Development of positron annihilation spectroscopy to test accelerated weathering of protective polymer coatings

A variable mono-energetic positron beam with a computer-controlled system has recently been constructed at the University of Missouri–Kansas City for weathering studies of polymeric coatings. The beam is designed to measure the S-parameter from Doppler-broadening energy spectra and the sub-nanometer defect properties from positron annihilation lifetimes (PAL). Significant variations of S-parameter and ortho-positronium intensity in coatings, as obtained from the newly built beam and from the Electrotechnical Laboratory’s beam, respectively, are observed as a function of depth and exposure time due to the Xe-light irradiation. A high sensitivity of positron annihilation signal response to the early stage of degradation is observed. Development of positron annihilation spectroscopy to test accelerated weathering of polymeric coatings is discussed.     

Elaboration of nano titania-magnetic reduced graphene oxide for degradation of tartrazine dye in aqueous solution

In this paper, magnetic nanocomposites are synthesized by loading reduced graphene oxide (RG) with two components of nanoparticles consisting of titanium dioxide (TiO₂) and magnetite (Fe₃O₄) with varying amounts. The structural and magnetic features of the prepared composite photocatalysts were investigated by powder X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (UV–vis/DRS), Raman and vibrating sample magnetometer (VSM). The resulting TiO₂/magnetite reduced graphene oxide (MRGT) composite demonstrated intrinsic visible light photocatalytic activity, on degradation of tartrazine (TZ) dye from a synthetic aqueous solution. Specifically, it exhibits higher photocatalytic activity than magnetite reduced graphene oxide (MRG) and TiO₂ nanoparticles. The photocatalytic degradation of TZ dye when using MRG and TiO₂ for 3 h under visible light was 35% and 10% respectively, whereas for MRGT it was more than 95%. The higher photocatalytic efficiency of MRGT is due to the existence of reduced graphene oxide and magnetite which enhances the photocatalytic efficiency of the composite in visible light towards the degradation of harmful soluble azo dye (tartrazine).     

Comparison of two methods for coating piezoelectric crystals

Quartz piezoelectric crystals were coated with triethanolamine using two procedures: spray and syringe methods. The reproducibility of both coating methods was evaluated comparing the relative standard deviation of batches of four crystals coated by each of the procedures. The relative standard deviation obtained with the spray method is a hyperbolic function of the coating frequency and is, at least, three times lower than the one calculated from results of the syringe method, which is not frequency dependent. An explanation for the poorer reproducibility of the syringe method is given, based on the radial sensitivity theory and the non-uniformity of the coating distribution over the crystal, as evidenced by microscopic photographs.     

Three-photon annihilation of positrons—Application to chemical characterization

The measurement of three-photon annihilation of positrons could provide not only an enhanced specificity in the determination of elements that have positron emitters as products, but also chemical (state) information on the environment of that element in the sample. The tree-photon yields have been determined for several compounds containing copper, in addition to the pure metal and a few alloys. The observations have been corraborated by angular correlation and Doppler broadening measurements and further substantiated by magnetic quenching in the angular correlation measurements.     

Interaction of slow positrons with H− ions in alkali metal hydrides

The probabilities of the fundamental interaction processes of slow positrons (in the energy interval from 0.025 to 5.46 eV) with H^– ions were calculated on the basis of the self-consistent field method. Curves of the angular correlation of gamma-quanta during two-photon annihilation of positrons moving in the field of H^– ions with energies of 0.025, 0.546, 1.365, 2.73, and 5.46 eV were obtained. Comparison of the probabilities of annihilation and radiation capture by H^– ions with the probabilities of processes occurring with thermalized positrons makes it possible to establish more clearly the annihilation mechanism of slow positrons in alkali metal hydrides.     

Assessment of polyelectrolyte coating stability under dynamic buffer conditions in CE

Dynamic buffer conditions are present in many electrophoretically driven separations. Polyelectrolyte multilayer coatings have been employed in CE because of their chemical and physical stability as well as their ease of application. The goal of this study is to measure the effect of dynamic changes in buffer pH on flow using a real-time method for measuring EOF. Polyelectrolyte multilayers (PEMs) were composed of pairs of strong or completely ionized polyelectrolytes including poly(diallyldimethylammonium) chloride and poly(styrene sulfonate) and weak or ionizable polyelectrolytes including poly(allylamine) and poly(methacrylic acid). Polyelectrolyte multilayers of varying thicknesses (3, 4, 7, 8, 15, or 16 layers) were also studied. While the magnitude of the EOF was monitored every 2 s, the buffer pH was exchanged from a relatively basic pH (7.1) to increasingly acidic pHs (6.6, 6.1, 5.5, and 5.1). Strong polyelectrolytes responded minimally to changes in buffer pH (<1%), whereas substantial (>10%) and sometimes irreversible changes were measured with weak polyelectrolytes. Thicker coatings resulted in a similar magnitude of response but were more likely to degrade in response to buffer pH changes. The most stable coatings were formed from thinner layers of strong polyelectrolytes.     

