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71.
THE AVAILABILITY OF SOLAR RADIATION BELOW 290 nm AND ITS IMPORTANCE IN PHOTOMODIFICATION OF POLYMERS
R. E. BARKER JR. 《Photochemistry and photobiology》1968,7(3):275-295
Abstract— On a percentage basis, ozone is a very minor component of the atmosphere; at STP it would make a layer only about 2 mm thick. On almost every other basis (biological, meteorological, paleontological, photochemical, etc.) it is a major component, due mainly to the tremendous reduction in solar ultraviolet flux which it causes in the 220–290 nm region. Since no data are available for Λ < 285 nm, a rational basis for estimating the flux reaching the earth's surface in this region is discussed. Variations in ozone concentration are of great importance, and it is possible to have more radiation with Λ < 270 nm fall on a surface in one extreme day than in several years of typical days. Often, persons involved in studies of polymer degradation by sunlight mention that a negligible fraction (1 ppm) of the radiant flux reaching the earth's surface is associated with wavelengths below 290 nm and infer that studies at shorter wavelengths will not be of much practical value. Such inferences are questionable for at least two reasons. (1) The quantum flux density below 290 nm is about 1016 photons cm-2 month-1 , so that considerable long-term damage is possible since most of the flux will be absorbed in a layer only a few microns thick. (2) Even if solar radiation below 290 nm were completely absent, the existence of correlations between absorption peaks in the near ultraviolet and visible, and in the infrared with ionization potentials typically 6–12 eV or 200-100 nm) is evidence that we may expect studies in the ultraviolet and extreme ultraviolet (EUV) to provide important clues to the problem of improving the resistance of polymers to sunlight. 相似文献
72.
EFFECT OF ULTRAVIOLET RADIATION ON THE ENERGY METABOLISM OF THE CORNEAL EPITHELIUM OF THE RABBIT 总被引:2,自引:0,他引:2
The present research was directed at quantifying possible alterations in corneal epithelial metabolic activity secondary to in vivo exposure to ultraviolet radiation (UVR). Microfluorometric energy metabolite assays on microgram (microgram) sized, freeze-dried tissue samples were used as an in vitro means of assessing overall metabolic activity in the epithelium of control rabbit corneas and in the epithelium of UVR-exposed rabbit corneas 2 min after discontinuation of exposure. The specific assays were for glucose, glycogen, adenosine triphosphate (ATP), and phosphocreatine (PCr). The radiant exposures were kept constant at 0.05 J cm-2 for all UVR wavelengths utilized (290, 300, 310 and 360 nm). Experimental UVR exposure conditions served to increase epithelial glucose and glycogen concentrations. Although the epithelial ATP concentrations were unchanged, the epithelial PCr concentrations (a high energy phosphate bond reservoir) decreased as a result of UVR exposure. Overall, the data demonstrate a decrease in corneal epithelial metabolic activity, which may be wavelength-dependent, as a result of UVR exposure. It is suggested that immediate metabolic stress can be responsible for the pattern of epithelial cell loss seen in photokeratitis. 相似文献
73.
Abstract— Total diffuse reflectance spectra of air-dried surfaces of free and neutralized amino acid preparations before and after irradiation in vitro are reported. It was found that some free or neutralized amino acid surfaces underwent modification which resulted in changes in the diffuse reflectance spectra after U.V. exposure. It is suggested that these reflectance changes result from transformations in the side chains of the amino acids and that the transformations may differ from those occurring when amino acids in solution are irradiated. Histidine, cystine, hydroxyproline and some protein surfaces showed changes in reflectance of 330–400 nm light similar to those reported in skin after U.V. irradiation in viuo. 相似文献
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Three-phase displacement experiments for a water-benzyl alcohol-decane system are simulated. Literature experimental three-phase relative permeabilities for the system are used to describe the relative permeabilities in the three-phase region for different three-phase relative permeability models. Saturation trajectories and elliptical regions are mapped in the three-phase region. Simulations are performed to model displacement experiments including breakthrough and the formation of multiple shocks. The model can be used to predict the results for other displacements. In an experiment where significant gravity segregation is present, the displacement is more accurately modeled by assuming a uniform initial condition than by using the actual vertical saturation and assuming no cross flow. It is shown how different residual saturation values can be measured in the laboratory depending on the initial saturation conditions in the core. The experimental residual saturations can be significantly different than the ‘theoretical’ or model values. 相似文献
78.
Swafford LA Weigand LA Bowers MJ McBride JR Rapaport JL Watt TL Dixit SK Feldman LC Rosenthal SJ 《Journal of the American Chemical Society》2006,128(37):12299-12306
Alloy nanocrystals provide an additional degree of freedom in selecting desirable properties for nanoscale engineering because their physical and optical properties depend on both size and composition. We report the pyrolytic synthesis of homogeneously alloyed CdS(x)Se(1-x) nanocrystals in all proportions. The nanocrystals are characterized using UV-visible absorption spectroscopy, transmission electron microscopy, X-ray diffractrometry, and Rutherford backscattering spectrometry to determine precisely structure, size, and composition. The dependence of band gap on nanocrystal size and composition is elucidated, yielding a bowing constant of 0.29, in agreement with bulk values. In addition, the morphology of the resultant nanocrystals can be altered by changing the reaction conditions, generating structures ranging from homogeneous, spherical nanocrystals to one-dimensional gradient nanorods. 相似文献
79.
Cláudio P. Ribeiro JR. Benny D. Freeman 《Journal of Polymer Science.Polymer Physics》2010,48(4):456-468
Experimental solubility and sorptive dilation data are reported for carbon dioxide and ethane in a crosslinked poly(ethylene oxide) (XLPEO) rubbery copolymer. Five different temperatures (253 ≤ T(K) ≤ 308) were considered, with a maximum gas pressure of 2.09 MPa (20.6 atm). The polymer was prepared by photopolymerization of a solution containing 70 wt % poly(ethylene glycol) methyl ether acrylate (PEGMEA) and 30 wt % poly(ethylene glycol) diacrylate (PEGDA). Sorption isotherms were described by the Flory‐Huggins model. For each gas, the Flory‐Huggins interaction parameter was a decreasing function of temperature and did not show a composition dependence. Dilation and sorption data were combined to calculate the partial molar volume (PMV) of the gases in the polymer, which was an increasing function of temperature. Based on a comparison with literature data for a XLPEO homopolymer prepared from pure PEGDA over the same range of operating conditions, an effect of the network composition on both gas solubility and PMV was found. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 456–468, 2010 相似文献
80.
Alves GA Amato S Anjos JC Appel JA Astorga J Bernard T Bracker SB Cremaldi LM Darling CL Dixon RL Errede D Gay C Green DR Jedicke R Karchin PE Kwan S Lueking LJ de Mello Neto JR Metheny J Milburn RH de Miranda JM da Motta Filho H Napier A Passmore D Rafatian A dos Reis AC Ross WR Santoro AF Sheaff M Souza MH Spalding WJ Stoughton C Streetman ME Summers DJ Takach SF Wallace A Wu Z 《Physical review letters》1994,72(6):812-815