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In Situ Thermal Generation of Silver Nanoparticles in 3D Printed Polymeric Structures Erika Fantino 1 , Annalisa Chiappone 2, *, Flaviana Calignano 2 , Marco Fontana 1 , Fabrizio Pirri 1,2 and Ignazio Roppolo 2 1 2

*

Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi, 24, Torino 10129, Italy; [email protected] (E.F.); [email protected] (M.F.); [email protected] (F.P.) Center for Sustainable Futures@PoliTo, Istituto Italiano di Tecnologia, Corso Trento, 21, Torino 10129, Italy; [email protected] (F.C.); [email protected] (I.R.) Correspondence: [email protected]; Tel.: +39-011-5091956

Academic Editor: Alessandra Vitale Received: 10 June 2016; Accepted: 12 July 2016; Published: 19 July 2016

Abstract: Polymer nanocomposites have always attracted the interest of researchers and industry because of their potential combination of properties from both the nanofillers and the hosting matrix. Gathering nanomaterials and 3D printing could offer clear advantages and numerous new opportunities in several application fields. Embedding nanofillers in a polymeric matrix could improve the final material properties but usually the printing process gets more difficult. Considering this drawback, in this paper we propose a method to obtain polymer nanocomposites by in situ generation of nanoparticles after the printing process. 3D structures were fabricated through a Digital Light Processing (DLP) system by disolving metal salts in the starting liquid formulation. The 3D fabrication is followed by a thermal treatment in order to induce in situ generation of metal nanoparticles (NPs) in the polymer matrix. Comprehensive studies were systematically performed on the thermo-mechanical characteristics, morphology and electrical properties of the 3D printed nanocomposites. Keywords: 3D printing; polymer-based nanocomposites; silver nanoparticles

1. Introduction Polymer-based nanocomposites (NCs) have been extensively studied in the last decades as a means to achieve improved properties via the control of the interactions between the polymeric host and the nanostructured filler [1,2]: They have become an important class of materials exploited in many different applications such as optics [3–6], microelectronics [7–9], bioactive materials [10,11] and others [12]. There are many nano- and micro-fabrication techniques available for the realization of such type of NCs, including electron beam lithography, photolithography, ink-jet printing, direct-write techniques, soft lithography and contact printing [13]. But these techniques do not offer simple approaches to fabricate three-dimensional (3D) structures. Currently, great efforts are being produced in the attempt to develop new nanocomposite processing techniques that may allow the production of highly reliable and precise 3D microstructures: A very promising and potentially cost-effective approach to manufacture such nanocomposite microdevices is represented by 3D printing [14–17]. A marriage of nanomaterials and 3D printing could offer clear advantages and numerous new opportunities [18–20]. 3D printing consists of the direct fabrication of a 3D object starting from a digital model. 3D printing is now routinely used in a variety of manufacturing sectors ranging from simple prototypes to direct part production, in both aerospace, automotive and bioengineering sectors [18,21–23].

Materials 2016, 9, 589; doi:10.3390/ma9070589

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In recent years, advances in materials science in conjunction with the existing 3D printing technologies are opening new areas of application: Researchers have investigated hybrid 3D printing processes and materials to create advanced products [15,16,24–29]. The possibility of coupling the 3D aspect with a low-cost fabrication would open up several possibilities in a broad range of fields, in particular in the electronic one [17,30,31]. Stereolithography (SLA) and Digital Light Processing (DLP) represent some of the most explored 3D techniques used for the fabrication of such microdevices [15,32]. The general procedure for building 3D structures with SLA involves the exposure of light (typically from a laser or light-emitting diode) to a photocurable resin (e.g., acrylated monomer or oligomer), which creates cross-linked regions where the light irradiates the matrix. The resolution of these techniques is influenced by many factors, depending both on the photocurable system (curing mechanism, kinetics, free radical diffusion, etc.) [33–35] and the optical system [36]. The throughput of the SLA process is slow due to the point-by-point scanning nature of the direct-write of the laser system while the DLP exploits a digital micro mirror-array device (DMD), to produce a dynamic digital mask: An entire part cross section can be cured at one time, resulting in a faster process than scanning a laser beam [36]. A significant amount of work on 3D printing of nanocomposites concerns the incorporation of different types of nanoparticles in a commercially available acrylate or epoxy resin. In most of the works the scope is to enhanced the properties of the matrix (electrical [37], magnetic [38], mechanical [39] and thermal properties [40]), instead in other papers the aim is to fabricate green bodies of ceramic components [41–43]. Nevertheless, in all the cases, the addition of nanofillers strongly affects the printing process: Solution viscosity, light penetration depth and nanoparticles dispersion and stability. The control over all these parameters can be really tedious and not easily achievable. Recently a new method to obtain 3D polymer-based NCs was proposed [44]: The results established a novel approach for the preparation of 3D nanocomposites by coupling the photoreduction of metal precursors with the DLP technology, allowing the fabrication of conductive 3D hybrid structures consisting of metal nanoparticles and organic polymers shaped in complex multilayered architectures. The main advantage of this technique is the combination of the DLP technology with the reduction of the silver precursor. Based on previous studies [44], we select the best formulation and in this paper we decided to exploit a thermal reduction of the metal precursor evaluating the possibility of contemporary sintering of the generated silver nanoparticles. The 3D structures were fabricated by embedding metal salts in the starting formulation, PEGDA oligomer and photoinitiator, and exposing them to the DLP system. The 3D fabrication is then followed by a thermal treatment (TT), in order to induce the in situ generation of metal nanoparticles (NPs) in the polymer matrix, (Figure 1). Comprehensive studies were systematically performed on the thermo-mechanical characteristics, the morphology and the electrical properties of the 3D printed nanocomposites.

