Article

Stereolithography of Semiconductor Silver and Acrylic-Based Nanocomposites

Luisa M. Valencia $^{1, *}$ , Miriam Herrera $^{1}$ , María de la Mata $^{1 \oplus}$ , Jesús Hernández-Saz $^{2} \oplus$ , Ismael Romero-Ocaña $^{1 \oplus}$ , Francisco J. Delgado $^{1}$ , Javier Benito $^{1 (D)}$ and Sergio I. Molina $^{1}$ (D) 1 Departamento de Ciencia de los Materiales e Ingeniería Metalúrgica y Química Inorgánica, IMEYMAT, Facultad de Ciencias, Universidad de Cádiz, Campus Río San Pedro, s/n, 11510 Cádiz, Spain 2 Departamento de Ingeniería y Ciencia de los Materiales y del Transporte, Universidad de Sevilla, Avda. Camino de los Descubrimientos s/n, 41092 Sevilla, Spain * Correspondence: luisamaria.valencia@uca.es; Tel.: +34-956-01-2028

Citation: Valencia, L.M.; Herrera, M.; de la Mata, M.; Hernández-Saz, J.; Romero-Ocaña, I.; Delgado, F.J.; Benito, J.; Molina, S.I. Stereolithography of Semiconductor Silver and Acrylic-Based Nanocomposites. Polymers 2022, 14, 5238. https:// doi.org/10.3390/polym14235238

Academic Editor: Yanqin Shi

Received: 26 October 2022
Accepted: 29 November 2022
Published: 1 December 2022
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Copyright: © 2022 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 (https:// creativecommons.org/licenses/by/ 4.0/).

Abstract

Polymer nanocomposites (PNCs) attract the attention of researchers and industry because of their potential properties in widespread fields. Specifically, electrically conductive and semiconductor PNCs are gaining interest as promising materials for biomedical, optoelectronic and sensing applications, among others. Here, metallic nanoparticles (NPs) are extensively used as nanoadditives to increase the electrical conductivity of mere acrylic resin. As the in situ formation of metallic NPs within the acrylic matrix is hindered by the solubility of the NP precursors, we propose a method to increase the density of Ag NPs by using different intermediate solvents, allowing preparation of Ag /acrylic resin nanocomposites with improved electrical behaviour. We fabricated 3D structures using stereolithography (SLA) by dissolving different quantities of metal precursor $({AgClO}_{4})$ in methanol and in $N, N$ -dimethylformamide (DMF) and adding these solutions to the acrylic resin. The high density of Ag NPs obtained notably increases the electrical conductivity of the nanocomposites, reaching the semiconductor regime. We analysed the effect of the auxiliary solvents during the printing process and the implications on the mechanical properties and the degree of cure of the fabricated nanocomposites. The good quality of the materials prepared by this method turn these nanocomposites into promising candidates for electronic applications.

Keywords: stereolithography; polymer-based nanocomposites; (semi)conducting polymeric nanocomposites; acrylic resin; silver nanoparticles; semiconductor

1. Introduction

Over the past few years, the synthesis of (semi)conducting polymer nanocomposites (PNCs) has undergone in-depth investigations with a careful selection of polymer matrixes and nanofillers [1-3]. With this aim, conductive nanomaterials are often dispersed in insulating polymer matrixes to fabricate (semi)conducting PNCs, which are promising resources for biomedical, optoelectronic and sensing applications [4-6]. Within this context, the incorporation of metallic nanoparticles (NPs) as fillers is an emerging approach to enhance the performance of (semi)conducting PNCs [7,8], since metallic NPs offer appealing physicochemical properties due to their high surface area to volume ratio.

In particular, Ag NPs are increasingly used as nanofillers in PNCs because of their excellent conductivity and antibacterial properties [9-12]. Ag NPs are commonly prepared by chemical reduction in an organic or aqueous medium containing stabilizers

[13, 14]

and then are added to a polymer solution to obtain the

Ag /

polymer composites [15-17]. As an interesting alternative, Sangermano et al. proposed the in situ synthesis of Ag-epoxy [18] and Au-acrylic [19] nanocomposites, where a radical photoinitiator can simultaneously form the metallic NPs as well as generate reactive species for crosslinking polymerization upon UV irradiation.

