Real-time Infrared Spectroscopy Research Articles

Photopolymerization of alicyclic methacrylate hydrogels for controlled release

AbstractAlicyclic hydroxy methacrylate monomer, o‐hydroxycyclohexyl methacrylate (HCMA), was synthesized and characterized by Fourier transformed infrared spectroscopy (FT‐IR) and proton nuclear magnetic resonance spectroscopy (1H‐NMR). Photopolymerization kinetics of HCMA was investigated via real‐time infrared spectroscopy (RT‐IR). Polymeric network hydrogels based on hydroxyethyl methacrylate (HEMA) and HCMA were prepared by using the photopolymerization technique. Mechanical strength, swelling characteristic, and controlled release behavior of hydrogels with various feed compositions were studied. Poly(HEMA‐co‐HCMA) hydrogel had higher storage modulus than that of poly(HEMA) hydrogel as investigated by dynamic mechanical analysis (DMA). Acid orange 8 was used as a model drug for the investigation of drug release behavior of copolymeric hydrogels. Results indicated that increase in HCMA ratio in hydrogel composition could reduce the swelling rate and prolong the release time. Scanning electron microscopy (SEM) was also utilized to study the surface morphology of hydrogels, and the results indicated that HCMA content influenced pore diameter on the hydrogel surface. Copyright © 2008 John Wiley & Sons, Ltd.

Synthesis and characterization of 4,4′-diacryloyloxybenzophenone

A copolymerizable type II photoinitiator 4,4′-diacryloyloxybenzophenone (DABP) based on 4,4′-dihydroxybenzophenone (DHBP) and acryloyl chloride was synthesized, and its structure was confirmed by 1H NMR and real-time infrared spectroscopy. UV–vis absorption spectroscopy of DABP exhibited red-shifted maximal absorption as compared with benzophenone (BP). The kinetics of photopolymerization was studied by real-time infrared spectroscopy. It showed that DABP was a more effective photoinitiator than benzophenone. When this photointiator and amine was used to efficiently initiate polymerization of acrylates and methacrylates, both rate of polymerization and final conversion increased and the induction period was shortened with increase of amine concentration, DABP concentration and light intensity.

Nanoclay dispersion effects on UV coatings curing

Clay-based nanocomposites have already shown their great potential. In this study, clay–acrylate nanocomposite formulations, for wood finishes, were prepared in four differents ways. High-speed mixing, ball milling, bead milling and three roll milling were used to disperse clay into the neat acrylate formulation. The viscosity of the formulations was determined. Small angle X-ray scattering and transmission electron microscopy experiments were performed on these samples in order to study the quality of the dispersion. Then, photo-calorimetry and real-time infrared spectroscopy were used in order to investigate the effects of clay loading and clay dispersion on the ultraviolet curing. These studies have shown that UV curing is related to the quality of clay dispersion. Presence of large clay aggregates decreases the conversion of the acrylate functional groups, even at a small clay loading. However, a small amount of clay, well dispersed, can help UV curing.

Cyclic acetal as coinitiator for bimolecular photoinitiating systems

Cyclic acetals were used to replace the conventional amines in bimolecular photoinitiating systems. The mixtures of benzophenone derivatives and cyclic acetals were used to initiate the UV photopolymerization of 1,6-hexanedioldiacrylate (HDDA). Camphorquinone (CQ)/1,3-benzodioxole (BDO) combinations were used to initiate the visible light photopolymerization of 2,2-bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (BisGMA)/triethylene glycol dimethacrylate (TEGDMA) (70/30 wt%) for dental application. The kinetics was recorded by real-time infrared spectroscopy (RTIR). Ethyl 4- N, N-dimethylaminobenzoate (EDMAB) was used as control in the same photocuring condition. The results showed that the addition of cyclic acetals greatly increased the rate of polymerization ( R p) and final double bond (DC) of HDDA. Combination of p-chlorobenzophenone (CBP)/BDO had the highest initiating reactivity. BDO also showed an effective coinitiator for camphorquinone-based initiator system. Comparing with EDMAB, CBP/BDO and CQ/BDO indicated comparable initiating reactivity. Moreover, the natural component characteristics of BDO made it a promising alternative to commercial amine in biomolecular photoinitiating system.