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Figure 1. Sketch of Digital Light Processing (DLP) setup that projects dynamic digital masks on the

Figure 1. Sketch of Digital Light Processing (DLP) setup that projects dynamic digital masks on the photocurable formulation featuring the formation of the polyethylene glycol diacrylate structure. photocurable formulation featuring the formation formationofof polyethylene glycol diacrylate structure. Subsequent Thermal treatment, with the thethe silver nanoparticles by reduction of the Subsequent Thermal treatment, with the formation of the silver nanoparticles by reduction of the metal precursors. metal precursors. 2. Experimental

2. Experimental 2.1. Materials

2.1. Materials

PEGDA with a molecular weight of 700 g·mol−1 and AgNO3 were purchased from Sigma–Aldrich SrlPEGDA (Milan, Italy) used as received. Bis-(2,4,6-trimethylbenzoyl) with aand molecular weight of 700 g¨mol´1 and AgNO3phenylphosphineoxide were purchased from(Irgacure Sigma–Aldrich 819, kindly provided by BASF, Kaisten, Switzerland) was selected for his fair absorbing Srl (Milan, Italy) and used as received. Bis-(2,4,6-trimethylbenzoyl) phenylphosphineoxide characteristics in the deep blue to near UV, and was added to the formulation (1 phr). The dye (Irgacure 819, kindly provided by BASF, Kaisten, Switzerland) was selected for his fair absorbing selected, Reactive Orange (RO), was purchased from Sigma–Aldrich and used as received.

characteristics in the deep blue to near UV, and was added to the formulation (1 phr). The dye selected, Reactive Orange of (RO), was purchased from Sigma–Aldrich and used as received. 2.2. Preparation the 3D Structures A 3DLPrinter-HD (Robot Factory, Mirano, Italy) was employed as printing equipment using 2.2. Preparation of the 3D2.0 Structures

a projector with a resolution of 50 μm (1920 × 480 × 1080 pixels). The build area is 100 × 56.25 × 150 A33DLPrinter-HD 2.0 (Robot Factory, from Mirano, wasThe employed astime printing equipment mm and the layer thickness is adjustable 10 toItaly) 100 μm. exposure was set at 1 s perusing a projector with a resolution of 50 µm (1920 ˆ 480 ˆ 1080 pixels). The build area is 100 ˆ 56.25 ˆ 150 layer for neat PEGDA and was increased up to 1.2 s per layer for the sample containing 15 phr of mm3 nitrate. andsilver the layer thickness is adjustable from 10 to 100 µm. The exposure time was set at 1 s per layer for

neat PEGDA and was increased up to 1.2 s per layer for the sample containing 15 phr of silver nitrate. 2.3. Thermal Treatmeant

2.3. Thermal Treatmeant The printed parts were submitted to thermal treatments in oven at 100, 150 and 200 °C; a Buchi Glass (BÜCHI AG, Flawil, Switzerland) was employed for100, the thermal TheOven printed partsLabortechnik were submitted to thermal treatments in oven at 150 andtreatments 200 ˝ C; a Buchi perfomed in vacuum.

