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排序方式: 共有262条查询结果,搜索用时 15 毫秒
91.
Adrin Leons Valentina Salaris Alicia Mujica-Garcia Marina P. Arrieta Daniel Lopez Marcela Lieblich Jos Maria Kenny Laura Peponi 《Molecules (Basel, Switzerland)》2021,26(16)
In this work, different poly (lactic acid) (PLA)-based nanocomposite electrospun fibers, reinforced with both organic and inorganic nanoparticles, were obtained. As organic fibers, cellulose nanocrystals, CNC, both neat and functionalized by “grafting from” reaction, chitosan and graphene were used; meanwhile, hydroxyapatite and silver nanoparticles were used as inorganic fibers. All of the nanoparticles were added at 1 wt% with respect to the PLA matrix in order to be able to compare their effect. The main aim of this work was to study the morphological, thermal and mechanical properties of the different systems, looking for differences between the effects of the addition of organic or inorganic nanoparticles. No differences were found in either the glass transition temperature or the melting temperature between the different electrospun systems. However, systems reinforced with both neat and functionalized CNC exhibited an enhanced degree of crystallinity of the electrospun fibers, by up to 12.3%. From a mechanical point of view, both organic and inorganic nanoparticles exhibited a decreased elastic modulus and tensile strength in comparison to neat electrospun PLA fibers, improving their elongation at break. Furthermore, all of the organic and inorganic reinforced systems disintegrated under composting conditions after 35 days. 相似文献
92.
N. M. Qureshi U. Arrieta C. Baudet A. Cartellier Y. Gagne M. Bourgoin 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(4):531-536
Turbulent transport of material inclusions plays an important role in many natural and industrial situations. Being able to
accurately model and predict the dynamics of dispersed particles transported by a turbulent carrier flow, remains a challenge.
One critical and difficult point is to develop models which correctly describe the dynamics of particles over a wide range
of sizes and densities. Our measurements show that acceleration statistics of particles dispersed in a turbulent flow do exhibit
specific, and so far unpredicted, size and density effects and that they preserve an extremely robust turbulent signature
with lognormal fluctuations, regardless of particles size and density. This has important consequences in terms of modeling
for the turbulent transport of dispersed inclusions. 相似文献
93.
L. Escobar-Alarcón A. Arrieta E. Camps S. Romero M. Fernandez E. Haro-Poniatowski 《Applied Physics A: Materials Science & Processing》2008,93(3):605-609
The plasma produced by the ablation of a high purity Al2O3 target, using the fundamental line (1064 nm) of a Nd:YAG laser, was characterized. The laser fluence was varied in order
to study its effect on the characteristics of the produced plasma as well as on the properties of the material deposited.
Optical emission spectroscopy (OES) was used to determine the type of excited species present in the plasma. The mean kinetic
energy of the ions and the maximum plasma density were determined from the time of flight (TOF) curves, obtained with a planar
Langmuir probe. The obtained results reveal that the fast peak in the probe curve could be attributed to Al III, while the
slow peak corresponds to the Al II. Aluminum oxide thin films were then deposited under the same conditions of the diagnosed
plasma, in an attempt to correlate the plasma parameters with the properties of the deposited material. It was found that
when Al II ion energies are lower than 461.0 eV the films deposited have structural characteristics similar to that of α-Al2O3, whereas at ion energies greater than 461.0 eV amorphous material was obtained. 相似文献
94.
95.
Cossio FP Arrieta A Lecea B Alajarin M Vidal A Tovar F 《The Journal of organic chemistry》2000,65(12):3633-3643
Highly stereocontrolled, intramolecular [2 + 2] cycloadditions between ketenimines and imines leading to 1,2-dihydroazeto[2, 1-b]quinazolines have been achieved. The source of stereocontrol is a chiral carbon atom adjacent either to the iminic carbon or nitrogen atom. In the first case, the stereocontrol stems from the preference for the axial conformer in the first transition structure. In the second case, the origin of the stereocontrol lies on the two-electron stabilizing interaction between the C-C bond being formed and the sigma orbital corresponding to the polar C-X bond, X being an electronegative atom. These models can be extended to other related systems for predicting the stereochemical outcome in this intramolecular reaction. 相似文献
96.