Synthesis and characterization of aliphatic amine co-initiator with different chain length for photopolymerization of dimethacrylate

Four kinds of aliphatic amines co-initiator with different chain length were synthesized via Michael-Addition reaction based on aliphatic amine and methyl acrylate. The addition products were characterized by Fourier transform infrared (FT-IR) and 1H NMR spectroscopy. UV–vis absorption and fluorescence spectroscopy were employed to investigate the photophysical process of combination of camphorquinone (CQ) and the newly synthesized amine. The activity of amine co-initiator was investigated by real-time infrared spectroscopy (RTIR). The results showed that the aliphatic amine with different chain length had no effect on the photoinitiation quantum yield of CQ and the formation of amine free radical. The double bond conversion and the rate of photopolymerization increased as the increase of amine concentration. Although the rate of photopolymerization had a slight decrease with the increase of chain length of aliphatic amine, it had only minor effect on the double bond conversion.

UV curable epoxy acrylate–clay nanocomposites

UV curable epoxy acrylates were reinforced with two different organically modified montmorillonites (MMTs) and an unmodified MMT. Conversion and rate of polymerization was monitored by real time infrared spectroscopy (RTIR) and photo-DSC. Microstructures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and optical clarity. Optical clarity of the films containing clay was quite good as only a slight decrease was observed. Physical properties of the reinforced films were examined by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), hardness and tensile testing. Enhancements in glass transition temperature ( T g), thermal stability and mechanical properties were observed. The films reinforced with the unmodified MMT exhibit the most significant enhancements in properties.

Synthesis and evaluation of N, N-dimethyl- N′, N′-di[2-(methylacryloyl)-ethoxycarbnylethyl] propyldiamine as a coinitiator

The common diluent and coinitiator for current dental resin composite were triethylene glycol dimethylacrylate (TEGDMA) and ethyl-4-dimethylaminobenzoate (EDMAB), respectively. In this study, a polymerisable coinitiator N, N-dimethyl- N′, N′-di[2-(methylacryloyl)-ethoxycarbnylethyl] propyldiamine (DMPDA) was synthesized via Michael-Addition reaction. A mixture of 2,2-bis[4-(2-hydroxy-3-methacryloxy propoxy)phenyl]-propane (Bis-GMA)/TEGDMA/camphorquinone (CQ) (75%:25%:0.5% w/w/w) with different coinitiators (i.e. DMPDA, EDMAB or 2-(dimethylamine)ethyl methacrylate (DMEM)) and concentrations were photopolymerized. The kinetics was monitored by real time infrared spectroscopy (RT-IR) and the mechanical properties were measured by dynamic mechanical analyzer (DMA). The results indicated that both the double bond conversion and the rate of polymerization of the resin mixtures increased as increasing the concentration of the DMPDA but were lower than that of EDMAB and DMEM at the same concentration. The modulus and T g of the cured samples were very close. Water sorption and solubility of the samples were almost the same. It was potential to be used as a coinitiator for dental composite.

Preparation of a Siloxane Acrylic Functional Siloxane Colloid for UV‐Curable Organic‐Inorganic Hybrid Films

AbstractSummary: A new siloxane colloid was developed, for the use in UV‐curable inorganic‐organic hybrid films. The UV‐crosslinkable silica‐colloids were prepared from vinyltrimethoxysilane (VTMS) via a sol‐gel method. The structure of silica‐colloids was characterized using 1H NMR, 29Si NMR, FT‐IR, and matrix‐assisted laser desorption/ionization (MALDI‐TOF) mass spectrometry (MS). The particle size of the siloxane colloid was evaluated using atomic force microscopy (AFM) and small angle light scattering (SALS). Organic phase was based on an acrylated polyester which was synthesized using 1,4‐cyclohexane dimethanol (1,4‐CHDM), neopentyl glycol (NPG), 1,6‐hexanediol (1,6‐HD), maleic anhydride (MA), adipic acid (ADA), and acrylic acid (AA). The acrylated polyester was characterized by gel‐permeation chromatography (GPC) and acid titration. A photo‐initiator was added to the formulation and the UV‐crosslinking reaction of hybrid film was monitored via photo‐differential scanning calorimetry (Photo‐DSC) and real‐time infrared spectroscopy (RT‐IR). The effect of the concentration of VTMS colloids, UV‐light intensity, and exposure time on the polymerization rate was investigated and compared with an inorganic‐organic hybrid film based on TEOS oligomers. Photo‐DSC and RT‐IR results indicated that VTMS colloids can greatly increase free radical polymerization rate and the VTMS colloids functioned effectively as cross‐linker and reactive diluent. AFM and small angle light scattering (SALS) data showed that the silica‐colloids were well dispersed in the organic phase. magnified image

