Photopolymer Matrix Research Articles

A dual-cure approach for the ultraviolet-assisted material extrusion of highly loaded opaque suspensions

Ultraviolet-Assisted Material Extrusion (UV-MEX) is an additive manufacturing (AM) technology in which a viscous ink, is selectively extruded and solidified via in-situ UV irradiation (photocuring) in a layer-by-layer fashion to create three-dimensional parts. UV-MEX is an attractive AM technology due to its ability to (i) extrude highly viscous inks that are not processable via other photo-enabled AM technologies, (ii) leverage the broad photopolymer material library and chemistries established for other photo-enabled AM technologies and (iii) process a wide range of loaded inks to enable fabrication of metal, ceramic, polymer, bio-based, and multi-material parts. UV-MEX is well positioned to process highly loaded and highly viscous photocurable inks, a class of materials that cannot be processed by other AM modalities due to their process-induced viscosity constraints. However, opaque particles in the highly loaded systems may absorb and scatter UV irradiation, which will hinder final part properties due to limited cure depth and degree of conversion of the photopolymer matrix. To enable the robust UV-MEX printing of highly loaded inks containing opaque fillers, this work introduces a framework for formulating and characterizing resins that are both photo (UV irradiation) and thermally (heat) curable. This dual-cure strategy is enabled by the facile addition of free radical photo- and thermal initiators into acrylate-based resins. The resin’s responsiveness to UV irradiation enables rapid solidification and stiffening of the resin to reduce the spreading and slumping of the extruded bead. However, due to the absorption of the UV irradiation by the opaque fillers, only a thin cured skin (20-100um) is formed on the surface of the extruded bead. Thus, the secondary thermal cure is used to fully cure the ink and develop the final properties of the printed part. Heating during the printing process is shown to provide in-situ thermal curing of the deposited ink, which further strengthens the deposited material and enables the printing of tall structures. In this work, the dual-cure ink formulation strategy is assessed for its ability to enable the printing of dimensionally stable and high-aspect ratio parts from inks highly loaded with opaque solids. A methodology for characterizing the curing of the highly loaded inks using rheological and differential scanning calorimetry experiments is presented, along with a framework for assessing their printing performance in UV-MEX. The dual-cure ink concept and screening methodology is demonstrated on a dual-cure urethane acrylate ink loaded with 60 vol% aluminum particles.

Quantum Dot Transfer from the Organic Phase to Acrylic Monomers for the Controlled Integration of Single-Photon Sources by Photopolymerization

This paper reports on a new strategy for obtaining homogeneous dispersion of grafted quantum dots (QDs) in a photopolymer matrix and their use for the integration of single-photon sources by two-photon polymerization (TPP) with nanoscale precision. The method is based on phase transfer of QDs from organic solvents to an acrylic matrix. The detailed protocol is described, and the corresponding mechanism is investigated and revealed. The phase transfer is done by ligand exchange through the introduction of mono-2-(methacryloyloxy) ethyl succinate (MES) that replaces oleic acid (OA). Infrared (IR) measurements show the replacement of OA on the QD surface by MES after ligand exchange. This allows QDs to move from the hexane phase to the pentaerythritol triacrylate (PETA) phase. The QDs that are homogeneously dispersed in the photopolymer without any clusterization do not show any significant broadening in their photoluminescence spectra even after more than 3 years. The ability of the hybrid photopolymer to create micro- and nanostructures by two-photon polymerization is demonstrated. The homogeneity of emission from 2D and 3D microstructures is confirmed by confocal photoluminescence microscopy. The fabrication and integration of a single-photon source in a spatially controlled manner by TPP is achieved and confirmed by auto-correlation measurements.