Different methods have been recently proposed to obtain Ag-PNCs using relevant additive manufacturing (AM) techniques. The main advantage of these methods is the in situ photoreduction of the Ag precursor during the printing process while the photopolymerization of the organic polymers takes place. Fantino et al. [20] reported the preparation of conductive 3D nanocomposites by coupling digital light processing (DLP) technology with the photoreduction of Ag precursors, generating Ag NPs during the UV post-curing process after the DLP. These authors also proposed an alternative thermal treatment to induce the formation of Ag NPs after the DLP process [21]. In situ approaches have also been considered in other AM technologies, such as in stereolithography (SLA). Customized SLA equipment has been proposed to fabricate acrylic-based structures with Ag-patterned surfaces by modifying the laser settings during the printing process, being suitable for resistive switching devices and bacteria proliferation control [22,23]. Additionally, Scancialepore et al. [24] reported the fabrication of 3D printed pieces with Ag NPs using low Ag precursor contents (Ag acetate), obtaining only a slight decrease in the electrical resistivity, in the same order of magnitude as that of pristine resin.

In a previous paper, the feasibility of fabricating nanocomposites based on acrylic photocurable formulations containing Ag NPs in situ generated by UV-induced reduction of

{AgNO}_{3}

and

{AgClO}_{4}

during the SLA printing process was proposed by the authors [25]. This approach involves the simultaneous polymerization of the acrylic resin and the reduction of Ag ions to metallic Ag by means of the UV laser action of an SLA printer. Although the electrical resistivity of the PNCs obtained was reduced four orders of magnitude, it still corresponded to insulator materials rather than to (semi)conductor materials, meaning that the amount of Ag NPs was not adequate to achieve the desired electrical conductivity. The solubility of the Ag precursors in the resin prevents the addition of more than

3 wt %

{AgClO}_{4}

to obtain the nanocomposites. In the present paper, we propose an approach to overcome such limitations, aiming to increase the density of Ag NPs by using an intermediate solvent to assist the salt dissolution, rendering higher amounts of precursor dispersed in the resin. Solvents like water, ethanol, methanol, and

N, N

-dimethylformamide (DMF) have been extensively used for the synthesis of PNCs in the literature, to dissolve and help in the dispersion of Ag NPs [15,16,26-29]. In particular, we evaluate the effect of using methanol and DMF to increase the amount of

{AgClO}_{4}

that can be dispersed in an acrylic resin used in SLA, to improve the electrical conductivity of Ag nanocomposites. The presence of the external solvent can affect the mechanical properties of the material due to pore formation during evaporation. The effect of the nature of the external solvents on the structural and mechanical properties of the resulting nanocomposites is also studied and discussed.

2. Materials and Methods

2.1. Materials

Clear photopolymer standard resin (a mixture of proprietary acrylic monomers and oligomers and phenylbis (2,4,6-trimethyl benzoyl)-phosphine oxide as photoinitiator) was purchased from XYZprinting, Inc. (XYZprinting, New Taipei City, Taiwan). Phenylbis (2,4,6-trimethyl benzoyl)-phosphine oxide and silver perchlorate

({AgClO}_{4})

were purchased from Alfa Aesar (Thermo Fisher, Kendal, Germany). Methanol, N,N-dimethylformamide (DMF), and isopropanol (IPA) were purchased from Scharlau (Scharlab, Sentmenat, Spain). All products were used as received.

