Photopolymerization System Research Articles

Modeling the nonlinear photoabsorptive behavior during self-written waveguide formation in a photopolymer

Photopolymer materials can be used as recording media for self-written waveguides (SWWs) as they can exhibit a large refractive index change and high photo-sensitivity. In free radical photo-polymerization systems, the dyes, functioning as the photosensitizer, strongly influence the material properties. During photo-illumination the spatial and temporal evolution of the dye concentration is an important factor leading to nonlinear absorption. In this paper, based on an investigation of the photochemical mechanisms, we analyze the nonlinear photo-absorptive effect during the photo-initiation processes. The time varying exposing light distribution is calculated and used to iteratively estimate the evolving cross-sectional refractive index and loss coefficient values. The model enables a more accurate and physical description of the optically induced growth of SWWs in such systems. Then SWWs formed in dry acrylamide/polyvinyl alcohol (AA/PVA) based photopolymer samples, containing different initial dye concentrations, are experimentally examined. The nonlinear absorptive behavior is quantified by comparing the model predictions and the experimental results.

Structure Design of Naphthalimide Derivatives: Toward Versatile Photoinitiators for Near-UV/Visible LEDs, 3D Printing, and Water-Soluble Photoinitiating Systems

Seven naphthalimide derivatives (NDP1–NDP7) with different substituents have been designed as versatile photoinitiators (PIs), and some of them when combined with an iodonium salt (and optionally N-vinylcarbazole) or an amine (and optionally chlorotriazine) are expected to exhibit an enhanced efficiency to initiate the cationic polymerization of epoxides and the free radical polymerization of acrylates under different irradiation sources (i.e., the LED at 385, 395, 405, 455, or 470 nm or the polychromatic visible light from the halogen lamp). Remarkably, some studied naphthalimide derivative based photoinitiating systems (PIS) are even more efficient than the commercial type I photoinitiator bisacylphosphine oxide and the well-known camphorquinone-based systems for cationic or radical photopolymerization. A good efficiency upon a LED projector at 405 nm used in 3D printers is also found: a 3D object can be easily created through an additive process where the final object is constructed by coating down suc...

Synthesis of Monodisperse PEG/SiO2 Hybrid Microspheres by Microfluidic Methods

In this paper, we reported an effective microfluidic-based method for fast continuous synthesis of monodisperse PEG/SiO2 hybrid microspheres. The T-junction microfluidic chip equipped with rounded channels and the online photopolymerization system were applied for the microspheres microfabrication. The size of the microspheres was well controlled by the rounded channels and flow rate ratios of immiscrible fluids. We can obtain porous PEG microspheres when the hybrid microspheres were corroded by hydrofluoric acid. What's more, the interpenetrating polymer network structures of SiO2 microspheres were obtained after the hybrid microspheres treated via a calcination process.

The effect of 4,4'-bis(N,N-diethylamino) benzophenone on the degree of conversion in liquid photopolymer for dental 3D printing.

PURPOSEThe purpose of this preliminary study was to investigate the effects of adding 4,4'-bis(N,N-diethylamino) benzophenone (DEABP) as a co-initiator to a binary photoinitiating system (camphorquinone-amine) to analyze on the degree of conversion (DC) of a light-cured resin for dental 3D printing.MATERIALS AND METHODSCylindrical specimens (N=60, n=30 per group, ø5 mm × 1 mm) were fabricated using bisphenol A glycerolate dimethacrylate (BisGMA) both with and without DEABP. The freshly mixed resins were exposed to light in a custom-made closed chamber with nine light-emitting diode lamps (wavelength: 405 nm; power: 840 mW/cm2) for polymerization at each incidence of light-irradiation at 10, 30, 60, 180, and 300 seconds, while five specimens at a time were evaluated at each given irradiation point. Fourier-transform infrared (FTIR) spectroscopy was used to measure the DC values of the resins. Two-way analysis of variance and the Duncan post hoc test were used to analyze statistically significant differences between the groups and given times (α=.05).RESULTSIn the DEABP-containing resin, the DC values were significantly higher at all points in time (P<.001), and also the initial polymerization velocity was faster than in the DEABP-free resin.CONCLUSIONThe addition of DEABP significantly enhanced the DC values and, thus, could potentially become an efficient photoinitiator when combined with a camphorquinone-amine system and may be utilized as a more advanced photopolymerization system for dental 3D printing.

