Photopolymer System Research Articles

Novel Photobase Generators Derived from Proazaphosphatrane–Aryl Borate for High‐Pressure Mercury Lamp Lithography

AbstractHerein, novel borate‐type photobase generators (PBGs) that generate a proazaphosphatrane known as Verkade's base (2,8,9‐triisobuthyl‐2,5,8,9‐tetraaza‐1‐phosphabicyclo[3.3.3]undecane (TTP)) were developed for photopatterning using a high‐pressure mercury lamp. Eight PBGs were synthesized, each featuring distinct borate substituents: phenyl, p‐fluorophenyl, p‐tolyl, p‐methoxyphenyl, m‐fluoro‐p‐methylphenyl, 4‐biphenyl, 2‐naphthyl, and 6‐methoxy‐2‐naphthyl. The relation between these substituents and their light absorption characteristics was analyzed via ultraviolet–visible spectroscopy and time‐dependent density functional theory calculations. The PBG with the 6‐methoxy‐2‐naphthylborate group (TTP–tetrakis[2‐(6‐methoxynaphthyl)]borate (TTP–TMNB)) demonstrated substantial light absorption near 365 nm (the i‐line of the high‐pressure mercury lamp), generating approximately 60 % of the corresponding TTP in deuterated tetrahydrofuran (THF) upon exposure to 650 mJ/cm2, as confirmed via 1H nuclear magnetic resonance spectroscopy. To evaluate its photolithographic potential, TTP–TMNB was mixed with epoxy resin (jER1001) and a poly(methacrylic acid)‐co‐poly(methyl methacrylate) copolymer (PMA1.6‐co‐PMMA100) in THF. The resulting mixture was spin‐coated onto a silicon wafer and irradiated with the i‐line through a photomask to create a negative‐tone image. The sensitivity and contrast of this photopolymer system were measured to be 45 mJ/cm2 and 1.0, respectively; moreover, a clear 10‐μm negative‐tone resolution was achieved.

Octaphenyl-POSS nanoparticles dispersed thick polyvinyl alcohol/acrylamide photopolymer with low volumetric shrinkage and high diffraction efficiency

Relatively high volumetric shrinkage induced by polymerization of photopolymer severely limits its applications in high-density holographic storage. In this paper, what we believe to be a novel organic-inorganic hybrid nanoparticles (NPs) of octaphenyl-polyhedral Oligomeric silsesquioxane (POSS) are employed into polyvinyl alcohol (PVA)/acrylamide photopolymer with 300µm thickness. By optimizing the concentration of octaphenyl-POSS, the volumetric shrinkage is successfully suppressed from 1.11% to 0.41%, and satisfies the requirements of commercial holographic data storage (HDS) material. Moreover, the introduction of POSS NPs does not sacrifice other holographic recording capacities. Contrarily, the diffraction efficiency of photopolymer after dispersing 2‰(w/v) POSS increases from 86% to 90%. The physical mechanism of enhanced capacities is discussed by using photopolymerization and NPs diffusion process. To our knowledge, this is the best results of PVA/acrylamide photopolymer with low volumetric shrinkage, high diffraction efficiency and angular selectivity. The POSS-dispersed photopolymer system is expected to have good application prospects in angular-multiplexing high-density 3-D holographic storage.

Light-matter interaction during and post polymerization in self-written polymer waveguide integrated with optical fibers

Self-written polymer waveguide is a polymer interconnection structure often integrated with optical fibers. The structure is fabricated by photopolymerization, through which a photon beam initiates a polymerization reaction to change liquid monomer into a solid polymer. During light-matter interaction photoinitiator molecules are excited to the singlet and then to the third state to form free radicals by attaching to the adjacent monomer molecules. The free radicals in turn form polymer chains and cross-linked structures. Photo-induced polymerization leads to a raised refractive index of the monomer and forms an optical waveguide. The change is permanent to allow the process of self-focusing and self-trapping of the propagation light beam and hence guide it to the aligned optical fiber. In this article, light matter interaction during polymer waveguide fabrication and post polymerization has been investigated. A photopolymer system of acrylate-based monomer, co-initiator amine and photo initiator is used to study the absorption of the monomer mixture at the spectral range of 450–550 nm. Then photobleaching was investigated after waveguide fabrication. The optical transmission efficiency of the polymer bridge increases as absorption ceases within the polymer structure such that optical loss reduces from 1.1 dB to about 0.65 dB.