Glass Oven (BÜCHI Labortechnik AG, Flawil, Switzerland) was employed for the thermal treatments perfomed in vacuum. 2.4. Characterization DSC measurements were performed with a DSC1 STARe System apparatus of TA Instruments 2.4. Characterization (TA Instruments Waters LLC, New Castle, DE, USA) equipped with a low temperature probe. The DSC measurements performed a DSC1 System apparatus −1. TA experiments were carriedwere out between −80 with and 60 °C withSTARe a heating rate of 10 °C·minof TGAInstruments was (TAperformed Instruments New 851e Castle, DE, USA) equipped with 25 a low temperature in airWaters using a LLC, TGA/SDTA instrument in the range between and 700 °C, with a probe. ˝ C¨min ´1out −1. The Theheating experiments were carried out between ´80 and 60 ˝ C with heating rate of 10was . TGA was rate of 10 °C·min morphological characterization of athe nanocomposite carried ˝ by FESEM Supra 40) (Carl Zeiss AG,instrument Jena, Germany). samples were25 prepared fracturing performed in (Zeiss air using a TGA/SDTA 851e in theThe range between and 700by C, with a heating ´1structures 3D in liquid nitrogen: Both surface andnanocomposite cross-section of was the cured materials ratethe of obtained 10 ˝ C¨min . The morphological characterization of the carried out by FESEM were analyzed. (Zeiss Supra 40) (Carl Zeiss AG, Jena, Germany). The samples were prepared by fracturing the obtained

3D structures in liquid nitrogen: Both surface and cross-section of the cured materials were analyzed. The UV–Visible spectra were recorded by means of a double beam Lambda 40 instrument (Perkin-Elmer Italia, Milano, Italy). The range between 280 and 800 nm was monitored with a scan rate of 480 nm/min. All the experiments were performed on 100 µm films coated on a glass slide.

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spectra were recorded by means of a double beam Lambda 40 instrument Materials The 2016, UV–Visible 9, 589 4 of 11 (Perkin-Elmer Italia, Milano, Italy). The range between 280 and 800 nm was monitored with a scan rate of 480 nm/min. All the experiments were performed on 100 μm films coated on a glass slide. Electrical conductivity byusing usingaaKeithley-238 Keithley-238High High Current Electrical conductivityofofthe the3D 3Dstructure structure was was measured measured by Current Source Measure Unit (Keithley Instruments, Cleveland, OH, USA) (voltage range ˘50 V, step 1 V) Source Measure Unit (Keithley Instruments, Cleveland, OH, USA) (voltage range ±50 V, step 1 V) realizing a two-point contact setup placing copper electrodes on theon twothe opposite sides of sides flat specimens realizing a two-point contact setup placing copper electrodes two opposite of flat 2). The best performing sample was measured in more complex geometries. specimens (area cmperforming (area 0.5 cm2 ). The0.5 best sample was measured in more complex geometries. The data The are dataobtained shown are obtainedmeasurements by multiple measurements on different for each shown by multiple on different samples (threesamples for each (three formulation). formulation). 3. Results and Discussion 3. Results and Discussion 3D conductive polymeric structures were fabricated by incorporating silver nanoparticles 3D (Silver conductive polymeric structures were fabricated by formulations incorporatingbased silveron nanoparticles precursor nitrate (AgNO3 )) into photocurable polymer polyethylene precursor (Silver nitrate (AgNO 3 )) into photocurable polymer formulations based on polyethylene glycol diacrylate (PEGDA) and exposing them to DLP system. The formulations were prepared by glycol diacrylate (PEGDA) and exposing to DLPand system. The formulations were prepared dissolving a fixed concentration of AgNOthem (15 phr), photoinitiator Irgacure 819 (1 phr) inbythe 3 dissolving a fixedThe concentration ofworks AgNOwith 3 (15 phr), and photoinitiator Irgacure 819 (1 phr) in the PEGDA oligomer. DLP system an illumination system in the UVA-visible range so PEGDA oligomer. The DLP system works an illumination system in the UVA-visible range so we properly selected a typical photoinitiatorwith working in the near UV-violet-blue spectrum: Irgacure we properly selected a typical photoinitiator working in the near UV-violet-blue spectrum: Irgacure 819 that belongs to BAPO (Bis-Acyl-Phosphine Oxide) family. A dye (0.2 phr) was also added to the 819 that belongs to BAPO (Bis-Acyl-Phosphine Oxide) family. A dye (0.2 phr) was also added to the formulation since the bright color of the dye can prevent the leaking out of light from the desired formulation since the bright color of the dye can prevent the leaking out of light from the desired illumination area and allows to control the thickness of each layer during the printing process. illumination area and allows to control the thickness of each layer during the printing process. Different computer-aided design (CAD) files were produced aiming to print different 3D objects, Different computer-aided design (CAD) files were produced aiming to print different 3D objects, ranging from simple honeycomband andhelicoidally helicoidally structures ranging from simplerectangular rectangularstructure structureto to more more complex complex honeycomb structures (Figure 2). (Figure 2).

