Study Of Photooxidation Research Articles

An in situ FT-IR study of photo-oxidation of alcohols over uranyl-anchored MCM-41: Possible reaction pathways

Photosensitive uranyl ions anchored onto MCM-41 mesoporous molecular sieves serve as remarkable photocatalysts in the degradation of alcohols, under ambient conditions of light, temperature, and air. The rates of conversion of alcohols to carbon dioxide was found to decrease in the order methanol > ethanol > 2-propanol > 1-propanol, with the difference in reactivity attributed to the stability of the carbon-centered radicals formed during photo-oxidation. Kinetics revealed that the photo-oxidation of alcohols followed a first-order reaction. A detailed in situ FT-IR analysis was used to identify the transient species formed during the photo-oxidation of ethanol and 2-propanol over uranyl-anchored photocatalyst. Acetic acid, ethyl acetate, and acetaldehyde were the intermediates obtained over UO 2+ 2/MCM-41 during photo-oxidation of ethanol, whereas acetate species, methyl acetate, and acetone were detected during photo-oxidation of 2-propanol. Based on the intermediate species formed, their growth with respect to irradiation time, and their intensities, appropriate reaction mechanisms were proposed to corroborate our observations.

Composition of PM 2.5 during the summer of 2003 in Research Triangle Park, North Carolina

A field study was carried out during the summer of 2003 to examine the overall composition of fine particulate matter (PM 2.5) in Research Triangle Park, North Carolina, USA, with particular emphasis on polar compounds from secondary organic aerosol (SOA). Collected samples were examined for gravimetric mass, organic and elemental carbon concentrations, inorganic ion concentrations, and detailed organic composition. On average, the ambient PM 2.5 was found to consist of 41% organic matter, 2% elemental carbon, 12% ammonium, 37% sulfate, and less than 1% nitrate and oxalate. Mass concentrations ranged from 6.4 to 31.4 μg m −3. The acidity of the aerosol was also estimated, and higher PM 2.5 and organic mass concentrations were generally observed under acidic conditions. A suite of chemical derivatization methods was used in conjunction with gas chromatography–mass spectrometry (GC–MS) to identify and quantify 29 polar organic compounds. Most of these compounds have been previously identified in laboratory photooxidation studies from hydrocarbon precursors, including isoprene, monoterpenes, β-caryophyllene, and toluene. From laboratory studies, several of these polar compounds have been proposed as tracers for SOA, and concentrations measured in this study indicate the contributions of the precursor hydrocarbons to ambient SOA could be important. Some of the organic tracers, particularly those associated with isoprene SOA, represented a greater fraction of the organic carbon when the aerosol was acidic.

A New Approach to Measuring the Action Spectrum for Singlet Oxygen Production by Human Retinal Lipofuscin

The human retinal pigment epithelial (RPE) layer contains a complex mixture of components called lipofuscin; this mixture forms with age and with various genetic disorders such as Stargardt's disease. Its presence may contribute to retinal deterioration via several mechanisms including photochemical processes. In the lipofuscin mixture, both type I and II mechanisms have been identified, with the latter consisting of the generation of singlet oxygen. Several components of that mixture have been identified, most notably a bis-retinoid pyridinium compound called A2E and its derivatives. Photooxidative studies on the compound A2E have revealed that its dominant photochemical mechanism is via free radical or type I processes. Because singlet oxygen is an important photooxidative intermediate in tissue, its generation in the RPE may contribute to retinal maculopathies. It is therefore necessary to determine which specific component(s) in the lipofuscin mixture produce singlet oxygen upon excitation with light. This was ascertained by evaluating the action spectrum for singlet oxygen production for the whole lipofuscin mixture using time-resolved spectroscopy. Singlet oxygen was generated by excitation of the sample at different wavelengths while maintaining a constant beam energy, and was directly detected by its phosphorescence decay at 1270 nm using a Ge photodiode. The action spectrum for singlet oxygen sensitization by the organic soluble portion of lipofuscin had an absorption maximum at ca 380 nm, which is to the blue of A2E (maximum at 430 nm). Compounds with a similar absorption maximum eluted in the HPLC earlier than A2E and were detected in human lipofuscin. The concentration of this component apparently increased in concentration in human RPE lipofuscin mixture as a function of age up to 90 years old.

