Efficient Photoinitiating Systems Research Articles

Graphene quantum dots–From spectroscopic performance to 3D printing applications and interaction studies with normal and cancer cells

In this study, different types of graphene quantum dots (GQDs) are investigated to understand their spectroscopic properties. Morphological and size analyses were performed. The mechanism of action of radical photopolymerization was evaluated. The obtained materials proved to be useful for developing efficient photoinitiating systems for 3D printing applications. In addition, the cytotoxicity of the GQDs was studied, and their effects on living cells were investigated. We demonstrated that GQDs can not only act as efficient photoinitiators but also as constituents of hydrogels that express very low toxicity and may interact with cells. The latter makes it possible to study them in the future as useful modern biosensors.

Towards safe phosphine oxides photoinitiators with good cytocompatibility for 3D printing of thermoplastics

Abstract3D printing is the technology of choice for the rapid production of objects with complex morphologies and controlled physical and chemical properties. 3D printing by photopolymerization is of great interest because of its ability to access very small scales and its high resolution. In the case of recent 3D printers, the preferred wavelength for light irradiation is 405 nm since it is a safe and energy‐economical irradiation. For 3D printing to be effective, it is therefore necessary to develop easily accessible, stable and highly efficient photoinitiating systems at the 405 nm wavelength. Beside, with the increasing use of photoinitiators, it is essential to develop structures with low cytotoxicity. Hence, the development of new photoinitiators is currently of major interest. To this end, the photochemical properties (triplet energy, bond dissociation energy, cleavage reaction enthalpy, and absorption properties) of several molecules have been calculated by molecular modeling and the most promising compounds have been synthesized. In this paper, four phosphine oxides photoinitiators (ADPO‐1, CPO‐2, CPO‐3, and FPO‐1) are synthesized and characterized. Subsequently, their absorption properties are measured and their efficiencies in photopolymerization under LED irradiation at 405 nm are evaluated. It is important to note that these structures have never been reported in 3D printing. Markedly, among the obtained photoinitiators, two of them show better efficiency than the commercially available and widely used phenylbis(2,4,6‐trimethylbenzoyl)phosphine oxide (BAPO) in photopolymerization (i.e., (Rp/[M0]) x100 values are 6.94, 12.95, and 4.93 s−1 for CPO‐2, ADPO‐1, and BAPO; with [M0] the initial acrylate function concentration). Furthermore, one of the synthesized photoinitiators is successfully used in 3D printing of thermoplastics for potential recycling ability. Finally, one of the obtained structures shows a lower cytotoxicity than the benchmark structure diphenyl(2,4,6‐trimethylbenzoyl)phosphine oxide (TPO).

N-ethyl carbazole-1-allylidene-based push-pull dyes as efficient light harvesting photoinitiators for sunlight induced polymerization

In this article, the free radical polymerization of acrylates was successfully achieved with a series of twelve push-pull chromophores comprising the N-ethyl carbazole-1-allylidene group as the electron-donating group. Using this strong donor, the resulting dyes, never synthesized before, could efficiently initiate the free radical photopolymerization of acrylates upon mild irradiation conditions e.g. using sunlight. These novel multi-component systems comprising the above mentioned push-pull dyes, a tertiary amine (ethyl dimethylaminobenzoate EDB) and an iodonium salt could act as efficient photoinitiating systems (PISs) in free radical polymerization (FRP), leading to the formation of 3D patterns with precise shapes under the direct laser write (DLW) approach. Among these new PISs, dyes 3, 6, 7 and 8 were selected due to their efficient photoinitiation performances for chemical mechanism studies including steady state photolysis, fluorescence quenching measurements, electron spin resonance (ESR) spin-trapping experiments and cyclic voltammetry. Markedly, their excellent photochemical reactivities prompted us to investigate the performance of these multi-component photoinitiating systems upon sunlight as an ecofriendly approach. As a result, this research proved that sunlight could be used as a potential light source that can advantageously replace LEDs when highly reactive push-pull dyes are used as photosensitizers for the free radical photopolymerization of acrylate resins. Finally, 3D patterns could be prepared with the new photocomposites, and silica fillers could be even incorporated within the photosensitive resins so that a gradient of resolution could be successfully demonstrated.