Figure 2. 3D objects produced by DLP technique from the formulation containing polyethylene glycol Figure 2. 3D objects produced by DLP technique from the formulation containing polyethylene glycol diacrylate (PEGDA) and 15 phr of silver nitrate. (a) Honeycomb structure as printed; (b,c) samples diacrylate (PEGDA) and 15 phr of silver nitrate. (a) Honeycomb structure as printed; (b,c) samples after the thermal treatments; the metallic aspect induced by the presence of the silver nanoparticles is after the thermal treatments; the metallic aspect induced by the presence of the silver nanoparticles is clearly visible. clearly visible.

After the realization of the 3D objects, different thermal treatments were performed in order to After the realization of the 3D objects, different thermal treatments were performed in order to induce the reduction of the metal precursor, and the formation of the metal nanoparticles inside the induce thestructures. reduction of the metal precursor, and the formation of the metal nanoparticles inside the printed printedAccording structures.to the literature, sintering process of silver NCs are commonly performed at a Accordingoftoabout the literature, process of silver are commonly at a temperature 200 °C [45];sintering the thermal treatments were NCs performed at differentperformed temperatures ˝ C [45]; the thermal treatments were performed at different temperatures up temperature of about 200 up to 200 °C both in air and in vacuum aiming to obtain the reduction and to help the possible ˝ C both in air and in vacuum aiming to obtain the reduction and to help the possible sintering to 200 sintering of the NPs. of the NPs. Firstly, in order to evaluate the best temperature window for the post process and to check its Firstly, in order the best temperature window for the post process andperformed to check its compatibility with to 3Devaluate printed nanocomposites, thermogravimetric analysis (TGA) was on just printed containing silver precursors in order toanalysis follow the response of the 3Don compatibility with samples 3D printed nanocomposites, thermogravimetric (TGA) was performed just printed samples containing silver precursors in order to follow the response of the 3D structure. Isothermal treatments performed in air confirmed that 200 ˝ C thermal process is not compatible with our structures since the polymer matrix undergoes thermo-oxidative degradation, showing a loss of

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structure. Isothermal treatments performed in air confirmed that 200 °C thermal process is not compatible our 3a). structures since the polymer matrix undergoes thermo-oxidative degradation, weight of 18%with (Figure Thermal treatments performed at lower temperatures showed considerably showing a loss of weight of 18% (Figure 3a). Thermal treatments performed at lower temperatures ˝ ˝ lower loss of weight (0.2% for 100 C treatment and 2.3% for 150 C treatment), confirming that showed considerably lower loss of weight (0.2% for 100 °C treatment and 2.3% for 150 °C treatment), between 150 and 200 ˝ C thermo-oxidative degradation occurs. TGA analyses were also conducted confirming that between 150 and 200 °C thermo-oxidative degradation occurs. TGA analyses were in inert atmosphere in order to simulate vacuum treatment since in vacuum or N2 atmosphere only also conducted in inert atmosphere in order to simulate vacuum treatment since in vacuum or N2 thermal degradation could occur. The nanocomposite showed thermal stability up to 220 ˝ C (Figure 3b) atmosphere only thermal degradation could occur. The nanocomposite showed thermal stability up ˝ C in vacuum. At higher temperature indicating the post treatmentthat could performed to 200 to 220 °Cthat (Figure 3b) indicating thebe post treatmentup could be performed up to 200 °C in vacuum. instead thermal polymerinstead degradation leaving a final occurs, residueleaving at higher temperature At higher temperature thermaloccurs, polymer degradation a final residue at related higher to silver salts dispersed the polymer matrix. in the polymer matrix. temperature related in to silver salts dispersed a)

100

b)

100

Weight Loss (%) iso@100°C

iso@100°C

99.5

iso@150°C iso@150°C 97.4

90

iso @200°C iso@200°C 82.6 85

Weight Loss (%)

Weight Loss (%)

80 95

60 40 20

80 0

10

20

30

40

Time (min)

50

60

0 0

100

200

300

400

500

600

700

Temperature (°C)

Figure3.3.(a) (a)Isothermal Isothermal treatments treatments performed (100 °C,˝ C, 150150 °C ˝and Figure performedin inair airatatdifferent differenttemperature temperature (100 C and 200 °C); (b) thermogravimetric analysis (TGA) plot of the sample PEGDA_15 phr AgNO3 heated in ˝ 200 C); (b) thermogravimetric analysis (TGA) plot of the sample PEGDA_15 phr AgNO3 heated in nitrogen at a rate of 10 °C/min. nitrogen at a rate of 10 ˝ C/min.

