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排序方式: 共有303条查询结果,搜索用时 46 毫秒
111.
The objective of the research is to investigate epoxy resins where crosslinks formed in curing reactions can be cleaved and reformed without significant sacrifice in resin properties. Experimental disulfide-containing crosslinking agents have been selected for the study, including dihydrazides of dithiodicarboxylic acids (aliphatic and aromatic) and dithioaromatic diamines. Conditions for curing reactions and for subsequent cleavage of crosslinks by reduction have been determined in model systems and confirmed for a prototype epoxy resin. A comparative evaluation of cure kinetics, crosslink density, and thermal and mechanical properties has been carried out for resin cured with aromatic curing agents (dithiodianiline and methylenedianiline) and with aliphatic compounds (3, 3′-dithiopropionic acid dihydrazide and hexamethylenediamine). It has been shown that in resin fully cured with disulfide-containing crosslinking agents, crosslinks are ruptured by reduction, and re-established by oxidation under mild conditions, or by reaction of thiol groups with bifunctional alkylating agents. 相似文献
112.
Giuliana Gorrasi Liberata Guadagno Concetta D'Aniello Carlo Naddeo Gennaro Romano Vittoria Vittoria 《Macromolecular Symposia》2003,203(1):285-294
Three syndiotactic polypropylene samples were crystallized under different conditions in order to obtain different polymorphs. A first sample was crystallized at high temperature, obtaining the helical form I; a second was crystallized from the melt at 0°C for many days obtaining the trans-planar mesophase; a third sample was obtained by solvent induced crystallization followed by annaeling of the trans-planar mesophase, leading to a mixture of both the helical forms I and II. In the dynamic-mechanical analysis the helical form I showed only one peak of tan δ corresponding to the amorphous glass transition. The other polymorphs also showed this transition centered at about the same temperature. Beside the peak corresponding to the Tg, the trans-planar mesophase was characterized by a peak appearing at 70°C, and the helical form II by a peak at 100°C. These peaks, unambiguously associated to transitions of the different forms, can be considered a distinctive evidence for the polymorphs obtained in different processing conditions. 相似文献
113.
Fulvia Felluga Maurizio Fermeglia Marco Ferrone Giuliana Pitacco Sabrina Pricl Ennio Valentin 《Helvetica chimica acta》2002,85(11):4046-4054
Enantiomerically pure methyl esters of (+)‐(2R,3S)‐ and (−)‐(2S,3R)‐5‐oxo‐2‐pentylpyrrolidine‐3‐carboxylic acid with 99% and 98% ee were obtained by enzymatic resolution of the corresponding racemic mixture using α‐chymotrypsin and pig‐liver acetone powder, respectively. Their absolute configurations were established by chemical methods, i.e., conversion of the trans‐γ‐lactam moiety to the corresponding γ‐lactone of known configuration. The favorable interactions between the trans‐γ‐lactam and α‐chymotrypsin were rationalized by molecular‐mechanics calculations, which suggest a different situation for the cis‐diastereoisomer. 相似文献
114.
Alain Favre Giuliana Moreno Christian Salet Francloise Vinzens 《Photochemistry and photobiology》1993,58(5):689-694
Monkey kidney cells (CV-1) grown for 4 h in the presence of 0.1 m M 4-thiouridine (s4 Urd) incorporate this photoactivable uridine analog in their RNA. A minor, 5–8%, thiolated RNA fraction can be isolated from bulk RNA by affinity chromatography. This RNA fraction contains 1.5–2.5 s4 Urd residues per 100 nucleotides and exhibits a broad chain length distribution ranging from 700 to 7000 nucleotides. It is essentially of nuclear origin and amounts to 30% of the RNA synthesized during exposure of cells to s4 Urd. Under the same s4 Urd labeling conditions, no thiolated pyrimidine residues have been detected in DNA.
Irradiation with 365 nm light (45 kJ/m2 ) of the cells immediately after s4 Urd exposure triggers long-term inhibition of DNA, RNA and protein synthesis accompanied by a linear decline (50% in 2 days) in the total cell mass of cultured cells. In contrast, exposure to s4 Urd alone results in moderate but reversible inhibitory effects. The available data suggest that s4 Urd-induced photolesions in newly synthesized RNA such as RNA-RNA cross-links as well as RNA-protein bridges are directly involved in impairment of essential cellular functions. 相似文献
Irradiation with 365 nm light (45 kJ/m
115.
Giuliana Borsa Stefania Castelletto Aldo Godone Carlo Novero Maria Luisa Rastello 《Optical Review》1997,4(4):484-489
This work presents a new technique for the measurement of second-order optical nonlinear coefficients from the absolute radiant power of the parametric fluorescence emission. It is based on calibrated detectors regarded as secondary radiometric standards. This technique makes it possible to skip a number of problems usually encountered with standard techniques based on calibrated neutral density filters and reference sources of light at much higher power levels. We infer a value for the d31 nonlinear coefficient of LiIO3 from spectral measurements of the fluorescence power integrated over all solid angles, by denning the wavelength bandwidths with interference filters. Moreover, by performing the measurements on the ultraviolet and visible emission lines of an argon ion laser in the range from 351 to 488 nm, the wavelength dependence of d31 is compared with the expected one obtained by Miller’s dispersion law. 相似文献
116.
117.
118.
Piero Zanello Giuliana OpromollaLuca PardiKeith H. Pannell Hemant K. Sharma 《Journal of organometallic chemistry》1993,450(1-2):193-196
The electrochemical behaviour of the ferrocenylacyl derivatives [FcCOER3] (E = C, Si or Ge; R = Me or Ph) is examined. One-electron oxidations to the substantially stable monocations [FcCOER3]+ occur at potentials significantly higher than that observed with ferrocene, but only minor differences hold within the series, independent of the nature of both E and R. In contrast the EPR spectra of the monocations for E = C show that the unpaired electron resides mainly on the iron, whereas for E = Si or Ge the electron density is essentially localized on the C5H4COER3 fragment. 相似文献
119.
Giuliana Palmieri 《Milan Journal of Mathematics》1995,65(1):335-356
Given a setS and a function σ:S x S→[0, +∞[ such that σ(x, x)=0, we define the generalizedp-energy of a curve γ: [a, b]→S, so that, ifS is a Hilbert space and σ(x, y)=‖x−y‖ we obtain
. Sufficient conditions for the equality of the defined energies are also given. Moreover the case in whichS is a set of measurable parts of ℝn and σr is a family of functions used in order to study the generalized minimizing motions, is discussed.
Conferenza tenuta il 30 ottobre 1995 相似文献
Conferenza tenuta il 30 ottobre 1995 相似文献
120.
Abstarct The Cp)2Ni2Fe(CO)3(µ3-C2H2) and Cp)2Ni2Fe(CO)3(µ3-C2H2) (B) complexes have been synthesized and spectroscopically characterized. An accurate X-ray study and a comparison with related structures shows that the substituents of the alkyne ligands exert considerable effects on the bonding parameters.Crystal data for complex A, monoclinic space group P21/n,a = 8.418(1),b = 15.779(2),c = 14,493(1) Å, = 91.64(1)°,Z = 4, 2753 observed reflections,R = 0.022; crystal data for complex B, monoclinic space group C2/c,a = 16.2189(7),b = 7.445(3),c = 25.745(5) Å, = 103.74(3),Z=8, 1853 observed reflections,R = 0.051. 相似文献