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31.
Christopher K. Ober Michael L. Hair 《Journal of polymer science. Part A, Polymer chemistry》1987,25(5):1395-1407
Considerable interest has been generated recently by the preparation of large monodisperse polymer latices by a multistage swelling process. Dispersion polymerization in organic media has been shown to be an alternative route to the preparation of large monodisperse polymer colloids. The size of the resulting particles is known to be dependent on the polarity of the reaction medium. Several other factors are also extremely important in determining both the size and size distribution of the resulting particles. These include the temperature of the reaction and the level of initiator. The temperature affects both the rate of free radical formation and the thermodynamic properties of the polymerization system. It is shown that monodispersity can be achieved over a considerable range of temperatures provided that a particular level of initiator is used at a given temperature. It is also shown that higher levels of initiator at a particular temperature actually produced larger particles. Interestingly, the level of initiator was not found to significantly change the overall rate of polymerization. 相似文献
32.
Osburn S Berden G Oomens J O'Hair RA Ryzhov V 《Journal of the American Society for Mass Spectrometry》2012,23(6):1019-1023
The radical cations of Cys-Gly and Gly-Cys were studied using ion-molecule reactions (IMR), infrared multiple-photon dissociation
(IRMPD) spectroscopy, and density functional theory (DFT) calculations. Homolytic cleavage of the S–NO bond of nitrosylated
precursors generated radical cations with the radical site initially located on the sulfur atom. Time-resolved ion-molecule
reactions showed that radical site migration via hydrogen atom transfer (HAT) occurred much more quickly in Gly-Cys•+ than in Cys-Gly•+. IRMPD and DFT calculations indicated that for Gly-Cys, the radical migrated from the sulfur atom to the α-carbon of glycine,
which is lower in energy than the sulfur radical (–53.5 kJ/mol). This migration does not occur for Cys-Gly because the glycine
α-carbon is higher in energy than the sulfur radical (10.3 kJ/mol). DFT calculations showed that the highest energy barriers
for rearrangement are 68.2 kJ/mol for Gly-Cys and 133.8 kJ/mol for Cys-Gly, which is in agreement with both the IMR and IRMPD
data and explains the HAT in Gly-Cys. 相似文献
33.
Hossein Tavana Dietmar Appelhans Rong-Chuan Zhuang Stefan Zschoche Karina Grundke Michael L. Hair A. Wilhelm Neumann 《Colloid and polymer science》2006,284(5):497-505
Surface energetics of two fluorinated maleimide copolymers containing fluorinated side chain, i.e., poly(ethene-alt-N-(4-(perfluoroheptylcarbonyl)aminobutyl)maleimide) (ETMF) and poly(octadecene-alt-N-(4-(perfluoroheptylcarbonyl)aminobutyl)maleimide) (ODMF), are studied by contact angle measurements with 10 liquids consisting
of fairly bulky molecules. Because of the inertness of octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane
(DMCPS) molecules, their contact angles are used to determine the surface tension of the two polymers. It is found that other
liquids show specific interactions with the ETMF films, and their contact angles deviate from a smooth curve that represents
the surface tension of ETMF, i.e., 11.00 mJ/m2. On ODMF surfaces, only OMCTS and DMCPS yield useful contact angles. Other liquids either dissolve the polymer film or show
a slip-stick pattern. This finding is discussed in terms of interactions between segments of the polymer chains and the test
liquids. OMCTS and DMCPS are suggested as the appropriate probe liquids, meeting specific criteria necessary for the determination
of accurate surface tension of fluoropolymers. 相似文献
34.
35.
External uniaxial stress effects on the radiation damage dynamics in a a-Fe b.c.c. structure are studied by means of the computer simulation method. Focused collision chains, defocused ones, and atom-atom collision cascades were simulated. The external uniaxial stresses affect the intervals between a vacancy and an interstitial atom produced due to a collision chain, i.e. changes a dynamical crowdion path. The external stresses change the number of stable point defects and their arrangement in the cascade development region if an atom-atom collision cascade develops in a stressed crystal. The studies of focused collision chains of energies close to the atom displacement threshold energies show that the latter can either increase or decrease due to the external stresses depending on the direction of the effect. 相似文献
36.