FEATURE ARTICLE: In Situ Monitoring of Ultrafast Photopolymerizations by Real-Time Infrared Spectroscopy

Abstract The photoinitiated polymerization of multifunctional monomers has been monitored in real time by infrared spectroscopy. Conversion versus time curves have been directly recorded for polymerization occurring within seconds upon intense illumination. The influence on the polymerization kinetics of the photoinitiator, the monomer, and the functionalized oligomer has been quantified for acrylate monomers undergoing radical polymerization and for epoxy monomers undergoing cationic polymerization. Real-time infrared (RTIR) spectroscopy proved to be particularly well suited to follow the polymerization of monomer mixtures leading to the formation of either crosslinked copolymers or interpenetrating polymer networks. It was also used to study the UV curing of filled acrylate resins containing colored pigments or clay particles which generate nanocomposite materials. Up to a few millimeter thick samples were analyzed by near IR spectroscopy, by following continuously the disappearance of the acrylate double bond overtone band on UV-exposure. Real-time monitoring, sensitivity, short response time, and versatility are among the prominent advantages of this powerful tool of investigation.

Organically modified montmorillonites in UV curable urethane acrylate films

UV curable films were reinforced with an organically modified montmorillonite (MMT). The organically modified MMTs were prepared by an ion exchange process, in which sodium ions were replaced by alkyl ammonium ions. Cetyltrimethylammonium bromide (CTMA) modified clays gave rise to better intercalated clay morphology. The microstructures were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Physical properties were examined by real time infrared spectroscopy (RTIR), differential scanning calorimetry (DSC), photo-DSC, dynamic mechanical thermal analysis (DMTA). Some tensile properties were also reported. Thermal and mechanical properties were generally improved with nanoclay reinforcements. The effects of nanoclays on the properties of UV curable films are critically evaluated in this paper.

Multifunctional photoinitiators (MFPIs): A new concept

The migration of unreacted photoinitiators and their by-products from UV-cured printing inks and coatings is an increasing concern for food packaging applications. This paper describes the synthesis of novel multifunctional photoinitiators (MFPI) for use in UV curing, with the aim of decreasing the amount of extractable components present in a UV-cured coating. The activity of these MFPIs was investigated using differential photocalorimetry (DPC) and real-time infrared spectroscopy (RTIR). These analytical techniques demonstrated that the increase in functionality and molecular weight of MFPIs did not affect the efficiency of the PI in initiating UV-curing polymerisation reactions. The extraction studies and high-performance liquid chromatography (HPLC) analysis demonstrated that the use of MFPI can bring about a considerable reduction in the amount of free PI extracted from the coating as the high functionality increased the likelihood of the PI being bound into the polymer coating.

Dual curing of cationic UV-curable clear and pigmented coating systems photosensitized by thioxanthone and anthracene

We investigated the effects of photosensitizers (2,4-diethylthioxanthone (DETX) and anthracene) and dual curing on the kinetic and physical properties of cationic photocurable clear and pigmented coatings based on cycloaliphatic diepoxide/ϵ-caprolactone polyol. The cationic photopolymerizations were investigated by Fourier-transform infrared spectroscopy with attenuated total internal reflection (FTIR-ATR), real-time infrared spectroscopy (RTIR), and differential scanning photocalorimetry (photo-DSC). The coatings were shown to polymerize in the dark due to the “living” character of cationic polymerization. Measurements of the yellowness index revealed that the yellowing of the coatings containing the blend of DETX with anthracene photosensitizer was less than that of the coatings containing DETX only. Depth-profile analyses of FTIR-ATR spectroscopy data showed that the dual curing significantly increased both surface and interior curing of the coating films. The RTIR and Photo-DSC analyses showed that the polymerization reactivity and the ultimate percentage conversion for the cationic clear and pigmented formulations containing photosensitizers were higher than the corresponding values for the formulation without photosensitizer.