Direct measurements of the OH and NO 2 evolutions in pulsed photo-oxidation studies of hydrocarbons : A method to quantify detailed and integral oxidation mechanisms in NO x containing air

A novel method has been developed to study the OH and NO 2 evolution in the laser pulse initiated photo-oxidation of hydrocarbons in NO x containing gas mixtures. In this method, long-path laser absorption (LPLA) for the time resolved detection of OH radicals in the (A 2∑ + − X 2∏) system at 308.417 nm is combined with cw-LIF for the time resolved detection of NO 2 after excitation at 488 nm at pressures around 50 mbar. It is shown that simultaneous measurements of these two species represent a detailed and sensitive signature of the elementary processes that occur following pulse initiated oxidation of hydrocarbons with photolytically generated OH radicals. The information contained in such profiles together with detailed numerical simulations permit: (i) to extract and to validate rate coefficients of otherwise difficult to access elementary processes of alkylperoxy and alkoxy radicals, (ii) to determine the extent of chain branching and OH regeneration and (iii) to derive the overall number of NO/NO 2 conversions (NOCON factors) in the complete oxidation chain of hydrocarbons. The latter are considered of substantial relevance to the assessment of individual hydrocarbons in the formation of ozone in photochemical smog mechanisms where they may be used to generate lumped oxidation schemes. The present paper describes the operation of the technique as well as the definition and derivation of NOCON factors. Because the species monitored (OH and NO 2) are not specific for any individual hydrocarbon, the technique is considered of wide applicability. A first application of the technique to the pulse initiated oxidation of propane by OH radicals at T = 298 K is presented.

Electrochemical Mass Spectroscopic and Surface Photovoltage Studies of Catalytic Water Photooxidation by Undoped and Carbon-Doped Titania

Carbon-doped TiO2, demonstrated as an efficient photocatalyst in visible light photooxidation of organic compounds, was prepared with different doping concentrations and investigated via differential electrochemical mass spectroscopy (DEMS) and capacitive surface photovoltage (SPV) measurements in the form of thin layer electrodes. In all cases the total photocurrent as well as the surface photovoltage of the doped materials decreased markedly in relation to the undoped one. No detectable oxygen evolved from the doped electrodes in acidic solution under UV-light excitation. Since an oxidation of formic acid is still apparent, it is concluded that this oxidation occurs via isolated, catalytically poorly active trap states within the forbidden energy region. The existence of these states is confirmed by capacitive SPV measurements.

Degradation and stabilisation of poly(ethylene- stat-vinyl acetate): 1 – Spectroscopic and rheological examination of thermal and thermo-oxidative degradation mechanisms

The degradation of ethylene vinyl acetate (EVA) copolymers was compared with low density polyethylene (LDPE), poly(vinyl acetate) (PVAc) and poly(vinyl chloride) (PVC) using FTIR, UV–visible and fluorescence spectroscopy as well as thermal and rheological analyses. Thermal, thermo-oxidative and photo-oxidative studies were conducted. Thermo-oxidation below 180 °C shows more similarities between EVA and LDPE. The luminescence spectra of degraded EVA and LDPE were almost identical but very different to that of PVAc. UV–vis analysis showed that the polyenes present in aged PVC were unlikely to be the same species responsible for the observed colour formation in aged EVA. It is suggested that they are polyconjugated carbonyl products. Rheological analysis also showed the evolution of crosslinking reactions during thermo-oxidation. FTIR studies after thermal degradation in inert conditions 290 °C showed complete loss of the ester functionality and associated lactone formation along with some evidence for ketonic and unsaturated carbonyl groups. Degradation in air at 180 °C, however, revealed that loss of the ester group was not so marked, with PVAc exhibiting the greatest stability. This was in line with the induction time to onset of autocatalytic carbonyl growth at 180 °C; the latter showed an apparent exponential decrease with increasing vinyl acetate content up to 28% w/w. Fluorescence analysis produced trends that complemented those of carbonyl index; the time to decomposition of initial fluorescent α,β-unsaturated carbonyl species coincided with the time to onset of carbonyl growth. Furthermore, the rate of formation of the new fluorescent species produced in EVA, and LDPE was similar to that of carbonyl growth. These new fluorescent species are therefore likely to be di- or tri-carbonyl products.

