Photopolymerization Rate Research Articles

Study of cationic ring‐opening photopolymerizations using optical pyrometry

AbstractStudies of the photoinitiated cationic ring‐opening polymerizations of epoxide and oxetane monomers were conducted using optical pyrometry. Using this technique, the temperature of these photopolymerizations was monitored as a function of time. The effects of photoinitiator type and monomer structure on the rates of photopolymerization were investigated. Optical pyrometry was also used to investigate the acceleration of the photopolymerizations of various epoxide and oxetane monomers. Certain mixtures of monomers displayed synergistic effects that markedly increased their overall rates of polymerizations. In all cases in which acceleration of polymerization rate was noted, it could be attributed to an increase in the speed of ring opening of the initially formed protonated cyclic ether. The effects of relative humidity on the rate of cationic ring‐opening photopolymerizations of cyclic ether monomers were also investigated. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3303–3319, 2004

Photopolymerization of pigmented thiol–ene systems

Photopolymerization kinetics and optical properties of pigmented thiol–ene coatings were investigated using photo-DSC, real-time FTIR, colorimetry, and AFM. Pigment has no deleterious effect on the unique ability of thiol–ene systems to photopolymerize in air. When trimethylolpropane tris-(3-mercaptopropionate) is incrementally added to tripropylene glycol diacrylate with and without calcium lithol rubine, a red organic pigment, the photopolymerization rate in nitrogen steadily decreases due to a shift in the polymerization mechanism from an acrylate homopolymerization to a thiol–ene copolymerization. However, the photopolymerization rate of pigmented and non-pigmented systems in air significantly increases with increasing thiol concentration, ultimately reaching a maximum at approximately 35 mole percent trifunctional thiol. The increase in rate is due to chain transfer from the non-reactive peroxy radical to the thiol. Thiol groups reduce oxygen inhibition to a greater degree than standard additives such as N-methyldiethanolamine, N-vinyl pyrrolidinone, and thioether containing trifunctional vinyl esters. For a typical acrylate based pigmented photocurable system, greater than 10 wt% photoinitiator is required to achieve a photopolymerization rate equivalent to a comparable thiol–ene system with 1 wt% photoinitiator in air. AFM and colorimetric data indicate that addition of trifunctional thiol has no deleterious effect on pigment dispersion and may in fact increase dispersion quality.

Effects of water contents and postcuring conditions on Bis-GMA/TEGDMA dental restorative composite resins

Dental restorative composite resins of the monomer bisphenol A glycerolate dimethacrylate (Bis-GMA/TEGDMA; 50/50, mass ratio), with different initial water contents in monomer mixtures and subject to various postcuring conditions, were prepared. The effects of water contents and postcuring conditions on photopolymerization rate, degree of methacrylate double bond conversion, and mechanical properties (flexural strength and elastic modulus) were systematically investigated. Within the scope of this investigation, the initial water content was not found to significantly affect the mechanical properties of the photocured composite resins. Nevertheless, the addition of distilled water into the monomer mixture slightly increased the degree of methacrylate double bond conversion, and slightly slowed down the photocuring process, especially when the amount of water was oversaturated. Postcuring conditions of 37°C vapor or water aging increased the degree of methacrylate double bond conversion, but made the mechanical properties of the composite resins less desirable, especially under water-aging conditions. Postcuring treatment of the composite resins in a 100°C vacuum (∼ 27.9 KPa) oven for 1 day improved both the flexural strength and the elastic modulus by at least 30%, but the improvements quickly diminished upon subsequent 37°C water aging. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 492–502, 2004

A study of the photoactivities and thermomechanical properties of epoxy resins using novel [cyclopentadien-Fe-arene]+PF6− photoinitiators

The cationic photopolymerization of epoxy resins, initiated by novel cyclopentadien-Fe-arene hexafluorophosphate ([cyclopentadien-Fe-arene]+PF6−), has been investigated. The influence of different photoinitiators and the presence of different epoxy monomers on the rate of photopolymerization and on the thermomechanical properties of the UV-cured film obtained had been studied. Using [cyclopentadien-Fe-biphenyl]+PF6− (CFB) as photoinitiator, the photoactivity increased obviously when cycloaliphatic epoxy monomer 3,4-epoxycyclohexylmethyl-3′,4′-epoxycyclohexane carboxylate (ERL-4221) was added in epoxy resin (E44), but the film obtained is micro-heterogeneous. However, in spite of the lower activity, the film from glycidylether 1,2,3-(2,3-epoxypropyl)-glycidylether (EPON-812) + E44 is homogeneous. The film obtained from di(2,3-epoxypropyl)3,4-epoxy-1,2-cyclohexanedioate (TDE-85) + E44 possesses the highest glass transition temperature Tg. Photoinitiators can influence the properties of the resulting epoxy resins, including the rigidity, crosslink density, Tg and homogeneity. The resulting epoxy resin initiated by [cyclopentadien-Fe-toluene]+PF6− (CFT) with lower photoactivity got the highest crosslink density. Initiated by [cyclopentadien-Fe-diphenylether]+PF6− (CFD) and [cyclopentadien-Fe-anisole]+PF6− (CFA) with greater solubility, the compatibility of epoxy ERL-4221 and E44 can be improved.

