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

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)

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) inin thethe microscopemicroscope thermionicthermionic emissionemission ZEISSZEISS EVO MA10 (ZEISS, Oberkochen, Germany);Germany); from the images obtained by secondary electronselectrons and and backscattered backscattered electrons, electrons, the the elements’ elements’ identification identification and and distribution distribution on the on image the image were wereacquired acquired by electrons by electrons and andenergy energy dispersive dispersive spec spectroscopytroscopy (EDS) (EDS) with with the the detector detector Oxford Oxford X-act (Oxford(Oxford Instruments,Instruments, Abingdon, Abingdon, UK) UK) coupled coupled with with SEM. SEM. Additionally, Additionally, the 3Dthe distribution 3D distribution of the of CuNi the NWsCuNi inNWs the polymericin the polymeric matrix matrix was performed was performed with a computedwith a computed tomography tomography (CT) system (CT) system XT H 160 XT CT H µ SCAN160 CT (Nikon,SCAN (Nikon, Tokyo, Japan);Tokyo, itJapan); used anit used energy an beamenergy of beam150 kV ofand 15050 kV Aand, and 50 the μA, working and the working distance betweendistance thebetween specimen the specimen and the detector and the wasdetector set to was optimize set to theoptimize phase the contrast phase between contrast the between NWs and the theNWs polymeric and the polymeric matrix. The matrix. exposure The timeexposure was 500 timems was, where 500 fourms, where frames four (each frames one of (each540 projections) one of 540 µ wereprojections) collected were by radiography.collected by radiography. Moreover, the Moreover, voxel size the was voxel fixed size at was10 mfixed, which at 10 was μm adequate, which was to imageadequate the to selected image region.the selected region. The mechanicalmechanical propertiesproperties were were characterized characterized by by nano-indentation nano-indentation testing testing with with a Berkovich-type a Berkovich- indenter,type indenter, a force a force of 10 ofmN 10 ,mN,and and a 10a 10s creep s creep in in the the IBIS-Authority IBIS-Authoritynano-indentation nano-indentation systemsystem (Fischer-Cripps(Fischer-Cripps LaboratoriesLaboratories PtyPty Limited,Limited, Sydney,Sydney, Australia).Australia). TheThe storagestorage modulimoduli ((EE0)) andand hardnesshardness (H)(H) were obtained from 3030 indentations indentations separated separated from from each each other other by by 30 30µm. μm. A dynamicdynamic mechanicalmechanical analysis analysis (DMA) (DMA) was was carried carried out out on on a Q800 a Q800 analyzer analyzer (TA (TA Instruments Instruments Inc., NewInc., New Castle, Castle, Delaware, Delaware, USA) USA) to characterize to characterize the viscoelastic the viscoelastic behavior behavior of the of substrates.the substrates. During During the sweep tests, the sample was clamped at both ends; a frequency of 1 Hz, a ramp rate of 5 ◦C/min, a temperature range of 30 ◦C to 90 ◦C, and a force of 1 N were used. The instrument was completely calibrated in accordance with the procedures of TA Instruments. In these tests, E0 is the storage Polymers 2020, 12, 2680 4 of 14 modulus—i.e., the elastic component that measures the energy stored during one oscillation cycle—and is related to the sample stiffness; E00 is the loss modulus—i.e., the viscous component that measures the mechanical energy dissipated through molecular motion in an oscillation cycle; and tan δ is the relationship between the elastic and inelastic components (phase lag referred to as loss tangent) that arises from any of the several molecular-level loss processes such as entanglement, slip, or friction between the monomers. Furthermore, tan δ has higher values for amorphous polymers and lower values for more crystalline polymers [19,23]. The thermal behavior of the specimens manufactured in the pure and CuNi NW-loaded resin was characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using an STA 449 F5 Jupiter system (NETZSCH, Selb, Germany) operated under a nitrogen atmosphere. Samples were analyzed between 30 and 600 ◦C with a heating rate of 10◦C/min to determine their glass transition temperatures and crystallographic properties. The Raman spectroscopy characterization of the substrates manufactured in pure and CuNi NW-loaded resin was performed to determine the vibration bands of the species on their surface. The Raman spectra were acquired using the Horiba Scientific confocal spectrometer LabRam HR (HORIBA, Kyoto, Japan) equipped with a 532 nm laser and a 100 microscope objective; all the Raman × spectra were obtained with a 15 mW laser power and a grating of 600g/mm (slit aperture) for 8 s acquisition times. The LabSpec 6 software Horiba Scientific (HORIBA, Kyoto, Japan) was used for 1 Raman spectra acquisition and analysis, where each sample was scanned in the range of 25–4000 cm− . The Fourier-transformed infrared (FTIR) spectra on substrates that were SLA-printed in pure and CuNi NW-loaded resin were obtained by the attenuated total reflection (ATR) technique in the Nicolet iS50 Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). The spectra were analyzed 1 1 in terms of transmittance in the wavenumber range of 400–4000 cm− , using a resolution of 4 cm− and an optical velocity of 0.1581cm/s; this analysis was used to determine the absorption bands of the organic functional groups on the surface of the substrates. X-ray diffraction (XRD) patterns of the substrates in pure and CuNi NWs-loaded resin were acquired in the powder diffractometer Bruker model D8 Advance with DaVinci geometry under the following conditions: a 40 kV voltage, a 0.6 mm divergence gap, a LynxEye linear detector, a 0.02035◦ step, an acquisition time of 0.6 s per step, and a range of 3.5–70◦. The qualitative analysis of the phases was performed through the comparison of the experimental XRD patterns with the database PDF-2 (2014) of the International Centre for Diffraction Data (ICDD).

3. Results and Discussion

3.1. Cost Considerations The manufacturing time of a cylindrical specimen, with or without CuNi NWs, was approximately 340 s—i.e., 11 s per layer of 3.6 cm2. In the present case, considering only the chemicals used for the synthesis and washing of the NWs, the cost for the CuNi NWs reaches 0.45 €/mg, whereas it is 0.14 €/mL for the pure resin. Hence, the manufacturing cost of a functional device containing CuNi NWs greatly depends on their content. In this study, with a load of 0.36% w/w, the cost of each specimen of pure resin was approximately 0.14 €, while it was 1.65 € for those manufactured with CuNi NWs.

3.2. Morphology The micrograph in Figure2a in secondary electron mode shows the topographical image of the surface of an SLA-printed, CuNi NW-loaded specimen, with yellow arrows pointing at CuNi NW aggregates embedded in the photopolymerized resin. These NW aggregates are even better highlighted in backscattered electron mode (Figure2b) thanks to the large atomic number di ﬀerence between Cu/Ni and the main elements in the resin, carbon (C) and oxygen (O) (the heavier the atomic number, Polymers 2020,, 1210,, 2680x FOR PEER REVIEW 5 of 14

between Cu/Ni and the main elements in the resin, carbon (C) and oxygen (O) (the heavier the atomic thenumber, brighter the thebrighter image). the Hence, image). SEM Hence, observations SEM obse evidencervations theevidence presence the of presence CuNi NW of aggregatesCuNi NW randomlyaggregates dispersed randomly in dispersed the polymeric in the matrix, polymeric which ma explainstrix, which the explains absence the of percolation. absence of percolation.