Photocatalytic Degradation of High Ammonia Concentration Water Solutions by TiO2

Recent studies reported the use of suspended TiO2 powder as photo catalyst in the degradation process of several organic and inorganic substances in water, like chlorinated hydrocarbons, alkyl- and alkanoloammines and ammonia. In order to evaluate the possibility of using this method for a further removal of ammonia from wastewater after a biological nitrification/denitrification treatment, a kinetic study of photo oxidation over TiO2 of NH3/NH4 in the high concentration range of 26 - 214 mg/I, has been carried out. Effects of pH, TiO2, ammonium and dissolved O2 concentration on the degradation efficiency and reaction products distribution (NO2-, NO3- e N2) were studied. The results have shown that, at a catalyst concentration of 0.012% (p/v), the photo catalytic oxidation of ammonia can be described with the Langumuir- Hinshelwood kinetic model, as reported by several authors.

Photocatalytic Coatings for Environmental Applications¶†

ABSTRACTA series of nano‐ and micronparticle‐grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their photocatalytic activities have been determined in a series of paint films by FTIR, chalking, color, gloss change and weight loss after artificial weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2‐propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of chargecarrier dynamics in the pigments. Pigment particle size, surface area and properties clearly play an important role in dispersion and any polymer‐pigment interactions. Photooxidation studies on several types of paint films show a clear demarcation between nanoparticle‐ and pigmentary‐grade titanium dioxide, with the former being more active because of their greater degree of catalytic surface activity. The photosensitivity of titanium dioxide is considered to arise from localized sites on the crystal surface (i.e. acidic OH), and occupation of these sites by surface treatments inhibits photoreduction of the pigment by ultraviolet radiation; hence, the destructive oxidation of the binder is inhibited. Coatings containing 2–5% by weight alumina or alumina and silica are satisfactory for generalpurpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7–10% weight. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminum phosphates of other metals. For example, the presence of hydrous alumina particles lowers van der Waals forces between pigments particles by several orders of magnitude, decreasing particle‐particle attractions. Hydrous aluminum oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilization in various water‐ and oilbased paint media in comparison with conventional organic stabilizers. Hindered piperidine stabilizers are shown to provide no additional benefits in this regard, often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self‐cleaning paints and microbiological surfaces is also demonstrated in this study. In the former case, surface erosion is shown to be controlled by varying the ratio of admixture of durable pigmentary‐grade rutile (heavily coated) and a catalytic‐grade anatase nanoparticle. For environmental applications in the development of coatings for destroying atmospheric pollutants such as nitrogen oxide gases (NOX), stable substrates are developed with photocatalytic nanoparticle‐grade anatase. In this study, porosity of the coatings through calcium carbonate doping is shown to be crucial in the control of the effective destruction of atmospheric NOx gases. For the development of microbiological substrates for the destruction of harmful bacteria, effective nanoparticle anatase titania is shown to be important, with hydrated high surface area particles giving the greatest activity.