Print parameter optimization and mechanical deformation analysis of alumina-nanoparticle doped photocurable nanocomposites fabricated using vat-photopolymerization based additive technology

As the recent trend of fabricating high-strength polymer has been gaining more attention in the market, researchers are exploring the possibility of using a 3D printing technique for the same. Vat-photopolymerization additive technology has good process capability and produces high-quality photopolymer parts with greater geometrical complexity. This paper proposes the vat-photopolymerization process to fabricate high-strength acrylate-based photopolymer composite with improved mechanical strength. The composite structures contain 1.0, 2.0, and 3.0 wt percentages of 13 nm average-sized alumina (Al2O3) nanoparticle (NP) as the reinforcement and acrylate photopolymer as matrix material. The viscosities of all the prepared suspensions were measured to determine the printing properties of the prepared nanocomposites, and these values are within the feasible printing limit. Furthermore, the depth of cure and time of exposure to UV light were optimized to achieve the desired level of printing performance. The thermogravimetric analysis (TGA) of the alumina nanocomposites reveals that the addition of alumina NP has minimal influence on the thermal stability of the photopolymer matrix. To ascertain the mechanical strength of the 3D printed products, tensile tests are performed, and it was witnessed that 1.0% weight fraction of alumina-NP loaded specimen experienced the highest tensile strength amongst all the weight fractions of alumina-NP considered, and its value increased by ∼48% when compared against the pure acrylate polymer sample. The increase in strength values for the alumina-NP loaded specimen was explained using dynamic mechanical analysis and optical and scanning electron micrographs. A constitutive elastic-viscoplastic isothermal non-linear time-dependent material model is also developed to predict the post-yield strain-softening behavior of these nanocomposites. The weight fraction and optimal weight fraction of alumina nanoparticles are correlated using semi-empirical equations of various model parameters.

Using supervised machine learning methods to predict microfiber alignment and electrical conductivity of polymer matrix composite materials fabricated with ultrasound directed self-assembly and stereolithography

Engineered polymer matrix composite materials with designer electrical properties are important for a myriad of engineering applications including flexible actuators and wearable sensors. We use stereolithography in combination with ultrasound directed self-assembly to align electrically conductive microfibers in a photopolymer matrix. We relate the fabrication process parameters to the resulting filler material alignment and corresponding electrical conductivity using supervised machine learning methods and quantify the prediction accuracy of data-driven models derived from different interpretable and non-interpretable algorithms. We determine that decision tree and artificial neural network algorithms result in data-driven models with R2 scores that are 79.8% and 83.2% higher, respectively, than a traditional multivariate regression analysis benchmark model in predicting the microfiber alignment. Similarly, random forest and artificial neural network algorithms result in data-driven models that predict composite material electrical conductivity 9.1% and 13.7% more accurately, respectively, than a logistic multivariate regression benchmark model. Relating the fabrication process parameters to the resulting electrical conductivity of the material is a crucial step towards fabricating polymer matrix composite materials with designer electrical properties for use in engineering applications.

Zn2+ responsive fluorescence enhancement for optical data storage.

In this paper, we put forward a new application in optical data storage (ODS) of tetraphenylethene (TPE)-doped photopolymer, which has an aggregation-induced emission attribute. The photopolymer host reacted with the excitation light at the focal point of a high numerical-aperture lens to enhance the fluorescence intensity mainly because of the function of the ${{\rm Zn}^{2 + }}$Zn2+ ion. We recorded data inside the photopolymer matrix by using this property and had distinct fluorescence intensity contrast between the photochemical regions and other regions. This attribute paves a new way for superresolution ODS and opens the way to exploring the possibility of utilizing TPE-doped photopolymers as chemical sensors in the future.

Optical tape for high capacity three-dimensional optical data storage

In this Letter, a new approach of optical tape for high capacity multilayer data storage is proposed. We show that a length of 5 cm and width of 2 cm of soft and transparent optical tape can be used for two-photon three-dimensional bit data storage. We successfully demonstrate writing and reading of six layers of data storage with a transverse bit separation of 2 μm and an axial separation of 2.5 μm in a tetraphenylethylene-doped photobleaching polymer. The fluorescence intensity is insensitive to the storage depth of the photopolymer matrix. Thus, the optical tape that we put forward in the experiment can help people realize true large data storage in the future, like magnetic tape. This method significantly paves a novel way for solving big data storage problems.

