The effects of oxygen diffusion on the surface photooxidation of polystyrene

This paper further illustrates the applicability of multiple internal reflectance infrared spectroscopy to the analysis of near-surface photooxidation. The results are compared with transmission infrared spectra to evaluate compositional gradients resulting from photooxidation and the influence of oxygen diffusion. The sample was a solvent-cast film of atactic, narrow distribution polystyrene, M_w of 100,000, that had been drawn to a ratio of 3.0 at 110°C by solid state coextrusion. Irradiation of these thin films, ～25μm thickness, was performed on exposure to air at 35°C for periods of up to 6 h using a mercury source emitting at 254 nm. On photooxidation, a board peak appears at 3200–3500 cm^?1, attributable to hydroperoxide formation. The most dramatic increase in the infrared spectra is found for a carbonyl band at 1730 cm^?1. It appears to result from an aromatic acid group since it is shifted to 1660 cm^?1 on immersion of the oxidized polystyrene films in aqueous ammonium hydroxide. It is confirmed that the photooxidation of polystyrene occurs preferentially at the surface and that this reaction rate is greatly reduced in the drawn polystyrene film.     

Photodegradation and stabilization of styrene–butadiene–styrene rubber

Photooxidative degradation and stabilization of a polystyrene–block–polybutadiene–block–polystyrene thermoplastic elastomer using a polychromatic UV light in air at 60°C has been studied by monitoring the appearance of the hydroxyl and carbonyl groups in Fourier transform infrared spectroscopy. The extent of photooxidative degradation in different samples has been compared. The rate of photooxidation was also estimated in the presence of different concentrations of 2,6‐di‐tert‐butyl‐4‐methylphenol [BHT], 2‐(2′‐hydroxy‐5′‐methylphenyl)benzotriazole [Tinuvin P] and tris(nonylphenyl) phosphite [Irgafos TNPP], and 1,2,2,6,6‐pentamethyl piperidinyl‐4‐acrylate was grafted onto the surface of the SBS film. The kinetic evolution of the oxidative reaction was determined. The morphological changes upon irradiation in the solution cast SBS films were studied by scanning electron microscopy. Based on the experimental data a suitable photooxidative degradation mechanism also has been proposed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1097–1102, 2000     

Long-term behavior of oil-based varnishes and paints. Photo- and thermooxidation of cured linseed oil

Thermooxidation at 100°C and photooxidation at wavelengths above 300 nm of dried oil films were evaluated. The chemical modifications of the networks were determined by infrared analysis coupled with gaseous treatments (NO, SF₄, and NH₃). The dried films are rather stable in thermooxidation, whereas in photooxidation, important degradation of the network occurs with many chain scissions. This photoinstability results from the presence of crosslinks that are sensitive to radical attack because of the lability of the hydrogen atom on the tertiary carbons. The photooxidation reactions are fully described in this paper. Yellowing of the cured samples, observed with ultraviolet-visible and fluorescence spectrometries, rapidly is decreased by irradiation because the oil contaminants that are mainly responsible for the yellowness are photooxidized. On the contrary, yellowing slowly but continuously increases during thermooxidation at 100°C.     

In situ detection of hindered amine stabilizer consumption in polymer through oxidation by indirect electron spin resonance

The stabilization mechanisms of hindered amine stabilizers (HAS) involve various oxidation products of the piperidine structure. Thus, we propose a novel method, based on electron spin resonance (ESR) spectroscopy, to check in situ the consumption of the total piperidyl species (intact HAS and all of its byproducts) in polypropylene (PP) films through photooxidation. First, the concentration of nitroxyl radicals produced upon irradiation in stabilized PP has been measured by direct ESR analysis. Then, the changes of concentration of the overall stabilizing species have been detected by indirect ESR, after conversion of the overall HAS derivatives into nitroxyl-free radicals by exposure of photooxidized PP to peracetic acid vapor at room temperature. Results were compared with those obtained by the conventional Fourier transform infrared method in the particular case of [2,2,6,6-tetramethyl piperidinyl]sebacate, and the reliability of both techniques was discussed. Thus, we assume that indirect ESR experiments consist of a easy, accurate, and very sensitive method to monitor the time evolution of a low-molecular weight HAS–NH concentration in PP upon photooxidation. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1107–1114, 1998     

