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1.
Sanchez-sanz M; Blyth MG 《The Quarterly Journal of Mechanics and Applied Mathematics》2007,60(2):125-138
Unsteady, axisymmetric stagnation flow about a circular cylinderis examined when the far-field flow is a periodic function oftime with a fixed time average and an oscillatory part of prescribedamplitude and frequency. Solutions are computed for arbitraryvalues of the Reynolds number, quantifying the effects of surfacecurvature, and a frequency parameter based on the period ofthe far-field flow. It is found that solutions remain regularand periodic provided that the far-field amplitude lies belowa critical value. Above this value, solutions terminate in afinite-time singularity. The blow-up time is delayed by increasingthe curvature of the surface. These results are corroboratedby asymptotic predictions valid in the limits of small and largeamplitude and frequency. For large Reynolds number, the problemreduces to the two-dimensional stagnation-point flow againsta plane wall studied by previous authors. 相似文献
2.
J. Escribano R. Masegosa D. Nava M. G. Prolongo Catalina Salom 《Journal of Thermal Analysis and Calorimetry》2006,83(3):675-679
The
mineral sabugalite (HAl)0.5[(UO2)2(PO4)]2⋅8H2O, has been studied using a combination of energy
dispersive X-ray analysis, X-ray diffraction, dynamic and controlled rate
thermal analysis techniques. X-ray diffraction shows that the starting material
in the thermal decomposition is sabugalite and the product of the thermal
treatment is a mixture of aluminium and uranyl phosphates. Four mass loss
steps are observed for the dehydration of sabugalite at 48°C (temperature
range 39 to 59°C), 84°C (temperature range 59 to 109°C), 127°C
(temperature range 109 to 165°C) and around 270°C (temperature range
175 to 525°C) with mass losses of 2.8, 6.5, 2.3 and 4.4%, respectively,
making a total mass loss of water of 16.0%. In the CRTA experiment mass loss
stages were found at 60, 97, 140 and 270°C which correspond to four dehydration
steps involving the loss of 2, 6, 6 and 2 moles of water. These mass losses
result in the formation of four phases namely meta(I)sabugalite, meta(II)sabugalite,
meta(III)sabugalite and finally uranyl phosphate and alumina phosphates. The
use of a combination of dynamic and controlled rate thermal analysis techniques
enabled a definitive study of the thermal decomposition of sabugalite. While
the temperature ranges and the mass losses vary due to the different experimental
conditions, the results of the CRTA analysis should be considered as standard
data due to the quasi-equilibrium nature of the thermal decomposition process.
The online version of the original article can be found at 相似文献
3.
Marta Sánchez-Cabezudo Margarita G. Prolongo Catalina Salom Rosa M. Masegosa 《Journal of Thermal Analysis and Calorimetry》2006,86(3):699-705
The cure kinetics
and morphology of diglycidyl ether of bisphenol A (DGEBA) modified with polyvinyl
acetate (PVAc) using diaminodiphenylmethane (DDM) as hardener were investigated
through differential scanning calorimetry (DSC) and environmental scanning
electron microscopy (ESEM). Isothermal curing measurements were carried out
at 150, 120 and 80°C. The kinetic parameters were obtained using the general
autocatalytic chemically controlled model. The comparison of the kinetic data
indicates that the presence of PVAc does not change the autocatalytic nature
of the cure reaction. Two T
g’s
were observed in the fully cured samples of the modified systems. ESEM micrographies
confirm the biphasic morphology. 相似文献
4.
R. M. Masegosa M. A. Llorente M. G. Prolongo I. Hernández-Fuentes A. Horta 《Colloid and polymer science》1984,262(11):862-866
Measurements of preferential sorption,(
3), determined by refractometry, and swelling equilibrium,
3
–1, of PMMA networks have been carried out in the cosolvent mixture MeCN+BuOH at 25 and 49 C. With an intermediate mixture composition,
3
–1 passes through a maximum at both temperatures. At 25 C MeCN is preferentially adsorbed by the network over most of the composition range, but a small inversion is detected. At 49 C MeCN is preferentially adsorbed over all the composition range.The behavior of the system crosslinked PMMA/MeCN + BuOH is compared with the results obtained for solutions of linear PMMA in mixtures formed by the same two solvents, MeCN and BuOH. 相似文献
5.
