Effect of film thickness on auto-oxidation in cobalt-catalyzed 1,4-polybutadiene films

Oxygen mass uptake was measured in 1,4-polybutadiene (PB) films undergoing cobalt-catalyzed oxidation in air. Films thicker than approximately 50 μm showed an increase in oxygen uptake per unit polymer mass as film thickness increased, while oxygen uptake per unit film area remained independent of thickness, suggesting that oxidation was heterogeneous and proceeded essentially as an oxidized front penetrating into the film from the surfaces exposed to oxygen. In contrast, oxidation in films thinner than about 28 μm proceeds homogeneously, with oxygen uptake per unit mass being essentially independent of thickness. In oxidized samples, oxygen and nitrogen permeability decreased by more than two orders of magnitude relative to permeability values in unoxidized samples. In thicker films, a two-phase model, based upon high levels of oxidation in a thin skin at the surface of PB and relatively low levels of oxidation in the core of the films, was used to describe gas permeability data and estimate the oxygen and nitrogen permeability in fully oxidized PB.     

Improved oxidative stability of sunflower oil in the presence of an oxygen-scavenging film

The oxidative stability of sunflower oil (SFO) was measured during storage at 23 and 37°C in the presence of a novel oxygen-scavenging film that contained polyfuryloxirane (PFO). Commercially refined and deodorized SFO was stored in a lighted room in sealed transparent packages containing either PFO film or an antioxidant, 0.02% butylated hydroxytoluene (BHT). Oxidative stability was evaluated by determination of peroxide values and gas-chromatographic measurement of headspace hexanal. SFO stored in the presence of the oxygen-scavenging film was more stable than oil stored without the film, or than film stored with 0.02% BHT. The PFO film scavenges oxygen through energy-transfer sensitization of singlet oxygen. The film is doped with eosin and the naturally-occurring dye, curcumin, which absorb over a wide range of visible wavelengths. Curcumin transfers its absorbed energy to eosin, which sensitizes the production of singlet oxygen. The singlet oxygen is scavenged by PFO. The use of two dyes increases the efficiency of the sensitization process, reducing the illumination time and intensity required for effective oxygen scavenging.     

Determination of permeability and diffusivity of oxygen in polymers by polarographic method with inert gas

A modified polarographic method with inert gas for determination of the oxygen permeability in polymers immersed in liquids is described. Owing to the stream of an inert gas towards polymer from the cathode side, lateral oxygen diffusion (edge effects) is minimized. Unlike the standard Fatt method, the method with inert gas is suitable also for thick samples and, therefore, for high-permeable materials. The method was tested for prediction of oxygen permeability in poly(1-vinyl 2-pyrrolidone) (PVP) and poly(2-hydroxyethyl methacrylate) (PHEMA). As an electrolyte, solution of potassium chloride was used. The effect of additional resistances and small lateral diffusion was taken into account. Unexpectedly, oxygen permeability in both polymers was greater for 0.1 M KCl than for 0.5 M KCl. The experimental setup was also used for diffusivity estimation in thick samples of PHEMA and PVP. Here, the oxygen flux response at one sample surface to the stepwise change in oxygen concentration at the other surface is measured and evaluated. The effect of the additional boundary layer on the oxygen transport is taken into account. A simple procedure for the diffusivity determination from the characteristic time of response as a function of the sample thickness is given. Solubility of oxygen in polymer is calculated from the obtained permeability and diffusivity.     

Molecular weight measurement in weathered polymers

Gel permeation chromatography has been used to monitor the molecular weight changes that occur when polymers are degraded by photooxidation. Methods of sample preparation and procedures for conducting the chromatography measurements and for analyzing the results are discussed. Examples are given of applications to studies of polystyrene (PS), polypropylene (PP), and glass fiber-reinforced polypropylene (GFPP) exposed to ultraviolet (UV) irradiation in the laboratory. The results show that the degradation rate is fastest near the exposed surface, but in PS and PP, degradation is much faster at the unexposed surface than in the center of the molding, where the UV intensity is greater than at the unexposed surface. It is deduced that degradation at the center is slowed by a shortage of oxygen. © 1994 John Wiley & Sons, Inc.     

