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91.
S,S-and R,R-warfarin alcohols 3 are prepared in good yield and in 99% d.e. (diastereomeric excess) by the reaction of S- or R-warfarin 1 with S or R-Alpine-Hidride 2. 相似文献
92.
Masao Kimura Roger S. Porter Gideon Salee 《Journal of Polymer Science.Polymer Physics》1983,21(3):367-378
Blends of poly(butylene terephthalate) (PBT) and a copolyester of bisphenol-A with 50% terephthalate-50% isophthalate (PAr), before and after transesterification, have been studied by thermal and dynamic mechanical tests to determine crystallinity and phase behavior. Blends without transesterification, as prepared by solution precipitation, show a single Tg, indicating amorphous miscibility of PBT and PAr. A melting-point depression for PBT crystals is not observed; this means that PBT crystallizes excluding PAr and the entropy of melting is small. The highest fractional crystallinity for PBT is obtained at 20-35% of PAr. Transesterified blends were obtained by holding the physical blends at 250°C for up to 16 h. The transesterified systems show higher Tg's than the corresponding physical blends and also show a marked melting-point depression and lesser PBT crystallinity at the corresponding increased PAr content. 相似文献
93.
94.
Peter G. Thompson Larry T. Taylor Bruce E. Richter Neil L. Porter John L. Ezzell 《Journal of separation science》1993,16(12):713-716
A polarity test mix consisting of acetophenone, N, N-dimethylaniline, naphthalene, decanoic acid, 2-naphthol, and n-tetracosane was spiked onto sand, and extracted with supercritical carbon dioxide, to evaluate the collection efficiency of various solvents and solvent mixtures. Nine single collection solvent systems and four mixed collection solvent systems were studied. When one-component collection solvents were employed, quantitative (above 90%) recovery of all analytes was not possible. With mixed collection solvents, recoveries of 90% or better with all analytes studied were possible. 相似文献
95.
B. E. Richter D. E. Knowles M. R. Andersen N. L. Porter E. R. Campbell D. W. Later 《Journal of separation science》1988,11(1):29-32
An analysis of the precision obtained using commercially available microvalve injectors is reported for three modes of injection: conventional split; timed-split; and direct. Results from this study show that good precision (< 3% RSD for external standard and < 1% RSD for internal standard methods) can be obtained with capillary supercritical fluid chromatography (SFC). However, particular attention must be paid to the type of valve used, the orientation of the column relative to the valve, the mode of interfacing or connecting the column to the valve, and the type of pressure or density programming used for the analysis as all of these factors will affect the reproducibility. 相似文献
96.
Billets of chain-extended polyethylene were prepared from Alathon 7050 (Mw 59,000, Mn 19,000) in an Instron capillary rheometer by crystallization at a constant pressure of 460 MPa, at a series of teimperatures from 198 to 221°C corresponding to varying degrees of undercooling. This gives chain-extended morphologies with a range of crystallinites and lamellar thicknesses. The billets were then solid-state extruded at 100°C through a conical die with 20° entrance angle up to an extrusion draw ration 23.4. Thermal behavior was studied with differential scanning calorimetry. The orientation function measured by wide-angle x-ray diffraction showed higher orientation function measured by wide-angle x-ray diffraction showed higher orientation at equivalent draw ratio when the initial billets were crystallized at lower temperatures. Drawing efficiency, defined as the ratio of molecular draw ratio (from shrinkage) to extrusion draw ratio correspondingly increases, reaching a maximum of 0.71 in our solid-state extrusion. These studies show that highly chain-extended polyethylene, i.e., with few chain entanglements, draws poorly. Drawability was improved by increasing chain entanglements by lowering the crystallization temperature. Electron micrographs of fracture surface replicas of extrudates revealed the coexistence of undeformed, tilted, partially drawn lamellae and fibrillar structure consistent with the cahange of morphologies in Peterlin's model of plastic deformation. 相似文献
97.
Joseph D. Danforth Scott R. Porter David Strickler 《Journal of solid state chemistry》1984,54(1):34-48
The acceleratory degradation of solids represents an area of research where extreme care must be exercised to obtain reproducible data. Different preparations of the same substance often show great variation in degradation patterns. Many kinetic equations have been developed to describe the kinetics of acceleratory degradations. Most of them are empirical and apply only to certain phases of a degradation. The non-steady-state kinetics which were developed to describe the zip degradation of poly(vinyl chloride) will be shown to describe effectively the unusual degradation behavior of inorganic solids that follow an acceleratory pattern. 相似文献
98.
R.P. Porter 《Optics Communications》1981,39(6):362-364
For a weak scatterer a three dimensional Fourier transform of the image formed by a generalized hologram is shown to yield directly the Fourier transform of the scattering potential evaluated on the Ewald sphere. The entire scattering potential can be approximately determined if the object is illuminated from all directions in a series of experiments and if the scattering potential is essentially bandlimited to within a sphere of radius 4π/λ. 相似文献
99.
The Nachtmann moments of deep inelastic scattering structure functions are required by kinematics to contain a so far neglected threshold factor which is dependent on both n and q2. Its presence significantly affects the “moment analysis” in the usual QCD phenomenology and it resolves the difficulties connected with improper threshold behavior of the “ξ-scaling” analysis of structure functions. 相似文献
100.
The viscoelastic (VE) response of freeze-dried blends of polystyrene (PS) and poly-(2,6-dimethyl phenylene oxide) (PPO) has been studied as a function of composition, frequency, and temperature to examine the degree of rheological compatibility. When blended together, the relaxation processes of both molecular species exhibit the same temperature dependence. However, the temperature dependence of the VE response is a function of composition. It is shown that this behavior can be predicted from the measured glass transition temperatures by assuming the additivity of the free volumes of the components. The properties of the blends are compared at equal free volumes. The effective segmental friction factor is found to be independent of composition while the modulus of the rubbery plateau increases with PPO concentration. This result is interpreted as a change in the entanglement molecular weight Me of the blends. When the changes in Me are considered, the relationship between the zero-shear viscosity η0 and the 3.4 power of the weight-average molecular weight, commonly found for high molecular weight homopolymers, predicts the compositional dependence of η0 for the PPO–PS blends. It is concluded that the PPO–PS system forms a rheologically compatible blend. 相似文献