S -nitrosothiols have many biological activities and may act as nitric oxide (NO) carriers and donors, prolonging NO half-life in vivo. In spite of their great potential as therapeutic agents, most S -nitrosothiols are too unstable to isolate. We have shown that the S -nitroso adduct of N -acetylcysteine (SNAC) can be synthesized directly in aqueous and polyethylene glycol (PEG) 400 matrix by using a reactive gaseous (NO/O2 ) mixture. Spectral monitoring of the S–N bond cleavage showed that SNAC, synthesized by this method, is relatively stable in nonbuf-fered aqueous solution at 25°C in the dark and that its stability is greatly increased in PEG matrix, resulting in a 28-fold decrease in its initial rate of thermal decomposition. Irradiation with UV light (λ= 333 nm) accelerated the rate of decomposition of SNAC to NO in both matrices, indicating that SNAC may find use for the photogeneration of NO. The quantum yield for SNAC decomposition decreased from 0.65 ± 0.15 in aqueous solution to 0.047 ± 0.005 in PEG 400 matrix. This increased stability in PEG matrix was assigned to a cage effect promoted by the PEG microenvironment that increases the rate of geminated radical pair recombination in the homolytic S–N bond cleavage process. This effect allowed for the storage of SNAC in PEG at −20°C in the dark for more than 10 weeks with negligible decomposition. Such stabilization may represent a viable option for the synthesis, storage and handling of S -nitrosothiol solutions for biomedical applications. 相似文献
6.
7.
M. A. García del Cid M. G. Prolongo C. Salom C. Arribas M. Sánchez-Cabezudo R. M. Masegosa 《Journal of Thermal Analysis and Calorimetry》2012,108(2):741-749
Epoxy–clay nanocomposites have been prepared with an organically modified montmorillonite. The epoxy network was based on
diglycidyl ether of bisphenol A (DGEBA) cured with diaminodiphenylmethane (DDM). The stoichiometry DGEBA–DDM was varied, the
molar ratio of amine hydrogen/epoxy groups, r, ranged from 0.85 to 1.15. The influence of stoichiometry on curing and properties of the nanocomposites was studied using
differential scanning calorimetry, dynamic mechanical thermal analysis and X-ray diffraction. All nanocomposites had intercalated
clay structures. The clays accelerated the curing reaction whose rate was also increased when increasing r. The heat of reaction, −ΔH (J/g epoxy), increased as r increased, reaching a constant value for r ≥ 1. In the presence of clays −ΔH was lower than in the neat DGEBA–DDM. The glass transition temperature (T
g) of the neat epoxy thermosets reached a maximum at r = 1; however, the nanocomposites showed the T
g maximum at 0.9 < r < 1. The presence of clay lowered the T
g for r > 0.94 and raised T
g for r ≤ 0.85. The elastic modulus of neat epoxy thermosets reached a maximum in the rubber state and a minimum in the glassy state
at r = 1. The nanocomposites showed similar behavior, but the maximum and the minimum values of the elastic modulus were reached
at stoichiometry r < 1. The comparison of the properties of neat epoxy with those of the nanocomposites varying the stoichiometry indicates
that the clay itself induces stoichiometric changes in the system. 相似文献
8.
Silvia G. Prolongo M. Burón A. Salazar A. Ureña J. Rodríguez 《Journal of Thermal Analysis and Calorimetry》2007,87(1):269-276
Blends based on epoxy resins and a random copolymer, poly(styrene-co-allylalcohol)
(PS-co-PA) were studied, analysing the effect of epoxy nature. The epoxy cross-linking
reaction was carried out by homopolymerisation, using an imidazole as initiator,
and by addition of several amine hardeners. The imidazole acts as initiator
of anionic epoxy etherification and as catalyser of epoxy-hydroxyl reaction.
Important differences were observed on the network structure and phase behaviour
of blends depending on the nature of epoxy matrix. These cause that the blends
present different morphologies and different dynamic mechanical properties. 相似文献
9.
C. Arribas Rosa M. Masegosa C. Salom E. Arévalo S. G. Prolongo Margarita G. Prolongo 《Journal of Thermal Analysis and Calorimetry》2006,86(3):693-698
Diglycidyl ether of bisfenol-A (DGEBA)/polybenzyl methacrylate (PBzMA)
blends cured with 4,4’-diaminodiphenylmethane (DDM) were studied. Miscibility,
phase separation, cure kinetics and morphology were investigated through differential
scanning calorimetry (DSC) and scanning electron microscopy (SEM). Non-reactive
DGEBA/PBzMA blends are miscible over the whole composition range. The addition
of PBzMA to the reactive (DGEBA+DDM) mixture slows down the curing rate, although
the reaction mechanism remains autocatalytic. On curing, initially miscible
(DGEBA+DDM)/PBzMA blends phase separate, arising two glass transition temperatures
that correspond to a PBzMA-rich phase and to epoxy network. Cured epoxy/PBzMA
blends present different morphologies as a function of the PBzMA content. 相似文献
10.
I. Hernández-Fuentes M.G. Prolongo R.M. Masegosa A. Horta 《European Polymer Journal》1982,18(1):29-35
The concentration (c′) marking the first deviation from linearity in the Huggins plot of specific viscosity ηsp/c vs c) has been determined for PMMA in chloroform, benzene (good solvents), acetonitrile, chlorobutane (poor solvents) and acetonitrile/chlorobutane mixtures (cosolvent). The dependence of c′ on polymer chain length and on solvent quality is given. The results are analysed in terms of the influence on c′ of incipient coil overlap, peripheral entanglements and other interactions, such as polymer association. 相似文献