Effect of UV light scattering during photopolymerization on UV spectroscopy measurements

A photopolymerization reaction involving 2-hydroxyethylmethacrylate (HEMA), a hydroxytelechelic polybutadiene (HTPB) and 2,2-dimethyl-2-hydroxyacetophenone (Darocur 1173) as photoinitiator has been investigated by Real Time UV and Real Time IR spectroscopies. For this system, RTUV analyses are completely opposite to the expected curves, namely a decrease in the sample absorbance (photobleaching phenomenon). This apparent contradiction has been explained by a scattering phenomenon inside the sample, which can be linked to the formation of sub-structures that can create local fluctuations of refractive index during the polymerization. A four-flux theory, which takes into account the polymer absorption and scattering properties, has allowed us to describe the UV light intensity variation inside the sample.     

Photo - DSC and real time - FT-IR kinetic study of a UV curable epoxy resin containing o-Boehmites

Nanocomposite coatings based on a cycloaliphatic epoxy resin (3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate - CE) with two different o-Boehmites (content ranging from 5 to 10 wt.%) were prepared by cationic photopolymerization. Two different in situ monitoring techniques, photocalorimetry (p-DSC) and real time FT-IR spectroscopy (RT-IR) were used in order to investigate the kinetics of the photopolymerization process. A theoretical approach for establishing the equivalence of the irradiation conditions in the two experimental set-ups was developed in order to compare the obtained results.A substantial correspondence of the two techniques was found for the qualitative trend of the final conversion of the epoxy groups of the filled formulations relatively to the pure CE resin. Conversely, the final conversion of the epoxy groups and the reaction rate were found to be quite different. Higher kinetic profiles were obtained in RT-IR experiments, in which were also observed lower final conversions of the epoxy groups relatively to the p-DSC measurements. The presence of the nanofillers resulted in a decrease in the reactivity of the epoxy system, which was attributed to the light absorbance of Boehmites due to scattering from the clusters in the micron-size range.All the nanocomposites exhibited a high level of transparency and high T_g values, which were found to decrease slightly with increasing the nanofiller content.     

Applicability of quinolizino-coumarins for monitoring free radical photopolymerization by fluorescence spectroscopy

Applicability of commercially available 2,3,5,6-1H,4H-tetrahydro-quinolizino[9,9a,1-gh]coumarin (Coumarin 6H) and its 9-methyl (Coumarin 102), 9-trifluoromethyl (Coumarin 153) and 10-carboxy (Coumarin 343) derivatives as fluorescent molecular probes for monitoring progress of free radical photopolymerization of several acrylic and methacrylic monomers by Fluorescence Probe Technique (FPT) has been tested. The progress of the photopolymerization was monitored using a specially designed cure monitoring system. It was found that all the quinolizino-coumarins shifted their fluorescence spectra towards shorter wavelengths with progress of polymerization, which enabled monitoring the progress in terms of fluorescence intensity ratios as the progress indicator. Coumarin 6H turned out to be the most sensitive to changes occurring during polymerization. Coumarin 102 and Coumarin 153 exhibit only about 20% lower sensitivity than that of Coumarin 6H, so those are also good enough for the cure monitoring of acrylic monomers, except for tetraethylene glycol diacrylate, where the quinolizino-coumarins response was disturbed by some fluorescent side product. Moreover, it has been found that the FPT technique has some limitations in the case of monofunctional monomers.     

Synthesis of spherical microcapsules by photopolymerization in aerosols

 The preparation of polymer microcapsules of well defined size in the range of 10–50 μm with different shell thickness to core diameter ratios is described. An aerosol of monodisperse droplets of a homogeneous ternary liquid system which contained a hydrophobic component and a hydrophilic component dissolved in a high-volatile mutual solvent, was produced by dispersing with a vibrating-orifice aerosol generator. After the evaporation of the solvent in a nitrogen atmosphere the particles demix and form a two-phase droplet of core-shell type. These droplets were illuminated with UV light and polymerized to highly monodisperse microcapsules with a solid polymer shell and a liquid core. The properties of the resulting particles (size, size distribution, shell thickness, shape and surface characteristics) were investigated by scanning electron microscopy, Raman spectroscopy on single optically levitated particles, and confocal Raman micro spectroscopy. The microcapsules were highly monodisperse and have spherical shape. Received: 24 July 1996 Accepted: 29 August 1996     

Study of the polymerization of acrylic resins by photocalorimetry: interactions between UV initiators and absorbers

Photocurable finishes should be extensively used for industrial wood joinery in the near future even if some technical problems have still to be solved and if the durability of the finishing system has to be enhanced. In a previous work, we studied the curing reaction of acrylic resins containing some particular photoinitiators by photocalorimetry and showed that curing can be achieved under daylight with these compounds (i.e. solving the problem of restoration on site). The aim of this work is to enhance the durability of these systems by incorporating UV absorbers into the formulations. The consequences of such an addition onto the curing kinetics are again investigated by photocalorimetry.     