Electrochemiluminescence of luminol in dimethyl sulfoxide at a polycrystalline gold electrode

The electrochemiluminescence (ECL) behavior of luminol in oxygen-saturated dimethyl sulfoxide (DMSO) solution at a polycrystalline gold electrode was studied under conventional cyclic voltammetric (CV) conditions. Corresponding to the reduction processes of oxygen, one ECL peak (ECL-1 at −1.40 V versus SCE) with two shoulders (S_1-1 at −0.55 V and S_1-2 at −0.90 V) was observed on the curve of ECL intensity versus potential. Corresponding to the subsequent oxidation processes of oxygen-containing species generated by the reduction of oxygen, one ECL peak (ECL-2 at −0.34 V) with a shoulder (S₂ at −0.67 V) was found. Among them, ECL-2 was strongest; it was 10 times stronger than ECL in pH 13.0 aqueous alkaline solution under the same conditions. These ECL peaks and shoulders were found to depend on the presence of N₂ and O₂, potential scan range, water concentration, supporting electrolyte, and the concentration of luminol. The emitter of both ECL peaks was identified as 3-aminophthalate (AP²⁻) by analyzing the ECL spectra. The origin for these ECL peaks is proposed to be related to the reactions of luminol with various oxygen-containing species such as O₂⁻, H₂O₂, OH⁻ and O₂ electrogenerated by the redox reactions of dissolved oxygen at different potentials. The results indicate that the ECL of luminol was correlated to the redox behavior of oxygen in DMSO. The present work reveals that the solvent plays an important role in the ECL behavior of luminol.     

An approach towards characterizing a reference sampler for culturable biological particle measurement

The first published account of a slit-to-agar sampler was of a system that used 10 cm petri plates. At some point after the 1950s samplers that collected biological aerosol particles on 15 cm plates appeared. More recently published patents describing slit samplers using 10 cm plates have cited economics as the prime motivation. Nonetheless, researchers have cautioned that the smaller plates can become saturated when heavy aerosol clouds are encountered, such as when a slit sampler is used as a reference collector when biological detectors are tested. This report describes how to determine the performance characteristics of slit samplers designed for either 10 or 15 cm plates. It also demonstrates the importance of controlled replicate measurements providing data suitable for rigorous statistical analysis. The results indicate that, for measuring biological clouds of between 5 and 30 agent containing particles per liter of air (ACPLA), the 15 cm plate sampler design is more efficient than those targeted for the smaller collection surface. A statistical method has been employed to test differences between regression slopes.     

Trialkylsilanes as reagents for the UV-induced surface modification of polybutadiene

A photochemical process for the modification of polymer surfaces using organosilane compounds has been developed. The process is based upon the UV irradiation of polybutadiene in the presence of liquid ethyldimethylsilane (C₂H₅)(CH₃)₂Si-H and gaseous trimethylsilane (CH₃)₃Si-H. UV irradiation was carried out with a medium pressure Hg lamp and a 193 nm ArF* excimer laser. The modified polymer surfaces were investigated by infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), contact angle measurements and atomic force microscopy (AFM). It is found that the photoassisted surface modification with trialkylsilanes leads to the introduction of trialkylsilyl groups onto the surface of the target polymer. From quantitative XPS data the composition of the modified polymer suface (C, O and Si) was determined. The surface modification with trialkylsilanes results in a significant lowering of surface tension γ of polybutadiene. The silane/UV process was found to be very sensitive to small amounts of oxygen in the process gas. Summing up, it is demonstrated that UV irradiation in the presence of gaseous silane compounds is a convenient way to introduce organosilicon groups onto the surface of technical polymers.     

Characterization of oxidized reticulated vitreous carbon electrode for oxygen reduction reaction in acid solutions

The oxidation of reticulated vitreous carbon (RVC) and its impact on the oxygen reduction reaction (ORR) in H₂SO₄ solutions has been studied. The results are compared with that of a planar glassy carbon (GC) electrode. The oxidation process was characterized by using different electrode configurations, GC (planar) and RVC electrodes both with flooded (batch process) and flow-through assembly. Cyclic voltammetry, potentiodynamic and rotating ring-disk electrode voltammetry were used for the characterization of the ORR. Anodically oxidized GC and flooded RVC are similar in that the ORR on both electrodes gave a more defined limiting current plateau. For the flow-through porous electrode, the oxidation process caused a distribution of the oxidation extent within the bed thickness, as evident from the SEM images, and only about half of the porous electrode was utilized in the oxidation process. X-ray photoelectron spectroscopy (XPS) measurements confirmed the above distribution and a gradient of the oxygen-to-carbon ratio was obtained within the porous bed. Oxidation of RVC led to an enhancement of its electrocatalytic properties towards ORR. H₂O₂ production was tested at the oxidized RVC from flowing acid solutions. The oxidation of RVC resulted in higher current efficiencies and higher outlet concentrations of the H₂O₂ acid solutions.     