Enhancement of holographic performance and stability of photopolymer materials by introducing epoxy resin and dyes

The use of a composite photopolymer system together with a photoinitiator have a significant effect on the optical properties of a volume phase holographic grating. This paper reports on the performance of a low refractive index monomer combined with a high refractive index epoxy resin. Performance improvements include the compatibility between the cross-linked network structure of the matrix and the recording medium. The effects of the exposure conditions and the internal composition of the materials on holographic performance were investigated. The results showed that the optimal refractive index modulation depends on the ratio of the epoxy resin and the pre-polymerization temperature. The use of Irgacure 784 as a photoinitiator gave better performance than Rose Bengal (RB) in terms of diffraction efficiency (∼92.5 %), transmittance (∼98 %) and slight shrinkage (0.314 %) was observed. The use of a dual-photoinitiator gave improvements in the radical generation rate and the stability, permitted recordings at an exposure energy of 50 mJ/cm2 and afforded high sensitivity (3.9 × 10-2 cm2/J) as a result of the synergy between the two photoinitiators. In other words, due to the highly reactive Irgacure 784 and the inhibiting polymerization characteristics of RB, enhancement of holographic performance and good stability were realized. It was shown in the photocuring and aging experiments that the photopolymer possessed anti-aging properties. The photopolymer can record multiplexed images at 532 nm and polarization holograms, demonstrating that it has the capability to perform high-density data recording.

Drum Tower‐Inspired Kirigami Structures for Rapid Fabrication of Multifunctional Shape‐Memory Smart Devices with Complex and Rigid 3D Geometry in a Two‐Stage Photopolymer

AbstractMechanically robust shape‐morphing materials that can transform from miniaturization into complex structures are highly desirable for real‐world device applications. However, fabricating mechanically robust yet geometrically complex 3D shape‐morphing structures with excellent functionality and high load‐bearing capacity remains a challenge. Inspired by drum tower buildings, a simple, rapid, and universal method is presented for fabricating multifunctional shape‐memory smart devices with complex and rigid 3D kirigami geometry in the two‐stage epoxy‐amine‐acrylate photopolymer systems. In the first‐stage reaction, transparent polymer films with a Tg of 29–49 °C are obtained by the epoxy‐amine chemistry. The shape‐memory programming process allows the first‐stage film with a drum tower‐inspired 2D kirigami pattern to be manipulated into an unsupported 3D structure. In the second‐stage reaction, UV‐induced free‐radical polymerization of methacrylate groups in the first‐stage network is employed to rapidly lock the programmed 3D kirigami structure with a Tg of 66–138 °C. The drum tower‐inspired 3D kirigami structure withstands 1000 times its own weight. The shape memory fluorescent 3D device and shape memory electronic 3D device are engineered by combining the two‐stage photopolymer system and a 3D kirigami structure. This work represents a versatile method to create multifunctional shape‐memory devices with rigid 3D geometry for potential applications.