Figure 2.2.SEM SEM images images on theon surfacethe surface of a CuNi of a NW-loaded CuNi NW-loaded substrate; substrate; (a) secondary (a) electronsecondary micrograph, electron withmicrograph, yellow arrows with yellow indicating arrows the position indicating of the the CuNi position NWs aggregates;of the CuNi and NWs (b) backscattered aggregates; electronand (b) micrograph,backscattered which electron shows micrograph, the regions which analyzed shows by the EDS. regions analyzed by EDS.

EDS was also used on the surfacesurface thethe CuNiCuNi NW-loadedNW-loaded substratesubstrate to analyzeanalyze its elementalelemental composition. The results are summarized in Table 1 1 and and thethe locationslocations ofof thethe EDSEDS analysesanalyses areare shownshown on FigureFigure2 2b,b, with with spectra spectra1 1to to4 corresponding4 corresponding to CuNito CuNi NW NW aggregates aggregates and and spectrum spectrum5 to pure5 to resin.pure Inresin. all theIn all spectra, the spectra, Cu and Cu Ni and correspond Ni correspond to the NWs,to the and NWs, C andand OC correspondand O correspond to the resin. to the Cu resin. and Cu Ni areand not Ni detectedare not detected in the region in the of spectrumregion of 5spectrum(pure resin), 5 (pure which resin), means which that the means NWs that were the not NWs degraded were andnot degraded dissolved and in the dissolved polymeric in the matrix polymeric during matrix the manufacturing during the manufacturing process (at least process notsigniﬁcantly, (at least not thesignificantly, EDS detection the EDS limit detection being 0.1 limit wt.%). being 0.1 wt. %).

Table 1. AtomicAtomic contents contents (in (in%) of% the) of elements the elements detected detected by EDS byon the EDS surface on the of surfacea CuNi NW-loaded of a CuNi NW-loadedsubstrate. substrate.

SpectrumSpectrum (At (At% %)) ElementElement 1𝟏 2𝟐 𝟑 3𝟒 4𝟓 5 C kC k 83.683.6 85.585.5 84.3 84.3 84.0 84.0 87.3 87.3 O kO k 14.914.9 12.712.7 14.2 14.2 9.1 9.112.7 12.7 Cu kCu k 1.01.0 1.41.4 1.0 1.0 5.7 5.7- - Ni kNi k 0.50.5 0.40.4 0.5 0.5 1.2 1.2- -

In Figure3 3,, thethe X-ray X-ray tomogram tomogram showsshows thethe 3D3D distributiondistribution ofof thethe CuNiCuNi NWNW aggregatesaggregates inin thethe photopolymerized resinresin matrixmatrix afterafter SLASLA printing.printing. TheThe brightness of an X-ray tomogram depends on the atomic number of thethe elementselements constitutingconstituting thethe sample, with a higher atomic number giving a brighter image [[19,24–26].19,24–26]. Given the large atomic number difference difference between CuCu/Ni/Ni and the main elements constituting the resin (C and O), it was possible to map a random 3D distribution of the CuNi NWs NWs in in the the polymeric polymeric matrix. matrix. In the In theanalyzed analyzed region region of 50.6 of 50.6 mmmm, the3 ,detected the detected volume volume of CuNi of CuNiNWs approximately NWs approximately corresponds corresponds to 0.06 to vol.0.06 %.vol Moreover,.%. Moreover, Figure Figure3 confirms3 confirms the SEM the results SEM results(Figure (Figure2) at a 23D) at level. a 3D level.The CuNi The CuNi NWs NWs form form randomly randomly dispersed dispersed aggregates aggregates in in the the polymeric polymeric matrix,matrix, preventing theirtheir percolation.percolation. To concludeconclude onon morphologicalmorphological considerations,considerations, duedue toto randomlyrandomly disperseddispersed aggregatesaggregates without percolation, the CuNi NWs are expected to influenceinfluence the thermo-mechanical properties [[19,27,28]19,27,28] of the polymer polymer matrix matrix but but not not its its electrical electrical properties properties [19,29]. [19,29 The]. The aggregation aggregation phenomenon phenomenon is likely is likely due dueto the to resin’s the resin’s high high viscosity viscosity and andthe presence the presence of po oflar polar groups groups on the on thepolymeric polymeric chains chains [19,24–26]. [19,24–26 In]. Inany any case, case, a homogeneous a homogeneous dispersion dispersion of the of theNWs NWs in the in thepolymeric polymeric matrix matrix without without aggregates aggregates is the is key to reach achieve percolation with a minimal NW content. Moreover, this would lead to an

Polymers 2020, 12, 2680 6 of 14 Polymers 2020, 10, x FOR PEER REVIEW 6 of 14 optimizedthe key to reachmanufacturability achieve percolation (closer with to athe minimal pure NWresin), content. a lower Moreover, cost (see this Section would lead3.1), toand an homogeneousoptimized manufacturability properties in the (closer 3D-printed to the pure composites resin), a lower[19]. cost (see Section 3.1), and homogeneous properties in the 3D-printed composites [19].

Polymers 2020, 10, x FOR PEER REVIEW 6 of 14

optimized manufacturability (closer to the pure resin), a lower cost (see Section 3.1), and homogeneous properties in the 3D-printed composites [19].

(a) (b)

Figure 3. (a) 3D3D renderingrendering ofof the the CuNi CuNi NW-loaded NW-loaded resin resin and and (b )(b magniﬁcation) magnification of theof the region region in the in redthe redrectangle rectangle in (a in). (a).

3.3. Mechanical Mechanical and Thermal Analysis Mappings ofof thethe mechanicalmechanical properties propertiesE 0E(Figure (Figure4a,b) 4a,b) and andH (Figure H (Figure4c,d) 4c,d) were obtainedwere obtained from fromnanoindentation nanoindentation tests on tests the on surfaces the surfaces of pure of and pure CuNi and NW-loaded, CuNi NW-loaded, SLA-printed SLA-printed substrates. substrates. The total 2 surface area of the mappings is 120 150(a)µ m , and the variations ( inb) the E0 and H values are represented The total surface area of the mappings× is 120 × 150 μm , and the variations in the E and H values areby colorrepresentedFigure scales, 3. (a with)by 3D color rendering a darker scales, colorof withthe correspondingCuNi a darker NW-loaded color to correspondingresin a higher and value.(b) magnification to The a higher advantage of value. the region such The mappings inadvantage the is suchto directly redmappings rectangle and visually is in to(a ).directly assess localand variationsvisually assess in the local mechanical variations properties. in the mechanical Moreover, theproperties. average valuesMoreover, extracted the average from these values mappings extracted can befrom representative these mappings of the can entire be specimens representative if the studiedof the entire areas specimensare3.3. large Mechanical enough if the and studied (Figure Thermal 5areas)[ Analysis19, 30are,31 la ].rge enough (Figure 5) [19,30,31]. In the case of the pure resin, the variations in E and H are likely due to the SLA process itself Mappings of the mechanical properties E (Figure0 4a,b) and H (Figure 4c,d) were obtained (surface roughness, surface defects, etc.). In the case of the CuNi NW-loaded substrate, two other from nanoindentation tests on the surfaces of pure and CuNi NW-loaded, SLA-printed substrates. parameters influence E and H: the NW aggregate concentration and the quality of the polymeric The total surface area of0 the mappings is 120 × 150 μm, and the variations in the E and H values matrix/NW aggregate bonding. Since there is a significant increase in E (57%) and H (50%) when are represented by color scales, with a darker color corresponding to a higher0 value. The advantage loading the resin with NWs (Figure5), it is assumed that the influence of the SLA process on E and such mappings is to directly and visually assess local variations in the mechanical properties.0 H can be neglected compared to the influence of the NWs. Moreover, as the dispersion of the NW Moreover, the average values extracted from these mappings can be representative of the entire aggregatesspecimens seemsif the studied random areas and rather are large uniform enough (Figure (Figure2), it5) can [19,30,31]. be assumed that the main factor driving the variation in E0 and H is the matrix/NW bonding.

