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Physical and Chemical Properties Characterization of 3D-Printed Substrates Loaded with Copper-Nickel Nanowires

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Physical and Chemical Properties Characterization of 3D-Printed Substrates Loaded with Copper-Nickel Nanowires polymers Article Physical and Chemical Properties Characterization of 3D-Printed Substrates Loaded with Copper-Nickel Nanowires Ely Dannier V-Niño 1,2,* , Quentin Lonne 3, Andrés Díaz Lantada 4, Enrique Mejía-Ospino 5, Hugo Armando Estupiñán Durán 6, Rafael Cabanzo Hernández 5, Gustavo Ramírez-Caballero 5 and José Luis Endrino 1,7,* 1 BCMaterials, Basque Center for Materials, Applications and Nanostructures, UPV/EHU Science Park, 48940 Leioa, Spain 2 Materials Science and Technology Research Group, Foundation of Researchers in Science and Technology of Materials, 680003 Bucaramanga, Colombia 3 School of Aerospace, Transport and Manufacturing, Cranfield University, Bedfordshire MK43 0AL, UK; [email protected] 4 Departamento de Ingeniería Mecánica, Universidad Politécnica de Madrid, 28006 Madrid, Spain; [email protected] 5 Escuelas de Química, Física e Ingeniería Química, Universidad Industrial de Santander, 680002 Bucaramanga, Colombia; [email protected] (E.M.-O.); [email protected] (R.C.H.); [email protected] (G.R.-C.) 6 Departamento de Materiales y Minerales, Universidad Nacional de Colombia, 050034 Medellín, Colombia; [email protected] 7 IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain * Correspondence: [email protected] (E.D.V.-N.); [email protected] (J.L.E.); Tel.: +34-94-465-1115 (E.D.V.-N. & J.L.E.) Received: 9 July 2020; Accepted: 11 November 2020; Published: 13 November 2020 Abstract: This study deals with the laser stereolithography manufacturing feasibility of copper-nickel nanowire-loaded photosensitive resins. The addition of nanowires resulted in a novel resin suitable for additive manufacturing technologies based on layer-by-layer photopolymerization. The pure and nanowire-loaded resin samples were 3D printed in a similar way. Their morphological, mechanical, thermal, and chemical properties were characterized. X-ray computed tomography revealed that 0.06 vol % of the composite resin was filled with nanowires forming randomly distributed aggregates. The increase of 57% in the storage modulus and 50% in the hardness when loading the resin with nanowire was attributed to the load transfer. Moreover, the decrease in the glass transition temperature from 57.9 ◦C to 52.8 ◦C in the polymeric matrix with nanowires evidenced a decrease in the cross-linking density, leading to a higher mobility of the polymer chains during glass transition. Consequently, this research demonstrates the successful dispersion and use of copper-nickel nanowires as a reinforcement material in a commercial resin for laser stereolithography. Keywords: nanofillers; bulk-functionalization; photopolymer resin; additive manufacturing; stereolithography 1. Introduction Additive manufacturing (AM) technologies, often employed for rapid prototyping (RP), directly manufacture complex three-dimensional (3D) objects layer-by-layer using the information of computer-aided design (CAD) files. Polymeric, metallic, ceramic, and even composite materials can be processed, employing a wide range of technologies, among which vat photopolymerization Polymers 2020, 12, 2680; doi:10.3390/polym12112680 www.mdpi.com/journal/polymers Polymers 2020, 12, 2680 2 of 14 stands out due to the degree of precision and throughput achieved [1–3]. Vat photopolymerization technologies, including laser stereolithography (SLA) and digital light processing (DLP), work with a vat of photopolymerizable resin. They use the power of a laser or a light projected through a digital mask to selectively activate the polymerization of the resin in the vat, layer by layer. SLA, a laser which usually works in the ultraviolet range, is used for precisely defining the contours and infill of the polymeric layers. Both bottom-up and top-down systems exist, depending on the movement of the 3D printing bed inside the vat. The top-down equipment offers an excellent process stability, while the bottom-up set-ups stand out for their design simplicity. Sometimes however, the latter present some limitations related to the lack of adhesion of the 3D-printed part with the printing bed [1–3]. SLA is a 3D printing technique that can produce complex 3D structures easily, rapidly, and precisely; the SLA technique allows the production of a wide range of shapes and components and is considered as a method of reference in rapid RP [2,4–7]. It is already used to manufacture microelectromechanical systems (MEMSs) and microfluidic devices [5,6,8–11], and loading the photopolymerizable resin matrix with functional fillers opens the possibility of new composite materials and devices with tailored properties [12–15]. Several studies already present