Photodegradation of poly(ether sulphone) Part 2. Wavelength and atmosphere dependence

AbstractThe photodegradation of poly(ether sulphone) (PES) was investigated systematically by time‐of‐flight SIMS (ToF‐SIMS) and x‐ray photoelectron spectroscopy (XPS). The effect of varying the irradiation dose, wavelength and the atmosphere was studied along with mechanistic photooxidation studies using 18O2. Photoinduced SO3H group formation was observed at 199, 260 and 315 nm for an oxygen‐containing atmosphere only. Photoinduced OH group formation is observed at all investigated wavelengths and atmospheres. Photoinduced cross‐linking was an important process in a vacuum. The oxygen reacts quickly with the PES surface during photoirradiation and chain scission was found to be the dominant process, and oxygen incorporation was found to be non‐specific but pronounced for SO3H group formation. It was found that for conditions where processes are affected by photoirradiation the extent is observed to follow the UV absorption and showed little dependence on the wavelength. The wavelength only affects the extent of the process in question, which is ascribed to a fast equilibrium of the absorbed energy. The photoinduced processes are very sensitive towards the presence of the smallest amounts of oxygen in the atmosphere. Copyright © 2004 John Wiley & Sons, Ltd.

Photooxidation studies with perylenediimides in solution, PVC and sol–gel thin films under concentrated sun light

The singlet oxygen quantum yields ( ϕ Δ), LUMO energy levels (eV), photo stabilities under Xe-lamp irradiation of N, N′-di-dodecyl-3,4,9,10-bis(dicarboximide)perylene and N, N′-di-(1-dehydroabiethyl)-3,4,9,10-bis(dicarboximide)perylene, N-DODEPER and ABIPER, respectively, in different host matrices (solution, PVC and sol–gel matrix) are determined. Quenching of their emission by KI is found to give k q values of 3.5–37.4 × 10 9 M −1 s −1. Quenching is more efficient in solution phase than it is in immobilized phases. ϕ Δ’s of ABIPER and N-DODEPER are in negligible range, 0.090 and 0.095, respectively. The LUMO energy levels determined by cyclic voltammetric studies are 3.65 and 3.49 eV for ABIPER and N-DODEPER, respectively. Neither ABIPER nor N-DODEPER showed degradation tendency in sol–gel matrix. Photooxidation of α-terpinene is performed with ABIPER and N-DODEPER as photosensitizers, both in acetonitrile solutions and immobilized phases (PVC and sol–gel host matrices) by the use of two different irradiation sources, Na lamp and concentrated sun light. The photosensitizers exhibited different activities on the product profile, which were strongly dependent on the irradiation source, period of irradiation and the phase that the sensitizers were placed (solution or immobilized phases). Product analysis results point that in immobilized phases of PVC films 3,6-dioxoheptanal is the major product. In solution and immobilized phase of sol–gel ascaridol is the most abundant product. Superoxide anion radical, forms as reactive intermediate at photoinduced electron transfer process, directs the primary mechanistic step of photooxidations. A secondary mechanistic step is being proposed for the clarification of product formations.

Degradation and stabilisation of polymers and coatings: nano versus pigmentary titania particles

A series of nano and micron particle size anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their thermal and photochemical activites have been determined in a metallocene polyethylene with a monomodal molar mass distribution and alkyd/acrylic based paint films. Their performance has been assessed by FTIR and hydroperoxide analysis, chalking, colour, gloss change and weight loss following artificial weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge carrier dynamics in the pigments. Photooxidation studies on polyethylene containing nano-particle and pigmentary grade titanium dioxide pigments show that in general the former are more photoactive with anatase and rutile forms exhibiting high activity. Pigment particle size and surface area clearly play an important role in dispersion and any polymer-pigment interactions. This relationship is also extrapolated to the thermal sensitisation activities of the pigments on processing and oven ageing. Nano-particle pigments induce oxidation of the polymer during processing and long-term thermal ageing accelerating the formation of hydroperoxide and carbonyl groups. This behaviour can play a major role in controlling the photoactivity of the pigments in a polymer matrix. Photooxidation studies on paint films show a clear demarcation between nano-particle and pigmentary grade titanium dioxide with the former being more active. Here nano and micron particle size anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. The photosensitivity of titanium dioxide is considered to arise from localised sites on the crystal surface, and occupation of these sites by surface treatments inhibits photo-reduction of the pigment by ultraviolet radiation and hence the destructive oxidation of the binder is inhibited. Coatings containing 2–5% w/w alumina or alumina and silica are satisfactory for general-purpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7–10% w/w. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminium phosphates of other metals. For example, the presence of hydrous aluminium oxide particles lowers Van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminium oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilisation in various water and oil based paint media in comparison with conventional organic stabilisers. Hindered piperidine stabilisers are shown to provide no additional benefits in this regard often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated.