Photopolymerization and toughening performance in polypropylene of hyperbranched polyurethane acrylate

A novel UV-curable hyperbranched polyurethane acrylate (HUA) was synthesized and found to polymerize rapidly in the presence of 5 wt.% benzophenone in N2 under UV exposure. The photopolymerization kinetics of HUA was studied by differential photocalorimetry (DPC). Its toughening effect for polypropylene (PP) was investigated by tensile and impact tests of the UV irradiated PP/HUA blends. The morphological structures and thermal behavior were determined by polarized optical microscopy, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The obtained results demonstrate that (1) the maximum photopolymerization rate increases with raising temperature up to 140 °C, whereas decreases at above 150 °C. The activation energy of 19 kJmol−1 for the photopolymerization was obtained at below 140 °C from the Arrhenius plot, while it is negative at above 150 °C. (2) The incorporation of 5 wt.% HUA greatly improved the notched impact strength of PP matrix with a slight improvement in the tensile strength and without obvious decline in breaking elongation. These results correlate well with SEM observation. (3) During the UV irradiation of PP/HUA blends, PP can be crosslinked/grafted with the cured HUA particles, resulting in the increase of the impact strength of PP matrix. (4) The cured HUA particles in the PP/HUA blends act as heterogeneous nucleation agent for PP, which results in the decrease of spherulite size and less perfection of PP crystals.

Pyrromethene 567 Dye as Visible Light Photoinitiator for Free Radical Polymerization

AbstractIn the present work we report on the photochemistry and photopolymerization activity of a new bimolecular photoinitiator system, which exhibits maximum sensitivity at 518 nm, consists of a sensitizer dye, pyrromethene 567 (PM567), and a radical generating reagent 3,3′,4,4′‐tetra(tert‐butylperoxycarbonyl)benzophenone (BTTB). The photosensitization of BTTB through the excitation of PM567 induces high polymerization rates, while different experiences have revealed that PM567 or BTTB, separately, are unable of initiating the polymerization of 2‐hydroxyethyl methacrylate (HEMA) over extensive periods of time at 40 °C, either in the dark or under irradiation. The polymerization efficiency of this photoinitiator system, PM567/BTTB, was analyzed following the bulk polymerization kinetics of monomer HEMA by differential scanning photo‐calorimetry. Photopolymerization rates and quantum yields were observed under isothermal conditions (40 °C) for continuous illumination polymerizations (at 518 nm) at different incident light intensities and different BTTB concentrations.Scheme of the photosensitization process of the bimolecular photoinitiator system of PM567/BTTB.magnified imageScheme of the photosensitization process of the bimolecular photoinitiator system of PM567/BTTB.

Photoinitiation of buthylmethacrylate polymerization by colloidal semiconductor nanoparticles

Nanoparticles of zinc and iron oxides as well as cadmium sulfide of different size were synthesized in 2-propanol, their band gaps and allowed bands positions were determined. The foregoing nanoparticles turned out to be efficient photoinitiators of butylmethacrylate polymerization. On the basis of the kinetic parameters of the process a free radical mechanism of the butylmethacrylate photopolymerization induced by investigated nanosized semiconductors was inferred. An increase in the specific rate of the butylmethacrylate photopolymerization with the decrease in the average CdS nanoparticles diameter was explained in terms of the theory of quantum confinement effects in semiconductors and was attributed to an increase in the probability of monomer reduction by photogenerated electrons of CdS nanocrystals conduction band with the diminishing of crystals size. It was shown that a number of xanthene dyes—fluorescein and its halogenated derivatives are efficient sensitizers of ZnO nanoparticles to the visible light. Butylmethacrylate photopolymerization rates grow at the presence of ZnO nanoparticles and sensitizers symbately with an increase in singlet and triplet-excited states energy of dyes studied. Such relation indicates the determining role of the process of electron transfer from photoexcited sensitizers to the ZnO nanoparticles conduction band in the initiation stage of polymerization. Flash photolysis was used for the investigation of the nature and decay characteristics of short-lived intermediates generated in systems consisting of ZnO nanoparticles, sensitizers and monomer. Schemes of the mechanism of the butylmethacrylate photopolymerization induced by investigated nanosized semiconductors were proposed.