The interaction of slow positrons with organic moecules above and below positronium formation thresholds

A review is given of the ionization of organic moecules by monoenergetic positrons having energies in the range of 0.5–15 eV. Two mechanisms, unique to positrons, are described. If the kinetic energy of the positron is above the positronium formation threshold, such that electrons can be removed from the molecules to form free positronium atoms, the ionization/fragmentation behavior can be explained qualitatively by a modification of the Ore gap theory. To explain how positrons can ionize and fragment molecules when their kinetic energies are below the positronium formation threshold, it is necessary to assume that energy is transferred to the molecule by the annihilation process. Ionization cross sections for positrons having kinetic energies below the positronium formation threshold are sensitive to molecular size, structure and bond types. Continuing work involves a search for positronium compound formation and measurements of the kinetic energy distributions of ions.     

Effect of coating electrolytes on two-tailed surfactant bilayer coatings in capillary electrophoresis

Surfactants such as dioctadecyldimethylammonium bromide (DODAB) form semi-permanent coatings that effectively prevent adsorption of cationic proteins onto the fused silica capillary in capillary electrophoresis (CE). The bilayer coating is generated by flushing the capillary with a 0.1 mM surfactant solution. However, formation of the bilayer is strongly dependent on the coating electrolyte. The effect of counter-ions, electrolyte concentrations and buffer co-ions were monitored based on: the separation of basic model proteins; the adsorption kinetics of DODA⁺ onto fused silica; and dynamic light scattering (DLS) to determine vesicle size. Low concentrations (≤10.0 mM) and/or weakly associating buffers such as phosphate (pH 3.0), acetate (pH 4.0) and chloride should be used for DODAB coating solutions. Dissolving the surfactant in strongly associating electrolyte, such as phosphate at pH 7.0, results in poor coating of the capillary surface. Effective cationic bilayer coatings are formed if the buffer conditions favor formation of vesicles with diameters < 300 nm. Monitoring turbidity at 400 nm provides a convenient means of verifying vesicle diameter variation of <5 nm; that is, that the coating solution is effective.     

Isothermal degradation and thermooxidative degradation of an epoxy powder coating

Summary Thermogravimetry was used to study the kinetics of isothermal degradation of an epoxy thermoset powder coating in a nitrogen atmosphere and in oxidizing atmospheres of air and pure oxygen. An integral isoconversional procedure was used to analyse how the activation energy varies depending on the degree of conversion and depending on the atmospheres used. In the case of degradation in a nitrogen atmosphere, in addition to the activation energy, the kinetic triplet was completed using an Avrami reaction model and the pre-exponential factor. With this atmosphere, the conclusion was reached that the isothermal and non-isothermal kinetics are equivalent. It was shown that the thermooxidative degradation process is more complex and consists of a two-stage process. The first stage of degradation is similar whether nitrogen, oxygen or air are present. Chain scission occurs and it seems that there is formation of thermally more stable compounds. The second stage of degradation, involving several phenomena, occurs only in the presence of oxygen or air and leads to the total disappearance of the organic material by thermooxidation. These stages are very similar under non-isothermal or isothermal conditions.     

A modified supported bilayer/diblock polymer--working towards a tunable coating for capillary electrophoresis

A surfactant bilayer/diblock polymer coating was previously developed for the separation of proteins. The coating consisted of a mixture of the cationic surfactant dioctadecyldimethylammonium bromide (DODAB) and the neutral polymer poly-oxyethylene (POE) 40 stearate (Journal of Chromatography A 1130 (2006) 265–271). Herein an improved method of generating DODAB/POE stearate coatings is demonstrated, which yields more predictable EOF, more stable coatings, greater average efficiencies and easier method development. In this sequential preparation method the DODAB is first flowed through the capillary, followed by a flow of the POE stearate (sequential method). A tunable EOF (−2.40 to −0.17 × 10⁻⁴ cm²/Vs) is achieved by varying the POE chain length (8, 40 and 100 oxyethylene units). Mixtures of POE 8 and POE 40 stearate enabled continuous variation in EOF from −2.44 to −0.42 × 10⁻⁴ cm²/Vs. Separations of basic proteins yielded efficiencies of 760 000–940 000 plates/m. Coatings formed using the sequential method were more stable over a larger number of runs (%RSD for migration times: 0.7–1.0% over 30 runs) than those formed using the original mixed method (%RSD: 2.4–4.6% over 14 runs). The ability to tune the EOF is important in maximizing the resolution of analytes with similar electrophoretic mobilities. Histone proteins are separated on a sequentially coated capillary with resolution of nine possible subtypes. Acidic proteins are separated on a sequentially coated capillary at pH 6.4.     