Translation of segment scale stiffening into macroscale reinforcement in polymer nanocomposites

Structuring of polymer nanocomposites (PNCs) with an aid of relatively weak external magnetic fields has been studied as a method for control of the nano‐ and microstructure. Magnetic nanoparticles (NPs) were assembled into high aspect ratio one‐dimensional strings and unidirectionally oriented with the magnetic field (B = 0–50 mT) within the photopolymer matrix. The effect of the anisotropic MNPs assemblies on the mechanical properties was studied over a wide temperature range for the first time. The impact of various reinforcing mechanisms was distinguished with respect to the position of the glass transition temperature (Tg). The reinforcing effect exhibits temperature dependency with a maximum ~65°C above the glass transition and only negligible effect below the Tg. In addition, significant directional anisotropy of stiffness was observed. Composite micromechanics was applied to interpret the orientation and size‐dependent reinforcement of PNCs, and temperature‐dependent stiffness of the polymer‐MNP structures was quantified. The presence of polymer chains with altered dynamics surrounding the MNPs inside the anisotropic assemblies was proposed to be an essential nanoscale mechanism mediating the stress transfer and contributing to mechanical robustness of the hybrid structures and PNCs. POLYM. ENG. SCI., 60:587–596, 2020. © 2019 Society of Plastics Engineers

Projection lithography patterned high-resolution quantum dots/thiol-ene photo-polymer pixels for color down conversion.

Pixelated color converters are envisioned to achieve full-color high-resolution display through down conversion of blue/ultraviolet(UV) micro-LEDs. Quantum dots (QDs) are promising narrow-band converters of high quantum efficiency and brightness enabling saturated colors with wide color gamut in displays. Here we demonstrate high-resolution pixelated red and green QDs/thiol-ene photo-polymer converters (single pixel down to 6 µm; converters array of 21 µm pixel, 30 µm pitch and sub 10 µm thickness) patterned through projection lithography. QDs capped with amine surface group are uniformly dispersed in thiol-ene photo-polymer matrix at high concentrations (up to 100 mg/mL), which reduces aggregation and improves conversion efficiency by 0.5-1 times compared to drop-cast QDs. Color cross-talk is also reduced through patterning light blocking walls between converter pixels.

Investigation of the strain‐rate‐dependent mechanical behavior of a photopolymer matrix composite with fumed nano‐silica filler

With the evolution of additive manufacturing, there is an increasing demand to produce high strength and stiffness polymers. Photopolymers are very commonly used in stereolithography and fused deposition modeling processes, but their application is limited due to their low strength and stiffness values. Nano‐sized fibers or particles are generally embedded in the polymer matrix to enhance their properties. In this study, we have studied the effect of fumed nano‐sized silica filler on the elastic and viscoelastic properties of the photopolymer. The uniaxial testing coupons with different concentrations of silica filler have been fabricated via casting. We observed improvement in mechanical properties by the addition of the nano‐sized filler. As polymers exhibit time‐dependent mechanical response, we have conducted tensile tests at different strain rates as it is one of the most common modes of deformation, and is commonly used to characterize the parameters of the rate‐dependent material. We noticed significant dependence of the mechanical properties on the strain rate. quasi‐linear viscoelastic (QLV) model, which combines hyperelastic and viscoelastic phenomena, has been employed to capture the response of the material at different strain rates. We found out that the QLV model with Yeoh strain energy density function adequately describes the rate‐dependent behavior of the material and has reasonable agreement with the experimental results. POLYM. ENG. SCI., 59:1695–1700 2019. © 2019 Society of Plastics Engineers

Microwave composite structures on the base of nickel-zinc ferrite Ni1−xZnxFe2O4 nanoparticles in the photopolymer matrix

The 3D printing compatible technology of “ferrite – nonmagnetic dielectric” composite structures preparation was proposed. Composite resonant elements for applications in the centimeter range of microwave frequencies comprising nonmagnetic dielectric BaTi4O9-ZnO and ferrite film made from spinel nickel-zinc ferrite Ni1−xZnxFe2O4 (x = 0; 0.25; 0.5; 0.75; 1) magnetic nanoparticles in the photopolymer matrix were manufactured.Microwave characteristics of the obtained composite resonators with different concentrations of ferrite nanoparticles in the film-forming suspension were investigated. It was shown, that magnetic field makes the influence on the transmission characteristics of composite resonators, namely the operating frequency, resonance linewidth and the resonant absorption level. The developed elements can be used to create the various microwave devices (solid-state oscillators, microwave filters and attenuators) with the magnetic field tunable parameters.