(a) (b)

Figure 4. Cont.

Polymers 2020, 10, x FOR PEER REVIEW 7 of 14

PolymersPolymers2020 2020, ,12 10,, 2680 x( FORc) PEER REVIEW (d) 7 7of of 14 14 Figure 4. Mappings of the mechanical properties E (a,b) and H (c,d), obtained from the nanoindentation tests on the surface of the pure and CuNi NW-loaded resin substrates, respectively. A total surface area of 120 × 150 μm was analyzed on each substrate.

In the case of the pure resin, the variations in E and H are likely due to the SLA process itself (surface roughness, surface defects, etc.). In the case of the CuNi NW-loaded substrate, two other parameters influence E and H: the NW aggregate concentration and the quality of the polymeric matrix/NW aggregate bonding. Since there is a significant increase in E (57%) and H (50%) when loading the resin with NWs (Figure 5), it is assumed that the influence of the SLA process on E and H can be neglected compared to the influence of the NWs. Moreover, as the dispersion of the NW aggregates seems random and(c ) rather uniform (Figure 2), it can be(d )assumed that the main factor driving the variationFigureFigure 4. in4.Mappings MappingsE and of the ofH mechanical the is mechanicalthe matrix/NW properties propertiesE0 (a,b ) bonding. andE H(a,(bc), d),and obtained H (c, fromd), obtained the nanoindentation from the testsnanoindentation on the surface tests of the on purethe surface and CuNi of the NW-loaded pure and resinCuNi substrates, NW-loaded respectively. resin substrates, A total respectively. surface area The CuNi ofNWA 120total aggregates surface150 µm 2areawas of analyzed 120seem × 150 onto μm each act was substrate. as analyzed harden oners each inside substrate. the matrix, and a higher matrix/NW × aggregate bonding quality provides a higher load transfer from the NWs to the matrix and, hence, The CuNi NW aggregates seem to act as hardenersE insideH the matrix, and a higher matrix/NW In the case of the pureE resin, theH variations in and are likely due to the SLA process itself higher mechanicalaggregate(surface propertiesroughness, bonding quality surface ( provides anddefects, ).etc.). a higherOn In the the load contrary,case transfer of the from CuNisome the NW-loaded NW NWs aggregates to the substrate, matrix and,maytwo other hence, be only slightly infiltrated byhigherparameters the mechanicalpolymer influence propertiesand E andconstitute H (:E the0 and NW Hlocal ).aggregate On defects the concentration contrary, with some low and NW mechanicalthe quality aggregates of the properties. may polymeric be only Obtaining well-dispersedslightlymatrix/NW fillers infiltrated aggregatein a polymeric by bonding. the polymer Sincematrix and there constitute(instead is a significant localof aggregates) defectsincrease within E lowis(57% consequently mechanical) and H (50% properties.) whena major focus in Obtainingloading the well-dispersed resin with NWs fillers (Figure in a polymeric5), it is assumed matrix that (instead the influence of aggregates) of the SLA is consequently process on E a majorand many studies, as this is the key to obtain an optimized matrix/filler bonding, a maximal load transfer, focusH can in be many neglected studies, compared as this is to the the key influence to obtain of an the optimized NWs. Moreover, matrix/filler as the bonding, dispersion a maximal of the NW load and enhancedtransfer,aggregates properties and seems enhanced [19,30–34]. random properties and rather [19,30 uniform–34]. (Figure 2), it can be assumed that the main factor driving the variation in E and H is the matrix/NW bonding. The CuNi NW aggregates seem to act as hardeners inside the matrix, and a higher matrix/NW aggregate bonding quality provides a higher load transfer from the NWs to the matrix and, hence, higher mechanical properties (E and H). On the contrary, some NW aggregates may be only slightly infiltrated by the polymer and constitute local defects with low mechanical properties. Obtaining well-dispersed fillers in a polymeric matrix (instead of aggregates) is consequently a major focus in many studies, as this is the key to obtain an optimized matrix/filler bonding, a maximal load transfer, and enhanced properties [19,30–34].

Figure 5. AverageFigure 5. valuesAverage of values E (green) of E0 (green) and and HH (brown)(brown) for for the the pure pure and CuNi and NW-loadedCuNi NW-loaded substrates, substrates, corresponding to the mappings in Figure4. corresponding to the mappings in Figure 4.

Figure6 presents the evolution of E0 and tan δ as a function of the temperature, between 30 ◦C Figure and6 presents90Figure◦C, for5. Averagethe pure evolution and values CuNi of E NW-loadedof (green) E andand substrates.H tan (brown) δ as for Figure athe function pure6a and shows CuNi ofthat theNW-loaded Etemperature,0 decreases substrates, for bothbetween 30 °C substrates when the temperature increases between 30 C and 60 C, highlighting a glass transition and 90 °C, for purecorresponding and CuNi to the mappings NW-loaded in Figure 4.substrates. ◦ Figure ◦6a shows that E decreases for both region. Moreover, E0 is significantly higher for the pure resin than for the CuNi NW-loaded one in this substrates whentemperatureFigure the temperature 6 range, presents and the the evolution maximumincreases of EE0 betweenvalues,and tan measured δ as30 a function °C at and30 ◦ofC ,60theare temperature,°C600.2, highlightingand 300.1 betweenMPa afor30 glass the°C transition region. Moreover,pureand and90 °C CuNiE, for is NW-loadedpure significantly and CuNi substrates, NW-loaded higher respectively. for substrates. the Furthermore,pure Figure re sin6a showsfactorsthan that suchfor Ethe as decreases the CuNi reaction NW-loadedfor degree both one in this temperatureandsubstrates the range, crosslinking when and the densitytemperature the maximum mainly increases influence E between values, the value 30 °Cmeasured of Eand0. The 60 °C glass, athighlighting transition30 °C, are processa glass 600.2 transition is thereby and 300.1 MPa confirmed,region. Moreover, suggesting E that is significantly the behavior higher of the for material the pure with resin and than without for the CuNi CuNi NWs NW-loaded can be evaluated one in throughthis temperature the rheological range, properties.and the maximum E values, measured at 30 °C, are 600.2 and 300.1 MPa

Polymers 2020, 10, x FOR PEER REVIEW 8 of 14 for the pure and CuNi NW-loaded substrates, respectively. Furthermore, factors such as the reaction degree and the crosslinking density mainly influence the value of E. The glass transition process is thereby confirmed, suggesting that the behavior of the material with and without CuNi NWs can be evaluated through the rheological properties.