Spectroscopic Studies of the Interactions between a Cationic Cyanine Dye and a Synthetic Phyllosilicate: From Photophysics to Hybrid Materials.

The interaction of the cationic organic dye Astrazon orange R (AO-R) with the synthetic phyllosilicate Laponite leads to very interesting hybrid materials. Indeed, the Laponite nanoparticles modify the photophysical properties of AO-R, inducing a stabilization of its excited emissive state by preventing ultrafast isomerization. The long-lived emissive clay-dye hybrid complex can be used to develop efficient photoinitiating systems, leading to organic-inorganic hybrid crosslinked polymer materials.

Photopolymerization of pyrrole/methacrylate mixtures using α-cleavage type photoinitiators in combination with iodonium salt

Hybrid systems formulated with pyrrole and methacrylate monomers were photopolymerized with both UV (λ=365nm) and visible (λ=470nm) light. It is known that conducting polymers produced by electrochemical or chemical synthesis are intractable solids or powders which display poor mechanical properties and low processability. An advantage of the photopolymerization process is that it allows processability and mechanical properties of final polymers to be optimized by incorporating flexibilizers and different additives into photopolymerizable formulations. Mixtures pyrrole/methacrylate were photoactivated with the iodonium salt p-(octyloxyphenyl)phenyliodonium hexafluoroantimonate (Ph2ISbF6), in combination with 2,2-dimethoxy-2-phenylacetophenone (DMPA), 2-methoxy-2-phenylacetophenone (BZME) or the pair camphorquinone (CQ)/ethyl-4-dimethylamino benzoate (EDMAB). The Ph2ISbF6 in combination with DMPA or BZME was an efficient photoinitiator system under irradiation at 365nm. Conversely, in mixtures photoactivated with Ph2ISbF6/CQ/EDMAB the polymerization of both pyrrole and methacrylate was comparatively slow. Microscopy studies revealed the absence of phase separation indicating that the mixtures pyrrole/methacrylate resulted in the formation of an interpenetrating polymer network. The electrical conductivity of the hybrid polymers increased markedly with the amount of polypyrrole as a result of the formation of a conducting polymer network in the insulating BisEMA matrix.

Photopolymerization Reactions: On the Way to a Green and Sustainable Chemistry

The present paper reviews some aspects concerned with the development of green technologies in the photopolymerization area: use of visible light sources (Xe and Hg-Xe lamps, diode lasers), soft irradiation conditions (household lamps: halogen lamp, fluorescence bulbs, LED bulbs), sunlight exposure, development of very efficient photoinitiating systems and use of renewable monomers. The drawbacks/breakthroughs encountered when going on the way of a greener approach are discussed. Examples of recent achievements are presented.

Polymerization of methacrylate resins photoinitiated by camphorquinone and bulky amine-functionalized silsesquioxanes

Polyhedral oligomeric silsesquioxanes (SSQO) functionalized with bulky amino groups were prepared by a two-stage synthesis procedure. In a first stage, a modified organotrimethoxysilane was synthesized by reacting 1 mol of N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane with 3 mol of 1,2-epoxy-3-phenoxypropane. The second stage consisted of the hydrolytic condensation of the modified silane catalyzed by formic acid. Methacrylate resins were activated for visible light polymerization by the addition of camphorquinone (CQ) in combination with different mass fractions of SSQO. The progress of monomer conversion versus irradiation time showed that the CQ/SSQO pair is an efficient photoinitiator system because a fast reaction and high conversion results from 60 s irradiation at 600 mW/cm2. The SSQO were incorporated up to 22 wt% in the methacrylate resin without compromising the flexural or the compressive properties of the final material.