Effect of hindered piperidine light stabilizer molecular structure and UV‐absorber addition on the oxidation of HDPE. Part 1: Long‐term thermal and photo‐oxidation studies

AbstractThis series of papers explores the effect of structural characteristics of 2,2,6,6‐tetramethylpiperidine‐based hindered amine light stabilizers (HALS) on the long‐term (40 months) thermal (110°C in air) and photo‐stabilization (Microscal unit wavelength >300 nm) performance characteristics of high‐density polyethylene formulations. Possible synergism with a triazine functional UV absorber is also explored. Under thermal degradation (measured by carbonyl index) the polymeric HALS performed best, mainly because of reduced volatilization. Additionally, >N‐methyl HALS generally showed superior performance under thermal degradation. There was no synergism between an N‐CH3 polymeric HALS and the UV1164 triazine additive. However, the equivalent N‐H polymeric HALS interacted in a complex manner with UV1164, giving synergism and antagonism, depending on HALS/UV1164 ratio. Strong synergism was evident with the monomeric HALS when the total stabilizer level was 0.2% w/w. Reduction in the overall stabilizer level to 0.05% w/w eliminated the synergism. The UV1164 alone led to rapid and intense yellowing; however, the rate and intensity of yellowing reduced dramatically upon combination with HALS, particularly when the UV1164 level was above 0.1% w/w. When the formulation was under UV attack, the molar mass and the type of N‐substitution had no influence on stabilization performance because of the relatively low temperature of testing (leading to reduced volatilization), and the similarly effective UV‐stabilization routes for N‐methyl HALS and N‐H HALS. Under UV attack, yellowing reached a maximum and then decreased to approximately the initial level, while HALS/UV1164 combinations generally showed weak antagonism. J. Vinyl Addit. Technol. 10:79–87, 2004. © 2004 Society of Plastics Engineers.

Studies on the Photochemical Oxidation of 1,3-Diphenylisobenzofuran and Cis-Cyclooctene in N,N-Dimethylformamide–Propanol Binary Mixture

Kinetic studies on the photochemical oxidation of 1,3-diphenylisobenzofuran have been carried out in binary liquid mixtures of N,N-dimethylformamide (DMF) with 1-propanol (PrOH). The strongest influence of solvating effects upon the course of this reaction was observed for compositions of the studied mixture corresponding respectively to ca. 50 mol% of PrOH. Additionally, we have carried out analogous studies of photooxidation of cis-cyclooctene. Similarly as previously, the noticeable influence of solvating effects upon the course of this reaction corresponded to ca. 50 mol% of DMF. We suspect that the mechanism of this effect is associated with the formation of the polar intermediate during the ene reaction being easier.

True quantum yields and adsorption constants as tools for a mechanistic study of the TiO 2-sensitized photooxidation of benzylic derivatives