Thiol−Ene Photopolymerization Mechanism and Rate Limiting Step Changes for Various Vinyl Functional Group Chemistries

The mechanism and kinetics of thiol−ene photopolymerizations utilizing a tetrafunctional thiol monomer copolymerized with acrylate, norbornene, vinyl ether, and vinyl silazane functionalized ene monomers are successfully modeled and experimentally characterized. Modeling predictions demonstrate that the reaction orders in thiol−ene systems are controlled by the ratio of thiyl radical propagation to chain transfer kinetic parameters (kp/kCT). Ratios of kinetic parameters (kp/kCT) were found to vary significantly with the ene functional group chemistry and to have a dramatic impact on polymerization kinetics. For high ratios of kp/kCT, polymerization rates are first order in thiol functional group concentration and nearly independent of ene functional group concentration. For kp/kCT values near unity, polymerization rates are approximately 1/2 order in both thiol and ene functional group concentrations. When kCT is much greater than kp, polymerization rates are first order in ene functional group concentration and nearly independent of the thiol functional group concentration. In thiol−allyl ether and thiol−acrylate systems, the step growth polymerization rates are first order in thiol functional group concentration (Rp ∝ [SH]). For thiol−norbornene and thiol−vinyl ether systems, polymerizations are nearly 1/2 order in both thiol and ene functional group concentrations (Rp ∝ [SH]1/2[CC]1/2). In thiol−vinyl silazane systems, polymerization rates are approximately first order in ene functional group concentration (Rp ∝ [CC]) and independent of thiol functional group concentration. A theory is proposed which states that the effect of functional group chemistry on kp/kCT is controlled primarily by ene functional group electron density (kp) and carbon radical stability (kCT).

Photopolymerization kinetics of hyperbranched acrylated aromatic polyester

AbstractThe influences of the irradiation temperature, comonomer content, sample thickness, and photoinitiator concentration on the polymerization kinetics of hyperbranched acrylated aromatic polyester (HAAPE) were investigated with photo‐differential scanning calorimetry and IR measurements. The maximum photopolymerization rate increased with the temperature rising up to 110°C but decreased beyond 110°C. An activation energy of 16 kJ mol−1 for the photopolymerization was obtained below 110°C from an Arrhenius plot, but it was negative beyond 120°C. A remarkable synergistic effect between HAAPE and the comonomer trimethylolpropane triacrylate with a molar fraction of around 0.4 was observed from a photopolymerization kinetic study of the resins. The final unsaturation conversion in an ultraviolet‐cured film decreased with the sample thickness, and this became more remarkable as the photoinitiator concentration increased. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1500–1504, 2003

Photosensitive 2,5-distyrylpyrazine particles produced from rapid expansion of supercritical solutions

Solvent-free, photoreactive particles of 2,5-distyrylpyrazine (DSP) monomer were developed by rapid precipitation from an expanding supercritical chlorodifluoromethane solution. DSP polymer particles were produced by solid-state photopolymerization. DSP particles below a critical diameter of about 0.5 μm were found to be mechanically stable and did not fragment upon photopolymerization. The rate of DSP photopolymerization was shown to be size-sensitive. Nano-scale particles demonstrated superior photoreactivity in the solid state in comparison to micro-scale crystals. UV spectra of DSP at different degrees of conversion were investigated and the extinction coefficients were calculated for the DSP monomer and polymer in sulfuric acid.

Cationic photopolymerization of carbazolyl-containing vinyl ethers

Photopolymerizations of 2-(9-carbazolyl) ethyl vinyl ether (CEVE) and (9-carbazolyl) methoxy ethyl vinyl ether (CMEVE) initiated with diphenyl iodonium tetrafluoroborate (DPIT), di( tert-butylphenyl)iodonium tetrafluoroborate (BPIT), cyclopropyldiphenylsulfonium tetrafluoroborate (CPS) and (η 5-2,4-cyclopentadien-1-yl)[(1,2,3,4,5,6-η)-(1-methyl ethyl) benzene]-iron (+)-hexafluorophosphate(-1) (I261) were studied. It was established that the efficiency of the iodonium salts and iron–arene complex is much higher in the cationic photopolymerization of the monomers than that of the sulfonium salt. CMEVE showed higher reactivity in cationic photopolymerization than CEVE. Higher degrees of polymerization were established for the products CEVE photopolymerization compared with those of the products of CMEVE reaction. The influence of the temperature on the rate of CEVE and CMEVE photopolymerization, molecular weight and conversion limit is discussed.

