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91.
R. C. Warren 《Rheologica Acta》1984,23(5):544-547
Solutions of Pyro grade nitrocellulose (NC) in dimethyl acetamide (DMA), containing between 42.5% and 60% NC (w/w), have been studied by differential scanning calorimetry and polarised light microscopy. The results showed that NC forms a lyotropic liquid crystal structure in DMA. A transition from the liquid crystal phase to an isotropic phase occurred over the temperature range 27 °C to 67 °C, and the enthalpy of transition increased with NC concentration. Rheological properties were determined using an extrusion rheometer with a slit die. The solutions were shown to have a yield stress for flow which increased with increasing NC concentration. The solutions were also found to be thixotropic. 相似文献
92.
Molecular dynamics simulations are used to investigate the effects of deformation on the segmental dynamics in an aging polymer glass. Individual particle trajectories are decomposed into a series of discontinuous hops, from which we obtain the full distribution of relaxation times and displacements under three deformation protocols: step stress (creep), step strain, and constant strain rate deformation. As in experiments, the dynamics can be accelerated by several orders of magnitude during deformation, and the history dependence is entirely erased during yield (mechanical rejuvenation). Aging can be explained as a result of the long tails in the relaxation time distribution of the glass, and similarly, mechanical rejuvenation is understood through the observed narrowing of this distribution during yield. Although the relaxation time distributions under deformation are highly protocol specific, in each case they may be described by a universal acceleration factor that depends only on the strain. 相似文献
93.
R. W. Warren D. S. Gates G. L. Driscoll 《Journal of polymer science. Part A, Polymer chemistry》1971,9(3):717-746
The composition of nonvolatile fluids obtained from thermally cracking and hydrogenating polyisobutylene was determined by using a combination of gas chromatography and nuclear magnetic resonance spectroscopy (NMR). This work involved the separation and characterization of a homologous series, C11–C40, of sixteen branched hydrocarbon species consisting of repeating isobutylene structures. As a result of this investigation, useful correlations between NMR spectra and molecular structure for highly branched hydrocarbons were developed. The data demonstrate that these hydrocarbons are unique species characterized by “crowded” and sterically hindered geminal methyl and isolated methylene groups. NMR solvent shift studies in benzene solutions indicate that it is possible to differentiate between maximally crowded geminal methyl groups and between maximally crowded methylene groups in these structures. Results of the benzene-induced solvent effects are discussed with respect to the stereochemistry of these molecules and related to existing solvent shift data. These results suggest that these hydrocarbons are polar or nearly polar materials. Successive losses of isobutylene units from stabilized tertiary radicals can account for the formation of the major species identified in these fluids. Higher carbon numbered species have lower refractive indices and densities and higher molal volumes than predicted by calculations. 相似文献
94.
95.
Tracy E. Scott Amin Khalili Brandon Newton Robert Warren Daniel P. Browe Joseph W. Freeman 《先进技术聚合物》2019,30(10):2604-2612
Hydrogels have been used for many applications in tissue engineering and regenerative medicine due to their versatile material properties and similarities to the native extracellular matrix. Poly (ethylene glycol) diacrylate (PEGDA) is an ionic electroactive polymer (EAP), a material that responds to an electric field with a change in size or shape while in an ionic solution, that may be used in the development of hydrogels. In this study, we have investigated a positively charged EAP that can bend without the need of external ions. PEGDA was modified with the positively charged molecule 2‐(methacryloyloxy)ethyl‐trimethylammonium chloride (MAETAC) to provide its own positive ions. This hydrogel was then characterized and optimized for bending and cellular biocompatibility with C2C12 mouse myoblast cells. Studies show that the polymer responds to an electric field and supports C2C12 viability. 相似文献
96.
Dalgarno SJ Hardie MJ Warren JE Raston CL 《Dalton transactions (Cambridge, England : 2003)》2004,(16):2413-2416
Two types of arrays are formed in water involving aza-crown ethers, p-sulfonatocalix[5]arene and europium(III) ions. One is a co-ordination polymer connecting calixarenes, sodium ions and lanthanide ions based on "ferris wheel" moieties incorporating aza-18-crown-6 and sodium ions. The second structure is a host-guest arrangement with di-protonated diaza-18-crown-6 in the cavity of the calixarenes as part of secondary coordination spheres of aquated europium(iii) ions. 相似文献
97.
Functional magnetic resonance imaging with intermolecular multiple-quantum coherences 总被引:4,自引:0,他引:4
Richter W Richter M Warren WS Merkle H Andersen P Adriany G Ugurbil K 《Magnetic resonance imaging》2000,18(5):489-494
For the first time, we demonstrate here functional magnetic resonance imaging (fMRI) using intermolecular multiple-quantum coherences (iMQCs). iMQCs are normally not observed in liquid-state NMR because dipolar interactions between spins average to zero. If the magnetic isotropy of the sample is broken through the use of magnetic field gradients, dipolar couplings can reappear, and hence iMQCs can be observed. Conventional (BOLD) fMRI measures susceptibility variations averaged over each voxel. In the experiment performed here, the sensitivity of iMQCs to frequency variations over mesoscopic and well-defined distances is exploited. We show that iMQC contrast is qualitatively and quantitatively different from BOLD contrast in a visual stimulation task. While the number of activated pixels is smaller in iMQC contrast, the intensity change in some pixels exceeds that of BOLD contrast severalfold. 相似文献
98.
Martin E. Keillor Craig E. Aalseth Anthony R. Day James E. Fast Eric W. Hoppe Brian J. Hyronimus Todd W. Hossbach Harry S. Miley Allen Seifert Glen A. Warren 《Journal of Radioanalytical and Nuclear Chemistry》2009,282(3):703-708
Physics experiments, environmental surveillance, and treaty verification techniques continue to require increased sensitivity
for detecting and quantifying radionuclides of interest. This can be done by detecting a greater fraction of gamma emissions
from a sample (higher detection efficiency) and reducing instrument backgrounds. A current effort for increased sensitivity
in high resolution gamma spectroscopy will produce an intrinsic germanium (HPGe) array designed for high detection efficiency,
ultra-low-background performance, and useful coincidence efficiencies. The system design is optimized to accommodate filter
paper samples, e.g. samples collected by the Radionuclide Aerosol Sampler/Analyzer (RASA). The system will provide high sensitivity
for weak collections on atmospheric filter samples, as well as offering the potential to gather additional information from
more active filters using gamma cascade coincidence detection. The current effort is constructing an ultra-low-background
HPGe crystal array consisting of two vacuum cryostats, each housing a hexagonal array of 7 crystals on the order of 70% relative
efficiency per crystal. Traditional methods for constructing ultra-low-background detectors are used, including use of materials
known to be low in radioactive contaminants, use of ultra pure reagents, clean room assembly, etc. The cryostat will be constructed
mainly from copper electroformed into near-final geometry at PNNL. Details of the detector design, simulation of efficiency
and coincidence performance, HPGe crystal testing, and progress on cryostat construction are presented. 相似文献
99.
We derive explicit, uniform, a priori interior Hessian and gradient estimates for special Lagrangian equations of all phases
in dimension two.
Y. Yuan is partially supported by an NSF grant. 相似文献
100.
Let K be a compact convex subset of a separated locally convex space (over R) and let Ap(K) denote the space of all continuous real-valued affine mappings defined on K, endowed with the topology of pointwise convergence on the extreme points of K. In this paper we shall examine some topological properties of Ap(K). For example, we shall consider when Ap(K) is monolithic and when separable compact subsets of Ap(K) are metrizable. 相似文献