In situ real-time monitoring of epoxy/amine kinetics by remote near infrared spectroscopy

An experimental set-up for in situ real-time monitoring of chemical reactions by remote fiber optic near-infrared spectroscopy was assembled in our laboratory and is described in this communication. A series of epoxy/amine formulations, nonpolymer-forming, and polymerforming alike, were prepared and their reaction kinetics investigated. Accurate and reproducible spectra were generated, the characteristic near-infrared peaks assigned, and the reaction kinetics evaluated by monitoring the rates of disappearance and/or appearance of the characteristic peaks. The effect of the chemical structure and functionality of the components were pointed out and a method suggested to correlate the spectral information with the chemophysical changes in multifunctional formulations.     

Synthesis of monodisperse polymer microspheres by photopolymerization of microdroplets

We have examined photopolymerization of highly monodisperse microdroplets of monomer solutions under UV-light radiation. Microdroplets were generated using a modified vibrating aerosol generator, and the diameter of the droplets can be tuned to any size between 5 to 100 m. Polymer particles derived from the droplets were characterized by optical microscopy and SEM. The results show that the polymer particles, under optimum conditions, can be highly spherical and monodisperse. The diameter and morphology of resulting microspheres depend on the diameter of the monomer solution droplets, monomer concentration, photopolymerization reaction temperature, residence time, and droplet dispersion.     

Influence of non-reactive solvent on optical performance, photopolymerization kinetics and morphology of nanoporous polymer gratings

A study of nanoporous polymer gratings, with controllable nanostructured porosity, as a function of grating performance, photopolymerization kinetics and morphology is presented. Modifying the standard holographic polymer dispersed liquid crystal (H-PDLC) system, by including a non-reactive solvent, results in a layered, nanoporous morphology and produces reflective optical elements with excellent optical performance of broadband reflection. The addition of the non-reactive solvent in the pre-polymer mixture results in a morphology typified by void/polymer layer-by-layer structures if sufficient optical energy is used during the holographic writing process. The duration and intensity of optical exposure necessary to form well-aligned nanoporous structures can only be obtained in the modified system by (a) illumination under longer time of holographic interference patterning (30 min) or (b) illumination under very short time of holographic interference patterning (30 s) and followed by post-curing using homogeneous optical exposure for 60 min. Comparatively, a typical H-PDLC is formed in less than 1 min. To further understand the differences in the formation of these two analogous materials, the temporal dynamics of the photoinitiation and polymerization (propagation) kinetics were examined. It is shown herein that the writing exposure gives a cross-linked polymer network that is denser in the bright regions. With 60% (or even 45%) acrylate conversion, almost no free monomer would be left after the writing. Continued exposure serves primarily to add cross-links. This has the tendency to collapse the network, especially the less dense portions, which in effect get glued down to the more dense parts. To the extent that the solvent increases the mobility of the polymer network, this process will be aided. Equally important, the size of the periodic nanopores can be varied from 10 to 50 nm by controlling either the LC concentration in the pre-polymer mixture or by controlling the time of the homogeneous post-cure.     

Novel preparation of monolithic imprinted columns for electrochromatographic separation by photopolymerization

A monolithic molecularly imprinted polymer with specific recognition ability for 4-hydroxybenzoic acid (4-HBA) was prepared by in situ photopolymerization, using methacrylic acid (MAA) as a functional monomer, ethylene glycol dimethacrylate (EDMA) as a cross-linking agent, toluene and isooctane as porogenic solvents and Irgacure 1800 as an initiator. Baseline separation of isomers of hydroxybenzoic acid was achieved in less than 8 min on this monolithic column using 4-HBA as template, but not on the blank polymer. Furthermore, some neutral compounds could also be baseline-separated on the imprinted polymer column in the mode of pressure-driven capillary electrochromatography.     

Monolithic microextraction tips by emulsion photopolymerization

Monoliths formed by photopolymerization are excellent means for fabricating functional elements in miniaturized microdevices such as microextraction tips which are becoming important for sample preparation. Various silica-based and polymer-based materials have been used to fabricate monoliths with through pores of several nm to 4 μm. However, the back pressure created by such methods is still considered to be high for microtips that use suction forces to deliver the liquid. In this study, we demonstrated that emulsion techniques such as oil-in-water can be used to form monoliths with large through pores (>20 μm), and with rigid structures on small (10 μL) and large (200 μL) pipette tips by photopolymerization. We further showed that, with minor modifications, various functionalized particles (5–20 μm) can be added to form stable emulsions and successfully encapsulated into the monoliths for qualitative and quantitative solid-phase microextractions for a diverse application. Due to high permeability and large surface area, quick equilibration can be achieved by pipetting to yield high recovery rates. Using tryptic digests of ovalbumin as the standard, we obtained a recovery yield of 90–109% (RSD: 10–16%) with a loading capacity of 3 μg for desalting tips immobilized with C18 beads. Using tryptic digests of β-casein and α-casein as standards, we showed that phosphopeptides were substantially enriched by tips immobilized with immobilized metal affinity chromatography or TiO₂ materials. Using estrogenic compounds as standards, we obtained a recovery yield of 95–108% (RSD: 10–12%) and linear calibration curves ranging from 5 to 100 ng (R² > 0.99) for Waters Oasis HLB tips immobilized with hydrophilic beads.     