Hyperbranched Acrylated Aromatic Polyester Used as a Modifier in UV‐Curable Epoxy Acrylate Resins

AbstractThe viscosity, the shrinkage degree and the photopolymerization rate of the epoxy acrylate (EB600) blended with hyperbranched acrylated aromatic polyester (HAAPE) were investigated. The addition of HAAPE into EB600 largely reduces the viscosity of the blend formulation and the shrinkage degree. For example, EB600 resin with 50% weight fraction of HAAPE has the 1250 cps of the viscosity and 2.0% of shrinkage degree, while the pure EB600 resin has 3000 cps of the viscosity and 10.5% of shrinkage degree. The photopolymerization rate of the resin is also promoted by HAAPE addition. The good miscibility between HAAPE and EB600 was also observed from the dynamic mechanical analysis. The tensile, flexural and compressive strength, and the thermal properties of the UV cured films are greatly improved.

Photo-DSC cure kinetics of vinyl ester resins II: influence of diluent concentration

The photopolymerization kinetics of two commercial vinyl ester resins (VERs) and a model VER photoinitiated by the camphorquinone/amine photoinitiator system were monitored using isothermal DSC. A decrease in styrene concentration in model VERs was found to raise the rate of photopolymerization. In contrast, when the styrene was replaced by a monomethacrylate diluent, the photopolymerization rate passed through a maximum near 70 wt% diluent monomer. This difference in the variation of the rate of polymerization with decreased monomer concentration was attributed to the competition of the effects of the higher reactivity of the methacrylyl radical relative to the styryl radical and the lower termination rate for divinyl-rich systems (both of which tend to raise the maximum polymerization rate) and the effects of the reduction in the initiation efficiency and decrease in k p due to increased fraction of pendant double bonds (which lower the polymerization rate) when the concentration of diluent monomer was reduced. Subsequent dark polymerization was observed during a temperature ramp and the onset of polymerization was independent of resin composition due to vitrification effects during the isothermal photocuring stage. The kinetics during the dark polymerization stage was discussed in terms of the radical concentration and the propagation rate constant. Increases in the concentration of either diluent monomer raised the extent of isothermal cure during the isothermal polymerization because vitrification was delayed by the lower crosslink density and the plasticizing effect of the diluent. Higher levels of diluent also raised the maximally attainable conversion due to reduced topological restrictions for reaction in networks of lower crosslink density.

Methacrylate derivatives incorporating pyroglutamic acid.

Methacrylates containing pyroglutamic acid were synthesized in good yields. Methyl alpha-pyroglutamyl methylacrylate (PyMM) and methyl alpha-pyroglutamidoundecanoyl methylacrylate (PyUM) give very fast photopolymerization rates both in homopolymerizations and with widely used commercial monomers N-vinyl pyrrolidinone (NVP) and hydroxyethyl methacrylate (HEMA). Soluble or cross-linked homopolymers can be obtained depending upon polymerization temperature. Pyroglutamic methacrylates polymerize without added initiator in the melt. Solution cast, photocured, and thermally cured coatings gave good to excellent adhesion to poly(ethylene terephthalate) and glass surfaces.

Photopolymerization of N,N‐dimethylaminoethylmethacrylate studied by photocalorimetry

AbstractPhotopolymerization of dimethylaminoethylmethacrylate (DMAEMA) is studied in bulk and in solutions in the presence of different photoinitiators using differential photocalorimetry (DPC). The rate of DMAEMA photopolymerization is slow compared to that of alkylmethacrylates. Bimodal DPC curves of DMAEMA photopolymerization in bulk are obtained. The type I photoinitiators (IRGACURE® 651 and IRGACURE® 1700), which produce free radicals by homolytic fragmentation of photoexcited molecules, are more effective in promoting photopolymerization of DMAEMA. The type II photoinitiators (benzophenone and IRGACURE® 500), which initiate DMAEMA photopolymerization through an H‐abstraction mechanism involving an amino group from the monomer (polymer), are less efficient and favor the formation of partly crosslinked products. The effects of the photoinitiator concentration, temperature, and solvent on the kinetic data are evaluated and discussed. A scheme of photopolymerization of DMAEMA, including the formation of intermediate DMAEMA based macromonomers, is proposed. The main point of the suggested scheme is a high chain transfer to the DMAEMA monomer. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 579–588, 2002