In Figure 6b, the maximum values of tan δ correspond to the glass transition temperatures (T), 57.9 and 52.8 °C, of the pure and CuNi NW-loaded resins, respectively (see also Table 2) [35].

Loading the polymeric matrix with CuNi NWs lowers the T, which could be due to a poor resin matrix/NW aggregate bonding. Indeed, a lack of chemical interaction between the resin and the CuNi NWs can decrease the crosslinking density and lead to a higher mobility of the polymeric chains in the composite, resulting in a lower T [19,35–39]; this is confirmed by a sharper peak around the maximum value of tan δ for the pure resin, which indicates a more ordered structure due to a higher crosslinking density. The addition of CuNi NWs highly influenced the molecular dynamics in the polymeric matrix, which resulted in a reduction in T and E [35,36]. The DMA results show a more elastic behavior of the CuNi NW-loaded substrate compared to the pure resin one, suggesting the enhanced stiffness Polymersof the material.2020, 12, 2680 8 of 14

(a)

(b)

Figure 6.6. DMA curves of the pure (0) and CuNiCuNi NW-loadedNW-loaded (1)(1) substratessubstrates forfor ((aa))E E0 and and (b ()btan) tanδ, δ as, as a functiona function of of temperature. temperature.

InThe Figure DSC 6curvesb, the in maximum Figure 7a values show a of broadtan δglasscorrespond transition to region the glass between transition 32 and temperatures 135 °C for

(theTg), pure57.9 andsubstrate52.8 ◦(CT, of=60.8 the pure °C) and and between CuNi NW-loaded 31 and 97 resins, °C for respectively the CuNi NW-loaded (see also Table one2)[ (T35 =]. Loading the polymeric matrix with CuNi NWs lowers the T , which could be due to a poor resin 51.6 °C) [35]; thus, the glass transition region and T slightly decreasedg when loading the resin with matrixCuNi NWs,/NW aggregate and the results bonding. were Indeed, in agreement a lack of with chemical the DMA interaction analysis between (see Table the resin 2). Moreover, and the CuNi the NWs can decrease the crosslinking density and lead to a higher mobility of the polymeric chains in the composite, resulting in a lower Tg [19,35–39]; this is conﬁrmed by a sharper peak around the maximum value of tan δ for the pure resin, which indicates a more ordered structure due to a higher crosslinking density.

Table 2. Values determined from the DMA, DSC, and TGA analysis.

Transition and Degradation Temperatures (◦C) CuNi NWs Content Tg DMA Tg DSC − − 0.00% w/w 57.9 60.8 0.36% w/w 52.8 51.6

The addition of CuNi NWs highly inﬂuenced the molecular dynamics in the polymeric matrix, which resulted in a reduction in Tg and E0 [35,36]. The DMA results show a more elastic behavior of the CuNi NW-loaded substrate compared to the pure resin one, suggesting the enhanced stiﬀness of the material. Polymers 2020, 10, x FOR PEER REVIEW 9 of 14

crystallization temperature, corresponding to the maximum of the first exothermal peak, is lower for the CuNi NW-loaded composite (ca. 121 °C) than for the pure resin (ca. 160 °C). The thermal stability of the samples was investigated by TGA between 32 and 600 °C under an

N atmosphere (Figure 7b); the slight mass loss between 80 and 100 °C may be due to the evaporation of adsorbed IPA in the resin, although most of it should have evaporated during the manufacturing process. The weight loss between 100 and 300 °C was mainly attributed to the evaporation of physiosorbed and chemisorbed water [35]. Finally, most of the thermal degradation of the samples occurring between 300 and 500 °C was likely due to the evaporation of organic Polymersspecies2020 from, 12 ,the 2680 resin (e.g., methacrylic acid and ester) [19,35–39]. Moreover, as the TGA curves9 of are 14 similar for both materials, it can be assumed that the NWs were not significantly degraded during the heating ramp and that the weight loss was mainly driven by the polymeric matrix degradation. TheA mechanical DSC curves and in Figurethermal7a analysis show a broad of pure glass and transition CuNi NW-loaded, region between SLA-printed32 and 135 samples◦C for was the purecarried substrate out. It ( Tseemsg = 60.8 that◦C the) and addition between of31 NWsand 97 to◦ Cthefor polymeric the CuNi NW-loaded matrix increased one (Tg the= 51.6 mechanical◦C)[35]; thus,surface the properties glass transition (Figures region 4 and and 5) thanksTg slightly to a load decreased transfer when phenomenon. loading the However, resin with it decreased CuNi NWs, the andthermomechanical the results were bulk in agreement properties with (Figures the DMA 6 and analysis 7) due (see to Table interfacial2). Moreover, defects the in crystallizationthe composite temperature,material (weak corresponding NW/matrix bonding). to the maximum of the ﬁrst exothermal peak, is lower for the CuNi NW-loaded composite (ca. 121 ◦C) than for the pure resin (ca. 160 ◦C).

(a) (b)

FigureFigure 7.7. ThermalThermal analysis of the pure (0) and CuNi NW-loaded (1)(1) substratessubstrates by (a)) DSC,DSC, ((b)) TGA,TGA, andand DTGADTGA (insert)(insert) asas aa functionfunction ofof thethe temperature.temperature.