Manipulating the Relative Rates of Reaction and Diffusion in a Holographic Photopolymer Based on Thiol–Ene Chemistry

Properly balanced reaction and diffusion kinetics are crucial for achieving optimal performance in holographic photopolymers. However, a comprehensive study on the effect of reaction and diffusion on the performance of thiol–ene-based holographic photopolymers has not been performed previously. To determine the relationship between reaction and diffusion, the refractive index modulation (Δn) of holographic gratings recorded in a model thiol–ene photopolymer system was evaluated with controlling the rates of reaction and diffusion processes. By changing the molecular weight of the polymer binder, the diffusion rate was varied over orders of magnitudes with the highest Δn of 0.026 achieved at a molecular weight of 2.9 × 104 Da. Meanwhile, the haze was significantly reduced in binders of higher molecular weight. Similarly, as the reaction rate was reduced in accordance with lowering the light intensity, the Δn reached a peak value of 0.023 at 7 mW/cm2 and was found to decrease at both higher (2500 lines/mm) and lower (1000 lines/mm) spatial frequencies. In particular, Δn approaching 0 was observed at a very low intensity (2 mW/cm2) and when the binder with a molecular weight as low as 0.5 × 104 Da was used. An analogous formulation incorporating a secondary thiol, which has slower reaction kinetics and a more stable thiyl radical relative to the primary thiol, exhibited a lower Δn , especially at higher spatial frequencies. Through one-step thiol-Michael addition, the functionality of a tetrathiol monomer was reduced to various extents to obtain a series of thiol–ene photopolymers with precise control over gel point conversions, among which the thiol with an average functionality of 3.5 realized the highest Δn of 0.028. An enhanced reactive binder with norbornene pendant groups was also synthesized, and holographic gratings recorded in it showed notably higher Δn at low light intensities compared to those recorded in non-reactive or less reactive binders.

Cyclic allylic sulfide based photopolymer for holographic recording showing high refractive index modulation

AbstractThe development and characterization of a new holographic photopolymer system showing high refractive index modulation (∆n) is presented. It exploits the ring‐opening polymerization reaction of a cyclic allylic sulfide (CAS) monomer, which is dispersed in a cellulose acetate butyrate (CAB) polymer matrix together with a blue sensitive radical photoinitiator. Volume Phase Holographic Gratings (VPHGs) obtained using these films show a very good fidelity of the transferred pattern, without deviations from the theoretical curves. A high ∆n is obtained due to the addition to this system of a tetrafunctional thiol crosslinker, which maximizes the material potential (formula limit) of the formulation. It also allows for a thermal post process of the gratings, further enhancing the ∆n. The photopolymer properties are evaluated as function of the monomer and thiol concentration to determine the optimal composition, at which a ∆n greater than 0.03 is obtained. This is a considerably high value in the field of photopolymers for holography and it enables the manufacturing of efficient holograms.

Optimization of Plasmonic Gold Nanoparticle Concentration in Green LED Light Active Dental Photopolymer.

Gold nanoparticles (AuNPs) display surface plasmon resonance (SPR) as a result of their irradiation at a targeted light frequency. SPR also results in heat production that increases the temperature of the surrounding environment, affecting polymerization. The aim was to investigate the SPR effect of AuNPs on a dimethacrylate-based photopolymer system. The tested composites were designed to overlap the illumination required for the polymerization and the plasmon effect. The 5 nm-sized dodecanethiol capped AuNPs were applied in different concentrations in the matrix that were irradiated with green light (λ = 532 nm), where the Irgacure 784 photoinitiator also absorbs the light. The plasmonic effect was investigated for the refractive index change by surface plasmon resonance imaging (SPRi) supplemented by ellipsometry. Moreover, optical transmission and transmission electron micrographs (TEM), diametral tensile stress (DTS), and confocal Raman spectroscopy was performed to determine the degree of conversion (DC) at 1.0, 1.4, and 2.0 mW/cm2 light intensities. It was found that the optimal conditions were at 0.0208 wt% AuNPs concentration and 1.4 mW/cm2 light intensity at which the refractive index change, DTS, and DC data were all maximal. The study confirmed that AuNPs are applicable to improve the polymerization efficiency of dental composite resin.

