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71.
González-Alvarez MJ Balbuena P Mellet CO Fernández JM Mendicuti F 《The journal of physical chemistry. B》2008,112(44):13717-13729
Steady-state and time-resolved fluorescence techniques were used to study the behavior of 2I,3I-O-(o-xylylene)-per-O-Me-alpha- and -beta-cyclodextrins in aqueous solution, based on the fluorescence of the bidentate xylylene moiety. Fluorescence decay profiles obtained upon excitation of the xylylene group were fitted to three-exponential decay functions. In addition to a fast component due to stray and/or scattered light, two other components ascribed to the monomer and dimer species, respectively, were identified. The dimer/monomer ratio increases with concentration and decreases with temperature, which is in agreement with an enthalpy-driven association process. The corresponding dimerization equilibrium constants (KD) were obtained from nonlinear regression analysis of the plots of tau against [CD] in the 5-45 degrees C range. A linear van't Hoff analysis for KD allows us to obtain the DeltaH and DeltaS associated to dimer formation. Molecular mechanics as well as molecular dynamics calculations in the presence of water were also employed to study the conformational behavior of such secondary-face-substituted cyclodextrins and rationalize the dimerization processes. 相似文献
72.
Sergio Bocchini Sandrine Morlat-Therias Giovanni Camino 《European Polymer Journal》2008,44(11):3473-3481
Nanocomposites containing hydrotalcite and prepared by melt compounding with polypropylene were UV-light irradiated in artificial accelerated conditions representative of solar irradiation (λ > 300 nm) at 60 °C in air. The chemical modifications resulting from photooxidation were followed by IR and UV-visible spectroscopies.The presence of hydrotalcite was shown to change the global rate of photooxidation of polypropylene by reducing the oxidation induction time and increasing the oxidation rate. The differences of the oxidation induction time disappeared after solvent extraction of the antioxidant. They were attributed to a quenching of the antioxidant activity resulting from a migration onto the filler surface induced by the preferential interaction with the polar hydrotalcite. Extracting the antioxidant did not change the oxidation rate at the permanent regime. The increase of the oxidation rate was attributed to transition metal ions, present as impurities in hydrotalcite, which can accelerate the oxidation of the polymer by various mechanisms including a catalysed decomposition of hydroperoxides. 相似文献
73.
74.
Dendrimer-tetrachloroplatinate precursor interactions. 2. Noncovalent binding in PAMAM outer pockets
Density functional theory is used to investigate the complexation ability of dendrimer outer pockets--in both tertiary amine protonated and unprotonated scenarios--toward molecular guests, particularly tetrachloroplatinate(II) and its mono- and diaquated derivatives as well as competing counterions. The effect of the outer pocket (host) on the binding affinity of guest molecules is analyzed and it is found that is more feasible for the host to accept species, particularly charged ones, inside an unprotonated pocket rather than outside; unlike the protonated pocket where the opposite is more likely to occur. Conformational changes triggered by the hosting of particular guests can have an impact in the global configuration of the larger dendrimer the pockets are part of. 相似文献
75.
Bartolomé C Espinet P Martín-Alvarez JM 《Chemical communications (Cambridge, England)》2007,(42):4384-4386
The reinvestigation of the title compounds, which are the only examples reported to show experimentally (by NMR) O-H...F-C bonds in solution proves that the NMR data were misinterpreted and the restrictions to rotation of one CF(3) group are due to crowding, not to intramolecular O-H...F-C bond. 相似文献
76.
Density functional theory is used to study oxygen adsorption and its effect on surface segregation in (2 1 1) surfaces of Pt(shell)/M(core) and Pt3M (M = Co, Ir) alloys. It is found that the most energetically favorable oxygen adsorption site is the bridge site over and parallel to the (1 0 0) step. Surface segregation phenomena is observed in Pt3Co, Pt3Ir and Pt/Co(core) systems. The Pt/Ir(core) structure was the only one, among the studied systems, that showed antisegregation behavior even in presence of oxygen adsorbed. 相似文献
77.
Marco Zanetti Giovanni Camino Peter Reichert Rolf Mülhaupt 《Macromolecular rapid communications》2001,22(3):176-180
The thermal degradation behaviour of nanocomposites based upon poly(propylene)/organoclay, modified with protonated octadecyl amine (C18) in comparison to that of non‐exfoliated microcomposites based upon organoclay, modified with protonated butyl amine (C4), was studied by thermogravimetry. In the case of the nanocomposite, the temperature at which volatilisation occurs increases as compared of the microcomposite. Moreover, the thermal oxidation process of the polymer is strongly slowed down in the nanocomposite with high char yield both by a physical barrier effect, enhanced by ablative reassembling of the silicate, and by a chemical catalytic action due to the silicate and to the strongly acid sites created by thermal decomposition of the protonated amine silicate modifier. 相似文献
78.
A. Fina D. Tabuani F. Carniato A. Frache E. Boccaleri G. Camino 《Thermochimica Acta》2006,440(1):36-42
The mechanism of thermal degradation of several substituted polyhedral oligomeric silsesquioxanes (POSS) cages is studied in this work.Hydrogen POSS and methyl POSS shows incomplete sublimation on heating, both in inert atmosphere and in air. Isobutyl and octyl substituted POSS undergo an almost complete evaporation when heated in inert atmosphere. In air, oxidation competes with volatilization, producing a considerable amount of silica-like residue on heating up to 800 °C.Phenyl POSS shows a higher thermal stability than saturated aliphatic POSS and limited volatility, producing a ceramic residue at high yield on heating in nitrogen, composed of a silica containing a considerable amount of free-carbon. A lower amount of residue is shown after heating in air, corresponding to the POSS Si-O fraction.A vinyl POSS cage/network resin is also studied, in comparison to above materials, showing the highest ceramic yield. 相似文献
79.
When polypropylene is thermally degraded in mixtures with a highly chlorinated fire retardant chloroparaffin, a smaller amount of the lighter hydrocarbon fraction, which is also modified in composition, is evolved compared with that evolved from pure polypropylene. A corresponding increase in the amount of high boiling chain fragments is obtained from the mixtures. This effect can be explained in terms of the interactions which occur between degrading polymer and additive. 相似文献
80.
Metal functionalized POSS as fire retardants in polypropylene 总被引:1,自引:0,他引:1
Alberto Fina Hendrikus C.L. Abbenhuis Giovanni Camino 《Polymer Degradation and Stability》2006,91(10):2275-2281
This paper deals with the study of the combustion properties of dimeric and oligomeric Al-and Zn-isobutyl silsesquioxane (POSS)/polypropylene (PP) composites, in comparison with neat PP and PP/octaisobutyl POSS.The presence of Al-POSS leads to a decrease in combustion rate with respect to PP, resulting in a decrease of Heat Release Rate (−43% at 10 wt% POSS loading) as well as reduction in CO and CO2 production rates, whereas a negative effect on the above parameters is obtained with octaisobutyl POSS. Zn-POSS does not significantly affect the PP combustion behaviour.The effect of Al-POSS is most likely related to its chemical activity, which favours the formation of a moderate amount of char residue, by catalysing secondary reactions in the polymer during combustion. 相似文献