Interaction of hydrogen-terminated Si(100), (110), and (111) surfaces with hydrogen plasma investigated byin situreal-time infrared absorption spectroscopy

We have used infrared absorption spectroscopy (IRAS) in the multiple internal reflection (MIR) geometry to investigate the interaction of hydrogen-terminated Si(100), (110), and (111) surfaces with hydrogen plasma at room temperature. We have measured infrared absorption spectra in the Si–H stretching vibration region of the hydrogen-terminated surfaces during H-plasma treatment. IRAS data show that at initial stages of H-plasma treatment, surface hydride species (SiHx,x=1–3) are removed from the surface. A long-term H-plasma treatment of Si(100) and (110) surfaces reproduces monohydride species and creates hydrogen-terminated Si vacancies (VHx) at subsurface regions, i.e., near the surface. On Si(111), no hydride species are reproduced even after a long-term H-plasma treatment. We suggest that monohydride is rather stable against attack of hydrogen radicals as compared to higher hydride species, SiH2 and SiH3. We find that formation of VHx depends on the crystallographic orientation of the Si surface: VHx formation is more favored on Si(110) than on Si(100), and no VHx form on Si(111).

Oxygen diffusion in ceramic suspensions for stereolithography

Optical profilometry and real-time infrared (RTIR) spectroscopy have been performed to determine the influence of oxygen diffusion in ceramics suspensions on stereolithography accuracy. As expected, oxygen tends to lower both polymerized depth and width by inhibiting the polymerization and scavenging the radicals. Researches concern the influence of process parameters (atmosphere composition and temperature) and dispersion formulation, through viscosity, its filler content or tertiary amine addition, on oxygen diffusion. Models, such as those of Wilke–Chang and Mackie and Meares, were used to fit the oxygen diffusivity in the piezoelectric ceramics suspensions. This study has led to the modification of the stereolithography process in order to allow piezocomposites fabrication with a higher accuracy.

An investigation of the surface properties and curing behavior of photocurable cationic films photosensitized by anthracene

We investigated differences in the curing behavior of cationic photopolymerizations photosensitized by anthracene by preparing various epoxy/vinyl-ether-based formulation systems with and without anthracene photosensitizer. Cationic photopolymerizations of formulations with different ratios of diglycidyl ether of bisphenol A epoxide (DGEBA), 1,4-cyclohexane dimethanol divinyl ether (CHVE), and anthracene were investigated using differential photocalorimetry (photo-DSC) and Fourier-transform infrared spectroscopy with attenuated total internal reflection (FTIR-ATR). In photo-DSC experiments, the formulations containing more than 5% (by weight) anthracene exhibited a two-stage exothermic reaction. As the concentration of anthracene photosensitizer was increased, the first exotherm decreased and the second one increased. Photo-DSC and real-time infrared spectroscopy analyses showed that the reaction rate increased as the anthracene photosensitizer concentration was increased. Depth profile analyses using FTIR-ATR spectroscopy showed that the films containing more than 0.5% anthracene exhibited significantly improved surface properties and more thorough curing for film thicknesses up to 100 μm.

Photoinitiated polymerization of vinyl ether-based systems

The photoinitiated polymerization of vinyl ether (VE)-based coatings has been studied by real-time infrared (RTIR) spectroscopy. In the presence of diaryliodonium or triarylsulfonium photoinitiators, the cationic polymerization occurs rapidly upon UV-exposure and continues to proceed upon storage in the dark. Increasing the formulation viscosity by introduction of telechelic VE oligomers was found to have a strong slowing down effect on the UV-curing, because of the reduced mobility of the reactive species. VEs proved to be very effective monomers to achieve a fast and extensive curing of dicycloaliphatic epoxides. Similar polymerization profiles have been recorded by RTIR spectroscopy for the two monomers upon UV-exposure, which argues in favor of a copolymerization process between VEs and epoxides. When associated to electron acceptor monomers, like unsaturated esters or N-substituted maleimides, VEs undergo a radical-type copolymerization upon UV-exposure. For the maleate/VE combination, the two monomers were found to disappear at the same rate, whatever the monomer feed composition. The resulting alternating copolymer was shown to be formed by homopolymerization of a donor/acceptor complex. A similar conclusion was drawn from the kinetic data obtained for the maleimide/VE combination which polymerizes readily upon UV-exposure, even in the absence of any added photoinitiator.