J. W. H. Schreurs A. M. Friedman D. J. Rokop M. W. Hair R. M. Walker 《辐射效应与固体损伤》2013,168(3-4):231-233
We have prepared a series of standard uranium glasses. A primary standard containing 41 ppm of added U had its concentration and isotope ratio verified by mass spectrometry and by fission track techniques. Secondary standards were compared to the primary standard using the fission track technique. These glasses are useful for measuring both Th and U in solids. The U concentration in an unknown is determined by comparing the fission track density induced by thermal neutron irradiation with the corresponding density in the standard. Th is measured by a cyclotron irradiation using the same U standards. The glasses are also useful for neutron dosimetry. Investigators may obtain small samples of most of these standards from R. H. Brill of the Corning Museum of Glass. Although most of the glasses were fabricated from depleted uranium, one standard containing natural uranium is available. A standard Th glass with concentration of 41 ppm was also made. 相似文献
37.
Structure of the Ligated Ag60 Nanoparticle [{Cl@Ag12}@Ag48(dppm)12] (where dppm=bis(diphenylphosphino)methane)
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Athanasios Zavras Antonija Mravak Margarita Bu?an?i? Jonathan M. White Vlasta Bona?i?-Koutecky Richard A. J. O Hair 《化学物理学报(中文版)》2019,32(2):182-186
A novel bisphosphine ligated Ag60 nanocluster, [{Cl@Ag12}@Ag48(dppm)12], has been dis-covered and characterized by X-ray crystallography. It consists of a central chloride located inside an icosahedral silver core layer, which is further encased by a second shell of 48 silver atoms/ions, which are capped with 12 bis(diphenylphosphino)methane (dppm) ligands. Due to lack of sufficient material the cluster could not be further characterized by other methods. DFT calculations were carried out on the cation [{Cl@Ag12}@Ag48(dppm)12]+ to determine if it corresponds to a superatom with a core count of n=58. The DFT optimized structure is in agreement with X-ray ndings, but the low value of the HOMO-LUMO gap does not support superatom stability. 相似文献
38.
With the aid of infrared spectroscopy it is shown that the bismuth ions in BaBiO3 are equivalent and that the charge distribution Ba2Bi3+Bi5+O6 is incorrect. In addition it is demonstrated that the bismuth ions are pentavalent in Ba2 TbBiO6, but tetravalent in Ba3PbBi2O9. 相似文献
39.
40.
Richard A. J. O’Hair Michelle L. Styles Gavin E. Reid 《Journal of the American Society for Mass Spectrometry》1998,9(12):1275-1284
The gas phase fragmentation reactions of protonated cysteine and cysteine-containing peptides have been studied using a combination of collisional activation in a tandem mass spectrometer and ab initio calculations [at the MP2(FC)/6-31G*//HF/6-31G* level of theory]. There are two major competing dissociation pathways for protonated cysteine involving: (i) loss of ammonia, and (ii) loss of the elements of [CH2O2]. MS/MS, MS/MS of selected ions formed by collisional activation in the electrospray ionization source as well as ab initio calculations have been carried out to determine the mechanisms of these reactions. The ab initio results reveal that the most stable [M + H − NH3]+ isomer is an episulfonium ion (A), whereas the most stable [M + H − CH2O2]+ isomer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H]+ ions of all the possible simple dipeptide and tripeptide methyl esters containing one cysteine (where all other residues are glycine) has also been investigated. When cysteine is at the N-terminal position, NH3 loss is observed, although the relative abundance of the resultant [M + H − NH3]+ ion decreases with increasing peptide size. In contrast, when cysteine is at any other position, water loss is observed. The proposed mechanism for loss of H2O is in competition with those channels leading to the formation of structurally relevant sequence ions. 相似文献