The anisotropy and pulse frequency dispersion of the spin–spin relaxation time TCP2E from Carr–Purcell–Meiboom–Gill pulse sequences is employed to evaluate the major contribution to transverse 2H spin relaxation in bilayer membranes. Analysis of the experiments is achieved in terms of a density operator formalism, employing the stochastic Liouville approach. From a comparison of the observed angular and frequency dependences of TCP2E with those predicted for order director fluctuations, we conclude that collective lipid motions constitute the dominant transverse relaxation process. Computer simulations provide the viscoelastic parameters of the lipid membranes. For 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayers at T=318 K an average elastic constant of K=2×10−11 N and an effective viscosity of η=0.1 P have been determined. Using the experimentally accessible value for the long wavelength cutoff of the elastic modes, one obtains the mean square amplitude of the director fluctuations 〈θ20〉=0.04. This corresponds to an order parameter of SOF=0.94. Apparently, the contributions of the collective motions to the measured order parameters are marginal.

No abstract

Molecular dynamics of a main chain thermotropic liquid crystal polymer in the smectic A phase has been investigated using multipulse dynamic nuclear magnetic resonance (NMR) techniques. Transverse deuteron spin relaxation times T~; from quadrupole echo pulse trains (modified Carr-Purcell-Meiboom-Gill sequence) measured for deuterons in the aromatic rings of the mesogenic units are obtained as a function of pulse spacing 7 sample orientation eN' and temperature. Just below the. nematic-smectic A phase transition, the relaxation times exhibit a linear dispersion regime ~; -7-1 consistent with smectic director fluctuations. At lower temperatures, the dispersion step gradually disappears, indicating that faster molecular motions are the dominant transverse relaxation process. The observed anisotropy in ~;, measured at short pulse spacings, approximately follows the (sin 4 e N) -I dependence expected for axial diffusion in a highly ordered medium. Analysis of the experiments is achieved employing a density operator treatment based on the stochastic Liouville equation. The intramolecular motion is identified with phenyl ring flips and is the fastest process studied, with correlation times varying from 10-10 to 10-7 s over the temperature range investigated. Intermolecular (individual molecule) dynamics are somewhat slower and have been interpreted as rotational diffusion in an orienting potential. The correlation times for intermolecular motion exhibit non-Arrhenius behavior approaching the glass transition, following a temperature dependence described by the Williams-Landel-Ferry equation over six orders of magnitude. This result indicates a strong coupling of the intermolecular motion to the glass transition process. The slowest motion affecting transverse deuteron spin relaxation is assigned to smectic director fluctuations or undulation waves. Analysis of the ~; dispersion yields information concerning the viscoelastic properties of the polymer. At T=418 K, a splay elastic constant of KI =2X lO-11 N has been estimated. Using the experimentally accessible value for the long wavelength cutoff of the elastic modes, the root mean square fluctuation 1/2 of the director is calculated to be 4°.

Three magnetometers based on dc superconducting quantum interference devices (SQUIDs) fabricated from YBa2Cu3O7−x have been operated in a magnetically shielded room using a flux-locked loop involving additional positive feedback with bias current reversal. Two of these devices, integrated multiloop dc SQUIDs with outer diameters of 7 mm, achieved white noise levels of 10 fT/√Hz for bicrystal junctions and 30 fT/√Hz for step-edge junctions. The third magnetometer involved a flux transformer with a 10×10 mm2 pickup coil connected to a 16-turn input coil which was inductively coupled to a bicrystal SQUID. This device achieved a white noise of 16.2 fT/√Hz. High quality magnetocardiograms were obtained without signal averaging.

No abstract