While 3D printed objects containing nitrate are heated, an irreversible color change is observed: While 3D printed objects containing nitrate are heated, an irreversible color change is observed: The object evolves from a red color, (Figure 2a), to a dark brown until, for higher treatment The object evolves from a red color, (Figure 2a), to a dark2b). brown higher temperatures, temperatures, it reaches a silver mirror aspect (Figure Thisuntil, can befor related totreatment a nucleation-growth it mechanism reaches a silver mirror (Figurewithin 2b). This be related of the silver aspect nanoparticles the can matrix [46]. to a nucleation-growth mechanism of the silver nanoparticles within the matrix [46]. The kinetics of formation of silver nanoparticles has been followed by UV-Vis measurements The kinetics of formation of silver nanoparticles been followed by UV-Vis measurements (Perkin-Elmer Italia, Milano, Italy) performed on curedhas films exposed at different temperatures for (Perkin-Elmer Italia, Milano, Italy) performed on cured films at different increasing times. The formulations were irradiated for 10 s byexposed visible light in ordertemperatures to mimic the for increasing times.that Theoccurs formulations wereprinting. irradiated for 10 s by visible light in order toonmimic the curing curing process during DLP Figure 4 reports the spectra obtained four samples process that occurs during DLP printing. Figure 4 reports the spectra obtained on four samples treated treated respectively at 100, 150, 200 °C in air and 200 °C in vacuum for different heating times, in ˝ ˝ order to follow the 150, silver200 nanoparticles formation. respectively at 100, C in air and 200 C in vacuum for different heating times, in order to it is possible to observe that for all the samples almost no silver reduction occurred follow At thefirst silver nanoparticles formation. during visible irradiation. For the samples at 100 and 150 °C min of heating we during can At first it is possible to observe that for treated all the samples almost noafter silver30reduction occurred ˝ clearly see the appearance of the typical signal around 450 nm due to the plasmon of resonance of the visible irradiation. For the samples treated at 100 and 150 C after 30 min of heating we can clearly metal NPs [47]. It isofpossible to evidence a progressive plasmonofofresonance resonanceofwith see the appearance the typical signal around 450 nmincrease due to of thethe plasmon the the metal increase of heating time which means that the silver nanoparticles nucleation/growth is ongoing. NPs [47]. It is possible to evidence a progressive increase of the plasmon of resonance with the increase fortime the sample 100the °Csilver the appearing peak isnucleation/growth relatively sharp, foristhe sample treated ofWhile heating which treated means at that nanoparticles ongoing. While for at 150 °C the peak is broader indicating the presence of bigger nanoparticles more closely packed; in ˝ the sample treated at 100 ˝ C the appearing peak is relatively sharp, for the sample treated at 150 C fact, it is known [48] that the Surface Plasmon Resonance (SPR) phenomenon is related to the size the peak is broader indicating the presence of bigger nanoparticles more closely packed; in fact, it is and spacing of the nanoparticles. known [48] that the Surface Plasmon Resonance (SPR) phenomenon is related to the size and spacing When the sample treated at 200 °C in air is considered, it is visible that 10 min of heating are of the nanoparticles. sufficient to obtain a broad SPR peak˝meaning that higher temperatures lead to a system in which When the sample treated at 200 C in air is considered, it is visible that 10 min of heating are silver NPs are numerous, with a broad dimensional distribution and closely packed. For longer sufficient obtain broad SPRplot peak meaning that to a system in heating to times the aabsorption increases also at higher higher temperatures wavelengths; lead this corresponds to which the silver NPs are numerous, with a broad dimensional distribution and closely packed. For longer heating appearance of the mirror-like aspect indicating the formation of a surface layer rich in NPs which times the absorption increases also The at higher this corresponds the appearance strongly absorbs in plot the visible range. same wavelengths; response is observable for the to sample treated in of the mirror-like indicating formation of a surface layer rich in NPs which strongly absorbs in vacuum even aspect for shorter heatingthe times (10 min). the visible range. The same response is observable for the sample treated in vacuum even for shorter heating times (10 min).

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Figure 4. UV-Vis plots of samples treated at (a) 100 ˝°C; (b) 150 °C; 200 °C˝in air and (d) 200 °C in˝ Figure 4. UV-Vis plots of samples treated at (a) 100 C; (b) 150 ˝ C;(c)(c) 200 C in air and (d) 200 C in vacuum after different heating times. Figure 4.obtained UV-Vis plots of samples treated at (a) 100 °C; (b) 150 °C; (c) 200 °C in air and (d) 200 °C in vacuum obtained after different heating times.

vacuum obtained after different heating times.