A system for accurate on-line measurement of total gas consumption or production rates in microbioreactors

A system has been developed, based on pressure controlled gas pumping, for accurate measurement of total gas consumption or production rates in the nmol/min range, applicable for on-line monitoring of bioconversions in microbioreactors. The system was validated by carrying out a bioconversion with known stoichiometric relation between gas consumption and substrate conversion, that is, the enzymatic oxidation of glucose to gluconic acid. The reaction was carried out in a stirred microreactor with a working volume of 100 μL, whereby the oxygen consumption was monitored on-line. Subsequently the system was applied to determine the oxygen transfer capacity of the microbioreactor. The dissolved oxygen concentration was measured with an optical dissolved oxygen sensor, which was integrated near the bottom of the reactor. Different stirrer sizes and geometries were investigated for their effect on the mass transfer of oxygen. A maximal k_La of 156±10 h⁻¹, allowing a maximal O₂-transfer rate up to 50 mmol O₂/L/h, was reached which is sufficient to grow cells aerobically in (fed-)batch mode at relatively high biomass concentrations.     

X-ray diffraction study of fibrous polymers. I. Degree of paracrystallinity—a new parameter for characterizing fibrous polymers

It has been recognized that paracrystallinity dominates a considerable portion of the fibrous polymers. In the general case, the folding of long chains in fibrous polymers result in several domains, some being crystalline, some amorphous, while the remainder are paracrystalline. Based on this model a new parameter-degree of paracrystallinity-has been proposed. This paper reports a method of determining this parameter. Native cellulose fibres of ramie, hemp and jute have been studied. It is found that degree of paracrystallinity in hemp and degree of crystallinity in ramie are greater than in the other fibres. Crystallinity index has been found to be less than that generally reported in the literature. This has been attributed to the intensity peaks previously considered in calculating the degree of crystallinity having components of paracrystallinity which have been evaluated and separated.     

Self-assembled nano-phase particles for enhanced oxygen barrier coatings on polymeric materials

Aqueous-based inorganic–organic hybrid coating materials comprising self-assembled nanophase particles (SNAP) were investigated for their potential to confer high gas barrier performance to flexible polymeric substrates, particularly to improve suitability of the substrates as encapsulation materials for organic photovoltaic (OPV) devices. Potential benefits of this approach include versatility in barrier coating formulation to achieve specific barrier properties, and application of coatings in a reel-to-reel process under ambient conditions. The present study focused on enhancement of the oxygen barrier performance of polypropylene (PP) substrates by applying SNAP-based coatings with and without the addition of an oxygen scavenging additive. SNAP particles were characterised using ²⁹Si NMR and dynamic light scattering, and coatings were analysed using atomic force microscopy and X-ray photoelectron spectroscopy. SNAP particle preparation and coating formulation was optimised with respect to oxygen transmission rate (OTR) of the coating on PP, and mechanical properties of the coating solution. In the absence of oxygen scavenger, the lowest OTR attained for the SNAP-based coatings was 0.87 cm³ mil m⁻² day⁻¹ atm⁻¹. The OTR was further reduced to 0.22 cm³ mil m⁻² day⁻¹ atm⁻¹ on addition of 9,10-anthraquinone-2,6-dissulfonic acid (AQDS) into the coating as an oxygen scavenger. These results represent a decrease in OTR by 4 orders of magnitude compared with uncoated PP, and the oxygen barrier obtained by addition of AQDS surpasses the performance of many plastic materials considered to be high oxygen barriers in the food and pharmaceutical industries.     

Doping-induced broadening of the hole density-of-states in conducting polymers

The density-of-states distribution in conducting polymers reflects the energy disorder caused by electrostatic and steric interactions resulting from the different environment in which each molecule is placed. In case of p-doping (oxidation), the highest occupied molecular orbital (HOMO) manifold spreads in energy following a distribution as a result of long-range electrostatic (dipolar) interactions with the surrounding disordered host. In this paper the repercussion of the dipolar disorder on electrochemical signals of standard polypyrrole films is explored. The analysis is based on the chemical capacitance variation with the applied potential in experiments performed in quasi-equilibrium conditions. In addition to the Gaussian shape of cyclic voltammograms at low-doping levels, the model is able to qualitatively account for the current plateau usually observed at high oxidation potentials. This approach allows to estimating the dipolar moment associated to the polymer/dopant complex.     