The oxidative degradation of styrenic copolymers: A comparison of photoproducts formation under natural and accelerated conditions

Natural and accelerated weathering of polystyrene and high‐impact polystyrene were carried out in the present investigation. The structural changes in the polymer samples were characterized by using FTIR spectroscopy, tensile strength testing, and SEM spectroscopy. The natural exposure was conducted throughout the year. Rates of photooxidation were determined by measuring the evaluation in hydroxyl and carbonyl regions. The surface deterioration was revealed from SEM micrographs. The drop in tensile strength was also monitored. A correlation between natural and artificial weathering was considered for lifetime prediction in a short exposure time. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1676–1682, 2002     

Photooxidation of polystyrene. Effect of stabilizers

The effect of commercially available stabilizers such as Tinuvin-327, Tinuvin-770, and Irganox-1024 on the photooxidation of polystyrene (PS) was investigated by following the changes in molecular weight and molecular weight distribution. Structural changes during degradation were studied by ¹H-NMR- and IR-Spectroscopy and by Electron Spectroscopy for Chemical Analysis (ESCA). Neat PS films became very brittle and yellow on exposure to UV light. Tinuvin-770 stabilized the PS films towards discolouration and brittleness. The ESCA studies of surface photooxidation of polystyrene indicate the formation of ketonic and methylene ether groups in the initial stages (? 100 h). At later stages ester and carbonate groups are formed.     

Surface photostabilization of polystyrene by Tinuvin 1577

The effect of a new hydroxyphenyltriazine additive (Tinuvin 1577) on the surface photooxidation of polystyrene films at various concentrations of the additive was studied using ESCA spectroscopy. The accelerated weathering was run in a Xenotest apparatus to mimic outdoor conditions. The presence of Tinuvin 1577 effectively inhibited the surface photooxidation of polystyrene. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1251–1256, 1998     

Photooxidation of the surfaces of polyphenylene oxide and polysulfone

Changes in the composition and structure of the surfaces of polyphenylene oxide (PPO) and polysulfone caused by UV photooxidation are followed using ESCA. The surface stoichiometry of PPO changes from C₈O₁ to C₈O_4.8 on photooxidation, the oxygen being added in the form of carbonyl and especially carboxyl groups. Surface photooxidation is rapid, the ESCA spectra showing substantial changes after a few minutes of irradiation, and no further changes being detected after 10 h of reaction. The surface photooxidation of polysulfone occurs in three stages. In the first 2 h rapid oxidation takes place at carbon atoms, the levels of C—O, carbonyl, and carboxyl groups increasing at comparable rates. This is followed by slower oxygen incorporation up to 15 h of reaction, during which time carbonyl and carboxyl levels increase and oxidation of the sulfone sulfur is observed. Some degradation occurs as groups containing carbon and oxygen are lost from the surface. Further oxidation then produces a decrease in the surface levels of C—O and carbonyl groups, and further degradation. The photooxidized surface has a stoichiometry of C₁₀O₆S, compared with C₂₇O₄S in the unreacted polymer.     

Surface properties of poly(dimethylsiloxane)-based inorganic/organic hybrid materials

Poly(dimethylsiloxane) (PDMS)-based hybrid materials were prepared by the sol-gel method on Si wafers, Al and polystyrene (PS) substrates. The reaction was monitored by attenuated total reflectance-infrared (ATR-IR) spectroscopy. The hybrid materials have always one surface made in contact with air and one with a substrate. These surfaces were investigated by using tapping mode atomic force microscopy (AFM), X-ray photo-electron spectroscopy (XPS), low-energy ion scattering (LEIS) and dynamic contact angle (DCA) analysis. The hybrid sample surfaces made in contact with air and substrates appeared to have different structures. The former have a silica-free PDMS top layer of ∼2 nm thick; while in the latter cases, SiO₂ are located at or just beneath the outermost atomic layer. In air and at room temperature, SiO₂ are likely beneath the outermost atomic layer. In contact with water, polar -OH groups at the surface of SiO₂ can easily stretch out to the outermost atomic layer. No correlation was found between the roughness of the surfaces and the amount of in situ formed SiO₂ present in the materials.     

