Small-Angle X-ray Scattering (SAXS) has been used as an analytical tool to study the development of microstructure with increasing Heat Treatment Temperature (HTT) of a carbonized phenolic resin, which is often used a...
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Small-Angle X-ray Scattering (SAXS) has been used as an analytical tool to study the development of microstructure with increasing Heat Treatment Temperature (HTT) of a carbonized phenolic resin, which is often used as matrix in commercial carbon-carbon composites. Processing parameters-heating rate and maximum heat treatment temperature-were the primary variables of interest. Results from SAXS have led to the consideration of a model for microstructure development in carbonized phenolic resins. Local density variations on the 10 angstrom-100 angstrom scale seem to be the predominant feature of these carbonized resins in the temperature range studied (450-900-degrees-C). Heat treatment results in a gradual destruction of crosslinks, which leads to clustering of the aromatic units. These clusters enclose pores, the high concentrations of which give rise to scattering maxima in SAXS data. Concentration and dimensions of pores are sensitive both to the heating rate used and the maximum carbonization temperature.
Thermal stability of [(CH3SiH)30(C6H5SiCH3)70]n a hydropolysilane copolymer, in vacuum and its crosslinking reactions with vinylic silanes as crosslinking agents was evaluated in order to obtain high yields of oxygen-...
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A nanostructured FeCrMoVM50 type steel was prepared via a chemical route. The process involved the thermal decomposition of organometallic precursors Fe(CO) 5 , Cr(CO) 6 , Mo(CO) 6 and V(CO) 6 , at 150°C ...
A nanostructured FeCrMoVM50 type steel was prepared via a chemical route. The process involved the thermal decomposition of organometallic precursors Fe(CO) 5 , Cr(CO) 6 , Mo(CO) 6 and V(CO) 6 , at 150°C for the formation of nanostructured M50 type steel powders. In addition to the thermal decomposition of these carbonyls, the results of the reduction of respective metal halides for the production of the same steel are also presented. The nanostructured steel powders obtained were also consolidated samples, were characterized using x-ray synthesized powders, as well as the consolidated samples, were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution electron microscopy (HRTEM). Possible mechanisms for the formation of nanostructured particles are also discussed.
The objectives of this research were (I) to determine whether the use of partially absorbable vascular grafts would improve their nonthrombogenic performance and whether they are more prone to aneurysmal dilation and ...
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The objectives of this research were (I) to determine whether the use of partially absorbable vascular grafts would improve their nonthrombogenic performance and whether they are more prone to aneurysmal dilation and subsequent failure in vivo. and (2) to find out the relationships, if any. between these in vivo and the previously reported in vitro data with an emphasis on how the in vitro changes in fabric structure and properties related to these in vivo data. Bicomponent vascular fabrics were made from Dacron and polyglycolic acid (PGA) yarns with a range of composition ratios of the PGA to Dacron. Both woven and single Jersey knit fabrics were made, and implanted in dogs for 4 months. The following findings and relationships were obtained. (1) The bicomponent vascular fabrics resulted in a full-wall healing in the thoracic aorta of dogs. All bicomponent vascular grafts in survived dogs exhibited 100% patency, no thrombus or aneurysmal formation, no hematoma or seroma around the grafts, and no fibrin coagula in the inner capsules. The gross morphology of the regenerated tissues was very similar visually to the adjacent original arterial tissue. Histologically, the luminal surface was lined with a layer of endothelial cells with myofibroblasts, fibroblasts. and collagens underneath. (2) The extent of the full-wall healing depended on the type of fabric structure. the concentration of absorbable yarns, the location of absorbable yarns (for the woven group only), and initial water permeability. It is believed that the concentration effect was related to the level of macrophage activation from the degradation products of the absorbable yarns, while the location effect was attributed to the various types of fabric structure change on the degradation of the absorbable yarns. (3) In general, the knitted group (K), was better than the woven group (W). K3 showed the best in vivo performance in the knitted group. (4) In the woven group, W3 was the best. The incorporation o
A series of liquid crystal polymer/polyethylene (LCP/PE) blends have been studied to determine the potential of such a system to produce a high modulus film material which retains fabrication and low temperature chara...
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A series of liquid crystal polymer/polyethylene (LCP/PE) blends have been studied to determine the potential of such a system to produce a high modulus film material which retains fabrication and low temperature characteristics of some current PE films. The subject of liquid crystalline polymer blends has been the focus of significant attention for the last decade due to the novel rheological and mechanical properties of this class of polymers. It has been demonstrated that if an LCP blend is processed under elongational flow conditions, the partially ordered LCP meso-phase intermediate allows the development of an oriented fibrillar morphology which is retained upon solidification. In this study, blown films of blends of 5 and 15% LCP in PE have been produced which show an enhancement in modulus over the neat PE matrix. These results are discussed in terms of processing conditions, LCP reinforcement aspect ratio, fibril diameter, and LCP/PE modulus ratio.
In an earlier publication we had presented a Monte Carlo method to calculate the incremental chemical potential between a chain of length nu and one of length nu+1 at any density (Kumar et al., 1991, Kumar, 1992a, 199...
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In an earlier publication we had presented a Monte Carlo method to calculate the incremental chemical potential between a chain of length nu and one of length nu+1 at any density (Kumar et al., 1991, Kumar, 1992a, 1992b). In this paper we begin by formally proving that the chemical potential of a whole chain of length nu can be obtained by sequentially summing the incremental chemical potentials of all chains whose lengths are smaller than nu under the same conditions. It was found that the incremental chemical potentials defined in this manner had a well defined chain length dependence, and that they were essentially independent of chain length in polymer melt systems. This allows us, in principle, to enumerate the phase diagrams for polymer blends using the incremental chemical potentials alone.
The first example of an aliphatic poly(enamino nitrile) analogous to a polyurea prepared by interfacial condensation of (dichloromethylene)propanedinitrile and 1,6-diaminohexane is described and characterized.
The first example of an aliphatic poly(enamino nitrile) analogous to a polyurea prepared by interfacial condensation of (dichloromethylene)propanedinitrile and 1,6-diaminohexane is described and characterized.
In an earlier publication we had presented a Monte Carlo method to calculate the incremental chemical potential between a chain of length nu and one of length nu+1 at any density. (Ref.9) In this paper we begin by for...
In an earlier publication we had presented a Monte Carlo method to calculate the incremental chemical potential between a chain of length nu and one of length nu+1 at any density. (Ref.9) In this paper we begin by formally proving that the chemical potential of a whole chain of length nu can be obtained by sequentially summing the incremental chemical potentials of all chains whose lengths are smaller than nu under the same conditions. We then compare the numerical value of chain chemical potentials computed in this fashion to those obtained by inserting whole chains into the simulation cell following the configurational bias procedure proposed by Smit and Frenkel. (Ref.5) It is found, for the two cases considered here, that die results obtained from the two different calculations agree within simulation uncertainty, thus verifying the validity of the proposed arguments.
Bis-electrophilic aromatic monomers were polymerized with hexamethylenediamine to high molecular weight polyenaminonitriles. All the polymers, except that containing a sulfone group, were soluble in polar, aprotic sol...
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Bis-electrophilic aromatic monomers were polymerized with hexamethylenediamine to high molecular weight polyenaminonitriles. All the polymers, except that containing a sulfone group, were soluble in polar, aprotic solvents and possessed film-forming properties. The polymers and their model compounds were characterized by spectroscopic methods. Some of the polymers possessed varying degrees of crystallinity as shown by differential scanning calorimetry. Thermogravimetry also revealed a surprising amount of thermal stability for macromolecules containing aliphatic segments.
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