Influence of Bi–Fe additive on properties of vanadium phosphate catalysts for n-butane oxidation to maleic anhydride

The physico-chemical and catalytic properties of three ways of modified catalysts were studied, i.e. (i) the addition of both Bi and Fe (nitrate form) during the refluxing VOPO₄·2H₂O with isobutanol (Catalyst A), (ii) the simultaneous addition of BiFe oxide powder in the course of the synthesis of precursor VOHPO₄·0.5H₂O (Catalyst B) and (iii) the mechanochemical treatment of precursor VOHPO₄·0.5H₂O and BiFe oxide in ethanol (Catalyst C). It was found that surface area of the modified catalysts has increased except Catalyst B. The reactivity of the oxygen species linked to V⁵⁺ and V⁴⁺ was studied by using H₂-TPR, which also affected the catalytic performance of the catalyst. The conversion of n-butane decreases with an increment of oxygen species associated with V⁵⁺.     

Adsorption of atomic and molecular oxygen on Cu(1 0 0)

We have studied the initial stages of the oxidation of the Cu(1 0 0) surface using ab initio calculations. Both atomic and molecular oxygen are addressed. We show that subsurface oxygen is not energetically favourable, but gets stabilized by on-surface O. We discuss the adsorption of molecular oxygen using elbow plots, which can be used in order to qualitatively understand the measured sticking coefficient at different oxygen coverages.     

Research Note: O3 or O2 and Ag: A New Catalyst Technology for Aqueous Phase Sanitation

Silver deposited on an inert surface with a very large area exhibits a strong catalytic interaction with oxygen which results in strong bactericidal activity. This catalytic process is fundamentally different from other known silver-based approaches which deliver low levels of silver ions into water.

Two factors appear to control the rate of this catalytic reaction process: (1) the size and dispersion of the silver crystals and surface area of the supporting bed; and (2) the volume of oxygen in solution. The source of the oxygen can be atmospheric oxygen dissolved in the water or, for a greatly enhanced reaction rate, dissolved ozone produced by an ozone generator. In this process, oxygen molecules absorbed onto the catalyst surface are subsequently transferred to other oxidizable substrates including bacteria and viruses.

These catalytic oxidizing reactions exhibit two properties of significance in the sanitation of water. First, because oxygen on the catalyst surface reacts with both living and nonliving substrates, bacteria or viruses which are in the water flowing over the catalyst-containing medium are killed or inactivated on contact by the oxidizing reactions (i.e., without requiring the release of metals into water). Second, oxygen also is transferred to oxidizable inorganics (such as bromide ion) to generate readily measurable and relatively stable “residual” oxidizers/disinfectants that continue to sanitize water downstream.

Results of this Author's experiments have been independently replicated by sources from the University of Arizona, and at Herbert V. Shuster, Inc. Laboratories in Massachusetts.     

X-ray measurement of chain orientation in non-crystalline polymers

A new method of orientation measurement using wide angle X-ray scattering (WAXS) is described and compared with methods based on birefringence and spectroscopy. It is applied to PMMA deformed in the glassy or rubbery states. For specimens plastically deformed in uniaxial tension, a comparison is made between orientation parameters measured from WAXS and similar parameters obtained from the same specimens using n.m.r. by Ward. The WAXS method is further used to measure orientation in material deformed in plane-strain compression. It also demonstrates the absence of chain alignment in elastically strained PMMA despite the high birefringence observed.     

A method for the dynamic measurement of diffusivities of gases in polymers

A method is presented for determining the diffusivity of a gas in a polymer from the response to a step concentration change in a continuous-flow permeation chamber. The outlined procedure has several advantages over techniques currently in use: it requires simple numerical integration rather than curve fitting; it utilizes the complete response, rather than a portion of the response which falls within the region of validity of a short-time asympotic solution of the diffusion equation; and it is applicable both to flat membranes and cylindrical tubes. An illustration of the method is provided by the measurement of the diffusivity of sulfur dioxide in a PTFE tube at several temperatures.     

An improved sample preparation method for monomer conversion measurement using headspace gas chromatography in emulsion polymerization research

This article reports an improved headspace gas chromatographic (HS‐GC) technique for the determination of monomers in polymer latex. The method is based on a HS‐GC technique reported previously (Chai et al., J Appl Polym Sci 2006, 99, 392) but includes a major modification in the method of sample preparation. By using a weight‐based sampling method, coupled with initial dissolution in dimethyl sulfoxide (DMSO), followed by water dilution, the uncertainty in the sample preparation associated with previous method can be significantly reduced. The improved method is reliable, practical, and suitable for use in the kinetic study during emulsion polymerization. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012