Initiating gradient photopolymerization and migration of a novel polymerizable polysiloxane α-hydroxy alkylphenones photoinitiator

This paper reported a photocleavage type polymerizable organosilicon macromolecular photoinitiator (HHMP-Si-CC), which was synthesized based on traditional photoinitiator 2-hydroxy-1-[4-(2-hydroxyethoxy) phenyl]-2-methyl propan-1-one (HHMP) and amino polysiloxane. HHMP-Si-CC cannot only spontaneously form a concentration gradient in the photopolymerization system, initiate gradient photopolymerization and effectively mitigate inhibition of oxygen, but also overcome the migration of photolysis fragments of the photoinitiator from UV-curable material. Its structure was confirmed by proton nuclear magnetic resonance (1H NMR), 13C NMR, 29Si NMR and Fourier transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC). The kinetics of photopolymerization of HHMP-Si-CC was studied by real-time infrared spectroscopy (RTIR). Moreover, it was proved by X-ray photoelectron spectroscopy (XPS) and UV absorption that HHMP-Si-CC had relatively good self-floating ability. The polymer initiated by HHMP-Si-CC presented a gradient change in the degree of polymerization, molecular weight, thermostability and glass transition temperature (Tg). The surface morphology of poly(triethylene glycol diacrylate) (PTPGDA) initiated by HHMP-Si-CC was studied by scanning electron microscopy (SEM). More importantly, the migration of photolysis fragments of the HHMP-Si-CC from UV-curable materials was also investigated by high performance liquid chromatography (HPLC). The results showed that the migration more was effectively mitigated by introducing polymerizable groups into the HHMP-Si-CC. HHMP-Si-CC should have potential applications for preparing more environmentally friendly gradient materials.

Visible light initiating systems for photopolymerization: status, development and challenges

A review of recent advances in visible light initiating systems using free radical, cationic and hybrid photoinitiators is presented.

Preparation of monodisperse PEG hydrogel composite microspheres via microfluidic chip with rounded channels

An effective microfluidic method to fabricate monodisperse polyethylene glycol (PEG) hydrogel composite microspheres with tunable dimensions and properties is reported in this paper. A T-junction microfluidic chip equipped with rounded channels and online photopolymerization system is applied for the microsphere microfabrication. The shape and size of the microspheres are well controlled by the rounded channels and PEG prepolymer/silicon oil flow rate ratios. The obtained PEG/aspirin composite microspheres exhibit a sustained release of aspirin for a wide time range; the obtained PEG/Fe3O4 nanocomposite microspheres exhibit excellent magnetic properties; and the obtained binary PEG/dye composite microspheres show the ability to synchronously load two functional components in the same peanut-shaped or Janus hydrogel particles.

Optimization of thick layers photopolymerization systems applying experimental and analytical approach

PurposeOver the last several years, the range of applications of photopolymerization process has been steadily increasing, especially in areas such as rapid prototyping, UV inks, UV coats and orthodontic applications. In spite of this, there are still several challenges to be overcome when the application concerns materials with thick layers. In this context, the main goal of this work is to outline a scheme to optimize the process of photopolymerizarion for thick layers, identifying its differences in relation to those applicable for thin layers.Design/methodology/approachFor this research, the authors have applied multivariable analysis methods which allow the identification of principal conclusions, based on analytical and experimental results. For analytical analysis, the authors applied numerical optimization for multivariables, while experimental analysis was done based on design of experiments. Both the analyses were based on methyl methacrylate as monomer and Omnirad 2500 as photoinitiator, with the adjustable variables being initiator concentration; power of light source; light wave length; and thickness of layer. The range of values chosen for initiator concentration was between 1 and 10 per cent, while for light power, the range was 5‐9 W. For light wave length, the authors selected 325 and 400 nm as limits for their study and 0.12 and 4 mm as the range for thickness of layers. For the analytical approach of their study, it was possible to identify optimum conditions for curing thick layers, besides looking at optimum condition at each step along the varying thickness. On the other hand, in the experimental approach, the authors just considered the initiator concentration and thickness as variables, applying gravimetric and photometric analysis to determine the conversion curve of material.FindingsIn conclusion based on these studies, it was possible to identify the influence of thickness and initiator concentration as function of penetration depth, polymerization rate and homogeneity of material, in addition to determining the effect of light power and light wave length over the process. As a result of these studies, it was possible to identify situations wherein the material will possibly undergo a high degree shrinkage in addition to showing consequences of high quantity of initiator. On the other hand, low concentration of initiator is shown to provide more homogeneous solution besides being more suitable for deep layers. It was also possible to compare analytical and experimental results, making it possible to predict the behaviour of material for other conditions.Originality/valueThe main value of this work is to show the possibility of optimizing photopolymerizable systems through an analytical approach. In addition, it emphasized the viability of the application of UV curable material for producing moulded parts.