On-line monitoring of the acrylate conversion in UV photopolymerization by near-infrared reflection spectroscopy

Near-infrared (NIR) reflection spectroscopy was used to determine the conversion in acrylate coatings after UV photopolymerization in order to test it as a method for process control in UV curing. A probe head was developed which is adapted to the specific requirements of UV curing and which is linked to a photodiode array spectrometer by an optical fiber. Reflection spectra from thin acrylate layers which were taken in intervals down to the millisecond range have shown an excellent signal-to noise ratio. Quantitative conversion data show good correlation with results from independent reference methods (FTIR, HPLC). Following thesebasic investigations, it was demonstrated that NIR reflection spectroscopy can be used for on-line monitoring of the acrylate conversion in thin coatings. Some examples of such investigations in pilot scale are presented.     

Use of functional microgels with vinyl groups to accelerate photopolymerization reaction

Three types of functional microgels with vinyl groups on their surface were prepared. For the first type, the counter anion from clorin was exchanged with β-methacryloylethyl sulfonic acid, styrene sulfonic acid or allyl sulfonic acid in a microgel with ammonium anions. For the second and third types, a quaternization with N,N-dimethylaminoethyl methacrylate of 3-chloro-2-hydroxypropyl methacrylate in the presence of microgel particles was prepared by emulsion copolymerization of styrene, chloromethylstyrene or N,N-dimethylamino-methylbenzene, and divinylbenzene. The resulting samples show good dispersibility in organic solvents without an emulsifier. A functional microgel-based photopolymer combined with an acrylate monomer and ultraviolet (UV) or visible (VIS) light-absorbing photoinitiators provides oleophilic images when exposed to UV or VIS light and developed in tap water. This photopolymer has a higher sensitivity than those of photopolymers based on microgels with an analogous composition but without vinyl groups. Photopolymers pepared by using functional microgels with a methacryloyl group exhibited a higher rate of polymerization (R_p) than that of photopolymers based on microgels without a vinyl group. The R_p of photopolymers prepared by using a functional microgel with either an allyl group or vinylphenyl group was nearly equal to that of photopolymers based on microgels with ammonium ions. Their high sensitivities are attributed to the rapid photopolymerization in the methacryloyl group. To determine how the photoreaction mechanism enhances sensitivity, the photoreaction products were investigated using a model photopolymerization system. It was found that the gelation reactions enhancing sensitivity are predominantly the polymerization and crosslinking ones when a microgel with the methacryloyl group is used, and the graft copolymerization with acrylate monomers when a microgel with either the allyl group or vinylphenyl group is used.     

Monitoring of visible light photopolymerization of an epoxy/dimethacrylate hybrid system by Raman and near-infrared spectroscopies

Hybrid systems formulated with epoxy and methacrylate monomers at mass fraction either 50:50 or 75:25 were studied. The individual monomer conversions during photopolymerization with visible light were monitored by Raman and near-infrared spectroscopies. The rate of polymerization and final degree of conversion of the methacrylate groups were raised when the epoxy monomer was present. This is attributed to enhancement of the mobility of the reactive species caused by the presence of the epoxy monomer. Conversely, the earlier vitrification of the system due to the faster polymerizing methacrylate network resulted in reduced conversion of epoxy rings.     

Aminophthalimide probes for monitoring of cationic photopolymerization by fluorescence probe technology and their effect on the polymerization kinetics

The performance of a series of phthalimide derivatives as fluorescent probes for monitoring of cationic photopolymerization of monomers by Fluorescence Probe Technology (FPT) has been studied in comparison to the performance of a 7-diethylamino-4-methylcoumarin (Coumarin 1) probe. It was found that the phthalimide probes are much more stable than Coumarin 1 under cationic polymerization conditions. Moreover, the aminophthalimide probes turned out to be long-wavelength sensitizers for the diaryliodonium photoinitiators used to initiate the polymerization process. The sensitizing effect of the probes compensates in excess the inhibiting effect of their amino groups on the cationic polymerization kinetics.     