Aliphatic silicone‐epoxy based hybrid photopolymers applied in stereolithography 3D printing

3D Printing has become a powerful technology for future advanced manufacturing, however the choice of materials with good performance is still limited. In this research, a type of hybrid photopolymer resin based on silicone epoxy with high UV curing rate and low viscosity was applied in stereolithography (SLA) technology. First, aliphatic silicone epoxy was synthesized through hydrosilylation reaction, and then compounding with acrylates, cycloaliphatic epoxy and photo‐initiators, finally the hybrid photopolymer system for SLA was obtained. The features of UV curing process were studied through photo‐differential scanning calorimetry and real‐time Fourier transform infrared measurements. Scanning electron microscopy test showed interpenetrating polymer network structure was formed in 3D objects. The performances of 3D printed sample such as mechanical properties, thermal mechanical and stability properties were also investigated in detail. Moreover, the 3D printed objects with complicated structures showed high printing accuracy. This new type of hybrid photopolymer system based on aliphatic silicone epoxy was with fabrication ease, good mechanical properties, good thermal stability and high printing resolution and has great potential of applications in industrial design and models making fields.

A Facile Fabrication Route of Poly(Ethylene Glycol Phenyl Ether Acrylate) Photopolymers with Efficient Optical Response for Holographic Storage

A series of photopolymers based on ethylene glycol phenyl ethyl arylate (EGPEA) monomers and poly(methyl methacrylate) (PMMA) matrix with varying initiator concentrations and sample thicknesses have been synthesized and their optical performance characterized in this study. The advantages of lowering the initiator concentration, including a rather short initiation time within a few seconds; a sharp rising optical response; and a stable saturated diffraction efficiency are demonstrated. The variation in the diffraction efficiency and response time with the exposure energy under various sample thickness and initiator concentrations is examined; a diffraction efficiency as high as 80% and a relatively short response time of 12–39 s are attainable. The dependence of the normalized optical parameter “sensitivity” on the exposure time is depicted, and the peak value of S ranges vastly from about 0.2 to 1.2 × 104 cm/J within a period of 15 s or so, with a maximum value of nearly 1.2 × 104. Favorable evidence of low initiator concentration can further be found when the dependence of the saturated diffraction efficiency with the exposure energy is examined. The data from this study using a low initiator concentration cover a range of exposure energy from 100 to 800 mJ/cm2 and a saturated diffraction efficiency from about 15% to 80%. The successful image reconstruction of 6-membered-ring metal nuts on a hologram based on this EGPEA/PMMA photopolymer system using a reflective holographic recording setup is demonstrated to verify its applicability to holographic storage.

Dexamethasone release from photopolymerised PEGDA700 for cochlea drug delivery

Abstract Sustained local drug release and high local drug concentrations can be achieved by specially designed DDSs, which can be created for different applications. This results in therapeutic drug amounts at the site of action, simultaneously providing a very low drug amount in total. Bodily compartments that contain liquids may be used to transport the drugs from DDS into the tissue, such as the cochlea with the perilymph in the case of cochlea implants. However, predominantly dry environments, such as the middle ear, are lacking such a medium but may deliver enough moisture for the use of swellable DDS through the surrounding mucus membranes. Therefore, DDS with new functionalities are needed to ensure a sustained drug release. In this study, the release of dexamethasone out of a photopolymer system is presented. The system is built from UV-polymerized PEGDA followed by the incorporation of dexamethasone via swelling. The drug release is tested in vitro with isotonic NaCl solution for specified time periods, showing two phases: a swelling phase and a release phase. After the swelling phase the concentration of dexamethasone in the release medium was not controlled by diffusion, although sink conditions were ensured. In contrast, this system can be used to release the drug until equilibrium with a final medium concentration that is far below the solubility of dexamethasone. Hence, such DDS may be useful for dexamethasone delivery into the cochlea through the round window membrane by ensuring a constant concentration in the perilymph.

3D Printing Latex: A Route to Complex Geometries of High Molecular Weight Polymers.