The thermal stability of the samples was investigated by TGA between 32 and 600 C under an Table 2. Values determined from the DMA, DSC, and TGA analysis. ◦ N2 atmosphere (Figure7b); the slight mass loss between 80 and 100 ◦C may be due to the evaporation of adsorbed IPA in the resin, althoughTransition most of it shouldand Degradation have evaporated Temperatures during the(°C) manufacturing CuNi NWs Content 𝐓 −𝐃𝐌𝐀 𝐓 −𝐃𝐒𝐂 process. The weight loss between 100 and 𝐠300 ◦C was mainly attributed𝐠 to the evaporation of physiosorbed and0.00% chemisorbed w⁄ w water [35]. Finally,57.9 most of the thermal degradation60.8 of the samples 0.36% w⁄ w 52.8 51.6 occurring between 300 and 500 ◦C was likely due to the evaporation of organic species from the resin (e.g., methacrylic acid and ester) [19,35–39]. Moreover, as the TGA curves are similar for both materials, 3.4. Spectroscopic Characterization it can be assumed that the NWs were not signiﬁcantly degraded during the heating ramp and that the weightThe loss Raman was mainly spectra driven obtained by the on polymericthe substrates matrix manufactured degradation. in the “Clear FLGPCL 02” pure resinA and mechanical CuNi NW-loaded and thermal resin analysis at 0.36% of w pure⁄ w areand presented CuNi NW-loaded, in Figure 8, SLA-printed where Cu and samples Ni present was carriedvibrational out. modes It seems at very that low the frequencies addition of (phonon NWs to thefrequencies, polymeric ~10 matrix cm increased). the mechanical surfaceThe properties spectrum (Figures of Figure4 and 8 from5) thanks the surface to a load of th transfere pure resin phenomenon. substrate reveals However, three it decreasedintense Raman the thermomechanicalpeaks at 1441 (strong), bulk properties 2928, and (Figures 29486 cm and7 )(very due tointense) interfacial that defects correspond in the compositeto the C−H material bond (weakvibrations. NW/ matrixConsequently, bonding). the medium and weak peaks at 1560 and 1590 cm are attributed to the C−N−H and C−NO bond vibrations, respectively. The peaks at 595 cm (medium) and 3.4.1608 Spectroscopic cm (weak) Characterization correspond to the C−C and C=C bond vibrations, respectively. The medium peaksThe at Raman1632 and spectra 1712 obtained cm refer on theto the substrates C=O symmetric manufactured and inantisymmetric the “Clear FLGPCL bond vibrations, 02” pure resinand the and one CuNi at NW-loaded1396 cm resinto the at CH0.36 %asymmetricw/w are presented bond stretching in Figure vibration.8, where CuThe and band Ni presentbetween vibrational785 and modes980 cm atvery corresponds low frequencies to the (phononC−O−C frequencies, bond vibrations,10 cm the1). one between 1000 and ∼ − The spectrum of Figure8 from the surface of the pure resin substrate reveals three intense

1 Raman peaks at 1441 (strong), 2928, and 2948 cm− (very intense) that correspond to the C-H bond 1 vibrations. Consequently, the medium and weak peaks at 1560 and 1590 cm− are attributed to the 1 1 C-N-H and C-NO2 bond vibrations, respectively. The peaks at 595 cm− (medium) and 1608 cm− (weak) correspond to the C-C and C = C bond vibrations, respectively. The medium peaks at 1 1632 and 1712 cm− refer to the C=O symmetric and antisymmetric bond vibrations, and the one at 1 1 1396 cm− to the CH3 asymmetric bond stretching vibration. The band between 785 and 980 cm− 1 corresponds to the C-O-C bond vibrations, the one between 1000 and 1160 cm− to the C-O-C bond 1 asymmetric stretching vibrations, and the one between 1300 and 1380 cm− to the C-CH3 bond Polymers 2020, 10, x FOR PEER REVIEW 10 of 14

1160 cm to the C−O−C bond asymmetric stretching vibrations, and the one between 1300 and 1380 cm to the C−CH bond vibrations. Additionally, the weak peaks at 2650 − 2810 and 3050 − 3500 cm are attributed to the O−C−H and O−H bond vibrations, respectively. Raman spectroscopy reveals the surface interaction between organic materials and metallic nanostructures through surface-enhanced Raman scattering. Figure 8 shows similar Raman spectra from the pure resin and CuNi NW-loaded resins; however, it is possible to observe intensification in vibrational signals corresponding to the C−H (2928 cm − 2948 cm) and O−H (3050 cm − 3500 cm) vibrational modes [19,35,40–42]. This is evidence of the surface interaction between the

Polymerspolymeric2020 ,resin12, 2680 and CuNi NWs through the C−H and O−H functional groups. Furthermore,10 of 14 metallic structures such as Cu and Ni do not present vibrational frequencies (phonon frequencies) in the range of wavenumbers observed in this study. Thus, this surface interaction can be responsible 1 vibrations.for the difference Additionally, in T before the weak and peaks after at doping.2650–2810 Additionally,and 3050–3500 thesecm studies− are attributedallowed us to to the identifyO-C-H anddifferences O-H bond in the vibrations, degree of respectively. crystallinity due to the influence of the doping agents.

Figure 8. RamanRaman spectra spectra acquired acquired on onthe the substrates substrates printed printed with with (0) pure (0) pure resin resin and (1) and CuNi (1) CuNiNW- NW-loadedloaded resin. resin.

RamanThe characteristic spectroscopy Raman reveals spectrum the surface of the photop interactionolymer between resin matrix organic shows materials a main and peak metallic with a nanostructuresmaximum intensity through of 1780 surface-enhanced counts in the Ramanpure resin scattering. and 1834 Figure counts8 shows in the similar one with Raman nanowires; spectra fromthe peak the purealso resinexhibits and CuNia wavelength NW-loaded shift resins; from however, 2926 to it is2930 possible cm to with observe the intensificationaddition of the in 1 1 1 1 vibrationalnanofillers. signalsThe increase corresponding in the intensification to the C-H (2928 of thecm peak− –2948 withcm nanowires− ) and O-H corresponds(3050 cm− to– 3500the energycm− ) vibrationaldifference in modes the vibrational [19,35,40–42 state]. This of the is evidence methacryla of thete surfacemolecule, interaction which is between related both the polymeric to the level resin of andpolymer CuNi structuring NWs through and the to C-Helectromagneticand O-H functional intensification groups. due Furthermore, to the plasmonic metallic effect structures produced such by as Cuthe andpresence Ni do of not the present nanowires vibrational in the polymer frequencies matrix (phonon. On the frequencies) other hand, in the the displacement range of wavenumbers of the peak observedto a greater in wavelength this study. Thus, indicates this a surface change interaction in the electronic can be dispersion responsible of for the the molecules difference on in theTg surfacebefore andof the after polymer, doping. which Additionally, implies evident these studies structural allowed changes us toin identifythe molecular differences bonds in and the degreea greater of crystallinitycrosslinking duethat tocorroborate the influence the ofthermo-mechanical the doping agents. results shown. TheIn order characteristic to determine Raman if the spectrum increasing of aggregate the photopolymer of nickel-copper resin matrix nanowires shows affects a main the peak chemical with astructure maximum of the intensity photopolymer of 1780 counts resin, inFTIR the puremeasurements resin and 1834were countsconducted in the to one determine with nanowires; whether 1 thealterations peak also occur exhibits in the a wavelength main chemical shift fromgroups2926 of tothe2930 polymer.cm− with The the results addition show of theno nanofillers.alterations Theattributable increase to in the the presence intensification of the CuNi of the NWs peak in with the nanowires concentration corresponds studied— to 0.36% the energy w⁄ w; ditherefore,fference inthe the polymer vibrational adequately state of fulfills the methacrylate its support function molecule, as a which nanomaterial is related (Figure both to 9). the level of polymer structuring and to electromagnetic intensification due to the plasmonic effect produced by the presence of the nanowires in the polymer matrix. On the other hand, the displacement of the peak to a greater wavelength indicates a change in the electronic dispersion of the molecules on the surface of the polymer, which implies evident structural changes in the molecular bonds and a greater crosslinking that corroborate the thermo-mechanical results shown. In order to determine if the increasing aggregate of nickel-copper nanowires affects the chemical structure of the photopolymer resin, FTIR measurements were conducted to determine whether alterations occur in the main chemical groups of the polymer. The results show no alterations attributable to the presence of the CuNi NWs in the concentration studied 0.36% w/w; therefore, the polymer adequately fulfills its support function as a nanomaterial (Figure9). Figure9 presents the FT-IR spectrum of the substrates manufactured in pure resin and CuNi 1 NW-loaded resin, where there are distinct absorption bands from 1150 to 1250 cm− , which can be 1 attributed to the C-O-C stretching vibrations. The two bands at 1386 and 748 cm− can be attributed to 1 the α-methyl group vibrations. Moreover, the bands at 865, 1297, 1367, and 1405 cm− can be attributed 1 to the C-C, O-CH2, C-H, and C-OH stretching vibrations, respectively. The band at 1700 cm− indicates 1 the presence of the ester group C=O stretching vibrations. The band at 1451 cm− can be attributed to the bending vibrations of the C-H bonds of the CH3 groups. The three bands at 2952, 2932, 1 − and 2862 cm− can be assigned to the C-H bond stretching vibrations. Furthermore, there are two weak Polymers 2020, 10, x FOR PEER REVIEW 11 of 14