Hydrophobic octadecylamine-polyphenol film coated slow released urea via one-step spraying co-deposition

Polymer coated fertilizer has been designed for gradual release of nutritional content of fertilizer to improve the utilization efficiency. However, the better slow release property of the coated fertilizer and simplified preparation process are still a huge challenge. The hydrophobic ODA-polyphenol film coated urea was prepared by one-step spraying for excellent nutrient release performance. Octadecylamine (ODA) was used to modify poly(tannic acid) (PTA) and poly(pyrogallol) (PPG) coating on substrates and prepare the hydrophobic ODA-PTA and ODA-PPG coated urea granules. The nutrient release rate of ODA-PTA and ODA-PPG coated fertilizers was much slower than PTA and PPG coated fertilizers in water and soil. The N release longevities of ODA-PTA coated fertilizers will be extended from 2 h to 30 days compared with PTA coated fertilizers. Moreover, pot experiments showed that coated fertilizer could effectively promote plant growth. This work will provide one-step spraying co-deposition approach to fabricate hydrophobic ODA-polyphenol film coated urea to improve the N release longevity, which can shine a light on the development of new slow release fertilizers (SRFs).     

Optimizing heavy-duty anticorrosive performances of coating films formed by acrylate-vinylidene chloride copolymer latexes through twice-painting technique

Twice-painting technique was adopted to prepare heavy-duty anticorrosive coating films formed by aqueous latexes of copolymers of vinylidene chloride(VDC) with an acrylate, namely methyl acrylate(MA), ethyl acrylate(EA), butyl acrylate(BA) or 2-ethylhexyl acrylate(EHA). Harsh salt-spray corrosion tests demonstrated that the optimized twicepainting technique was that the acidic latex solution was adjusted to p H 5-6 for the first painting, while it was utilized directly for the second painting. The test of 600 h of harsh salt-spray corrosion showed that MA-VDC85 coating could protect the steel excellently, whereas the other acrylate-VDC coatings with 75%-90% VDC content could not protect the steel so effectively. Further corrosion test showed that(1) MA-VDC85 coating protected steel from loss of metallic luster for at least 1000 h of salt-spray corrosion;(2) adhesion of MA-VDC85 coating to steel was excellent for at least 800 h of saltspray corrosion, but became very poor after 1000 h. Differential scanning calorimetry, thermogravimetric analysis, X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy were used to evaluate the corroded MA-VDC85 film.     

Influence of N sources on the photocatalytic activity of N-doped TiO2

Influence of nitrogen precursors urea, semicarbazide and N,N’-dimethyl urea on the photocatalytic activity of the N-doped TiO₂ were studied by a simple decomposition method. The nano N-TiO₂ catalysts were synthesized via two different modified approaches by calcination at 500 °C. The synthesized samples were characterized by IR, UV-DRS, Raman, TG-DTA, XRD, EDX, XPS, SEM, TEM and BET analysis. Of the synthesized six samples of N-TiO₂ five samples showed better photocatalytic activity towards direct sunlight photo-degradation of methylene blue (MB) and rhodamine B (RhB) than Degussa P25. The catalysts obtained using semicarbazide samples F3 and F4 having large surface area of 76 and 85.8 m²/g displayed maximum photocatalytic activity. The sample F4 was 1.5 times more active than Degussa P25 for the decolourisation of MB and 1.9 times more active for the decolourisation of RhB. The presence of nitrogen, large surface area and coupling of rutile-anatase phases were found to be the main responsible factors for the enhanced photocatalytic activity. The exclusive formation of the anatase phase in the case of urea precursor is attributed to the slow evaporation of urea due to the formation of melamine derived products. The calcination temperature is the deciding factor responsible for the photocatalytic activity of the N-TiO₂ samples prepared from precursors which can potentially form the melamine and its oligomerized products on the surface of TiO₂.     