New recording material with AgBr nanoparticles for optical holography

A new recording material with silver bromide nanoparticles and photopolymerization system has been prepared and tested as a recording medium for optical holography. Through the method of the real-time measurement of a diffraction grating growth, we have shown that an efficient volume phase grating is formed during a holographic exposure. The material response on different exposure parameters has been measured and obtained results have been discussed. The transfer of nanoparticles within the recording layer, which is the main cause of the grating growth, has been experimentally verified through the direct observation of the grating micro-structure with the scanning electron microscope.

Optimization of a photopolymerizable material based on a photocyclic initiating system using holographic recording

A set of photoinitiating systems (PIS) for free radical photopolymerization was studied using time-resolved spectroscopic experiments, real-time FTIR and holographic recording. It is shown that the efficiency of the photoinitiating system can be drastically increased when a redox additive is added to the conventional dye/coinitiator system by virtue of photocyclic behaviour. The homogeneous photopolymerization process was found to reach a fast vitrification, limiting the conversion at about 55%. By contrast, holographic recording underlines the differences in photoinitiating system reactivity, allowing diffraction efficiencies close to unity for the most reactive PIS.

Quantitative Comparison of Five Different Photosensitizers for Use in a Photopolymer

Several studies of the time varying photon absorption effects, which occur during the photoinitiation process involving in photopolymer materials, have been presented. Three primary mechanisms have been identified: (i) the dye absorption, (ii) recovery, and (iii) bleaching. Based on an analysis of these mechanisms, the production of primary radicals can be physically described and modelled. In free radical photopolymerization systems, the excited dye molecules induce the production of the primary radicals, R•, which are key in determining how many monomers are polymerized. This, in turn, is closely related to the refractive index modulation formed during holographic recording. In this paper, to avoid the complexities involved in estimating the rate constant of intersystem crossing, kst, in going from the excited singlet state dye to the excited triplet state dye, we introduce two rates, kaS and kaT these are the proposed rate constants of photon absorption in going from the ground state to the singlet and triplet states, respectively. Using the resulting model, four kinds of Xanthene dyes: Erythrosin B; Eosin Y; Phloxine B, Rose Bengal, and one Thiazine dye: Methylene Blue, are experimentally characterised for use in an AA/PVA photopolymer.

A Black Beam Borne by an Incandescent Field Self-Traps in a Photopolymerizing Medium

We report that a self-trapped black optical beam that is spatially and temporally incoherent forms spontaneously in a nascent photopolymerization system. The black beam inscribes a permanent cylindrical channel, which prevents the propagation of visible light even under passive conditions (in the absence of polymerization). The finding opens a powerful new mechanism to manipulate light signals from incoherent sources such as LEDs through selective suppression of light propagation. This contrasts with approaches employed by photonic crystals and optical waveguides, which concentrate and guide light intensity within spatially localized regions. The self-trapped black beam forms when a broad incandescent beam bearing a negligible depression was launched into a photopolymerizable medium. Because of refractive index changes caused by polymerization, the depression narrows, deepens, and continually rejects the visible spectrum of light until it stabilizes as a black beam that propagates over long distances (≫ effective Rayleigh range) without significant divergence. As refractive index changes due to polymerization are irreversible, the cylindrical region occupied by the self-trapped black beam is inscribed as a black channel waveguide in the medium.

Photochemically Mediated Atom Transfer Radical Polymerization of Methyl Methacrylate Using ppm Amounts of Catalyst

Well-controlled polymerization of methyl methacrylate at 35 °C was achieved by photochemically mediated atom transfer radical polymerization using a copper catalyst concentration as low as 50–100 ppm. Irradiation at λ > 350 nm provided both a reduction of initially added copper(II) catalyst complexed with either PMDETA or TPMA ligand to a copper(I) activator and a sufficient rate of polymerization. Poly(methyl methacrylate) with a narrow dispersity and predictable molar mass was obtained when an initiator, such as 2-bromopropionitrile, was used. Successful chain-extension polymerization confirmed the living character of the photopolymerization system.

