Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.

Magnetic relaxation in blood and blood clots.

Abstract: Nuclear magnetic relaxation rates are measured for whole blood, blood plasma, whole blood clots, and plasma clots in vitro. Relaxation rates are linear in the hematocrit and transverse relaxation rates are significantly greater than longitudinal relaxation rates. Longitudinal relaxation rates measured from 0.01 to 42 MHz for proton Larmor frequencies are found to decline monotonically with increasing magnetic field strength; however, the dispersion curves do not follow a simple Lorentzian behavior, which is anticipated in a suspension of particles in a solution of proteins having a distribution of molecular weights. The transverse relaxation rate is a function of the acquisition parameters, in particular, the choice of TE in either Hahn echo experiments or in echo-train experiments. The origin of this dependence of T2 on TE or the interpulse spacing in an echo train is identified with the exchange of water from inside the red blood cell to the outside and is only an important relaxation mechanism in the case where the blood cell membrane is intact and the cell contains deoxygenated hemoglobin. The dependence of the apparent transverse relaxation rate on the interpulse spacing in a Meiboom-Gill-Carr-Purcell pulse sequence provides the estimate that the mean residence time of water inside the blood cell is about 10 ms. These data provide a sound basis for understanding the dependence of magnetic images on magnetic field strength and the choices of the image acquisition parameters, TE and TR.

Rotation of Molecules and Nuclear Spin Relaxation

Abstract: Nuclear spin relaxation has been developed as a standard method for studying molecular motions in liquids, solids, polymers, and—to a lesser extent—gases, staring with the pioneering work of Bloembergen, Purcell, and Pound [1]. Of the great variety of molecular motions possible (e.g., translations, rotations, vibrations) rotations are particularly important for nuclear spin relaxation. Conversely, nuclear spin relaxation can be especially successful if information about rotational motions is desired. In this case nuclear spin relaxation can yield quantitative information over an extraordinary wide range of characteristic frequencies, from about 1 Hz to 1014Hz. It shoud be noted that, typically, the nuclear spin relaxation times actually observed are much longer than the characteristic times of rotation of molecules. Therefore, a theory is needed which links the time constants of nuclear spin relaxation to the correlation functions describing the rotational motion, and so a considerable part of this volume is devoted to this goal. For rapid motions in liquids of low viscosity nuclear spin relaxation thus in principle, is inferior to scattering experiments (e.g., light and neutron scattering) which allow the determination of correlation functions.

Superconductivity and NMR investigations of KTl1.5-doped C60

Abstract: Doping of C60 fullerene with KTl1.5 results in superconductivity withTconset=17.6 K as shown by a dc magnetization measurement. Preliminary13C NMR studies on a doped and undoped sample are reported. Compared to original C60 the linewidth increases by a factor of 3, the resonance frequency is shifted by 42 ppm, and the relaxation rateT1−1 is an order of magnitude larger in the intercalated material at room temperature. Both observations support the picture of a metallic sample.

Relaxation Phenomena in Vitrifying Polymers and Molecular Liquids

Abstract: Recent experimental results on the dynamics of glass-forming materials, particularly polymers, are surveyed. The focus is on aspects of the behavior that are connected to or correlated with structural relaxation. These results include the invariance to thermodynamic conditions (temperature, pressure, volume) of a number of properties—breadth of the relaxation dispersion, number of dynamically correlating molecules, Johari−Goldstein secondary relaxation time, onset of the dynamic crossover, and the product of temperature and specific volume with the latter raised to a material constant—provided the structural relaxation time is maintained constant. Additional salient experimental findings include the correlation of various high-frequency processes, usually measured in the glassy state, with properties of the equilibrium material above Tg. These correlations indicate that the glass transition, although conventionally defined by the relaxation time becoming larger than experimental time scales (>100 s), has ...

Relaxation Patterns of Nearly Critical Gels

Abstract: We measure the complex rheological behavior of nearly critical gels and analyze the data by searching for characteristic patterns and abstracting those patterns into a self-consistent model. The sample is a linear, flexible, nearly monodisperse polybutadiene which gets cross-linked on its vinyl side groups. The dynamic mechanical storage and loss moduli of cross-linking polymers change smoothly during the liquid-solid transition, while equilibrium rheological properties (e.g., zero-shear viscosity and equilibrium compliance) diverge. During gelation, the relaxation occurs in a distinct pattern which can be described in a quantitative way with a minimum number of parameters. The pattern can be understood as a combination of the BSW spectrum (representing the precursor relaxation behavior) and the self- similar Chambon-Winter gel spectrum (modeling the terminal relaxation due to growing clusters). The spectrum is cut off at the material's longest relaxation time, Imax. Our model parameters, Imax and Ge (equilibrium modulus), exhibit characteristic scaling behavior with respect to the distance from the gel point,jp - pcj. The relaxation exponent, n, in the terminal zone is a function of the extent of reaction. Hence, dynamic scaling (requires constant n values) is not valid for our system. The proposed model passes the self-consistency test by predicting the mechanical behavior (at different frequencies) as a function of the extent of reaction and other rheological observations during the sol-gel transition.