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81.
Subramanian Sundarrajan Mahadevan Surianarayanan Kalathur Sabdham Vangepuram Srinivasan 《Journal of polymer science. Part A, Polymer chemistry》2005,43(3):638-649
The synthesis of polysulfide polymers with unsaturated and saturated units in the backbone and their characterization by Fourier transform infrared, NMR, gel permeation chromatography, thermogravimetric analysis, and differential scanning calorimetry are reported. This is the first report on an analysis of the thermal degradation of an unsaturated polysulfide polymer [poly(2‐butene sulfide)] carried out by pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS). A unique phenomenon of exothermic degradation has been detected by differential thermal analysis and has been attributed to the energetics of the unsaturated polysulfide linkage during degradation. The thermal degradation products studied by Py–GC/MS indicate that the formation of sulfur‐containing products is more favored than the formation of non‐sulfur‐containing products. Furthermore, a comparative study of the thermal degradation of unsaturated and saturated polysulfide polymers has been conducted with thermogravimetry and Py–GC/MS analyses. These analyses have shown that the mechanisms of degradation of these polymers are different, and the lower number of pyrolysis products indicates a selective cleavage of the polymer during degradation in the saturated polysulfide polymers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 638–649, 2005 相似文献
82.
83.
Nighswander-Rempel SP Mahadevan IB Rubinsztein-Dunlop H Meredith P 《Photochemistry and photobiology》2007,83(6):1449-1454
Eumelanin plays a variety of important physiological roles in human skin. However, its structure and fundamental properties still remain poorly understood. Although the absorbance of eumelanin is broad and reveals little about its structure, a variety of techniques have revealed the presence of a disordered array of chromophores within the melanin compound. In order to examine the fluorescence decay dynamics of these chromophores, time-resolved spectroscopy was applied to solutions of synthetic eumelanin and a melanin-like polymer of N-methyl,5-hydroxy,6-methoxyindole (N-Me-5H6MI). Solutions were excited with 80 fs laser pulses at 355, 370, 390 and 400 nm, and decay time courses were acquired at 20 nm intervals between 400 and 600 nm for each excitation wavelength. Decay profiles for both eumelanin and the polymer exhibited a characteristic multiexponential behavior with decay times between 0.5 and 15 ns, although steady-state spectra for the polymer exhibited only two peaks. The long-decay component in the polymer showed a significant decrease in both amplitude (30-5%) and decay time (14-6 ns) with increasing emission wavelength. In contrast, the amplitude and decay time in melanin increased slightly (10-15% and 7-10 ns, respectively) from 400 to 520 nm emission, at which point they leveled off. These trends were consistent for all excitation wavelengths. These results suggest that the multiexponential behavior of melanin fluorescence is characteristic of each oligomer within the eumelanin compound, and is consistent with the assertion that the diversity of constituents within eumelanin provides it with a robustness in spectral properties. 相似文献
84.
We study the dynamics of a flexible foil immersed in a fluid and moving close to a rigid wall. Lubrication theory allows us to derive equations of motion for the foil and thus examine the passive settling and the active swimming of a foil. This also allows us to partly answer the long-standing question in cartoon physics--can carpets fly? Our analysis suggests a region in parameter space where one may realize this dream and move the virtual towards reality. 相似文献
85.
86.
I. Navas R. Vinodkumar V. P. Mahadevan?Pillai 《Applied Physics A: Materials Science & Processing》2011,103(2):373-380
We report on the synthesis of self-assembled hillock shaped MoO3 nanoparticles on thin films exhibiting intense photoluminescence (PL) by RF magnetron sputtering and subsequent oxidation.
MoO3 nanocrystals of size ∼29 nm are self-assembled into uniform nanoparticles with diameter ∼174 nm. The mechanism of the intense
PL behaviour from MoO3 nanoparticles is investigated and systematically discussed. The films exhibit two bands; a near-band-edge UV emission and
a defect related deep level visible emission. The enhancement in PL intensity with annealing is not only by the improvement
in crystallinity and grain size but also by the increase in the rms surface roughness and porosity of the films. The PL intensity
is thermally activated with activation energy 1.07 and 0.87 eV respectively for the UV and visible emissions. The UV band
exhibits a blue shift according to the band gap with increasing post-annealing temperatures, which suggests the possibility
to tune the UV photoluminescence band by varying the oxidation temperature. 相似文献
87.
Suthan T Rajesh NP Mahadevan CK Bhagavannarayana G 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,78(2):771-776
The 2-hydroxy-4-methoxybenzophenone single crystal has been grown by the modified vertical Bridgman technique using the double wall ampoule. The grown crystal was confirmed by single and powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy analyses. High resolution X-ray diffraction (HRXRD) analysis indicates the crystalline perfection of the grown crystal. The cutoff wavelength of the grown crystal was analyzed by optical studies. The dielectric measurements were carried out and the results indicate an increase in dielectric and conductivity parameters with the increase of temperature at all frequencies. The thermal property of the grown crystal was studied by thermo gravimetric (TG) and differential thermal analyses (DTA). 相似文献
88.
The positions of the semicore Ga d levels in GaX semiconductors (X=N,P, and As) are underestimated in density functional calculations using either the local density approximation LDA or the generalized gradient approximation GGA for the exchange functional. Correcting for this inaccuracy within LDA+U calculations with an on-site Coulumb interaction U on the semicore d-states results in a modest enhancement of the band gap. We show that this modest enhancement of the band-gap energy comes from the movement of the valence-band maximum alone, thus not affecting the conduction-band states. Further, the localization of the charge on Ga d states with U leads to a regulation of charge on Ga. This yields a shift of 1–2 eV of the core levels on the Ga atom while the anion core levels remain unchanged. 相似文献
89.
In analogy with the definition of resonant or quasi-bound states used in three-dimensional quantal scattering, we define the
quasi-bound states that occur in one-dimensional transmission generated by twin symmetric potential barriers and evaluate
their energies and widths using two typical examples: (i) twin rectangular barrier and (ii) twin Gaussian-type barrier. The
energies at which reflectionless transmission occurs correspond to these states and the widths of the transmission peaks are
also the same as those of quasi-bound states. We compare the behaviour of the magnitude of wave functions of quasi-bound states
with those for bound states and with the above-barrier state wave function. We deduce a Breit-Wigner-type resonance formula
which neatly describes the variation of transmission coefficient as a function of energy at below-barrier energies. Similar
formula with additional empirical term explains approximately the peaks of transmission coefficients at above-barrier energies
as well. Further, we study the variation of tunnelling time as a function of energy and compare the same with transmission,
reflection time and Breit-Wigner delay time around a quasi-bound state energy. We also find that tunnelling time is of the
same order of magnitude as lifetime of the quasi-bound state, but somewhat larger. 相似文献
90.
Aravind Surendran Lathika Paramashivan Sivapirakasam Suthangathan Mahadevan Surianarayanan 《Journal of Thermal Analysis and Calorimetry》2019,136(5):2183-2193
Journal of Thermal Analysis and Calorimetry - This paper addresses the thermal behavior of commonly used NaN3/KNO3 gas generant mixtures under different heating rates using simultaneous thermal... 相似文献