No abstract

Magnetic field cycling in nuclear magnetic resonance (NMR) experiments has been used since the early days of NMR. Originally such time-dependent magnetic field experiments were motivated to study cross relaxation, spin system thermodynamics and indirect detection of quadrupolar resonance. The first apparatus used mechanical or pneumatic systems to shoot the sample between two magnets, the typical "flying time" being a few hundreds of milliseconds. As a natural evolution of the experimental technique and the need to extend its application to samples with higher relaxation rates, faster magnetic field switching devices were developed during the last years. Special electric networks combined with sophisticated air core magnets allowed one to switch magnetic fields between zero and fields of the order of 0.5 T in a few milliseconds. Today we refer to this new generation of instruments as "fast-field-cycling" devices. The technique has been successfully used during the last years to obtain information on the molecular dynamics and order in different materials, ranging from organic solids, metals, polymers, liquid crystals, porous media to biological systems. At present it is also turning to be an important tool for the design of contrast agents for magnetic resonance imaging. Fast field cycling was mainly oriented to T, relaxometry as a unique technique offering a dynamic window of several decades, ranging from few kilohertz to several megahertz. However, there exist less conventional applications of the technique that can also provide relevant information concerning molecular dynamics, structure and molecular order. In this article we will briefly deal with basic aspects of the technique, its evolution, present-day relevant applications and the last improvements concerning specialized instrumentation.

For Abstract see ChemInform Abstract in Full Text.

No abstract

Linear polyethylene oxides with molecular weights M~ of 1665 and 10170 confined in pores with variable diameters in a solid methacrylate matrix were studied by proton field-cycting nuclear magnetic resonance relaxometry. The pote diameter was varied in the range of 9-57 nm. In all cases, the spin-lattice relaxation time shows a frequency dependence close to T~ ce v ~.4 in the range of v= 3. l0 -L-2.10 ~ MHz as predicted by the tube-reptation model. This proton T r disoersion essentially reproduces that found in a previous deuteron ~~udy (R. Kimmich, R.-O. Seirter, U. Beginn, M. M611er, N. Fatkullin: Chem. Phys. Lett. 307, 147, 1999). Asa feature particularly chamcteristic fbr reptation, this finding suggests that reptation is the dominating chain dynamics mechanism under pore confinement in the corresponding time range. The absolute values of the spin-lattice relaxation times indieate that the diameter of the effective tubes in which reptation occurs is much smaller than the pote diameters on the time scale of spin-lattice rela.xation experiments. An estimation leads to a value d" ~ 0.5 nm. The impenetrability of the solid pore walls, the uncrossability of polymer chains ("excluded volume") and the low value of the compressibi[[ty in polymer melts ereate the "eorset et'fect" which reduces the lateral motions of polymer chains to a microscopic scale of only a few tenths of a nanometer. i lntroductionIn previous papers [1-3] it was shown that polymer melts confined in porous media ate subject to chain dynamics considembly deviating from the behavior in bulk. These studies were performed by field-cycling nuclear magnetic resonance (NMR) relaxometry and field-gradient NMR diffusometry in the fringefield variant [4,5]. It tumed out that the dynamical limits predicted for the tubereptation model [6,7] can be well reproduced with experiments in polymer melts confined in nanoporous solid matrices. The motivation of the present work is the attempt to detect experimentally the crossover from confined to bulk behavior by varying the pore diameter, or more precisely, to probe the length scale