Systematic Studies of UV Stability and Photopolymerization Efficiency of DNA‐Based Nanomaterials

DNA has gained acceptance as an attractive nanomaterial building block due to its versatility and manipulability in design and synthesis. Our group has synthesized a variety of branched DNA monomers including X-shaped DNA (X-DNA) and Y-shaped DNA (Y-DNA), which have well-defined nanoscale geometries. 7] Various functional moieties can also be conjugated onto the DNA monomers, allowing them to have diverse applications. In particular, a photoreactive group can be covalently attached to each arm to allow for UV-initiated polymerization (photopolymerization). Photopolymerization provides a rapid, one-step route for polymerization at room temperature in an aqueous environment, thus it can be easily controlled spatially and temporally. 9] Using photopolymerization, we have successfully constructed DNA nanospheres from XDNA monomers and multifunctional DNA polymers from anisotropic, branched, and crosslinkable (ABC) DNA monomers. Despite the advantages of photopolymerization, reactive oxygen species (ROS) created during the photopolymerization process, including singlet oxygen (O2) and hydroxyl radicals (·OH), cause DNA damage, which may compromise the structural integrity of the photopolymerized DNA nanomaterials. Photoinitiators, commonly used to initiate photopolymerization, are more prone to radical formation, and these free radicals create more ROS that may further cause DNA damage. The mechanisms of DNA damage by ROS include cleavage of sugar-phosphate backbone, disruption of hydrogen bonds by oxidative base modification, cleavage of the nitrogenous base from the 2’ carbon in the deoxyribose sugar, and cleavage of the phosphodiester bonds in the DNA backbone. These disrupted bonds compromise the DNA material integrity. To address these issues, several photoreaction parameters have been studied in terms of their effects on DNA damage; however, most of these studies have only focused on plasmid DNA. For example, Anseth and co-workers reported that wavelength of UV affected DNA damage: short wavelengths (254 nm) turned some intact plasmids into the open-circle nicked form, but long wavelengths (365 nm) had minimal impact on DNA integrity. Park and colleagues revealed that photoreaction time also affected DNA damage: with longer irradiation time, plasmid DNA suffered more damage. Many other studies have investigated the relationshiop between UV light and DNA exclusively on the genetic and cellular level. To the best of our knowledge, this report is the first systematic study of UV damage on DNA building blocks (monomers), rather than plasmids, during photopolymerization. To establish optimal conditions for our DNA photopolymerization system, we first quantified damage on DNA monomers during photopolymerization and second studied the polymerization efficiency of photopolymerizable DNA monomers. More specifically, we focused on parameters relating to: 1) the DNA buildling blocks and 2) the photoreaction conditions. With respect to the DNA building blocks, we varied DNA concentration, structure, and condition of a protectant [i.e. poly(ethylene glycol)] ; for the photoreaction conditions, we examined UV reaction time, photoinitiator concentration, and UV intensity (Scheme 1).

Free Radical Shadow Cure Initiated Using Two-Component and Three-Component Initiator Systems

In photopolymerization systems, “shadow cure” may be defined as polymerization which extends into regions which are not illuminated by the incident initiating light source. The objective of this study is to evaluate the use of fluorescent additives for polymerization in masked regions that are unilluminated by the incident initiating light. Two different fluorescent dyes are investigated: fluorescein (FL) and eosin Y spirit soluble (EYss). A systematic series of studies was performed to characterize the effects of fluorescence intensity, the incident light intensity, and the presence of a diphenyl iodonium salt on the observed degree of shadow cure. It was concluded that shadow cure may be enhanced if one or more fluorescent compounds emit fluorescent light at wavelengths absorbed by the dye in a two- or three-component photoinitiator system. The addition of DPI to the two-component systems containing MDEA and FL or EYss led to a significant enhancement in the observed shadow cure. This result was attributed to the fact that DPI will increase both the number of active centers and the mobility of the active centers as a result of the electron transfer reactions in which it participates.

Control of concentration gradient and initiating gradient photopolymerization of polysiloxane benzophenone photoinitiator

This paper reported a polysiloxane benzophenone photoinitiator (HBP-Si-A), which was synthesized based on traditional photoinitiator 4-Hydroxybenzophenone (HBP) and epoxy polysiloxane. Its structure was confirmed by proton nuclear magnetic resonance (1H-NMR), fourier transform infrared spectroscopy (FT-IR) and gel permeation chromatography (GPC). The kinetics of photopolymerization initiated by HBP-Si-A was studied by real-time infrared spectroscopy (RTIR). More importantly, it is proved by X-ray photoelectron spectroscopy (XPS), UV absorption and GPC measurements that the polysiloxane benzophenone photoinitiator (HBP-Si-A) had good ability to float up. As a result, the HBP-Si-A could easily form concentration gradient in photopolymerization system and initiate gradient photopolymerization. A convenient and simple method for preparing gradient polymer initiated by HBP-Si-A was successfully developed. The surface morphology and elemental composition of each segment of poly(tripropylene glycol diacrylate) (PTPGDA) were studied by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) respectively. In addition, HBP-Si-A could also more effectively mitigate inhibition of oxygen in radical photopolymerization.