Thermal properties and ionic conductivity of a polymer prepared by the in situ photopolymerization of a vinylimidazole monomer containing a mesogenic group

We have synthesized liquid crystalline polymers containing an imidazolium salt moiety and a mesogenic group by the in situ photopolymerization of a liquid crystalline vinylimidazole monomer in order to investigate the relationship between their thermal properties and ionic conductivity. A smectic phase was shown by the vinylimidazole monomer. The in situ photopolymerization of the monomer was carried out in the temperature range of the smectic phase. The polymer thus prepared displayed a highly ordered smectic phase in the temperature range between room temperature and about 200°C. The ionic conductivity of the polymer increased with increasing temperature. Anisotropic ionic conductivity behavior was observed for the polymer. The ionic conductivity of the polymer aligned homogeneously is larger than when homeotropically aligned.     

Numerical simulation of laser-induced breakdown spectroscopy: Modeling of aerosol analysis with finite diffusion and vaporization effects

The application of laser-induced breakdown spectroscopy (LIBS) to aerosol systems has been shown to provide quantitative analysis of particle-derived species; however, the exact nature of the plasma/particle interactions remains to be fully understood. Although the plasma/particle interaction may be idealized within a framework of instantaneous vaporization and analyte diffusion throughout the plasma volume, experimental evidence suggests that these processes actually occur on finite time scales relative to the plasma decay times at which measurements are frequently taken. In the present work, a numerical simulation of the temperature and species concentration fields of a plasma containing a single particle, including dissociation and diffusion on semi-empirical finite time scales, is developed. Using these results, the intensity of analyte emission is calculated as a function of time, and the standard ion/neutral ratios typical of aerosol-derived LIBS signals are calculated. Furthermore, the ratio of ion/neutral ratios for two different species was used to assess the temperature homogeneity of the particle-derived analytes in comparison to the overall plasma temperature field. From this numerical study, it is shown that the finite time scale of evaporation and diffusion of aerosol material results in a non-uniform spatial distribution in concentration. This results, in turn, in temperature and free electron density gradients within the plasma, leading to variation between the local conditions surrounding aerosol mass and the bulk conditions of the plasma as a whole.     

Gel formation and photopolymerization during supramolecular self-assemblies of α-CDs with LA-PEG-LA copolymer end-capped with methacryloyl groups

A novel gelation occurs in water during supramolecular self-assemblies of α-cyclodextrins being threaded onto amphiphilic LA-PEG-LA copolymer end-capped with methacryloyl groups. The rheologic studies show that the gels are thixotropic and reversible. While exposed to UV irradiation with a photoinitiator added in advance, they can be photopolymerized in situ to give rise to chemically cross-linked biodegradable hydrogels with the markedly improved mechanical strength. The gels formed prior to and after UV irradiation are characterized using FTIR, ¹H NMR, WAXD and TGA techniques. The swelling ratio and in vitro degradation of the photocured hydrogels are also investigated. It appears that both physical and chemical gels have the potential to be used as injectable biomaterials.     

Novel polymeric photoinitiators comprising of side-chain benzophenone and coinitiator amine: Photochemical and photopolymerization behaviors

Three kinds of macrophotoinitiators, PBP-P, PBP-E and PBP-B, were synthesized by step-polymerization of benzophenone and different coinitiator amino monomers. The low molecular weight analogue, 2,4-di(3-(diethyl amino)-2-hydroxypropoxy)-benzophenone (DAHBP), was also synthesized as a low molecular weight model compound. The UV-vis spectra of PBP-P, PBP-E, PBP-B and DAHBP are similar with large red-shifted maximum absorption comparing with BP. ESR spectra of PBP-P and DAHBP possess the same initiation mechanism with DEBP/TEA systems. The photopolymerization of two monomers with different functionality poly(propylene glycol)diacrylate (PPGDA) and trimethylolpropane triacrylate (TMPTA), initiated by macrophotoinitiators and low molecular weight analogs, was studied through photo-DSC. The results show that different efficiency of photoinitiators towards monomers: first, polymeric photoinitiators are more efficient than low molecular weight analogs; then PBP-E is the most efficient for PPGDA; lastly, PBP-B is the most efficient for TMPTA. The efficiency of the photopolymerization is mainly affected by structure of amine in macrophotoinitiator.     

Photoinduced coupling and adsorption of caffeic acid on silver surface studied by surface-enhanced Raman spectroscopy

The effect of light on the caffeic acid (CA) oxidative coupling is studied in aqueous solution and on silver by surface-enhanced Raman spectroscopy (SERS). CA can polymerize in aqueous solution or on a metal surface through an oxidative mechanism involving the formation of the corresponding quinone giving rise to characteristic Raman features in each case. We show here that the effect of light in relation to this oxidative coupling is crucial taking place mainly in the solution bulk. The products derived from such polymerization can then adsorb on the silver surface employed for SERS measurements, thus allowing its detection by Raman spectroscopy. The influence of irradiation time and the wavelength of the light employed for the photoinduced coupling was investigated.