3D Printing of Cantilever-Type Microstructures by Stereolithography of Ferromagnetic Photopolymers.

In the present work, prototypes of polymeric cantilever-based magnetic microstructures were fabricated by means of stereolithography (SL). To this end, a UV-curable system suitable for high-resolution SL-processing was formulated by blending a bifunctional acrylic monomer with photoinitiator and visible dye whose content was tuned to tailor resin SL sensitivity. Subsequently, to confer ferromagnetic properties to the photopolymer, two different strategies were implemented. A two-step approach involved selective deposition of a metal layer on photopolymer SL-cured surfaces through an electroless plating process. On the other hand, SL-processable ferromagnetically responsive nanocomposites (FRCs) were obtained by directly loading magnetite nanoparticles within the photopolymer matrix. In order to achieve high-printing resolution, resin SL sensitivities were studied as a function of the various additives contents. Photocalorimetric analyses were also performed to investigate the photopolymer conversion efficiency upon light exposure. High-performing formulations were characterized by reduced penetration depth (<50 μm) and small critical energies thus enabling for fast printing of micrometric structures. Finally, the self-standing characteristics of the resin combined with the layered-fashion deposition typical of the 3D printing technologies were exploited for the fabrication of cantilever (CL)-based beams presented as possible magnetic sensors. As a demonstration of the feasibility of the two approaches, the magnetic beams were successfully actuated and their sensing performances in terms of static deflection vs applied magnetic field applied were qualitatively studied. Being not restricted to CL-based geometries, the combination of SL-printing with the formulation of novel smart photopolymers open the way toward the fabrication of high-customized complex 3D models integrating functional microstructures.

An efficient photochemical route to Pd nanoparticles; application to the one-step synthesis of Pd@polymer nanocomposite films

We have developed a facile, efficient, low cost and ‘green’ photochemical approach to preparing surfactant-free Pd nanoparticles and Pd-immobilized@acrylate photo-polymer films at room temperature, under air and without any additional treatment. The reaction system only includes a photo-initiator used as a generator of free radicals and a Pd(II) salt. In ethanol solution, the photochemical reduction of Pd(II) to Pd(0) generates very small metal particles with a narrow size distribution (2–4 nm). Furthermore, we have shown that the formation of Pd nanoparticles from a Pd(II) salt can be reversible thus allowing easy handling and safe storage with the possibility of generating the nanoparticles just before use. In the presence of an acrylate bifunctional monomer, Pd@polymer film was obtained through a ‘one-pot, one-step’ process resulting from a simultaneous photo-reduction of Pd(II) and photo-polymerization of acrylate units. The simultaneous generation of a 3D polymer network and of metal particles leads to a homogeneous distribution of Pd nanoparticles in the photo-polymer matrix with an average diameter of approximately 3.7 ± 1.1 nm. Such as-prepared Pd@polymer films were found to efficiently catalyze the Mizoroki–Heck reaction in the presence of only 0.9 mequiv. of supported palladium. The major interest of this arrangement is its recoverability and reusability, which makes it very attractive both from a practical and economical viewpoint. Finally, it is worth noting that this innovation offers a great advantage over concurrent methods in that it is simply generated within minutes, it is highly stable, and there is sharp monodispersity in the size of the Pd nanoparticles that can be stored for months without alteration of their physico-chemical properties and catalytic activity.