Characteristics of photocatalytic oxidation of gaseous 2‐propanol using thin‐film TiO2 photocatalyst

This work presents a photocatalysis‐based method to treat and purify air because of its broad applicability to common, oxidizable, air contaminants. The effect of oxygen content, temperature, water vapor, and 2‐propanol concentration on the TiO₂ surface was investigated. The rate of 2‐propanol decomposition increased with increasing the oxygen content, but was reduced at temperatures higher than 100°C. When water vapor concentration was in the range of 10–355 mmol m^?3, the rate of 2‐propanol decomposition was proportional to the water content. However, an opposite result was observed at a higher concentration of water vapor. 2‐Propanol was photooxidized to acetone, and eventually to carbon dioxide and water. The kinetic model of 2‐propanol photooxidation was successfully developed by the competitive Langmuir–Hinshelwood rate form, incorporating the inhibition effect coming from the formation of acetone. Copyright © 2004 Society of Chemical Industry     

Effects of coupling agents on the natural aging behavior and oxidation profile of high‐density polyethylene/sericite composites

The natural photooxidation of high‐density polyethylene (HDPE)/sericite composites was carried out outdoors for 180 days. The oxidative products, oxidation profile, and section morphology were characterized with Fourier transform infrared spectroscopy, infrared microscopy, and scanning electron microscopy, respectively. The results showed that HDPE/sericite had a higher oxidation degree than HDPE. All the coupling agents (CAs) accelerated the oxidation reaction of HDPE and made the final carbonyl index higher than that of an untreated one. The effect of sericite might have been due to the increased ultraviolet absorbance, whereas the effects of CAs might have been due to the active functional groups. Furthermore, cracks in the sections of test bars of HDPE/sericite composites after natural exposure were also observed. Their average lengths and densities were different for various CA treatments. This difference had a direct correspondence with the oxidation profile along the depth. This demonstrated that significant oxidation and consequently chain scission in the surface layer were responsible for the formation of cracks. Comparing the natural aging behavior of the films with test bars, we obtained more information about the propagation of the photooxidation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Chemical depth profiling of automotive coating systems using slab microtome sectioning with IR/UV-VIS spectroscopy and optical microscopy

Chemical surface/near-surface analysis and depth profiling of high performance automotive coating systems can be done efficiently by commonly available infrared (IR), ultraviolet-visible (UV-VIS) and optical microscopy (OM) measurement technologies. The different stages of mass loss for acrylic/melamine-crosslinked clearcoats modeled after those used in high performance automotive coating systems were explored as a function of exposure time/conditions. Two approaches were used. Transmission mode IR experiments were conducted to monitor mass loss in isolated clearcoats. OM was used to monitor mass loss in clearcoats that are part of actual multilayered coating systems. UV-screener (UVA) permanence and interphase mixing (i.e., between clearcoat and basecoat layers) as well as general chemistry depth profiling studies of model acrylic/melamine-crosslinked and acrylic/melamine+silane-crosslinked clearcoats were done as a function of system preparation and exposure time/conditions. UVA depth profiles were obtained using slab microtomy sectioning co-planar to the surface, followed by solvent extraction and UV-VIS solution analysis to monitor UVA content as a function of section mass and thickness. General chemistry depth profiles were obtained using slab microtomy sectioning (as mentioned previously), followed by ATR (IR) analysis to monitor changes in the photooxidation index values and the spectral envelop in the 2600–3800 cm⁻¹ range. The approaches described herein do not require expertise/equipment that is available to only a few, thus increasing the potential utility of such tools. Marshall R&D Laboratory, Philadelphia, PA, 19146.     

