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排序方式: 共有361条查询结果,搜索用时 31 毫秒
1.
P. Guenoun M. Delsanti D. Gazeau J.W. Mays D.C. Cook M. Tirrell M. Tirrell L. Auvray 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(1):77-86
Aqueous micellar solutions of ionic/neutral block copolymers have been studied by light scattering, small angle neutron scattering
and small angle X-ray scattering. We made use of a polymer comprised of a short hydrophobic block (polyethylene-propylene)
PEP and of a long polyelectrolytic block (polystyrene-sulfonate) PSSNa which has been shown previously to micellize in water.
The apparent polydispersity of these micelles is studied in detail, showing the existence of a few large aggregates coexisting
with the population of micelles. Solutions of micelles are found to order above some threshold in polymer concentration. The
order is liquid-like, as demonstrated by the evolution with concentration of the peak observed in the structure factor (), and the degree of order is found to be identical over a large range of concentrations (up to 20 wt%). Consistent values
of the aggregation number of the micelles are found by independent methods. The effect of salt addition on the order is found
to be weak.
Received: 19 June 1997 / Received in final form: 4 September 1997 / Accepted: 9 October 1997 相似文献
2.
Summary We deal with two diffusion problems: Space-integrated conserved entities characterizing very fast - diffusion - controlled reactions, such as time lags, etc. are universal. They are given by relationships which do not reflect the failure of the mean field hydrodynamic equations. We present another application which does not reflect this failure, for determining the surface flux via a diffusion controlled reaction producing a colored product. Another anomalous diffusion process we considered is transport through cellular materials whose cell sizes are highly nonuniform. We have analyzed the effects of extreme nonuniformity by considering fractal-like models of cellular solids. The diffusion current through these models can exhibit anomalous time-dependencies which are not predicted by the diffusion equation. In particular, it is shown that the initial diffusion current can be characterized by a power-law dependence on the time. Furthermore, the exponent of the power law is given in terms of the distribution of cell sizes in the fractal-like cellular solid. 相似文献
3.
YU Ming XU Qiang School of Materials Science Engineering East China University of Science Technology Shanghai China 《Chinese Journal of Reactive Polymers》2006,(1)
1. INTRODUCTIONThese years, HCFC-141b may be the most used blowing agent for its useful properties. But, because of its high global warming effect leading to the destruction of the ozone layer, production of HCFC141b has been forbidden. However, there are… 相似文献
4.
聚四氟乙烯微胞孔膜的结构和表征 总被引:1,自引:1,他引:1
使用多种方法制备了聚四氟乙烯微胞孔膜,以扫描电子显微镜、化学分析电子能谱及鼓泡测孔径法表征,结果表明,通过等离子体聚合,乙烯-四氟乙烯枝沉淀于基体膜上的方法能可控地获得良好的表征。 相似文献
5.
Tarek M. Madkour Rasha A. Azzam 《Journal of polymer science. Part A, Polymer chemistry》2002,40(14):2526-2536
Polyurethane elastomers of a controlled molecular architecture were synthesized using a two‐step polymerization technique. The building blocks of the elastomeric materials included urea–urethane prepolymers end‐capped with diisocyanate groups and had an exact number of urea groups at both ends. Two‐dimensional bifurcated hydrogen‐bonding networks incorporating the urea groups were, with differential scanning calorimetric and dynamic mechanical thermal analyzer techniques, responsible for the increase in the glass‐transition temperature (Tg) of the hard block and sharp interface morphology between the pure “hard” domains and pure “soft” domains. The higher extent of the phase separation between the two phases contributed to higher elastic moduli for the hard blocks and higher tensile strength for the elastomeric samples. Higher elongation values were attributed to the liberation of the elastomeric chain ends that otherwise would have been constrained in the interface region. The higher Tg values of the hard blocks corresponded to an increase in the hardness values and a decrease in the tear‐strength values. The increase in the amount of urea groups within the hard segments, as a result of the increased amount of water and blowing catalyst, resulted in elastomeric foams with higher open‐cell content. This resulted in lower resilience values as measured using the pendulum rebound test and was attributed to the ability of the open cells to absorb and dissipate energy. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2526–2536, 2002 相似文献
6.
Ceramic Foams from a Preceramic Polymer and Polyurethanes: Preparation and Morphological Investigations 总被引:1,自引:0,他引:1
Colombo Paolo Griffoni Martina Modesti Michele 《Journal of Sol-Gel Science and Technology》1998,13(1-3):195-199
Open-cell ceramic foams were obtained from a preceramic polymer (silicone resin) and blown polyurethanes. The preceramic polymer, which is crosslinked by condensation of silanol groups, was dissolved in CH2Cl2 and added to a liquid polyol containing the surfactant and the amine catalyst. Isocyanate was then added to the mixture and the foam was obtained through a twofold blowing mechanism (physical and chemical blowing). The morphology of the expanded polyurethane, which can be flexible or semirigid, characterized the final structure of the ceramic foam. The materials obtained were pyrolyzed in a nitrogen flux at temperatures of 1000–1200°C, thus allowing for the polymer-to-ceramic transformation to occur in the preceramic polymer. The ceramic foams produced in this way consisted of an amorphous silicon oxycarbide ceramic (SiOC). They presented a density ranging from 0.1 to 0.3 g/cm3. The average pore diameter ranged from 200 to 400 m and they possessed 80 to 90% open porosity. 相似文献
7.