2.2. Sample Preparation

Two solvents, methanol and DMF, were used to disperse different quantities of

{AgClO}_{4}

( 5,10 , and

15 wt %

) in an acrylic resin ( 1 mL solvent

/ 20 mL

acrylic resin) to fabricate the nanocomposites. Based on the results obtained in our previous work [25],

2 wt %

photoinitiator (phenylbis (2,4,6-trimethyl benzoyl)-phosphine oxide) was added to the solutions to improve the degree of cure of the acrylic resin. An Ultrasonic Cleaner USC500T provided
by VWR (VWR International, Radnor, PA, USA) and working at 45 kHz was used for the sonication processes ( 30 min ). Solid specimens were obtained by two different techniques:

Using a UV chamber with a light source of 405 nm and a power of $1.25 mW / {cm}^{2}$ (FormCure, Formlabs, Somerville, MA, USA)) for 90 min . Previous to that, some solutions were left in vacuum during 24 h to evaporate the solvent.
Printing by SLA with Nobel 1.0 equipment, XYZprinting, Inc. (XYZprinting, New Taipei City, Taiwan)., using a 405 nm laser with an output power of 100 mW and a spot size that allows an XY resolution of $300 μ m$ . All samples were printed with a layer height of $100 μ m$ . Once printed, the samples were washed in IPA for several minutes. Post-processing of the samples was also performed inside a UV chamber with a light source of 405 nm and a power of $1.25 mW / {cm}^{2}$ (FormCure, Formlabs) for 60 min .
Flat discs with a thickness of 2 mm and diameter of 65 mm were fabricated for electrical and mechanical measurements. Moreover, complex cubic structures with hollows and curved parts ( $2 \times 2 \times 2 {cm}^{3}$ ) were also printed.

2.3. Characterization

Surface and cross-section structural analyses were performed using a Scios 2 DualBeam (Thermo Fisher Scientific, Waltham, MA, USA) focused ion beam-scanning electron microscope (FIB-SEM) working at 30 kV ion aceleration voltage. This equipment was also used to obtain electron-transparent thin films of the nanocomposites for transmission electron microscopy (TEM) analyses. The thin films were cleaned at low voltage ( 2 kV ).

Samples were coated prior to FIB-SEM analyses with a layer of Au in a SCD 004 Sputter Coater (BAL-TEC, Balzers, Lichtenstein).

High angle annular dark field scanning (HAADF-S-) TEM and Energy-dispersive X-ray (EDX) measurements were performed using a Thermo Scientific TALOS F200S (Thermo Fisher Scientific, Waltham, MA, USA) working at 200 kV .

Shore D hardness was carried out using a Durometer (Sauter HB (&TI), Vitoria-Gasteiz, Spain) according to ASTM D2240. At least five measurements were performed on each test specimen; the results were averaged, and standard deviations were determined.

Differential scanning calorimetry (DSC) was used to determine the curing enthalpy of the different samples with a Q20 (TA Instruments, New Castle, DE, USA). DSC curves were obtained by performing a temperature sweep from room temperature

(25^{\circ} C)

320^{\circ} C

10^{\circ} C / \min

under a nitrogen atmosphere. A subsequent cooling and heating sweep at

10^{\circ} C / \min

was performed to confirm the complete polymerization of the resin in the first sweep.

Electrical resistivity was measured following ASTM D257 using a Keithley 6517B electrometer (Keithley, Cleveland, OH, USA) with a voltage of 500 V . At least three measurements were performed for each composite. Results were averaged, with standard deviations presented as error bars.

3. Results

3.1. Fabrication of Nanocomposites: Photopolymerization in UV Chamber vs. SLA Printer

For comparative purposes, nanocomposites obtained by a conventional procedure, using a UV curing chamber for the photopolymerization (schematized in Figure 1a), were manufactured. Initially, the formulations were prepared by dissolving the desired concentrations of

{AgClO}_{4}

(including a reference formulation with no

{AgClO}_{4}

) with a

2 wt %

of photoinitiator in the two solvents considered, methanol and DMF, stirring the mixture to help the dissolution. After that, the mixture was added to the acrylic resin, and the new solution was sonicated to disperse and homogenize the solution. Once the formulations were prepared, they were poured into different molds and cured using the UV chamber to obtain the solid pieces. The photo-reactive process started by UV irradiation, triggering the formation of transient radical species able to initiate the radical polymerization of acrylic monomers to solidify the resin, while also initiating the reduction of

{Ag}^{+}

{Ag}^{0}

to create