Evaluation of UV-induced embrittlement of PE-HD by Charpy impact test

The impact fracture behavior of two common high-density polyethylene grades for container applications were intensively studied by the instrumented Charpy impact test after well-defined exposure to UV-irradiation. Individual stages of the impact event, such as crack initiation and crack propagation energy as well as maximum impact load, were investigated from the recorded load–deflection curves. UV-induced material property changes were further investigated by infrared spectroscopy, differential scanning calorimetry, and dynamic-mechanical analysis as well as density measurements. Based on the results of the Charpy impact test, three indicators were identified to describe the extend of photooxidation on high-density polyethylene: (a) a reduced Charpy impact strength—at least to half of its initial value for a distinctly brittle impact fracture, (b) a marked decrease in the crack propagation contribution to the impact strength, and (c) an increase of the brittle features of the fracture surface.     

Syntheses and properties of photodegradable polystyrene-containing carbonyl group

The copolymerizations of styrene (St) and methyl vinyl ketone (MVK) or phenyl vinyl ketone (PVK) were carried out in benzene by using azobisisobutylonitrile (AIBN). The synthesized copolymers, poly(St-co-MVK) and poly(St-co-PVK), were identified by infrared (IR) spectroscopy. Under the presence of oxygen, the copolymers containing carbonyl group after UV irradiation showed photodegradable properties that caused by Norrish Type II reaction. The new IR characteristic peaks, such as carbonyl and vinyl groups of the photooxidized copolymers, increased by increasing UV irradiation time. In the photodegradation property, the color differences of the synthesized copolymers were higher than those of polystyrene with increasing UV irradiation time, which was caused by the inherent photodegradable property of carbonyl unit in copolymers. The average molecular weights of samples after UV irradiation for 300 h decreased, as compared with polymers before irradiation. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1237–1242, 1998     

Photo- and thermal-oxidation of the nonwoven polypropylene fabric studied by FT-IR photoacoustic spectroscopy

The photo- and thermal-oxidation of a nonwoven polypropylene fabric was investigated using Fourier transform infrared photoacoustic spectroscopy (FT-IR/PAS). The infrared spectroscopic data indicate that alcohol, peroxide, aldehyde, ketone, carboxylic acid, and anhydride were formed as the products of the thermal oxidation and photooxidation of the polypropylene fabric. Ester was also formed during the thermal oxidation process. The thermal oxidation was accelerated after an initiation period. It was found that thermal oxidation occurred homogeneously between the surfaces of the fabric and its bulk. For the polypropylene fabric irradiated by ultraviolet (UV) radiation, however, the highest degree of oxidation occurred in the surface of the fabric facing the UV radiation source. It was also found that the UV radiation at 254 nm caused photooxidation of polyropylene. No photooxidation was observed in the fabric exposed to the UV radiation at 350 nm under the same conditions. © 1994 John Wiley & Sons, Inc.     

Photooxidation and biotrickling filtration for controlling industrial emissions of trichloroethylene and perchloroethylene

A two-stage process integrating ultraviolet oxidation and biotrickling filtration (UV-BTF) was developed for treatment of industrial air streams contaminated with trichloroethylene (TCE) and perchloroethylene (PCE). Laboratory-scale studies demonstrated that the UV-BTF system consistently achieved contaminant removal efficiencies of 99-100% under optimal conditions. The UV-BTF process employs an advanced oxidation process (AOP) such as UV photooxidation as pretreatment for alleviating biological toxicity attributed to high and/or fluctuating contaminant concentrations, and for transforming the contaminants to more easily biodegradable products. The UV-BTF process showed significantly better performance than the BTF process without UV photooxidation pretreatment. Free-radical reaction mechanisms, pathways, and product formations were proposed for the UV photolysis of TCE and PCE. The photooxidation was mediated by chlorine atom reactions, and the principal products for TCE were phosgene, dichloroacetyl chloride (DCAC), low levels of trichloroacetyl chloride (TCAC), carbon monoxide, and hydrochloric acid; while those for PCE were phosgene, TCAC, low levels of DCAC, carbon dioxide, and hydrochloric acid. These products were identified and quantified by a combination of techniques, namely, gas chromatography-mass spectrometry (GC-MS) and Fourier transform infrared (FTIR) spectroscopy. These photooxidation products were completely removed in the BTF stage of the UV-BTF process, as indicated by subsequent GC-MS analyses. The destruction of photolysis products in the BTF stage occurred by a combination of hydrolysis, adsorption and microbial degradation.     