The magnetic properties of mixed-valent compounds of general formula Ru2Cl(mu-O2CR)4 [R = CH2-CH3 (1), C(Me)=CHEt) (2)] have been studied in the 2-300 K temperature range. This magnetic study also includes a revision of the magnetic properties of the complex Ru2Cl(mu-O2CCMePh2)4 (3). Compounds 1-3 show a linear structure and a strong antiferromagnetic coupling between the diruthenium units through the chlorine atoms according to previous studies. Two fitting models to explain the magnetic properties of these complexes that incorporate a large zero-field splitting together with a strong antiferromagnetic coupling are described. These models consider that each diruthenium unit (S = 3/2) is magnetically coupled to the nearest diruthenium unit and ignores the longer distance magnetic coupling. The fitting models were found to be successful in fitting the magnetic data of the linear diruthenium(II,III) complexes. The zero-field splitting, D, and the antiferromagnetic coupling, zJ, vary from 37.8 to 48.0 cm-1 and from -7.43 to -13.30 cm-1, respectively, for complexes. The D values are similar to those calculated for the nonlinear diruthenium(II,III) compounds and confirm the validity of the proposed fitting models. 相似文献
97.
Lecea B Arrastia I Arrieta A Roa G Lopez X Arriortua MI Ugalde JM Cossío FP 《The Journal of organic chemistry》1996,61(9):3070-3079
The outcome of the cycloaddition between activated ketenes and alpha,beta-unsaturated imines has been investigated both experimentally and theoretically. Our results indicate that activated monosubstituted ketenes yield exclusively [2 + 2] cycloadducts. Disubstituted activated ketenes yield [2 + 2] and/or [4 + 2] cycloadducts. In one case, an unexpected piperidin-2-one has been obtained, although its relative abundance with respect to the corresponding [2 + 2] or [4 + 2] cycloadducts can be minimized by the proper choice of experimental conditions. The ability of different ab initio and semiempirical methods to account for these results has been tested. The best agreement between theory and experiment is achieved at the MP2/6-31G level of theory, with solvent effects taken into account. The semiempirical hamiltonian AM1, at the RHF level, tends to overestimate the stability of the transition structures leading to six-membered cycloadducts, whereas 3 x 3CI-HE/AM1 and CASSCF(2,2)/6-31G methods tend to overestimate the stability and the biradical character of the transition structures leading to [2 + 2] cycloadducts. 相似文献
98.
99.
Immobilized penicillin acylase is a moderately priced versatile enzyme, that is able to catalyze the synthesis of derived
penicillins and cephalosporins from the corresponding β-lactam nuclei and proper side-chain precursors. Kinetically controlled
synthesis is a better strategy when product yield is a key issue. Yield should increase at reduced water activity by depressing
the competing hydrolytic reactions in favor of synthesis; therefore, organic cosolvents can be a suitable reaction media for
synthesis. Using response surface methodology and product yield as objective function, temperature and pH were optimized in
the kinetically controlled synthesis of ampicillin using previously screened cosolvents and reaction conditions. Optimum pH
was 6.0 for ethylene glycol (EG) and glycerol (GL) and 6.6 for 1–2 propanediol (PD); optimum temperature was 30°C for GL and
for EG and PD was in the lower extreme of the range studied, optimum lying below 26°C. Maximum molar yields predicted by the
model were 58,51, and 46% for EG, GL, and PD, respectively, which were experimentally validated. Highest yield in aqueous
buffer was always <40%. Molar yields about 60% compare favorably with values reported for the kinetically and thermodynamically
controlled synthesis of ampicillin and other derived penicillins. 相似文献
100.