Monitoring cell culture media degradation using surface enhanced Raman scattering (SERS) spectroscopy

The quality of the cell culture media used in biopharmaceutical manufacturing is a crucial factor affecting bioprocess performance and the quality of the final product. Due to their complex composition these media are inherently unstable, and significant compositional variations can occur particularly when in the prepared liquid state. For example photo-degradation of cell culture media can have adverse effects on cell viability and thus process performance. There is therefore, from quality control, quality assurance and process management view points, an urgent demand for the development of rapid and inexpensive tools for the stability monitoring of these complex mixtures. Spectroscopic methods, based on fluorescence or Raman measurements, have now become viable alternatives to more time-consuming and expensive (on a unit analysis cost) chromatographic and/or mass spectrometry based methods for routine analysis of media. Here we demonstrate the application of surface enhanced Raman scattering (SERS) spectroscopy for the simple, fast, analysis of cell culture media degradation. Once stringent reproducibility controls are implemented, chemometric data analysis methods can then be used to rapidly monitor the compositional changes in chemically defined media. SERS shows clearly that even when media are stored at low temperature (2–8 °C) and in the dark, significant chemical changes occur, particularly with regard to cysteine/cystine concentration.     

Controlled degradation of biocomposites ENR/PCL containing natural antioxidants

The 21st century is witnessing a return to green polymers made from renewable raw materials due to crude oil shortage and to ecological reasons. The main polymers meeting such conditions include aliphatic polyesters or biopolymers, the interest in which has been increasing over the last decade. The aim of our study was to obtain pro-ecological composites with increased resistance to aging using a biopolymer, epoxidized natural rubber (ENR) and polyester polycaprolactone (PCL). To improve the resistance of these composites to aging, we used natural anti-aging substances, mainly flavonoids and amino acids. After simulating climatic, UV, and thermo-oxidative conditions, we assessed the effect of the antioxidants used on the degradation capability of ENR/PCL composites. Based on the tensile strength test, cross-link density and calorimetric measurements, we observed a considerable increase in the aging resistance of ENR/PCL composites. At the same time, the ENR/PCL composites showed a high susceptibility to biodegradation by microorganisms, which allowed us to obtain polymeric products with controlled degradation capabilities.     

聚丙烯酸酯类毛细管整体柱的制备及其在加压毛细管电色谱中的应用

毛细管电色谱(CEC)是毛细管电泳(CE)和微径液相色谱(micro—HPLC)技术的结合,是集CE的电子迁移机制和micro-HPLC的分配分离机理发展起来的一种高效微分离技术．CEC以塞子流型的电渗流代替抛物线流型的压力流,具有CE的高效性,能够分离电中性化合物而具有HPLC的高选择性．     

The effect of a magnetic tape coating process on the structure and properties of a semi-crystalline polymer

A combination of structure identifying and bulk property experiments were combined with a two-phase analytical methodology to elucidate the influence of a magnetic tape coating process on the substrate polymer film. Employing a typical tape manufacturing process that utilizes coating, drying, and calendering stages, polyethylenete-rephthalate polymer film samples with and without the magnetic coating were prepared.Experiments and modeling studies performed on these samples demonstrate that the drying and calendering processes may increase the crystallinity and noncrystalline orientation of the substrate film. In addition, dynamic mechanical experiments identified a viscoelastic transition at 50 °C for the PET substrate film sample which is near the glass transition of the magnetic coating utilized. Overall, the results of this investigation provide a basis for evaluating structure property interrelations of polymer-based magnetic tapes.     

A statistical degradation model for the service life prediction of aircraft coatings: With a comparison to an existing methodology

An advance on the model used by Guseva et al. [1] for estimating the service life of organic coatings under service conditions from accelerated test results has been developed. Instead of modelling just the times to failure, this new approach uses the complete degradation curve and allows for a more general specification of the failure time distribution. This modified model was then applied to the estimation of the service life (defined as gloss loss) for aircraft coatings and the results were compared to those obtained by Guseva et al. It was found that when this model was applied to the naturally weathered data, gloss loss (and thus failure times at a given level of gloss loss) followed a generalised gamma distribution, rather than the Weibull distribution identified by Guseva et al. Further, the new approach suggested a reduction in the warranty time of about one month. When the model was applied to the accelerated test data, it produced more accurate extrapolations of the median failure time associated with the naturally weathered data - (49.3 months compared to a measured 52 months obtained at the naturally weathered site). In fact, the extrapolated distribution obtained by this new approach was much closer to the distribution for the naturally weathered data than the extrapolated distribution obtained by Guseva et al.