Polymers 2020, 12, 2680 11 of 14 absorption bands at 3375 and 1637 cm 1, which can be attributed to the OH group stretching and − − bending vibrations, respectively. Notwithstanding this, there are no appreciable differences between the FTIR spectra of the pure resin and the CuNi NW-loaded resin because the resin does not form any covalentFigure bond 9. FTIR with spectra this type acquired of nano-additives; on the substrates additionally, printed with the (0) pu nanowiresre resin and do (1) not CuNi add NW-loaded new functional groups,resin. which can be observed with infrared. According to the above discussions, it can be concluded that the photoreactive polymer resin is composed of a macromolecular methacrylic acid (MAA) and a methylPolymersFigure 2020 methacrylate, 109 ,presents x FOR PEER (MMA) the REVIEW FT-IR mixture spectrum [19,35 ,of39 ,the43,44 substrates]. manufactured in pure resin and 11CuNi of 14 NW-loaded resin, where there are distinct absorption bands from 1150 to 1250 cm, which can be attributed to the C−O−C stretching vibrations. The two bands at 1386 and 748 cm can be attributed to the α -methyl group vibrations. Moreover, the bands at 865 , 1297 , 1367 , and 1405 cm can be attributed to the C−C, O−CH , C−H, and C−OH stretching vibrations, respectively. The band at 1700 cm indicates the presence of the ester group C=O stretching vibrations. The band at 1451 cm can be attributed to the bending vibrations of the C−H bonds of the −CH groups. The three bands at 2952, 2932, and 2862 cm can be assigned to the C−H bond stretching vibrations. Furthermore, there are two weak absorption bands at 3375 and 1637 cm , which can be attributed to the −OH group stretching and bending vibrations, respectively. Notwithstanding this, there are no appreciable differences between the FTIR spectra of the pure resin and the CuNi NW-loaded resin because the resin does not form any covalent bond with this type of nano-additives; additionally, the nanowires do not add new functional groups, which can be observed with infrared. According to the above discussions, it can be concluded that Figure 9. 9. FTIRFTIR spectra spectra acquired acquired on the on substrates the substrates printed printed with (0) withpure resin (0) pure and (1) resin CuNi and NW-loaded (1) CuNi the photoreactive polymer resin is composed of a macromolecular methacrylic acid (MAA) and a NW-loadedresin. resin. methyl methacrylate (MMA) mixture [19,35,39,43,44]. TheFigure semiquantitative 9 presents the analysis FT-IR spectrum of the phases of the foundsubstrates in the manufactured specimensspecimens was in pure performed resin and using CuNi the RietveldNW-loaded method. resin, whereThe The percentages percentages there are distinct determined absorption co correspondrrespond bands from to the 1150 polycrystalline to 1250 cm phases, which without can be consideringattributed to the the percentage C−O−C ofstretching amorphous vibrations. material. The The The two XRD XRD bands patterns patterns at 1386 of of Figure and 10748 prove cm thatcan thebe CuNiattributed NWs to contained the α -methyl in some group of the vibrations. substratessubstrates manufacturedmaMoreover,nufactured the bybands 3D printing at 865 have, 1297 a face-centeredface-centered, 1367 , and cubic structure (in agreement with the database PDF-02 of the ICDD). Additionally, the contents of 1405 cm can be attributed to the C−C, O−CH , C−H, and C−OH stretching vibrations, 59.8%respectively. coppercopper The andand band 40.2%40.2% atnickel nickel1700 were cmwere evaluated evaluatedindicates in thein a a semiquantitative presencesemiquantitative of the esteranalysis. analysis. group NoNo C=O didiffractionffraction stretching peak wasvibrations. observed The on band the the atXRD XRD 1451 pattern pattern cm of can of the the be “Clear attributed “Clear FL FLGPCLGPCL to the 02” bending 02” resin resin vibrationswithout without na of nanowiresnowires the C−H because because bonds it ofis it isamorphous. amorphous. the −CH groups. The three bands at 2952, 2932, and 2862 cm can be assigned to the C−H bond stretching vibrations. Furthermore, there are two weak absorption bands at 3375 and 1637 cm , which can be attributed to the −OH group stretching and bending vibrations, respectively. Notwithstanding this, there are no appreciable differences between the FTIR spectra of the pure resin and the CuNi NW-loaded resin because the resin does not form any covalent bond with this type of nano-additives; additionally, the nanowires do not add new functional groups, which can be observed with infrared. According to the above discussions, it can be concluded that the photoreactive polymer resin is composed of a macromolecular methacrylic acid (MAA) and a methyl methacrylate (MMA) mixture [19,35,39,43,44]. The semiquantitative analysis of the phases found in the specimens was performed using the Rietveld method. The percentages determined correspond to the polycrystalline phases without considering the percentage of amorphous material. The XRD patterns of Figure 10 prove that the CuNi NWs contained in some of the substrates manufactured by 3D printing have a face-centered cubicFigure structure 10. XRD (inXRD agreement patterns patterns acquired acquiredwith the on ondatabasethe the substrates substrates PDF-02 printed printed of thewith ICDD). with (0) pure (0) Additionally, pure resin resin and (1) and theCuNi (1) contents CuNiNW- of 59.8%NW-loadedloaded copper resin. and resin. 40.2% nickel were evaluated in a semiquantitative analysis. No diffraction peak was observed on the XRD pattern of the “Clear FLGPCL 02” resin without nanowires because it is The spectroscopy analysis clearly indicates some structural changes, which might be due to an amorphous. intermolecular transformation; thus, the incorporation of the nanowires affects the polymerization process, decreasing the scattering and crosslinking points. The results obtained allowed us to know the morphologic, mechanical, thermal, and spectrometric behavior of photopolymers resins, with and without CuNi NWs, used for 3D printing by the SLA technique. Notwithstanding this, the effect of CuNi NWs was not as predominant as expected. The reason for this is that many parameters

Figure 10. XRD patterns acquired on the substrates printed with (0) pure resin and (1) CuNi NW- loaded resin.