Gel time prediction of multifunctional acrylates using a kinetics model

A kinetics model for prediction of double-bond conversion and gel time in the photopolymerization of multifunctional acrylates is presented. The system consisting of a trifunctional acrylate, trimethylolpropane triacrylate (TMPTA), and a photoinitiator, 2,2-dimethoxy-1,2-diphenylethan-1-one (DMPA), was studied using Fourier-transform infrared spectroscopy (FTIR) measurements to monitor double-bond conversion and microrheology techniques to quantify the gel time for this system. Rate constants for the kinetics model were first estimated by fitting the model only to the FTIR double-bond conversion data, and later to both the FTIR and microrheology data. The measured gel time correlated with both the calculated initial rate of radical generation and a constant value of the predicted double-bond conversion, over a broad range of conditions. The model allows for materials formulation and exposure source intensity variables to be included in stereolithography inverse problem solutions, and could be applied to other cross-linking based photopolymerization systems.

Optical characterization of photopolymers materials: theoretical and experimental examination of primary radical generation

Several studies of the time varying photon absorption effects, which occur during the photo-initiation process in photopolymer materials, have been presented. Three primary mechanisms have been identified: (i) the dye absorption, (ii) recovery, and (iii) bleaching. Based on an analysis of these mechanisms, the production of primary radicals can be physically described and modelled. In free radical photo-polymerization systems, the excited dye molecules induce the production of the primary radicals, R •, which is a key factor in determining how much monomer is polymerized. This, in turn, is closely related to the refractive index modulation formed during holographic recording. In this article, by modifying the composition of a polyvinylalcohol/acrylamide based photopolymer material, i.e. excluding any co-initiator, the photo-kinetic behaviour of the material is greatly simplified. In this way, the rate constant of intersystem crossing, k st, in going from the excited singlet state dye to the excited triple state dye can be determined. k st is then available to be applied in a full model of the photo-initiation process making it possible to accurately predict the time varying concentration of primary radicals generated during exposure.

Pyrromethene derivatives in three‐component photoinitiating systems for free radical photopolymerization

Abstract1,3,5,7,8‐pentamethyl pyrromethene difluoroborate complex (HMP) and 2,6‐diethyl‐8‐phenyl‐1,3,5,7‐tetramethylpyrromethene difluoroborate complex (EPP) were used to initiate the polymerization of a diacrylate in a two‐ and a three‐component photoinitiating system (PIS), together with an amine (ethyl‐4‐dimethylaminobenzoate, EDB) and triazine A (2‐(4‐methoxyphenyl)‐4,6‐bis(trichloromethyl)‐1,3,5‐triazine, TA) as coinitiators. For both pyrromethene dyes, the highest conversion was achieved with the three‐component PIS. As these dyes have high‐fluorescence quantum yields, steady state and time‐resolved techniques were used to study the possible fluorescence quenching by the amine and the triazine, as well as laser flash photolysis to investigate the electron transfer process that occurs in these PIS from either the singlet or triplet excited states. The electron transfer reaction is evidenced by using time‐resolved photoconductivity. Experiments show that the main interaction between the dye and both coinitiators is through its excited singlet state and the process is more efficient when TA is present. The beneficial effect noted when both coinitiators are used in a three‐component system is ascribed to secondary reactions between the coinitiators and intermediates that lead to the generation of higher amount of initiating species and the recovery of the initial dye. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2594–2603, 2010

Visible Light Photoinitiating Systems: Toward a Good Control of the Photopolymerization Efficiency

This paper discusses the photochemistry of three-component photoinitiating systems (3K-PIS) for free radical photopolymerization and the efforts made during the last decades to propose efficient systems applied to the laser imaging area. A special focus is devoted to a new 3K-PIS working in the green region. It is based on a pyrromethene dye which is reduced or oxidized by a coinitiator. A third redox component is used that leads to the recovery of the initial dye and the formation of additional initiating species, preventing a fast photobleaching of the dye. The beneficial effect on the photopolymerization rates and the final monomer conversion is clearly noticed. Laser flash photolysis was used to understand the reaction mechanisms, and detailed photopolymerization kinetics allow the study of polymeric network formation.