the development of functional composites with improved performance related to the manufacturing of flexible component materials [16–18]. Besides this, both the surface and bulk functionalization of the essential elements may lead to final devices with enhanced properties and innovative functionalities [7–15,19,20]. This study presents the evaluation of the physicochemical properties of substrates printed by SLA using a photosensitive resin with dispersed copper-nickel nanowires (CuNi NWs) as reinforcing agents. Additionally, the methods and materials used in the design and manufacture of the selected specimens are described. Hence, the analysis and discussion of the results obtained provide useful information regarding the potential benefits of the present approach. 2. Experimental Details 2.1. Materials and Methods The base material used to manufacture the specimens was the commercial photoreactive resin “Clear FLGPCL 02” (Form 1+, Formlabs, Somerville, MA, USA) [10,11,15,19,21], which is a mixture of methacrylic acids esters (methacrylate oligomers and monomers) and photoinitiators, according to the manufacturer’s datasheet. The CuNi NWs for mass-functionalization were provided by the Surface Engineering and Nanotechnology Institute (Cranfield University, Bedford, UK) in a solution of isopropyl alcohol (IPA). The total mass of this IPA/NW ink was 9.38 g, including 0.16 g of CuNi NWs. The process used to synthesize pure Cu NWs has been previously reported by some of the authors [22]. Typically, copper chloride dihydrate and nickel acetate hexahydrate were mixed and heated at 2+ 0 190 ◦C in oleylamine. The nickel ions Ni were reduced by the oleylamine in Ni , and, subsequently, Ni0 reduced the Cu2+ ions in Cu0 in a galvanic reaction. The nickel only had a catalytic role, and pure Cu NWs were obtained. In this study, however, the temperature of the synthesis was increased to above 200 ◦C to enhance the reduction kinetics of the catalytic nickel, and, hence, some nickel was incorporated into the NWs ( 20 mol%). Consequently, CuNi alloy NWs were obtained instead of ∼ pure Cu NWs [22]; they were washed and stored in IPA to form an IPA/NW ink. The IPA/NW ink was mixed with an incline of 30◦ at high speed in the MPC 004ST vacuum casting machine (SLM Solutions, Lübeck, Germany) for 30 min, and then with a magnetic stirrer bar for 90 min to 1000 rpm in the stirring hot plates model SPA 1020B (Thermolyne Corporation, Dubuque, IA, USA); Figure1a shows the pure resin while Figure1b shows the CuNi NW-loaded resin. Cylindrical substrates with a diameter of 21.3 mm and a thickness of 3 mm were designed using the FreeCAD v0.16 software and the CAD designs were exported to Standard Tessellation Language/STereoLithography (STL) format, which is a standard file format for additive manufacturing technologies and 3D printers [19]. The fabrication of the substrates with and without CuNi NWs was Polymers 2020, 10, x FOR PEER REVIEW 3 of 14 Cylindrical substrates with a diameter of 21.3 mm and a thickness of 3 mm were designed using the FreeCAD v0.16 software and the CAD designs were exported to Standard Tessellation PolymersLanguage/STereoLithography2020, 12, 2680 (STL) format, which is a standard file format for additive3 of 14 manufacturing technologies and 3D printers [19]. The fabrication of the substrates with and without CuNi NWs was then carried out using the SLA technique with a Formlabs Form 1+ printer then[7,15,19,21]. carried It out followed using the a bottom-up SLA technique approach, with a starting Formlabs with Form the 1 printing+ printer bed [7,15 completely,19,21]. It followedimmersed a bottom-upin the resin approach, vat, and used starting a laser with to the photopolymer printing bedize completely the resin immersedlayer-by-layer in the from resin below, vat, and through used athe laser transparent to photopolymerize bottom of the the vat resin [3,15,19]. layer-by-layer Three substrates from below, of pure through resin (Figure the transparent 1c) and 3 bottomsubstrates of theof CuNi vat [ 3NW-loaded,15,19]. Three resin substrates (Figure 1d) of were pure SLA-printed, resin (Figure1 withc) and a resolution3 substrates of 100 of CuNiµm per NW-loaded layer and µ resina total (Figure printing1d) time were of SLA-printed, 34 min [19]. with a resolution of 100 m per layer and a total printing time of 34 min [19]. (a) (b) (c) (d) Figure 1.1. Pictures of (a) the pure resin, (b) the resin mixed with the IPA/NW IPA/NW ink, the SLA-printed (c) purepure resinresin substrate,substrate, andand ((dd)) thethe CuNiCuNi NW-loadedNW-loaded substrate.substrate. 2.2. Characterization Techniques The surfacesurface morphologymorphology andand elementalelemental compositioncomposition ofof thethe SLA-printed,SLA-printed, CuNiCuNi NW-loadedNW-loaded substratessubstrates were analyzedanalyzed byby scanningscanning electronelectron microscopymicroscopy (SEM)(SEM)
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