nanocomposite is extruded through a micronozzle and exposed to UV radiation using a set of optical fibers linked with the extrusion device. The fast curing allows the production of a spiral-shaped structure along the path of the UV-DW tool.

2 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–5 Final page numbers not assigned w.MaterialsViews.com w.advmat.de flux measured for the nanocomposite ink exposedto50mWcm 2ofUVradiation.Upon UV exposure, the cross-linking reaction generates heat until most of the available reaction sites are joined to other molecules. The exothermic peak occurs 2.7 s after the beginning of the UV exposure and the heat flux subsequently diminishes towards the end of the reaction. The total heat of reaction after a 30 s exposure is 346 15J g 1. Although the UV-curing conditions of an extruded filament during the UV-DW fabrication slightly differ from those of the UV-DSC characterizations, the latter provides clear indication that the developed nanocomposite rapidly polymerizes under UV. On the other hand, the presence of residual solvent (dichloromethane) and different nanoparticules (C-SWNTs and fumed silica) might reduce the overall curing speed of the nanocomposite compared with the pure resin. However, the rapid exothermic rise and subsequent drop shown in Figure 1b indicate a relatively fast curing reaction that is believed to facilitate the UV-DW fabrication process.

The nanocomposite ink was used to fabricate microcoiled structures. Figure 2a shows top and isometric views from a computer-aided design (CAD) image of the programmed deposition path of the UV-DW apparatus to manufacture a microcoil using six 0.8mm diameter loops for a total height of 2mm. Figure 2b presents scanning electron microscopy (SEM) top and isometric images of the coil actually deposited. Comparison between the CAD (Fig. 2a) and the SEM (Fig. 2b) images demonstrates a high fidelity of the manufacturing process. Figure 2c shows the relatively smooth surface of the microstructure with few features probably due to the presence of C-SWNT or other carbon aggregates. This repeatable process enables the integration of several coils side-by-side. Networks of three springs were deposited for mechanical solicitation. Figure 2d shows an isometric SEM image of a three-coil triangular network similar to those mechanically tested. Figure 2e shows the force versus the compression distance of a typical network of nanocomposite coils. The mechanical response under compressive force was quasi-linear over the 1.5mm compression distance with a microcoil network rigidity of 1.5 0.4 mN m 1. This specific rigidity could be decreased or increased by changing the C-SWNT loading or the coil geometry (e.g., pitch and diameter). The high flexibility of the UV-DW process could allow the integration of microcoils on MEMS for their microload-bearing potential.

The UV-DW was also used to manufacture a nanocomposite microcoil connected by the top end to an aluminum electrode exhibiting the capability of connecting two electrical components with a tortuous path. Figure 3a shows a CAD image of the five-turn coil specified as the program path for the UV-DW apparatus. Figure 3b shows the actual deposited nanocomposite coil linking a filament deposited on a glass substrate with a 2.1mm high aluminum electrode. Current measurements were made upon voltage application between the two ends of the coil.

Figure 2. Microcoil spring manufactured by UV-DW. a) Virtual image of the programmed UV-DW fabrication of the coil. b) SEM image of the actual deposited nanocomposite coils. c) Representative SEM image of the nanocomposite filament surface. d) Typical triangular microcoil spring network. e) Mechanical solicitation of a triangular microcoil spring network.

Figure 3. Nanocomposite coil fabrication and electrical characterization. a) Virtual image showing the programmed UV-DW fabrication of the coil. b) Optical microscope image of the actual deposited coil. c) Measured current circulating in the coil upon voltage application.

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The measurements are shown in Figure 3c where a linear correspondence can be observed between the applied voltage and the measured current, corresponding to a conductivity of 10 6Sc m 1. The measured conductivity could be increased by improving the C-SWNT load and dispersion, or using other material systems, leading potentially to the UV-DW fabrication of organic inductors for radio frequency filters.

The UV-DW microfabrication method demonstrated here opens new prospects for the manufacturing of complex 3D nanocomposite structures in many technologic applications. Different UV-curable systems are possible to accelerate manufacturing times and the utilization of different micro- or nanoparticles (for functionalizing the UV-curable host polymer) combined with improved dispersion techniques could enable product manufacturing with enhanced electrical, mechanical, optical, and thermal properties. For example, the UV-DW process could be used for the creation of complex components for MEMS (e.g., gears and springs). It could also be used to create flexible electrical connections, freestanding electromagnetic shielding meshes, or inductance components in organic microelectronics, as well as microprosthetic devices or tissue engineering scaffolds in the biomedical field.