Vat photopolymerization (VP) additive manufacturing fabricates intricate geometries with excellent resolution; however, high molecular weight polymers are not amenable to VP due to concomitant high solution and melt viscosities. Thus, a challenging paradox arises between printability and mechanical performance. This report describes concurrent photopolymer and VP system design to navigate this paradox with the unprecedented use of polymeric colloids (latexes) that effectively decouple the dependency of viscosity on molecular weight. Photocrosslinking of a continuous-phase scaffold, which surrounds the latex particles, combined with in situ computer-vision print parameter optimization, which compensates for light scattering, enables high-resolution VP of high molecular weight polymer latexes as particle-embedded green bodies. Thermal post-processing promotes coalescence of the dispersed particles throughout the scaffold, forming a semi-interpenetrating polymer network without loss in part resolution. Printing a styrene-butadiene rubber latex, a previously inaccessible elastomer composition for VP, exemplified this approach and yielded printed elastomers with precise geometry and tensile extensibilities exceeding 500%.

Serialized holography for brand protection and authentication.

The problems presented by counterfeit products and documentation are discussed. Limitations of existing holograms for anti-counterfeit applications are described. We describe the advantages of full holographic serialization and the requirements in terms of materials and techniques for mass production of true serialized holograms. These requirements having been met, we report for the first time the mass production of fully serialized holograms. The novelty of the approach consists of the direct use of the product manufacturer's information as the object in a holographic recording system along with a self-processing photopolymer and modular optical system to facilitate mass production of truly serialized volume holograms. Various types of serialized holograms for overt and covert authentication are described. We discuss briefly the application of Optrace's manufacturing methods for future generation holographic devices.

Theoretical modeling of the effect of polymer chain immobilization rates on holographic recording in photopolymers

This paper introduces an improved mathematical model for holographic grating formation in an acrylamide-based photopolymer, which consists of partial differential equations derived from physical laws. The model is based on the two-way diffusion theory of [Appl. Opt.43, 2900 (2004)10.1364/AO.43.002900APOPAI1559-128X], which assumes short polymer chains are free to diffuse, and generalizes a similar model presented in [J. Opt. Soc. Am. B27, 197 (2010)10.1364/JOSAB.27.000197JOBPDE0740-3224] by introducing an immobilization rate governed by chain growth and cross-linking. Numerical simulations were carried out in order to investigate the behavior of the photopolymer system for short and long exposures, with particular emphasis on the effect of recording parameters (such as illumination frequency and intensity), as well as material permeability, on refractive index modulation, refractive index profile, and grating distortion. The model reproduces many well-known experimental observations, such as the decrease of refractive index modulation at high spatial frequencies and appearance of higher harmonics in the refractive index profile when the diffusion rate is much slower than the polymerization rate. These properties are supported by a theoretical investigation which uses perturbation techniques to approximate the solution over various time scales.

Improving the performance of methylene blue sensitized photopolymer by doping with nickel ion

Holographic performance of an economically cheap metal ion doped photopolymer material is presented. We investigated the effect of incorporation of nickel ion into the methylene blue sensitized poly (vinyl alcohol)/acrylamide (MBPVA/AA) photopolymer system. The composition and preliminary characterization of the developed photopolymer material is reported. The presence of nickel ion improves the diffraction efficiency, stability of the material and it operates in a wide range of spatial frequencies (550–2000lines/mm) at exposure energy of 100mJ/cm2. When nickel ion concentration was 0.01mM, maximum diffraction efficiency of 84% at exposure energy of 100mJ/cm2 with spatial frequency 1335lines/mm could be achieved for gratings recorded using wavelength of 632.8nm. The material showed panchromaticity with more than 70% diffraction efficiency in both blue and green regions. Effects of humidity and temperature on the stored gratings were studied by keeping films in different environmental conditions. Suitability of recording large area holograms was also explored.

