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
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.
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.
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. (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.
In this paper we explore the crystallization kinetics of poly(butylene terephthalate) (PBT), poly(ethylene terephthalate) (PET), and miscible blends of PBT with a polyarylate based on Bisphenol A and a 75/25 mole rati...
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In this paper we explore the crystallization kinetics of poly(butylene terephthalate) (PBT), poly(ethylene terephthalate) (PET), and miscible blends of PBT with a polyarylate based on Bisphenol A and a 75/25 mole ratio of isophthalic and terephthalic acids. For PBT, a regime II --> III transition is indicated near 210-degrees-C (DELTA-T approximately 30-degrees-C) and the fold surface free energy (sigma(e)) was found to be in the range of 57-75 erg/cm2. This leads to a work of chain folding (q) of approximately 3-5 kcal/mol folds. Analysis of the crystallization data of van Antwerpen and van Krevelen for PET shows that crystallization occurs in regime II across a wide range of T(c). Sigma(e) was estimated to be approximately 140 erg/cm2 and q approximately 10 kcal/mol from their data. These values are roughly twice that of PBT, consistent with the longer flexible segments in the PBT repeat unit. Crystallization data for three PBT/PAr blends was analyzed using a modified Lauritzen-Hoffman method and compared to that of pure PBT.
In a previous paper we illustrated a Monte Carlo method to calculate the incremental chemical potential between polymer chains of length nu and nu + 1 at all densities. In this document we present a statistical mechan...
In a previous paper we illustrated a Monte Carlo method to calculate the incremental chemical potential between polymer chains of length nu and nu + 1 at all densities. In this document we present a statistical mechanics model that allows for its quantitative evaluation as a function of chain length and temperature for macromolecular systems at zero density. Through this approach the incremental chemical potential at zero density is shown to be a strong function of chain length at low temperatures, but becomes chain length independent even for short chains (nu greater-than-or-equal-to 3) at higher temperatures. The temperature where the chemical potential becomes essentially chain length independent is a definition of the Boyle transition for finite chain length polymers, and we show that this transition temperature has a well defined chain length dependence. Chain dimensions and incremental entropies associated with chain segments are also found to decrease significantly when the temperature is reduced below this theta-temperature thus providing that chain segments are localized in the vicinity of other segments in the collapsed state. The consequences of these results on the modeling of the phase equilibria of polymer solutions are also examined.
Poly(enamino nitrile) (PEAN) forms miscible blends with several commercial polymers including poly(ethylene oxide) (PEO), poly(4-vinylpyridine) (P4VP), and tertiary amide polymers such as poly(ethyloxazoline) (PEOX), ...
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Poly(enamino nitrile) (PEAN) forms miscible blends with several commercial polymers including poly(ethylene oxide) (PEO), poly(4-vinylpyridine) (P4VP), and tertiary amide polymers such as poly(ethyloxazoline) (PEOX), poly(N-vinylpyrrolidine) (PNVP), and poly(N,N-dimethylacrylamide) (PDMA) as determined by the transparency of blend films and the observation of a single glass transition temperature which changes smoothly with composition. Binary blends containing more than 50 wt % of PEO showed PEO melting endotherms indicating the development of PEO crystallites in the homogeneous amorphous mixture. At higher temperatures PEO/PEAN blends undergo phase separation which signals the existence of a lower critical solution temperature. An endothermic transition associated with phase separation was clearly discernible by calorimetry, as was an exothermic transition associated with redissolution. These reversible phase separation and redissolution processes were observed by hot-stage optical microscopy. Strong hydrogen-bonding interaction between enamine hydrogen atoms and electronegative oxygen or nitrogen atoms on the other component polymers was evidenced by FT-IR and is suggested as a specific interaction which favors miscibilities in these blends.
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