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81.
Indra K. Varma George M. Fohlen John A. Parker 《Journal of polymer science. Part A, Polymer chemistry》1982,20(2):283-297
Ten structurally different bisimide resins were prepared by reacting maleic anhydride/citraconic anhydride and benzophenone tetracarboxylic dianhydride with aromatic diamines and fused aromatic structures or heterocyclic groups. The amines included were 1,5-diaminonaphthalene, 2,5-bis(p-aminophenyl)1,3,4-oxadiazole, 3,3-bis(p-aminophenyl)phthalide, 9,9-bis(p-aminophenyl)fluorene. and 10,10-bis(p-aminophenyl)anthrone. These monomers were characterized by infrared (IR). 1H-NMR, mass spectroscopy, and elemental analysis. Thermal polymerization of these monomers was investigated by differential scanning calorimetry. Broad exothermic peaks were observed for a temperature range of 225–380°C. Temperature of exothermic peak position was influenced by the presence of substituents at the olefinic bond, and in biscitraconimides it was 40–50°C lower than in the corresponding bismaleimides. Anaerobic char yields of cured bisimide resins ranged from 44 to 64%. Oxadiazole-containing bisimides had low thermal stability. Increase in formula weight between the imide groups did not influence the char yields in a systematic manner. Graphite cloth laminates with two of these bisimide resins were fabricated and tested for a number of physical properties. Their limiting oxygen index was 70–72%. 相似文献
82.
The effect of certain aromatic compounds on the PMR spectrum of methyl methacrylate (MMA) was investigated. The magnitude of observed aromatic-induced shifts decreased in the order benzene ? styrene > chlorobenzene ≈ bromobenzene.Assuming that the interaction arises from a stoichiometric 1:1 complex, equilibrium parameters for the MMA-benzene interaction have been estimated. ΔH ± S.E. (ΔH) = ?(8 ± 4) kJ mol?1. These effects are likely to have a small influence on the kinetics of copolymerization with aromatic monomers and polymerization in aromatic solvent. The stereochemistry of the solute-solvent interactions suggests that MMA takes a cis-conformation in solution, which is relevant to the mechanism of stereoregular polymerizations of this monomer. 相似文献
83.
Pawlukojć A Leciejewicz J Tomkinso J Parker SF 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2002,58(13):2897-2904
Inelastic incoherent neutron scattering (IINS) spectra were obtained at 10 K for normal and deuterated L-serine. The geometry of L-serine molecule was optimized for the zwitterion form using ab initio HF, MP2 and DFT (B3LYP) levels with 6-31G* and 6-311 + +G4** basis sets. The theoretical frequencies of normal and d4-L-serine were compared with IINS spectra. Normal coordinate analysis and band assignments based on ab initio calculations and experimental data were presented. IINS frequencies due to the out-of-plane gamma(N-H...O) hydrogen bond motions were observed and identified. 相似文献
84.
D. Fennell Evans Martin Allen B. W. Ninham Abdel Fouda 《Journal of solution chemistry》1984,13(2):87-101
Critical micelle concentrations were determined by conductance measurements for decyl-, dodecyl-, tetradecyl- and hexadecyltrimethylammonium bromide in water at 25, 60, 95, 130, and 160°C. The results are discussed in terms of the equilibrium model and the nonlinear Poisson-Boltzmann model for micelle formation. The free energies of transferring a methylene group from water to the oil-like interior of the micelle are found to be –781 at 25°C, –796 at 60°C, –819 at 95°C, –815 at 130°C, and –787 at 160°C cal-mol–1. 相似文献
85.
Food-induced demineralization (erosion) is one of the key factors in surface structural changes of tooth enamel, with soft drinks being a significant etiological agent. The objective of this study was to measure early stages of enamel loss with high accuracy on native enamel surfaces combined with qualitative observations of changes in the surface morphology using the atomic force microscope (AFM). Native unerupted third molar surfaces were partly covered with a gold reference layer. Samples were imaged with the AFM before dissolution (at baseline) and after exposure to three different drinks (mineral water, a "toothkind" blackcurrant drink, and a lemon and lime juice drink) at five different exposure times (15 min, 30 min, 1 h, 2 h, and 3 h). The changes in the surface morphology were investigated qualitatively as well as quantitatively. This study showed that the maximum material loss occurred at the aprismatic parts of the enamel close to the perikymata. The maximum enamel loss was greatest for the lemon and lime juice drink and lowest for water. A two-way ANOVA of the transformed data, employing the natural logarithm, showed a statistically significant difference between both the drinks and the exposure time at a 95% confidence level (P=0.000). This demonstrates that the AFM is a suitable tool for measuring early stages of enamel demineralization. Copyright 2000 Academic Press. 相似文献
86.
T. Melvin M. A. Plumb S. W. Botchway P. O'Neill A. W. Parker 《Photochemistry and photobiology》1995,61(6):584-591
Irradiation of DNA with 193 nm light results in monophotonic photoionization, with the formation of a base radical cation and a hydrated electron (φP1 = 0.048–0.065). Although >50% of the photoionization events initially occur at guanine in DNA, migration of the “hole” from the other bases to guanine occurs to yield predominantly its radical cation or its deprotonated form. From sequence analysis, the data reveal that 193 nm light induces single strand breaks (ssb) in double-stranded DNA preferential 3’ to a guanine residue. However, it has previously been reported that 193 nm light yields very low yields of ssb (<2% of the yield of eaq). The distribution of these ssb at guanine is nonrandom, showing a dependence on the neighboring base moiety. The efficiency of ssb formation at nonguanine sites is estimated to be at least one order of magnitude lower. The preferred cleavage at guanine is consistent with migration and localization of the electron loss center at guanine. It is argued that singlet oxygen and the photoionized phosphate group of the sugar moiety are not major precursors to ssb. At present, the mechanisms of strand breakage are not known although a guanine radical or one of its products remain potential precursors. 相似文献
87.
Abe K Abt I Ahn CJ Akagi T Allen NJ Ash WW Aston D Baird KG Baltay C Band HR Barakat MB Baranko G Bardon O Barklow T Bazarko AO Ben-David R Benvenuti AC Bienz T Bilei GM Bisello D Blaylock G Bogart JR Bolton T Bower GR Brau JE Breidenbach M Bugg WM Burke D Burnett TH Burrows PN Busza W Calcaterra A Caldwell DO Calloway D Camanzi B Carpinelli M Cassell R Castaldi R Castro A Cavalli-Sforza M Church E Cohn HO Coller JA Cook V Cotton R Cowan RF Coyne DG D'Oliveira A Damerell CJ Daoudi M 《Physical review D: Particles and fields》1995,52(7):4240-4244
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