Styrylpyridinium borate salts as dye photoinitiators of free‐radical polymerization

AbstractStyrylpyridinium borate salts photoinitiate free‐radical polymerization. The rate of photopolymerization depends on the ΔGo of electron transfer between a borate anion and a styrypyridinium cation. This latter value was estimated for a series of styrylpyridinium borate salts, and the relationship between the rate of polymerization and the free energy of activation gives the dependence predicted by the classical theory of electron transfer. This relation was independently observed for the two series of styrylpyridinium borate salts tested—one for the photoredox pair with an iodine atom and the second without. Styrylpyridinium borate salts were stable at ambient temperature in the formulations prepared for the photopolymerization experiments. Photopolymerization initiated by the photoredox pairs tested proceeded by the conventional mechanism in which bimolecular termination occurs by a reaction between two macroradicals. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1433–1440, 2002

Cyanine borates revisited. Study of the kinetics of photoinitiated free radical polymerization via intermolecular electron transfer process

Three series of cyanine butyl triphenyl borate salts were prepared and tested as initiators of free radical polymerization photoinitiated via the photoinduced electron transfer process. To illustrate the influence of the primary process on the rate of photoinitiated polymerization the fluorescence lifetime of the tested dye containing non-electron donating counterion as well as electron donating borate ion were measured. The fluorescence lifetime measurements were performed using a pico/femtosecond spectrometer allowing a successful analysis of fluorescence decay signals in the range of single picoseconds. As obtained results show, for cyanine dyes bi-exponential decay of fluorescence was observed, while for cyanine with a substituent at the meso position, for the majority of dyes, mono-exponential decays were obtained. Based on fluorescence measurements the rate of the electron transfer process between excited dyes and borate anions was calculated and a short discussion on photophysics of cyanine dyes is given. Several important conclusions arrive from the theoretical and experimental data: (i) the experimental data reveal that the relationship between the rate of the electron transfer and the free energy for this reaction display typical “normal Marcus region” kinetic behavior; (ii) the experimental results show that the relationship between the logarithm of the rate of polymerization and the logarithm of the rate constant for the electron transfer is linear in character. This finding permits the conclusion that the rate of photopolymerization is a function of the free energy change of the electron transfer (ΔGel). This type of relationship is observed only for cyanines with no substituent in the meso position. These observations allow the conclusion that in the case of meso-cyanines the fluorescing state of the dye is not the state that is reduced by the borate. The main conclusion derived from the data presented in the paper is that the Marcus theory of electron transfer can be extended for the description of certain types of photoinitiated polymerization.

Effect of Silicon Oxide Fillers on Photochemical Curing of Compounds Based on Acrylic Monomers and Oligomers

The course of curing of photocurable compounds was studied as influenced by filling with disperse silicon oxide substances. The effect of the filler concentration on the conversion and rate of photopolymerization was examined.

Coatings obtained through cationic UV curing of epoxide systems in the presence of epoxy functionalized polybutadiene

The cationic photopolymerization of a biscycloaliphatic and a di-glycidyl epoxide monomer was investigated in the presence of an epoxy hydroxy functionalized polybutadiene (PBE). Triphenylsulfonium hexafluoroantimonate was used as photoinitiator. The kinetics of UV curing, in the presence of the additive, was determined via FT-IR analysis, showing an increase of the rate of photopolymerization and of the final epoxy groups conversion. Comparison experiments were performed with hydroxy functionalized polybutadiene. Evidences are obtained that PBE is incorporated via a copolymerization reaction involving the epoxy groups and via a chain transfer reaction involving the OH groups. The investigation of the properties reveals a flexibilization effect due to the presence of the additive, evidenced by a sharp decrease of Tg of the cured films.

Photo-DSC cure kinetics of vinyl ester resins. I. Influence of temperature

Isothermal DSC was employed to monitor the photopolymerization kinetics of a commercial vinyl ester resin (VER) photoinitiated by the camphorquinone/amine photoinitiator system. The maximum rate of photopolymerization was found to increase tenfold as the isothermal cure temperature was varied from −30 to 90 °C due to faster propagation and greater initiator efficiency counterbalanced by a faster termination rate. After photopolymerization, the samples were temperature ramped in the DSC to monitor the dark polymerization reaction. The reduced polymerization rate increased with the scan temperature due to an increase in propagation rate. The summation of the isothermal and residual polymerization heats indicated that the final degree of cure after postcure was independent of the isothermal cure temperature. The onset temperatures for the recommencement of cure during the dark temperature scanning experiments were found to correspond closely with their respective isothermal cure temperatures as expected due to vitrification during the isothermal cure stage.