Shape and distribution of the silver nanoparticles into the printed polymeric matrices were

Shape and distribution of the Scanning silver nanoparticles into the printed (Carl polymeric matrices were investigated by distribution Field Emission Electron Microscopy Zeiss AG, Jena, Shape and of the silver nanoparticles into the (FESEM) printed polymeric matrices were investigated by FESEM Field Emission Electron Microscopy (FESEM) (Carl3Zeiss AG, Jena, Germany). Germany). images ofScanning the 3D printed samples containing 15 phr AgNO after thermal investigated by Field Emission Scanning Electron Microscopy (FESEM) (Carldifferent Zeiss AG, Jena, FESEM images of the 3D printed samples containing 15 phr AgNO after different thermal treatments treatments are displayed in Figure 5; the formation of nanoparticles in the 3D structures is confirmed 3 Germany). FESEM images of the 3D printed samples containing 15 phr AgNO3 after different thermal are displayed Figure 5; the ofthe nanoparticles in the 3D is confirmed and, as can and, as canin be observed from the figure, silver nanoparticles are structures generally inisshape and treatments are displayed information Figure 5; the formation of nanoparticles in the 3Dspherical structures confirmed relatively uniformly-dispersed inside the matrix. It is interesting to note that an important be observed from the figure, thethe silver nanoparticles are generally sphericalspherical in shapeinand relatively and, as can be observed from figure, the silver nanoparticles are generally shape and morphological difference appears between the It samples: Size,that shape distribution of silver uniformly-dispersed inside clearly the matrix. is matrix. interesting note anand important relatively uniformly-dispersed inside It the istointeresting to note that anmorphological important nanoparticles the matrix are different when the samples submitted to a post treatment at of 200silver °C morphological difference clearly appears between the samples: Size, shape and distribution difference clearly into appears between the samples: Size, shape and distribution of silver nanoparticles in air (Figureinto 5a) the or in vacuum (Figure 5b) arethe considered: In the first case, large unstructured ˝ nanoparticles matrix are different when samples submitted to a post treatment at 200 °C 5a) into the matrix are different when the samples submitted to a post treatment at 200 C in air (Figure aggregates are 5a) present at vacuum the surface, probably due to thermal oxidation (Figure 5a)large whileunstructured in the latter in air (Figure or in (Figure 5b) are considered: In the first case, or in vacuum (Figure 5b) are considered: In the first case, large unstructured aggregates are present at case smaller and more at homogeneously dispersed nanoparticles are visible (Figure 5b). aggregates are present surface,oxidation probably due to thermal oxidation while in theand latter the surface, probably due cases tothe thermal (Figure 5a) while in the(Figure latter 5a) case smaller more Moreover, in both a relative enrichment of nanoparticles concentration could case smaller and more homogeneously dispersed nanoparticles are visible (Figure 5b). be observed homogeneously dispersed nanoparticles are visible (Figure 5b). at 3DMoreover, printed structure surface (Figure enrichment 5b) with respect to the core of the structurecould (Figure This in both cases a relative of nanoparticles concentration be5c). observed Moreover, both cases relative enrichment of nanoparticles couldthe be general observed at was alreadyinobserved in asome previous work [44,46] and could concentration be explained with at 3D printed structure surface (Figure 5b) with respect to the core of the structure (Figure 5c). This diffusion laws. surface (Figure 5b) with respect to the core of the structure (Figure 5c). This was 3D printed structure was already observed in some previous work [44,46] and could be explained with the general already observed diffusion laws.in some previous work [44,46] and could be explained with the general diffusion laws.

Figure 5. Cont.

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Figure 5. (a) Cross section of a sample treated 1 h at 150 °C in air; (b) Cross section of a sample treated

Figure 5. (a) Cross section of a sample treated 1 h at 150 ˝ C in air; (b) Cross section of a sample treated 1 h at 150 °C in vacuum; (c) Cross section of the core of a sample treated 1 h at 150 °C in vacuum. 1 h at 150 ˝ C in vacuum; (c) Cross section of the core of a sample treated 1 h at 150 ˝ C in vacuum.

Differential scanning calorimetry (DSC) was also performed; the Tg values measured are Differential scanning (DSC)that was also performed; Tg values measuredfor are reported reported in Table 1. It is calorimetry possible to notice a thermal treatmentthe at higher temperatures longer in Table It is possible toofnotice that to a thermal higher temperatures for longer times times1. induces a decrease Tg, related polymer treatment degradationatobserved in TGA experiments. In fact, induces a decrease of T , related to polymer degradation observed in TGA experiments. In the degradation leads to g a general breakage of C–C bonds, reducing the chemical cross-linking points fact, the degradation to a of general breakage of C–C bonds, reducing the chemical cross-linking and thus to anleads increase the mobility of the polymeric chains which corresponds to a decrease points of temperature. On the contrary, a thermalchains treatment in harsh conditions performed andglass thus transition to an increase of the mobility of the polymeric which corresponds to abut decrease of glass in nitrogen atmosphere not inducea thermal a decrease of thermal properties since no degradation transition temperature. Ondoes the contrary, treatment in harsh conditions but performed in occurred, as demonstrated also in TGA experiments. nitrogen atmosphere does not induce a decrease of thermal properties since no degradation occurred,

as demonstrated also in TGA experiments.