Photoinitiated cross-linking of a thiol–methacrylate system

The photoinitiated thiol–ene cross-linking of a dimethacrylate polyether of Bisphenol A and pentaerythritol tetrakis 2-mercaptopropionate was studied in the presence of 2,2-dimethyl-2-hydroxy acetophenone (Darocur 1173). Two complementary techniques were used: photocalorimetry and real-time infrared spectroscopy. In the first part, the kinetic reaction was characterized by a stoichiometric mixture in reactive functions. The influence of temperature, photoinitiator concentration and ultraviolet (UV) light intensity was investigated. The results mainly show that the methacrylate homopolymerization is faster than the thiol–ene addition so that the reaction is usually stopped because of the complete consumption of the methacrylate double bonds. A theoretical approach has allowed us to determine a transfer constant of 0.26. Moreover, an increase in the autoacceleration rate was observed in the presence of the thiol monomer. In the second part, the molar fraction of methacrylate double bonds and thiol functions was changed to determine the effect on the kinetic reaction and the glass transition temperature of the final material.

Simultaneous Measurement of Polymerization Kinetics and Stress Development in Radiation-Cured Coatings: A New Experimental Approach and Relationship between the Degree of Conversion and Stress

Stress buildup and polymerization kinetics in UV-cured multifunctional (functionality 4−8) acrylic monomers in coating fabrications have been studied simultaneously using a newly developed combined cantilever deflection method and real-time infrared spectroscopy technique. The macroscopic phenomenon in terms of stress in the film to the molecular architecture in terms of cross-link density has been correlated. Stress buildup was observed to not occur instantaneously after radiation was introduced but rather after a certain time interval. This time interval with respect to onset of polymerization is attributed to the buildup of cross-links, leading to a gel state. Furthermore, it is possible to directly associate stress development with degree of chemical reaction. These experimental observations regarding the gelation point are compared to a number of models presented in the literature. In addition, we have developed a different approach incorporating the probability of finding a relative concentration of reacted or partially reacted monomers relating the stress and degree of conversion of vinyl groups. The predicted stress−conversion relationship based on percolation models is not in good agreement with our experimental data.

Recent Development in Free Radical Photopolymerization Direct and Sensitized Excitation of Maleimides

In several recent reports, the efficient photoinitiated polymerization by excited state maleimides have been shown in numerous acceptor/donor pair combinations [1-10] The strong hydrogen abstractability of the excited triplet state MI has also been evaluated in initiation of acrylate polymerization and the importance of available abstractable hydrogens has been verified [11]. The unique feature of maleimides in general is its dual effect of simultaneously acting as a photoinitiator and as a polymerizable monomer. The mechanistic evaluation of the photoexcitation process was accomplished by laser flash quenching studies. Rates of polymerization and degrees of monomer conversion as a function of irradiance were calculated from photo-DSC and Real Time Infrared Spectroscopy (RTIR) measurements

Photocuring activity of several commercial, near UV activated photoinitiators in clear and pigmented systems

Some of the most important commercial near UV photoinitiators have been analyzed by UV spectroscopy to evaluate the type of electronic transitions occurring upon absorption of light. Photocuring studies have been undertaken by real time infrared spectroscopy (RTIR) in clear and pigmented (black, magenta, cyan, and yellow) systems with UV and visible light at different photoinitiator concentrations in the presence of air. Generally, the Type I photofragmenting photoinitiators appear to operate more effectively under UV excitation when compared with the Type II thioxanthones, especially in pigmented systems. There is a reasonable correlation between the UV and visible absorption properties of the respective initiators and their overlap with the excitation source.