The quantum yields ( Φ) of the colloidal TiO 2-sensitized photooxidation of 4- ( 1) and 3-methoxybenzyl alcohol ( 2) together with 4-methoxybenzyltrimethyl- ( 3) and 4-methoxybenzyltriisopropylsilane ( 4) were determined in CH 3CN, in the presence of HClO 4 for 3 and 4. The true quantum yields ( Φ 0) of 1, 2 and 3, obtained from a Langmuir–Hinshelwood isotherm treatment of Φ at different substrate concentrations, are linearly correlated with I A −1/2, where I A is the light intensity. According to the previously suggested mechanisms for alcohols and silanes, a kinetic scheme that justifies this correlation is suggested. It is shown that the ratio of the slopes (from the Φ 0 versus I A −1/2 plots) for 1 and 2 is equal to the Φ 0 1 /Φ 0 2 ratio at any I A; this ratio depends on the rate constants in the kinetic scheme, in this case principally on the electron transfer constant, k et. On the contrary, the Φ 0 3 /Φ 0 4 ratio depends on k p, the cation radical desilylation rate constant, confirming a steric hindrance to nucleophylic assistance in the CSi fragmentation by the bulky isopropyl group in the SiR 3 moiety. Differently from Φ 0, the adsorption constants on the semiconductor under irradiation ( K D, obtained from the above isotherm treatment) are independent of I A. Moreover, as K D 1 =K D 2 and K D 3 =K D 4 , the structural modifications within the two alcohols and within the two silanes should be far enough away from the adsorption site. For all the substrates, K (the dark adsorption constant) is five times greater than K D, showing that this change does not depend on the substrate structure but is the result of different experimental conditions.

Biodegradation and Photooxidation Studies of Model Organic Pollutants

In the paper we present the results of biodegradation and photooxidation studies of halosubstituted benzyl alcohols. The basic goal of our research was to examine the effect of the type and position of the halogen atom in the aromatic ring on the degradation rate and to use the experimental data for the development of HQSAR models for predicting degradation rates. Biodegradation results of 2-halosubstituted benzyl alcohols correlate with the size of the halogen atom, while for 4-halosubstituted benzyl alcohols, a good correlation with the energy of C—X bond was observed. In the photooxidation process, the impact of position of the halogen on the rate of reaction is smaller. The HQSAR model was derived only for photooxidation data sets, while for the biodegradation it was not possible to obtain a model with satisfactory statistical characteristics.

ESR study of photooxidation of phenothiazines in aqueous micellar solutions

The mechanism of photooxidation of phenothiazine in liquid and frozen (77 K) aqueous micellar solutions of the surfactant (sodium dodecyl sulfate) was studied by ESR. The main reaction of electrons formed by the photochemical oxidation of phenothiazine in a liquid micellar solution of the anionic surfactant is the reduction of molecular oxygen dissolved in the aqueous bulk phase. 10-Methylphenothiazine was used as a stable radical cation probe in a liquid solution. The influence of electrolytes on the photoionization yield in both frozen and liquid solutions of the surfactant was studied.

Fluorescence emission and photooxidation studies with 5,6- and 6,7-benzocoumarins and a 5,6-benzochromone under direct and concentrated sun light

4-Methyl-8-hydroxy-benzo(6,7)coumarin, I, 4-methyl-6-hydroxy-benzo(5,6)coumarin, II, and 2-methyl-6-hydroxy-benzo(5,6)chromone, III, have shown similar absorption and fluorescence emission spectra. Fluorescence emission quantum yields for I and III are found to be very low, φ f=0.02, but 4-methyl-6-hydroxy-benzo(5,6)coumarin, II, has a eight-fold higher fluorescence quantum yield of the other two specie, in acetonitrile solution, φ f=0.16. Quenching of anthracene fluorescence emission by I, II and III are found to give k q values of 1.0×10 7–1.2×10 9 M −1 s −1. Benzo(5,6)coumarin, II, which gives the most intense fluorescence also presents the highest quenching rate, k q=1.2×10 9 M −1 s −1. Experimentally determined k q values are seen to correlate well with the free energy of electron transfer (Δ G ET) which are calculated to be in the range of −8.0 to −9.4 kcal/mol, where benzo(5,6)coumarin, II, gives the lowest free energy of electron transfer Δ G ET=−9.4 kcal/mol. These results indicate that I–III behave as electron acceptor moieties toward a condensed aromatic ring, anthracene. The Stokes shift values of 88–105 nm and broad fluorescence emission bands respect to absorption–excitation bands, indicates a molecular structure change in the excited states of I–III. Fluorescence lifetimes of 0.1–0.9 ns in I–III, singlet oxygen quantum yields of 0.15 and 0.40 for I and II, respectively, may be taken as evidence of singlet–triplet intersystem crossings. The photooxidation products of α-terpinene, sensitised by II, under direct and concentrated sun light conditions that are mainly p-cymene and ascaridole. In accordance with literature data on coumarin derivatives, benzocoumarins also seem to produce singlet oxygen and beside singlet oxygen, in addition super oxide anion radical production appear to be dominant especially under concentrated sun light. Under direct sun light conditions ascaridole is the major product. Some by-products of α-terpinene photooxidation are also determined at GC–MS analysis. Those by-products are assumed to be generated from ascaridole decomposition.