Photopolymerization of methacrylate monomers using polyhedral silsesquioxanes bearing side-chain amines as photoinitiator

Organotriethoxysilanes (APS-PGE2) were synthesized by reacting 1 mol of 3-(aminopropyl)triethoxysilane with 2 mol of 1,2-epoxy-3-phenoxypropane (PGE). Polyhedral oligomeric silsesquioxanes functionalized with bulky amino groups (ASSQO) were prepared by hydrolytic condensation of APS-PGE2 catalyzed by formic acid. Methacrylate resins were activated for visible light polymerization by the addition of 1 wt.% CQ in combination with the synthesized ASSQO at loadings between 0 and 30 wt.%. The progress of monomer conversion versus irradiation time showed that the CQ/ASSQO pair is an efficient photoinitiator system because a fast reaction and high conversion result from 60 s irradiation at 600 mW/cm 2. The lack of methacrylate groups in the ASSQO, able to polymerize with the methacrylate resin, results in the absence of chemical bond between the ASSQO cages and the matrix. Debonding of ASSQO cages from the polymer give rise to nanovoids; which allows the methacrylate matrix to yield and deform plastically. Consequently, the final effect is a decrease in the flexural modulus and compressive strength with increasing amounts of ASSQO. The present study highlights the surface effect on the overall properties in nanostructured materials.

The synthesis, spectroscopic and electrochemical properties, and application of new dyeing photoinitiator systems for acrylate monomers polymerization

New symmetrical bicationic polymethine dyes were synthesized and their spectroscopic and electrochemical properties were described. The bichromophoric dyes (benzothiazole, benzoxazole, indolinium derivatives) were investigated as sensitizers in the free radical photopolymerization initiated by their borate salts. The obtained kinetic results shown that bicationic polymethine dyes as the organoborate salts are much efficient photoinitiating systems of acrylate monomers polymerization than monocationic parent dyes. The rate of polymerization depends on Δ G ET of electron transfer from borate anion to the excited singlet state of bicationic polymethine dye. The relationship between the rate of polymerization and the free energy of electron transfer process shows the dependence predicted by the classical theory of electron transfer.

Non-extractable photoinitiators based on thiol-functionalized benzophenones and thioxanthones

Non-extractable photoinitiators could be advantageous for use in biological, electronic, and food packaging applications. Therefore, thioxanthone and benzophenone derivatives were synthesized from 1,6-hexane dithiol and chlorinated benzophenone or thioxanthone. The efficiency of thiol-functionalized photoinitiators in combination with amine co-initiators was compared to benzophenone and isopropylthioxanthone with amine co-initiators, and the cleavage of photoinitiator 2,2-dimethoxy-1,2-diphenylethan-1-one in acrylic resins. The reaction kinetics were analyzed using photo-differential scanning calorimetry and real-time FTIR. Coating physical properties were evaluated by pendulum and pencil hardness, steel–wool scratch and mandrel bend tests. The non-extractable photoinitiators had higher absorbance than their benzophenone or isopropylthioxanthone counterparts due to the sulfide substitution on the phenyl ring, and the free thiol groups reacted with the acrylate by either an amine catalyzed Michael addition or a free-radical chain process. The combination of thiol-functionalized photoinitiators with secondary amines provides an efficient photoinitiator system that is locked into the photopolymerized network and cannot be extracted with typical solvents.

Three‐cationic carbocyanine dyes as sensitizers in very efficient photoinitiating systems for multifunctional monomer polymerization

AbstractThis article is focused on the description of the synthesis, spectroscopic, electrochemical properties, and the free radical photoinitiation abilities of new three‐cationic carbocyanine dyes. A new synthestic strategy for preparation of symmetrical cyanine dyes has been developed based on the 2‐methylbenzothiazole, 2‐methylbenzoxazole and 2,3,3‐trimethylindolinium quaternization by α‐bromo‐ω‐tertiary ammonium salts followed by the condensation of obtained product with triethyl ortho‐formate. Resulted dyes possess in one molecule three quaternary nitrogen atoms, for example, they are three‐cationic in character. Novel three‐cationic symmetric carbocyanine dyes paired with n‐butyltriphenylborate anions are very efficient photoinitiators of free radical polymerization of acrylate monomers, for example, (TMPTA) when irradiated with the visible emission of an argon‐ion laser. The photoinitiating abilities of the novel photoredox pairs were compared to the photoinitiation properties of their monocationic equivalents, for example, N,N′‐diethylthia‐, N,N′‐diethyloxa‐ and N,N′‐dimethylindocarbocyanine n‐butyltriphenylborates (SB2, OB2, and IB2) and also compared to the initiation ability of Rose Bengal derivative, for example, typical triplet state photoinitiator. The kinetic studies clearly demonstrated that the three‐cationic carbocyanine borate photoredox pairs exhibit a significant increase in the efficiency of TMPTA free radical polymerization in comparison to the photoredox pairs containing structurally related, mono‐cationic carbocyanine borates. It was also shown that, the addition of a second coinitiator to the cyanine borate salts enhances their photoinitiating abilities. The best results were obtained for three cationic carbocyanine borates in the presence of second coinitiator being N‐methoxypyridinium n‐butyltriphenylborate. The photoinitiating abilities of the systems of these types are more effective than those composed of carbocyanine cation/borate anion exclusively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4636–4654, 2009