Table 1. Tg values measured in DSC for PEGDA containing 15 phr of AgNO3 after the thermal post treatment at different temperature (100 °C, 150 °C and 200 °C) at different time (10’, 30’ and 60’).

Table 1. Tg values measured in DSC for PEGDA containing 15 phr of AgNO3 after the thermal post Treatment g VALUE treatment at different temperature (100 ˝ C, 150 ˝ CTand 200 ˝ C)(°C) at different time (101 , 301 and 601 ). No TT −28 VALUE C) °C Treatment TT_Air @100 °CT g@150 °C (˝@200 −25 −25 −25 No TT10’ ´28 ˝ ˝ −25 30’ −26 −38 TT_Air @100 C @150 C @200 ˝ C −26 60’ −30 −37 ´25 ´25 ´25 101 TT_Vacuum @150 °C @200 °C ´25´26 ´38 301 ´26 ´30 601 30’ −26 −26´37 TT_Vacuum @150 @200 ˝ C 60’ −27 ˝ C −28 ´26 ´26 301 ´27then evaluated; ´28 firstly, the resistivity of The electrical properties 60 of1 the printed materials were flat printed specimens was measured by sandwiching them between copper electrodes. Figure 6a shows the relationship between theprinted measured resistance and heating temperatures, are then The electrical properties of the materials were then evaluated; firstly, the the data resistivity of flat reported in Figure 6b. The NC containing silver nitrate is two orders of magnitude more conductive printed specimens was measured by sandwiching them between copper electrodes. Figure 6a shows than the neat polymeric matrix. This could be related to the presence of the conductive fillers the relationship between the measured resistance and heating temperatures, the data are then reported dispersed in the polymeric matrix. Consequent thermal treatments in air condition up to a in Figure 6b. The NC containing silver nitrate is two orders of magnitude more conductive than the temperature of 150 °C, even for long periods of time, do not influence NCs resistivity, always keeping neatvalues polymeric matrix. This could be related to the presence of the conductive fillers dispersed in of the order of some MΩcm. Those values are compatible with a hopping-controlled the polymeric Consequent thermal in air condition up to a temperature of the 150 ˝ C, conduction matrix. mechanism [49], meaning thattreatments the NPs nucleation-growth mechanism induced by eventemperature for long periods of time, do not influence resistivity, always keeping valuesdecrease of the order decreases the medium distance NCs between the NPs, causing a moderate of of some MΩcm.but Those values are compatible a hopping-controlled conduction mechanism [49], resistivity not sufficient to guarantee an with efficient percolating path. meaningIt that the NPs to nucleation-growth mechanism induced temperature decreases is important observe that a thermal treatment at 200 by °C the performed in air involves a the dramatically increase of the resistivity, this could be relateddecrease to a largeof oxidation of silver NPs.sufficient On the to medium distance between the NPs, causing a moderate resistivity but not contrary, thermal treatments performed in vacuum lead to the lower resistivity values, nearly guarantee ansimilar efficient percolating path. two orders of magnitude lower than the corresponding sample. The obtained values It is important to observe that a thermal treatmentuntreated at 200 ˝ C performed in air involves a indicate that no sintering has occurred. dramatically increase of the resistivity, this could be related to a large oxidation of silver NPs. In a recent work we reported about the fabrication of 3D printed object containing silver salts in On the contrary, similar thermal treatments performed in vacuum lead to the lower resistivity values, which the subsequent Ag NPs reduction was performed by means of UV irradiation. The resistivity

nearly two orders of magnitude lower than the corresponding untreated sample. The obtained values indicate that no sintering has occurred.