Photooxidative degradation of styrenic polymers: 13C-NMR and morphological changes upon irradiation

Thin films of high impact polystyrene (HIPS 8350) and styrene–butadiene–styrene triblock copolymers (SBS) were exposed to polychromatic light for different time intervals in SEPAP 12/24 at 55°C in atmospheric air. The photooxidized samples were analyzed by ¹³C-NMR spectroscopy in the liquid state. Epoxides and alcohols were photoproducts in the SBS. Epoxides were not observed in the HIPS. The morphological changes of the solution cast HIPS and SBS films caused by irradiation were studied by scanning electron microscopy. The property deterioration was explained in terms of scission reactions in the HIPS and SBS matrix. The micrographs indicate that both materials are of a two phasic system. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 637–645, 1998     

Nano-scale friction of polystyrene in air and in vacuum

Using atomic force microscopy (AFM), we measured friction between an AFM tip and a polystyrene surface at 25 °C, as a function of the sliding velocity and the applied normal load, both in air and under vacuum conditions. The objective was to analyze the influence of humidity on the frictional behavior of polystyrene. Our experimental results as a function of sliding velocity revealed a logarithmic increase of the friction force in air whereas a logarithmic decrease of this force is found in vacuum. Our comparative results unveil that two different dissipation mechanisms are dominating the frictional behavior of polystyrene in air and in vacuum. We propose a tentative explanation.     

Role of minor constituents in the photooxidation of virgin olive oil

Virgin olive oil was photooxidized at 2 and 40°C and at fluorescent light intensities of 0, 620, 2710, and 5340 lux. As expected, higher fluorescent light intensities induced higher peroxide formation in the oil. The thiobarbituric acid reactive substances (TBARS) were found to be good indicators of photooxidation during the early stage of the reaction. Pheophytin A and β-carotene were light- and temperature-sensitive, whereas α-tocopherol and total polyphenols were mostly affected by light. Pheophytin A functioned as a photosensitizer, resulting in rapid oxidation of the oil. β-Carotene was a strong natural inhibitor of photooxidation for all light intensities at 2°C, suggesting quenching properties for singlet oxygen. However, β-carotene antioxidant activity was reduced at 40°C because of its rapid destruction.     

Phototransformation of water-soluble polymers. Part II: photooxidation of poly(ethylene oxide) in aqueous solution

Poly(ethylene oxide) (PEO) was irradiated in aqueous solution under long wavelengths (λ>300 nm, 20 °C) and in presence of oxygen. The photooxidation of PEO was studied by IR spectrophotometry, viscometry and size exclusion chromatography. The formation of the oxidation photoproducts was studied by infrared analysis of films obtained by evaporation of aliquots of irradiated aqueous solutions. The photoproducts were identified by chemical derivatization treatments coupled with infrared measurements. Viscosimetry and SEC analysis showed that photooxidation was leading to a dramatic decrease of the molecular weights. The influence of the pH of the aqueous solutions was also examined. Unexpected results were obtained for the pH 12 solutions, indicating a strong inhibition of the oxidation.

Comparison with the results obtained in the case of PEO irradiated in the solid state showed that no direct transposition of the knowledge concerning the behavior of the solid polymer could be made.