Polymers 2020, 12, 2680 12 of 14 involved in the manufacturing process, such as the concentration and dispersion of the CuNi NWs, the specimens’ size, and the polymerizing process interaction between the laser of the printer and the CuNi NWs, caused the physicochemical properties to not improve signiﬁcantly with regard to the pure resin. Moreover, the highlight of this research was that specimens of cylindrical geometry reinforced with CuNi NWs were successfully manufactured by 3D printing through the SLA technique.

4. Conclusions This paper presents the manufacturing of new polymeric resin substrates functionalized with CuNi NWs by the SLA technique. Copper-nickel alloy nanowires were successfully dispersed in the “Clear FLGPCL 02” resin, which is a typical resin used in 3D printing by the stereolithography technique. Besides the functionalization improvements, the manufacturability of three-dimensional objects by the SLA technique (3D printing) using a mass-functionalized photopolymer with CuNi NWs is demonstrated. Moreover, a homogeneous 3D distribution of the CuNi NWs inside the polymeric matrix was achieved, and the presence of the nanowires correlated with the modification of the mechanical and thermal properties of the resin. It was also demonstrated that the addition of CuNi NWs reduces the crosslinking density and leads to the higher mobility of the polymeric chains during the glass transition. Furthermore, both the melting and degradation temperatures remained unchanged below 600 ◦C, which was expected as Cu and Ni have higher melting points (1085 and 1455 ◦C, respectively). This study underscores the possible use of the SLA technique, in combination with functional nanomaterials, in the fabrication of new precision devices and for wearable electronics such as advanced transducers, flexible sensors, circuit/devices using self-assembly approaches, robotic parts, microfluidic devices, and microelectromechanical systems.

Author Contributions: E.D.V.-N. and A.D.L. conceived, designed, and performed the experiments; analyzed the data; and wrote the paper. Q.L. supported with the synthesis of the nanowires. E.M.-O., R.C.H., and G.R.-C. worked on X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy, dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). H.A.E.D. worked on the Raman spectroscopy and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). J.L.E. managed and coordinated the overall research. All the authors contributed to the discussion of the results. All authors have read and agreed to the published version of the manuscript. Funding: This work was partially financed by the Colombian agency Colciencias through doctoral scholarship 617. This work was partly funded by the Spanish MINECO Retos project SUSANA (PID2019-108103RB-C31). The support from project Frontiers2020 funded by the Basque Government under the ELKARTEK call is also gratefully acknowledged. Acknowledgments: The authors express their gratitude to the Laboratorio de Difracción de Rayos X, Laboratorio de Espectroscopia Atómica y Molecular, Centro de Investigación Científica y Tecnológica en Materiales y Nanociencia, Parque Tecnológico Guatiguará, Universidad Industrial de Santander. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Ford, S.; Despeisse, M. Additive manufacturing and sustainability: An exploratory study of the advantages and challenges. J. Clean. Prod. 2016, 137, 1573–1587. [CrossRef] 2. Jacobs, P.F. Stereolithography and Other RP&M Technologies: From Rapid Prototyping to Rapid Tooling; Society of Manufacturing Engineers: Dearborn, MI, USA, 1996. 3. Bhushan, B.; Caspers, M. An overview of additive manufacturing (3D printing) for microfabrication. Microsyst. Technol. 2017, 23, 1117–1124. [CrossRef] 4. Brennan-Craddock, J.; Brackett, D.; Wildman, R.; Hague, R. The design of impact absorbing structures for additive manufacture. J. Phys. Conf. Ser. 2012, 382, 1–7. [CrossRef] 5. Ho, K.S.; Bradley, R.J.; Billson, D.R.; Hutchins, D.A. Micro-stereolithography as a transducer design method. Ultrasonics 2008, 48, 1–5. [CrossRef] 6. Maruo, S.; Ikuta, K. Submicron stereolithography for the production of freely movable mechanisms by using single-photon polymerization. Sens. Actuators A Phys. 2002, 100, 70–76. [CrossRef] Polymers 2020, 12, 2680 13 of 14

