Small-angle X-ray scattering (SAXS) was employed to study the nanostructural properties of poly(ethylene glycol)(PEG)/silica
hybrid wet gels prepared from hydrolysis of tetraethoxysilane (TEOS) in mixtures with PEG/(PEG + TEOS) molar ratio in the
nominal range between 0 and 0.8. The SAXS pattern was found to be well fitted by the scattering from a polymeric particle
of radius of gyration Rg with an internal structure of primary silica particles described by a polymeric constraint of a mass-fractal of dimension
D. Rg increases with the PEG quantity while D first increases from 2.24 for the pure TEOS-derived gel to reach values between 2.46 and 2.40 with the additions of PEG.
The correlation volume Vc as determined by SAXS for such a polymeric structure behaves as a volume-fractal and accordingly scales with Rg as Vc ~ RgD. An overall mean value of about 1 nm could be estimated as an upper limit for the radius of the primary silica particle building
up the structure of all the wet gels, independent of the PEG addition. 相似文献
LiCrxMn2-xO4(x=0, 0.02, 0.05, 0.08, 0.10) compounds with a spinel crystal structure have been prepared by a novel ultrasonic co-precipitation method. The effects of the calcination temperature and the citric acid-to-metal ion molar ratio (R) on powder characteristics and electrochemical performance are evaluated. It is found that the optimum R and sintering temperature for LiCrxMn2-xO4 materials by the ultrasonic co-precipitation method are R= 5/6 and 800°C, respectively. The calcined powders are loosely bound agglomerates of abnormally coarsened particles with a narrow range of particle sizes. The effect of Cr doping was also explored. Electrochemical studies show that optimum materials synthesized by the ultrasonic co-precipitation method demonstrate good cycling performance. 相似文献
The influences of nanoparticle size and concentration on the thermodynamic behaviour of epoxy/polystyrene blends are evaluated in the framework of Ginzburg's simple analytical theory. Two approaches have been employed: NPEPO (for particles coated with epoxy groups) and NPFEN (for particles coated with phenyl groups). Using NPEPO, the particles are found to prefer the phase richer in epoxy, whereas the opposite occurs for NPFEN. The particles size significantly influences blend compatibility. When the particle radius Rp is about the same size as the radius of gyration Rg of PS, the compatibility of blends increases with particle concentration, whereas for Rp > Rg, higher particle concentrations do not clearly stabilize the polymeric blends.
Dilute water–salt solutions of compositionally heterogeneous κ-carrageenan (κ-CG) containing 3% of ι-CG units are studied by combined static and dynamic (DLS) light scattering in 0.1M NaCl at 25°C. The structure-sensitive ratio Rg/Rh?=?2.2 of the fast DLS mode corresponds to the coils of κ-CG homopolymer. The irreversible core–shell aggregates (Rg/Rh?=?0.4) give rise to a slow mode. The optical rotation reveals that the aggregates originate from helical fragments of contaminating multi-block κ/ι-copolymers. Comparison with capillary viscometry shows that the weight fraction of aggregates is as small as the fraction of contaminating κ/ι-hybrids. 相似文献
Reduction of zirconium dioxide with boron carbide and nanofibrous carbon in argon yielded a highly dispersed powder of zirconium diboride. Characteristics of zirconium diboride powders were examined by various analytical methods. The material obtained is represented by a single phase, zirconium diboride. Powder particles are for the most part aggregated. The average size of particles and aggregates is 10.9–12.9 μm with a wide size distribution. The specific surface area of the samples is 1.8–3.6 m2 g–1. The oxidation of zirconium diboride begins at a temperature of 640°C The optimal synthesis parameters were determined: ZrO2: B4C: C molar ratio of 2: 1: 3 (in accordance with stoichiometry), process temperature 1600–1700°C, synthesis duration 20 min. 相似文献
An exotherm, observed in differential scanning calorimetry (DSC) scans of amorphous food materials above their glass transition temperature,Tg, may occur due to sugar crystallization, nonenzymatic browning, or both. In the present study, this exothermal phenomenon in initially anhydrous skim milk and lactose-hydrolyzed skim milk was considered to occur due to browning during isothermal holding at various temperatures above the initialTg. The nonenzymatic, Maillard browning reaction produces water that in amorphous foods, may plasticize the material and reduceTg. The assumption was that quantification of formation of water from theTg depression, which should not be observed as a result of crystallization under anhydrous conditions, can be used to determine kinetics of the nonenzymatic browning reaction. The formation of water was found to be substantial, and the amount formed could be quantified from theTg measured after isothermal treatment at various temperatures using DSC. The rate of water formation followed zero-order kinetics, and its temperature dependence well aboveTg was Arrhenius-type. Although water plasticization of the material occurred during the reaction, and there was a dynamic change in the temperature differenceT–Tg, the browning reaction was probably diffusioncontrolled in anhydrous skim milk in the vicinity of theTg of lactose. This could be observed from a significant increase in activation energy. The kinetics and temperature dependence of the Maillard reaction in skim milk and lactose-hydrolyzed skim milk were of similar type well above the initialTg. The difference in temperature dependence in theTg region of lactose, but above that of lactose-hydrolyzed skim milk, became significant, as the rate in skim milk, but not in lactose-hydrolyzed skim milk, became diffusion-controlled. The results showed that rates of diffusion-controlled reactions may follow the Williams-Landel-Ferry (WLF) equation, as kinetic restrictions become apparent within amorphous materials in reactions exhibiting high rates at the same temperature under non-diffusion-controlled conditions. 相似文献
Summary: A unified model is developed for the finite size‐effect on the glass‐transition temperature of polymers, Tg(D), where D denotes the diameter of particles or thickness of films. In terms of this model, Tg depends on both the size and interface conditions. The predicted results are consistent with the experimental evidence for polystyrene (PS) particles and films with different interface situations.