Study on microstructure and mechanical properties relationship of short fibers/rubber foam composites 总被引:1,自引:0,他引:1
Research on short fibers/rubber foam composites is rarely found in the literature. In this paper, microcellular rubber foams unfilled (MF), strengthened by pretreated short fibers (MFPS) and untreated short fibers (MFUS) are prepared, respectively. The microstructure and mechanical properties of the three composites have been studied via scanning electron microscope (SEM) and mechanical testing, respectively. The SEM results show that both pretreated and untreated short fibers disperse uniformly in the composites and in bidimensional orientation. Moreover, the pretreated short fibers have much better adhesion with the rubber matrix than untreated ones. The experimental results also indicate that the introduction of short fibers is mainly responsible for the great enhancement of most mechanical properties of the microcellular rubber foams, and the good interfacial adhesion of the short fibers with the matrix contributes to the more extensive improvement in the mechanical properties. It is also found that the reinforcement effect of short fibers to compressive modulus strongly depends on the density of microcellular rubber foams, the orientation of short fiber and the deformation ratio. The compressive modulus of microcellular rubber foams at the normalized density less than 0.70 and beyond 0.70 is predicted by the modified Simple Blending Model and the Halpin-Kerner Model, respectively. The theoretically predicted values are in good accordance with the experimental results. 相似文献
8.
Francesco Galvagnini Andrea Dorigato Luca Fambri Alessandro Pegoretti 《Molecules (Basel, Switzerland)》2022,27(23)
polypropylene (PP) syntactic foams (SFs) containing hollow glass microspheres (HGMs) possess low density and elevated mechanical properties, which can be tuned according to the specific application. A possible way to improve their multifunctionality could be the incorporation of organic Phase Change Materials (PCMs), widely used for thermal energy storage (TES) applications. In the present work, a PCM constituted by encapsulated paraffin, having a melting temperature of 57 °C, was embedded in a compatibilized polypropylene SF by melt compounding and hot pressing at different relative amounts. The rheological, morphological, thermal, and mechanical properties of the prepared materials were systematically investigated. Rheological properties in the molten state were strongly affected by the introduction of both PCMs and HGMs. As expected, the introduction of HGMs reduced both the foam density and thermal conductivity, while the enthalpy of fusion (representing the TES capability) was proportional to the PCM concentration. The mechanical properties of these foams were improved by the incorporation of HGMs, while they were reduced by addition of PCMs. Therefore, the combination of PCMs and HGMs in a PP matrix generated multifunctional materials with tunable thermo-mechanical properties, with a wide range of applications in the automotive, oil, textile, electronics, and aerospace fields. 相似文献
9.
《Journal of Polymer Science.Polymer Physics》2018,56(10):803-813
Superheated water (shH2O) is investigated as a process aid in conventional aliphatic polyamide (PA) systems. The polymers investigated include PA 6 (PA6), PA 6,6 (PA66), PA 6,12 (PA612), and PA 12 (PA12). It is shown that the PA melting and crystallization temperatures are significantly reduced when exposed to shH2O. For example, the melting temperature of PA6 is depressed from 206 to 153 °C in the presence of shH2O. A relationship between amide group density and thermal transition temperature reduction is observed. Processing these materials in shH2O has led to a variety of materials ranging from low‐density foams to higher density locally anisotropic foamed morphologies. In situ observations of PAs melting in the presence of shH2O are performed using a specially designed reactor. Results from these experiments are used to estimate the diffusion coefficient of shH2O in PA6. Finally, low‐temperature extrusion is performed with PA6 and shH2O at temperatures as low as 180 °C and mixture viscosity is estimated. A 20‐fold depression in the melt viscosity of PA6 is observed at 240 °C with shH2O. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 803–813 相似文献
10.
The main objective of this study was to evaluate the sound absorption properties of rigid polyurethane foams (PUFs) produced from crude glycerol (CG) and/or liquefied coffee grounds derived polyol (POL). The lignin content of POL proved to have a major influence on the structure and mechanical properties of the foams. Indeed, the POL content increased the cell size of the foams and their stiffness, which subsequently influenced the sound absorption coefficients. The POL derived foam has slightly higher sound absorption coefficient values at lower frequencies, while the CG foam has higher sound absorption coefficient values at higher frequencies. In turn, the foam prepared using a 50/50 mixture of polyols presents slightly higher sound absorption coefficient values in the medium frequencies range due to a balance between the cell structure and the mechanical properties. The results obtained seem to suggest that the mechanisms involved in sound wave absorption depend on the formulation used to prepare the foams. Additionally higher POL contents improved the thermal stability of PUFs as well as their mechanical properties. From this work the suitability of CG and/or POL derived PUFs as sound absorbing materials has been proven. 相似文献