Experimental

The UV-DW apparatus was actuated with a moving stage along the x-axis and a robot head (I&J220-4, I&J Fisnar) moving in the y–z plane that was computer controlled with commercial software (JR Points for Dispensing, Janome Sewing Machine). The custom-designed UV-DW tool was installed with a receptacle for a syringe containing the deposited material. Six optical fibers, fitted through a ring surrounding the extrusion nozzle, directed the UV radiation close to the extrusion point of the material, at the tip of a capillary nozzle (Precision Stainless Steel Tips, EFD). The optical fibers collected the light from two high-intensity UV light-emitting diodes (LED, NCSU033A, Nichia) having a wavelength centered at 365nm. The height of the optical fiber ring was adjusted using a micrometer head. The intensity of the UV radiation was 50mW cm 2. This result was obtained by using a UV intensity probe (UV Intensity meter, model 100, Karl Suss) placed at a distance ( 1cm) similar to that of the extruded filament to the end of the optical fibers. The pressure was applied on the nanocomposite ink using an air-powered dispensing system (HP-7X, EFD). All the coils manufactured in this study were deposited using a pressure of 2 MPa and a moving speed of 0.3mm s 1. The C-SWNTs were produced by a pulsed laser vaporization technique, using an excimer KrF laser [17]. A reflux of 5h in a 3M HNO3 (Sigma–Aldrich) solution was used to purify the as-grown material. A commercially available UV-curable polyurethane matrix containing mercapto esters and low concentration of acrylic monomers was used (NEA123MB, Norland Products). Zinc protoporphyrin IX (ZnPP, Sigma–Aldrich) was used to facilitate the dispersion of the C-SWNTs. A specific amount of C-SWNT was sonicated (Ultrasonic cleaner 8891, Cole-Parmer) in a solution of 0.1mM of ZnPP in dichloromethane (DCM, Sigma–Aldrich) for 30min. Polyurethane was added to the C-SWNT solution while mixing at 800rpm for 30min over a stirring hot plate (Model SP131825, Barnstead international). After complete solvent evaporation, the nanocomposite mixture was passed several times through a three-roll mixer mill (80E, Exakt) with the following procedure: five passes at a gap of 25mmandaspeedof200rpm,fivepassesatagapof15mmandaspeedof 200rpm,andninepassesata gapof5mmanda speedof250rpm[18].The nanocomposite was slowly added to a fumed silica nanoparticle (Aerosil 200, Degussa) solution in the DCM while mixing over a stirring hot plate at 408C. Finally, the mixture was poured into syringe barrels (3cm3, EFD) where subsequently the DCM solvent was evaporated at 208C. The whole procedure produced UV-curable polyurethane nanocomposite ink containing 0.5wt % C-SWNTs and 5wt % of fumed silica.

An experimental method based on capillary viscosimetry was used to measure the process-related apparent viscosity of the nanocomposite ink. Lines of material were extruded over glass substrates at five different pressures and calibrated displacement speeds to obtain different strain rate and shear stress combinations. The flow rate, apparent wall shear stress, and process-related apparent viscosity were calculated using experimental conditions and cross-section measurements (high resolution optical sensor, CHR 150, Stil) of the deposited filaments [19].

The UV-curing behavior of the nanocomposite ink was evaluated with a

Photo-DSC (DSC822e, METTLER Toledo). The UV source was a Mercury–Xenon lamp (Lightningcure L8333, Hamamatsu) that provided UV radiation in the 240–400nm wavelength range. Nanocomposite samples were prepared at 2.0 0.1mg and were cured at an intensity of 50mW cm 2 and a thickness of 200mm. Experiments were performed under an air flow of 50mL min 1 at 308C. The heat of reaction was obtained from exotherms.

Scanning electron microscopy (Quanta FEG 200, FEI Instruments) was performed on a six-turn microcoil having a 800mm diameter and a 500mm pitch that was manufactured using a 100mm extrusion nozzle (5132-0.25-B, EFD) on a silicon substrate. The first and last turns had a null pitch.

The nanocomposite mechanical coupons consisted of a network of threeidentical1mmdiametercoils.Theywere depositedona400mmthick plastic laminated cardboard substrate in a triangle layout having an inter-coil distance of 3mm. The extrusion nozzle diameter was 150mm (5127-0.25-B, EFD). The micro-coils comprised 6 turns for a total height of 5mm with the middle coils having a pitch of 1mm and the first and last turns being flat to provide support for mechanical solicitation. Before mechanical testing, the spring networks were post-cured at 908C for 3h. The compression tests were done using a dynamic mechanical analyzer (DMA Q800, Texas Instruments) equipped with a compression fixture. A preload of 4 mN was applied for 1.5min to reach equilibrium at a temperature of 208C. The load was increased at a rate of 5 mN min 2 until the compression reached 1.5mm. The rigidity of the spring network was averaged from a sample of three coupons.

Nanocomposite microcoils having 5 turns were deposited on a glass substrate. The top-end wire was deposited on a 2.1mm thick aluminum block. The pitch was 0.5mm and the radius was 0.6mm. The coil image was acquired with an optical microscope (BX-61, Olympus and using the software Image-Pro plus V5, Media Cybernetics). The electrical properties of the nanocomposite microcoils were characterized at room temperature using a Hewlett-Packard 4140B semiconductor parameter analyzer with Ag electrodes deposited on the microcoil ends.

[A video of the UV-DW fabrication of a microcoil is available at: http:// w.polymtl.ca/lm2/en/video/].

Acknowledgements

The authors acknowledge the financial support from FQRNT (le Fond Quebecois de la Recherche sur la Nature et les Technologies) and NSERC (Natural Sciences and Engineering Research Council of Canada). They also wish to thank Prof. Reidl of Laval University for assistance with the UV-DSC characterization.

Received: June 30, 2009 Published online:

V. Schmidt, L. Kuna, A. Haase, F. Varga, H. Lichtenegger, J. Stampfl, J. Coat. Technol. Res. 2007, 4, 505. [4] S. Kawata, H.-B. Sun, T. Tanaka, K. Takada, Nature 2001, 412, 697.

4 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 1–5 Final page numbers not assigned

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[9] G. M. Gratson, F. Garcia-Santamaria, V. Lousse, M. Xu, S. Fan, J. A. Lewis,

[12] R. Landers, A. Pfister, U. Hubner, H. John, R. Schmelzeisen, R. Mulhaupt, J.

[15] N. G. Sahoo, Y. C. Jung, H. J. Yoo, J. W. Cho, Macromol. Chem. Phys. 2006, 207, 1773. [16] S. R. Raghavan, H. J. Walls, S. A. Khan, Langmuir 2000, 16, 7920.

[19] J. Bruneaux, D. Therriault, M. C. Heuzey, J. Micromech. Microeng. 2008, 18, 115020-1.

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