Effect of coinitiator type on initiation efficiency of two‐component photoinitiator systems based on Eosin

AbstractEffect of coinitiator structure on relative initiation efficiency of two‐component Eosin/coinitiator systems has been evaluated quantitatively in polymerization of poly(ethylene glycol) diacrylate as an example monomer. The initiation efficiency has been measured by the Fluorescence Probe Technique (FPT), using Eosin both as a photoinitiator component and as a fluorescent probe. A LED/fiber optic‐based measurement system has been developed and applied in this study. It has been found that from among 17 compounds tested, the following coinitiators form most efficient photoinitiating systems in combination with Eosin, when exposed to visible light: coinitiator, relative efficiency = triethanolamine, 1.0; 2,6‐diisopropyl‐N,N‐dimethylaniline, 0.70; 2‐benzyl‐2‐(dimethylamino)‐4′‐morpholinobutyrophenone (Irgacure 369), 0.91; carbon tetrabromide, 2.1; [4‐[(2‐hydroxytetradecyl)oxy]phenyl]phenyliodonium hexafluoroantimonate (SarCat 1012), 28. These relative efficiencies refer to the following component concentrations: [Eosin] = 8.6 × 10−4 M, (0.05% by weight); [coinitiator] = 4.3 × 10−2 M. The factors affecting the initiation efficiency of the systems studied are discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3519–3532, 2008

The role of the photoinitiating systems towards the inhibiting effect of phenolic compounds in the photopolymerization of acrylates

Photopolymerization reactions of wood coatings under UV and visible light have been carried out. The influence of model phenolic derivatives found in wood extractives on the polymerization kinetics as well as the coating properties of clear coating formulations exposed to light in laboratory and industrial type conditions has been discussed as a function of various UV/visible or visible photoinitiating systems based on ketones and dyes. The properties of suitable formulations are discussed. Several kinds of efficient photoinitiating systems are particularly investigated such as isopropylthioxanthone/amine; bis-acylphosphine oxide derivative/isopropylthioxanthone/amine and Rose Bengal/amine/additive.

Vitrification and curing studies of a photopolymerizable semi‐interpenetrating polymer network. II

AbstractThe photopolymerizable semi‐interpenetrating polymer network‐II being studied consists of a linear copolyester, a crosslinked network of multifunctional acrylate monomers and a very efficient photoinitiator system. Using both Raman and dynamic mechanical spectroscopy (DMS), it has been determined that the polymerizing network vitrifies after a very low level of crosslinking is attained. This results in a sample that has a Tg which is substantially lower than would be expected from knowledge of the values of Tg for the individual components of the blend. The extent of reaction and, therefore, the glass transition temperature can be increased by heating the sample during photopolymerization. In addition, the glass transition temperature of the network can be increased by thermally curing the sample after photopolymerization. It has been found that once the network has been fully cured, the glass transition temperature is much higher than would be predicted. This has been interpreted in terms of synergistic effects; conceivably, the acrylates form a tight matrix around the linear copolyester and effectively reduce the molecular mobility of the components. The results also show that the acrylates and the copolyester are not inherently miscible. However, if a sufficient concentration of acrylates is present, it is possible to prevent the copolyester from phase separating. Dynamic mechanical spectroscopy data do indicate that these semi‐inter‐penetrating polymer networks‐II (semi‐IPN‐II) samples are somewhat heterogeneous; however, there does not seem to be any gross phase separation. Transmission, electron microscopy (TEM) results show that the most fully cured sample has domains on the order of 50–250 Å associated with the copolyester phase.