The formation of gold nanoparticles in photopolymerized networks

Photopolymer networks containing a phosphonium polyelectrolyte were used as scaffolds for the synthesis of gold nanoparticles (AuNPs). Chloride anions electrostatically bound to the phosphonium salt were exchanged with AuCl4− and then reduced with NaBH4 to form AuNPs. The stiffness of the photopolymer matrix had a pronounced effect on the concentration of AuNPs formed within the material, where softer, more swellable networks provided greater anion-exchange sites and therefore more AuNP formation. Numerous loading/reduction cycles with AuCl4− and NaBH4 increased the concentration of AuNPs upon each cycling step without significantly changing AuNP size, thus providing a means to control AuNP concentration within the material. SEM and TEM analysis revealed particles sizes ranging from 10 to 30 nm with significant microscale heterogeneity likely arising from phase separation of the phosphonium polyelectrolyte from the photopolymer matrix followed by AuNP synthesis within those regions. We also demonstrate how this methodology can be applied to patterned photopolymer networks, providing the means to explore this approach for photolithographic applications.

Laser induced birefringence in La–Ga–S–O–Gd glass polymer nanocomposites

In this work, we demonstrate a possibility to use La–Ga–S–O–Gd glass polymer nanocomposites as effective materials for photo-induced birefringence. Here we chose PVA as a photopolymer matrix. The photo-induced effects were studied using 8 ns Nd: YAG laser generating bicolor coherent light beams with wavelengths 1064, and 532 nm. The detection of the photo-induced birefringence was carried out using cw He–Ne laser at 1150 nm. The optimal concentration of the nanoglass favoring maximal changes of refractive indices is established. The photo-induced laser power density was changed up to 0.9 GW/cm2. The photo-induced beams were incident at angles varying within the 32° and 52° with respect to the nanocomposite planes. The polarizations of the beams did not play principal role. We discovered an appearance of maximal birefringence equal to about 0.078. The effect is strongly dependent on the nanoparticle sizes and is completely reversible after switching off the laser treatment within several milliseconds. Such features are useful for the recording of optical information and production of gratings with desirable periods.

Laser induced third harmonic generation in δ-Bi1−xNdxB3O6 nanocomposites

A possibility of optically operated third harmonic generation (THG) in polymer nanocomposites based on orthorhombic δ-Bi1−xNdxB3O6 powders (where x=0.025÷0.100) is presented. The nanoparticles were fabricated using polymeric precursor method. The particles were acoustically milled and then they were embedded into polyvinyl alcohol (PVA) photopolymer matrix. After solidification the additional photoinducing treatment was performed by two coherent 1064nm Nd:YAG laser beams. The angle between the photoinducing beams and their polarization was varied in order to achieve the maximum of THG. THG efficiency was monitored immediately after Nd:YAG laser treatment at different temperatures. The photoinduced THG was explored versus the Nd3+ content and temperature. Origin of the effect is discussed within a framework of phenomenological description.

Unclonable security features for additive manufacturing

Leveraging the technology's unique ability to selectively place multiple materials throughout a part volume, the authors demonstrate a new approach for the fabrication of a new physical security feature for additively manufactured parts. Specifically, the authors create photopolymer suspensions featuring quantum dots – a nanoparticle that absorbs ultraviolet light and emits light in the visible spectrum – that are then embedded into objects created by PolyJet material jetting. While the quantum dots appear ordered at the macroscale, their stochastic arrangement at the microscale (via the inkjetted droplet) provide the randomness necessary to serve as the key element of a Physical Unclonable Function, essentially transforming the 3D printed object itself as an anti-counterfeiting system. In this work the authors explore the effects of quantum dot loading on optical signatures of the nanoparticles in the photopolymer matrix. Quantum dot loadings as low as 5×10−3wt.% can be detected inside the object with a fluorescent microscope, while this same concentration is invisible to the naked eye. By adjusting the magnification of the fluorescent microscope, we demonstrate the feasibility of a new paradigm for three-dimensional security patterns.