Interrelationship of spectroscopic properties with the thermal and photochemical behaviour of titanium dioxide pigments in metallocene polyethylene and alkyd based paint films: micron versus nanoparticles

A series of nano and micron particle grade anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their thermal and photochemical activites have been determined in monomodal metallocene polyethylene (with a monomodal mass distribution) and alkyd based paint films. Their performance has been assessed by FTIR and hydroperoxide analysis, chalking, gloss change and weight loss following artifical weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, luminescence, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge carrier dynamics in the pigments. Photo-oxidation studies on polyethylene containing nano-particle and pigmentary grade titanium dioxide pigments show that in general the former are more photo-active with anatase and rutile forms exhibiting high activity. Pigment particle size and surface area clearly play an important role in dispersion and any polymer–pigment interactions. This relationship is also extrapolated to the thermal sensitisation activities of the pigments on processing and oven ageing. Nano-particle pigments induce oxidation of the polymer during processing and long-term thermal ageing accelerating the formation of hydroperoxide and carbonyl groups. This behaviour can play a major role in controlling the photo-activity of the pigments in a polymer matrix. Photo-oxidation studies on paint films show a clear demarcation between nano-particle and pigmentary grade titanium dioxide with the former being more active. SEM analysis shows the formation of pits and holes emanating out from the particles. Model system studies based on 2-propanol oxidation and hydroxyl analysis go some way to predicting pigment activities but clear correlations do not exist. On the other hand the real-time microwave cavity perturbation method can be used as a gauge of photo-activity in titanium dioxide pigments. This, we believe, is a useful application, which could form the basis of an effective characterisation technique for commercial systems.

Photooxidation study of polymer light-emitting devices

In this paper, the photooxidation effect on the organic electroluminescent devices based on a π-conjugated polymer, poly(2-methoxy-5-dodecyloxy- p-phenylene vinylene) (MDOPPV) is reported. The devices used have a semitransparent Al cathode, which enables atmospheric oxygen to penetrate into the polymer layer in the device, and therefore extensive photooxidation takes place upon photoirradiation in air. It was confirmed that when the Al cathode is sufficiently thin (with an optical transmittance of ∼30% at λ=500 nm), the photooxidation rate of the polymer is independent of whether the Al film exists or not. Therefore, the electroluminescence (EL) data obtained from such a device irradiated for a period of time can be directly compared with the photoluminescence (PL) data obtained from a naked polymer film irradiated for a similar period. We have found that the external quantum efficiency of EL for the device based on MDOPPV does not change on photooxidation in air for a few minutes, while the quantum efficiency of PL rapidly decreases upon such photooxidation. This means that the optically generated luminescent species are efficiently quenched by photooxidized defects, but the electrically generated luminescent species are not, suggesting that the recombination zone for the electroluminescent process is spatially separated from the photooxidized defects, or excitons ‘dodge’ the defects.

Photo-oxidation studies on polyurethane coating: effect of additives on yellowing of polyurethane

Urethane clear coatings based on castor oil and MDI, develop extensive yellowing in the sunlight. The photo-oxidation and yellowing in the urethane coatings upon polychromatic irradiation was monitored by FT–IR spectroscopy, UV/visible spectroscopy and Colormate HDS colorimetry. Several quinoidal oxidation products have been evaluated. The photo-oxidation and yellowing have also been evaluated for coatings modified with different additives and/or their synergistic mixtures.