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In a recent work we reported about the fabrication of 3D printed object containing silver salts in Materials 2016, 9, 589 8 of 11 which the subsequent Ag NPs reduction was performed by means of UV irradiation. The resistivity ˝ values obtained after 1 h of thermal treatment at 200 C are in line with the values obtained for a values obtained after 1 h of thermal treatment at 200 °C are in line with the values obtained for a sample UV irradiated for 10 min, indicating that with this NCs UV reduction seems more efficient sample UV irradiated for 10 min, indicating that with this NCs UV reduction seems more efficient in in the realization of conductive 3D structures. At last we performed electrical measurements also on the realization of conductive 3D structures. At last we performed electrical measurements also on 3D 3D structures, as example we reported the value for the sample heated at 200 ˝ C for 1 h in vacuum, structures, as example we reported the value for the sample heated at 200 °C for 1 h in vacuum, Figure 7. Figure The current-voltage (I–V) curve resultslinear, perfectly linear, showingofa 3.6 resistance 3.6 MΩ. The 7. current-voltage (I–V) curve results perfectly showing a resistance MΩ. Thisofvalue, Thisalthough value, although not was verysufficient high, was sufficient achieve the a led (Figure 6), not very high, to achieve the to illumination of aillumination led (Figure 6),ofthus envisaging thusa envisaging a possible application in the market of electronics as dissipating material [50]. possible application in the market of electronics as dissipating material [50]. a)

Conductivity (S/cm)

1E-6

1E-7

PEGDA PEGDA 15phr AgNO3

1E-8

100°C Heating 150°C Heating 200°C Heating 150°C Heating vacuum 200°C Heating vacuum

1E-9

1E-10 10

20

30

40

50

60

Heating time (min)

b) Sample

Heating Temperature (°C)

Heating Time (min)

σ (μS/cm)

ρ (MΩ/cm)

PEGDA

-

-

0.003

260

-

-

0.10

10

100

10

0.09

11

100

30

0.13

7.4

100

60

0.22

4.6

150

10

0.11

9.2

150

30

0.22

4.5

150

60

0.31

3.2

150 (vacuum)

30

0.43

2.2

150 (vacuum)

60

0.99

1

200

10

0.0005

1756

200

30

0.0001

10,200

200

60

0.00005

18,000

200 (vacuum)

30

2.3

0.43

200 (vacuum)

60

2.4

0.41

PEGDA 15 phr AgNO3

Figure 6. (a) Relationship between resistance and heating temperatures measured on flat printed

Figure 6. (a) Relationship between resistance and heating temperatures measured on flat printed specimens; (b) Conductivity and resistance values measured on flat samples treated at different specimens; (b) Conductivity and resistance values measured on flat samples treated at different temperatures for different times. temperatures for different times.

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Figure 7. I–V plot obtained by contating an honeycomb structure treated at 200 °C in vacuum. Inset:

Figure 7. I–V plot obtained by contating an honeycomb structure treated at 200 ˝ C in vacuum. the current flowing trough the structure was sufficient to achieve the illumination of a led. Inset: the current flowing trough the structure was sufficient to achieve the illumination of a led.

4. Conclusions 4. Conclusions The method here proposed allows to obtain 3D structures with complex geometries presenting

The method here proposed allows to obtain 3D structures with complex geometries presenting promising electrical properties. The in situ thermal generation of silver NPs performed on the printed promising electrical properties. situ thermal of silver NPs the printed samples does not influence The the in stability of the generation polymeric structures and performed represents aonpossible samples does not the stability of the polymeric structures and represents a possible alternative alternative toinfluence the UV generation already proposed. to the UV generation already proposed. Author Contributions: Erika Fantino, Annalisa Chiappone, Ignazio Roppolo followed the whole work

Erika Annalisa Chiappone, IgnazioCalignano Roppoloprovided followedthe theCAD whole workforpreparing Author Contributions: preparing the samples andFantino, performing the measuremnts. Flaviana models the the samples andMarco performing the measuremnts. Flaviana Calignano provided the CAD modelsoffor 3D printer, 3D printer, Fontana performed SEM analyses, Fabrizio Pirri contributed in the discussion thethe results. Marco Fontana performed SEM analyses, Fabrizio Pirri contributed in the discussion of the results. Conflicts of Interest: The authors declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations

Abbreviations

The following abbreviations are used in this manuscript:

The following abbreviations are used in this manuscript: NPs

nanoparticles

NPs TT nanoparticles thermal treatment TT treatment 3D thermal three-dimensional 3D NCs three-dimensional nanocomposites NCs SLA nanocomposites stereolithography SLA DLP stereolithography digital light processing DLP DMD digital light processing digital micromirror-array device DMD digital micromirror-array device DSC differential scanning calorimetry DSC differential scanning calorimetry TGA thermogravimetric analysis TGA thermogravimetric analysis computer-aided design CAD CAD computer-aided design BAPO bis-acyl-phosphine oxide BAPO bis-acyl-phosphine oxide PEGDA polyethylene glycol diacrylate PEGDA polyethylene glycol diacrylate emission scanning electronmicroscopy microscopy FESEMFESEM fieldfield emission scanning electron

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