Morphology and properties of a photopolymer/clay nanocomposite prepared by a rapid prototyping system

AbstractThis study represents an in situ polymerization by preparation of tetrafunction polyester acrylate mixed with 1,6-hexanediol diacrylate/clay nanocomposite by digital light processor rapid prototyping. The morphology of nano-clay fillers and the dispersing agent in the photopolymer matrix are investigated by scanning electron microscopy (SEM). Degradation temperature, tensile strength, impact strength, and hardness are characterized by using thermogravimetric analysis, universal tensile machine, Izod impact tester, and hardness shore A tester, respectively. Results show that the effect of clay loading with an appropriate amount of dispersant tends to significantly increase not only the tensile strength and hardness but also the degradation temperature of photopolymer/clay nanocomposite; however, the impact strength is not affected. In the same conditions, as visualized on SEM images, the nanocomposite tends to form the exfoliated structure with agglomeration of clay, which is caused by uneven distribution of nano-clay in the photopolymer matrix.

Research Summary of an Additive Manufacturing Technology for the Fabrication of 3D Composites with Tailored Internal Structure

A novel additive manufacturing technology is used to create micro-composites, which can be tailored for specific end-use applications. The Field-Aided Laminar Composite (FALCom) process uses specifically focused electric fields to align nano- to micro-sized particles into chain-like structures, which are referred to as pseudo-fibers. These pseudo-fibers are then immediately frozen into place by incident ultraviolet radiation on the photopolymer matrix. The pseudo-fibers are arranged by design, and they are used to create three-dimensional composite structures. Multiple filler materials have been evaluated for use in the FALCom system; however, this report describes aluminum micro-particles that are aligned and oriented in an acrylic photopolymer matrix. A description of the technology and a review of experimental processing are shown, and conclusions, as well as, future work are discussed.

Spectroscopic studies of the size effects in the absorption spectra of (NH2(C2H5)2)2CuCl4 nanocrystals incorporated into the PMMA photopolymer matrix

Abstract The absorption spectra of (NH 2 (C 2 H 5 ) 2 ) 2 CuCl 4 (DEACC) single crystals and nanocrystals (NC) incorporated into the polymethyl methacrylate (PMMA) polymer matrices were investigated both experimentally and theoretically. It was established that the crystal field spectra of Cu 2+ ion detect clearly the quantum size effects. The observed spectra were analyzed using first principle crystal field quantum chemical calculations. It was shown that incorporation of NCs into the polymer matrix allows to identify the charge-transfer (CT) bands in the spectra of DEACC crystals.

Polymer-dispersed liquid crystal/layered double hydroxide nanocomposite: A new emerging optical application

Abstract This paper describes the preparation and characterization of polymer-dispersed liquid crystal (PDLC)/layered double hydroxides (LDHs) nanocomposites. In this study, Mg–Al LDH was applied as two-dimensional host material, with the unique properties of higher anionic exchange capacity (AEC) and higher aspect ratio. Preparation of nanomaterials with a uniform, higher crystalline and surface charge density could be performed by a hydrothermal method. SEM and particle diameter analysis measurement, successfully obtained affords smaller crystallites with a higher aspect ratio, having a very narrow distribution of crystallite size. The structure formula [ Mg 2 Al ( OH ) 6 ] NO 3 · 1.2 H 2 O were carried out by ICP-AES, EA and TGA. The major applications of LDHs based on the intercalation or exchange of specific guests as absorbents have been reported. However, it is seldom discussed to apply as optical materials. In this work, we have studied the Mg–Al LDH mixed with nematic liquid crystal and photo-polymer matrix. Photo-polymerization was induced by argon ion laser irradiation on the filled cells, which contain photo-initiator, monomers, liquid crystals and Mg–Al LDHs. Diffraction efficiency makes apparently increasing from 1.4 to 5.6%. When the sample was added 1 phr of Mg–Al LDH, the diffraction efficiency rise to 5.6 from 1.4%, compared with original PDLCs. Polymerization induced the phase separation of polymer and liquid crystal in the PDLC film. Higher diffraction efficiency was observed when PDLC hybridized with inorganic LDH, which will change the polymerization rate of PDLC, and help the formation of the grating effect; therefore, the diffraction efficiency of the whole system was improved. The noteworthy factor of improving diffraction efficiency will be discussed in this paper.