Advances of Near Infrared Sensitized Radical and Cationic Photopolymerization: from Graphic Industry to Traditional Coatings

This contribution summarizes recent progress in the field of near-infrared (NIR) initiated photopolymerization. The photoinitiator system consists of a cyanine as sensitizer (Sens) and an iodonium salt with distinct structural pattern of both the cation and anion as radical initiator. Both, photonic and thermal events are discussed as the main source for formation of initiating species. Electron transfer between the excited state of Sens (Sens*) and the iodonium salt can be seen as the main source for formation of initiating species such as radicals and protons/electrophiles. Furthermore, the ion mobility as probed by the electric conductivity possesses a major function to tune the reactivity of the photopolymer system. The reactivity of these systems was studied in different applications such as Computer to Plate (CtP), LED curing, photonic baking, and curing of powder coatings with NIR lasers exhibiting line shape focus.

Reflection mode holographic recording in methylene blue-sensitized polyvinyl alcohol acrylamide films

Photopolymer systems can produce good image quality holograms that does not require any post-processing and are environmentally stable with good diffraction efficiency. The present work reports the development of a methylene blue-sensitized polyvinyl alcohol acrylamide (MBPVA/AA) photopolymer system for recording white light reflection holograms. Reflection gratings were recorded in the photopolymer films with different concentrations of methylene blue (MB). Various parameters affecting the holographic properties of the samples were also studied. The holographic performance of the material is found to depend on its chemical composition and the recording parameters.

A simple, inexpensive, real‐time interferometric cure monitoring system for optically cured polymers

AbstractOptically cured polymers play a crucial role in many commercial applications, ranging from UV‐cured inks, coatings, and dental fillings to the optical fabrication of three‐dimensional prototypes by means of stereolithography. However, our understanding of the detailed processes that occur in the curing of these materials is often limited by the practical difficulty of observing the cure process in real time, under “real‐world” conditions. This article reports the simple implementation of a free‐space Mach‐Zehnder interferometer that can be constructed inexpensively from readily available components, and demonstrates its utility in monitoring the UV curing of a typical acrylate photopolymer system in real time. In particular, it is shown that this system can sensitively track the induction period, the light‐induced photopolymerization reaction, and the subsequent dark reaction that continues after the irradiation has been halted. This Interferometric Cure Monitor, or “ICM,” thus provides a valuable addition to other analytical methods, both for improving the quality control of existing commercial processes and for aiding the development of improved photopolymer formulations. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Improved shelf-life stability of holographic photopolymer containing monomer stabilizer

Herein we present an environmentally robust photopolymer system containing monomer stabilizer and thereby can improve the shelf-life stability of conventional photopolymer media. In this report, we conducted systematic study of the radical scavenger effect on shelf-life time in terms of achievable diffraction efficiency of photopolymers as a function of storage time. Furthermore, the stability of photopolymerizable monomer was also discussed regarding to monomer conversion by FT-Raman analysis during the storage.

Evaluation of 3D structures fabricated with two-photon-photopolymerization by using FTIR spectroscopy

Two-photon-induced photopolymerization (2PP) has gained increased interest due to the capability of manufacturing three-dimensional structures with very high feature resolution. To assess the suitability of photopolymer systems for 2PP, methods have to be developed that allow a screening of the efficiency of monomer-initiator combinations in the context of high throughput, large processing window and geometric quality of the final parts. In this paper, a method for evaluating 2PP structures is described. For this purpose, the double-bond conversion of fabricated 2PP structures was measured giving quantifiable results about the efficiency of the photoinitiator. The method is based on local measurement of the double-bond conversion of the photopolymer using a microscope in combination with infrared spectroscopy. The obtained double-bond conversion is a measure for the efficiency of the photopolymer system (initiator in combination with monomer), and thus allows to compare different photopolymers in a quantitative way. Beside this evaluation of 2PP structures, fabrication of complex 3D structures was done to determine the limits of the 2PP technology for miscellaneous components.