7. Wang, J.; Goyanes, A.; Gaisford, S.; Basit, A.W. Stereolithographic (SLA) 3D printing of oral modified-release dosage forms. Int. J. Pharm. 2016, 503, 207–212. [CrossRef] 8. Zhang, X.; Jiang, X.N.; Sun, C. Micro-stereolithography for MEMS. In Proceedings of the 1998 ASME International Mechanical Engineering Congress and Exposition, Anaheim, CA, USA, 15–20 November 1998. 9. Wicker, R.B.; Medina, F.; Ranade, A.; Palmer, J.A. Embedded micro-channel fabrication using lines can stereolithography. Assem. Autom. 2005, 25, 316–329. [CrossRef] 10. Díaz Lantada, A.; Rebollo, M. Towards low-cost effective and homogeneous thermal activation of shape memory polymers. Materials 2013, 6, 5447–5465. [CrossRef] 11. Díaz Lantada, A.; de Blas Romero, A.; Tanarro, E.C. Micro-vascular shape-memory polymer actuators with complex geometries obtained by laser stereolithography. Smart Mater. Struct. 2016, 25, 1–10. [CrossRef] 12. de Leon, A.C.; Chen, Q.; Palaganas, N.B.; Palaganas, J.O.; Manapat, J.; Advincula, R.C. High performance polymer nanocomposites for additive manufacturing applications. React. Funct. Polym. 2016, 103, 141–155. [CrossRef] 13. Kumar, S.; Hofmann, M.; Steinmann, B.; Foster, E.J.; Weder, C. Reinforcement of stereolithographic resins for rapid prototyping with cellulose nanocrystals. ACS Appl. Mater. Interfaces 2012, 4, 5399–5407. [CrossRef] [PubMed] 14. Sandoval, J.H.; Wicker, R.B. Functionalizing stereolithography resins: Effects of dispersed multi-walled carbon, nanotubes on physical properties. Rapid Prototyp. J. 2006, 12, 292–303. [CrossRef] 15. V-Niño, E.D.; Endrino-Armenteros, J.L.; Estupiñan-Duran, H.A.; Pérez-Gutiérrez, B.; Díaz Lantada, A. Caracterización microscópica de texturas superficiales fabricadas aditivamente mediante estereolitografía laser. Respuestas 2016, 21, 37–47. [CrossRef] 16. Corrado, A.; Polini, W. Measurement of high flexibility components in composite material by touch probe and force sensing resistors. J. Manuf. Process. 2019, 45, 520–531. [CrossRef] 17. Li, H.; Shin, K.; Henkelman, G. Effects of ensembles, ligand, and strain on adsorbate binding to alloy surfaces. J. Chem. Phys. 2018, 149, 1–8. [CrossRef][PubMed] 18. Wang, M.; Xie, R.; Chen, Y.; Pu, X.; Jiang, W.; Yao, L. A novel mesoporous zeolite-activated carbon composite as an effective adsorbent for removal of ammonia-nitrogen and methylene blue from aqueous solution. Bioresour. Technol. 2018, 268, 726–732. [CrossRef] 19. V-Niño, E.D.; Díaz Lantada, A.; Lonne, Q.; Estupiñán Durán, H.A.; Mejía-Ospino, E.; Ramírez-Caballero, G.; Endrino, J.L. Manufacturing of polymeric substrates with copper nanofillers through laser stereolithography technique. Polymers 2018, 10, 1325. [CrossRef] 20. Kirihara, S.; Miyamoto, Y.; Takenaga, K.; Wada Takeda, M.; Kajiyama, K. Fabrication of electromagnetic crystals with a complete diamond structure by stereolithography. Solid State Commun. 2002, 121, 435–439. [CrossRef] 21. Inverardi, N.; Pandini, S.; Bignotti, F.; Scalet, G.; Marconi, S.; Auricchio, F. Temperature-memory effect in 3D printed photopolymers with broad glass transition. AIP Conf. Proc. 2018, 1981, 1–10. 22. Lonne, Q.; Endrino, J.L.; Huang, Z. UV treatment of flexible copper nanowire mesh films for transparent conductor applications. Nanoscale Res. Lett. 2017, 12, 1–8. [CrossRef] 23. Cohen, S.R.; Kalfon-Cohen, E. Dynamic nano-indentation by instrumented nano-indentation and force microscopy: A comparative review. Beilstein J. Nanotechnol. 2013, 4, 815–833. [CrossRef][PubMed] 24. Cnudde, V.; Jacobs, P.J.S. Monitoring of weathering and conservation of building materials through non-destructive X-ray computed microtomography. Environ. Geol. 2004, 46, 477–485. [CrossRef] 25. Hong, G.; Choi, S. Rapid self-sealing of cracks in cementitious materials incorporating superabsorbent polymers. Constr. Build. Mater. 2017, 143, 366–375. [CrossRef] 26. Garcea, S.C.; Sinclair, I.; Spearing, S.M.; Withers, P.J. Mapping fibre failure in situ in carbon fibre reinforced polymers by fast synchrotron X-ray computed tomography. Compos. Sci. Technol. 2017, 149, 81–89. [CrossRef] 27. Crosby, A.J.; Lee, J.-Y. Polymer nanocomposites: The “nano” effect on mechanical properties. Polym. Rev. 2007, 47, 217–229. [CrossRef] 28. Bansal, A.; Yang, H.; Li, C.; Benicewicz, B.C.; Kumar, S.K.; Schadler, L.S. Controlling the thermomechanical properties of polymer nanocomposites by tailoring the polymer-particle interface. J. Polym. Sci. Part B Polym. Phys. 2006, 44, 2944–2950. [CrossRef] 29. Ye, S.; Rathmell, A.R.; Chen, Z.; Stewart, I.E.; Wiley, B.J. Metal nanowire networks: The next generation of transparent conductors. Adv. Mater. 2014, 26, 6670–6687. [CrossRef] Polymers 2020, 12, 2680 14 of 14

30. Chakraborty, A.K.; Plyhm, T.; Barbezat, M.; Necola, A.; Terrasi, G.P. Carbon nanotube (CNT)–epoxy nanocomposites: A systematic investigation of CNT dispersion. J. Nanopart. Res. 2011, 13, 6493–6506. [CrossRef] 31. Gayle, A.J.; Cook, R.F. Mapping viscoelastic and plastic properties of polymers and polymer-nanotube composites using instrumented indentation. J. Mater. Res. 2016, 31, 2347–2360. [CrossRef] 32. Sandler, J.; Shaffer, M.S.P.; Prasse, T.; Bauhofer, W.; Schulte, K.; Windle, A.H. Development of a dispersion process for carbon nanotubes in an epoxy matrix and the resulting electrical properties. Polymer 1999, 40, 5967–5971. [CrossRef] 33. Xie, X.L.; Mai, Y.W.; Zhou, X.P. Dispersion and alignment of carbon nanotubes in polymer matrix: A review. Mater. Sci. Eng. R 2005, 49, 89–112. [CrossRef] 34. Ma, P.C.; Siddiqui, N.A.; Marom, G.; Kim, J.K. Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review. Compos. Part A 2010, 41, 1345–1367. [CrossRef] 35. V.-Niño, E.D. Systematic Research of Physicochemical Properties of Functional Photopolymers for Additive Manufacturing; Universidad Politécnica de Madrid: Madrid, Spain, 2019. 36. Vadukumpully, S.; Paul, J.; Mahanta, N.; Valiyaveettil, S. Flexible conductive graphene/poly(vinyl chloride) composite thin films with high mechanical strength and thermal stability. Carbon 2011, 49, 198–205. [CrossRef] 37. Jyotishkumar, P.; Pionteck, J.; Hassler, R.; George, S.M.; Cvelbar, U.; Thomas, S. Studies on stress relaxation and thermomechanical properties of Poly(acrylonitrile-butadiene-styrene) modified epoxy-amine systems. Ind. Eng. Chem. Res. 2011, 50, 4432–4440. 38. Wei, X.; Li, D.; Jiang, W.; Gu, Z.; Wang, X.; Zhang, Z.; Sun, Z. 3D printable graphene composite. Sci. Rep. 2015, 5, 1–7. [CrossRef] 39. Doganay, D.; Coskun, S.; Kaynak, C.; Unalan, H.E. Electrical, mechanical and thermal properties of aligned silver nanowire/polylactide nanocomposite films. Compos. Part B 2016, 99, 288–296. [CrossRef] 40. Hua, M.Z.; Feng, S.; Wang, S.; Lu, X. Rapid detection and quantification of 2,4-dichlorophenoxyacetic acid in milk using molecularly imprinted polymers–surface-enhanced Raman spectroscopy. Food Chem. 2018, 258, 254–259. [CrossRef] 41. Sessa, C.; Weiss, R.; Niessner, R.; Ivleva, N.P.; Stege, H. Towards a surface enhanced Raman scattering (SERS) spectra database for synthetic organic colourants in cultural heritage. The effect of using different metal substrates on the spectra. Microchem. J. 2018, 138, 209–225. [CrossRef] 42. Garcia-Rico, E.; Alvarez-Puebla, R.A.; Guerrini, L. Direct surface-enhanced Raman scattering (SERS) spectroscopy of nucleic acids: From fundamental studies to real-life applications. Chem. Soc. Rev. 2018, 47, 4909–4923. [CrossRef] 43. Zhao, T.; Li, X.; Yu, R.; Zhang, Y.; Yang, X.; Zhao, X.; Wang, L.; Huang, W. Silicone–epoxy-based hybrid photopolymers for 3D printing. Macromol. Chem. Phys. 2018, 219, 1–10. [CrossRef] 44. Weng, Z.; Zhou, Y.; Lin, W.; Senthil, T.; Wu, L. Structure-property relationship of nano enhanced stereolithography resin for desktop SLA 3D printer. Compos. Part A 2016, 88, 234–242. [CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).