Neutron quasielastic

The dynamics of bulk polymers have been approached in two different ways. On one hand, models of localized conformational jumps have been proposed to interpret numerous NMR experiments (see e.g. Ref. or . These models, which are specific of a given polymer assume that a short chain sequence performs conforpia-tional jumps between a few number of sites, the rest of the chain being immobile. Such localized jumps would lead to a well separated elastic peak in neutron quasielastic scattering experiments, in contradiction with all the experimental data obtained from polymer meltsIndeed, these models can in some cases be invoked to describe secondary relaxations in glassy polymers, but they are not sufficient to account for the numerous liquid-like properties of polymer melts. 

The results presented here show that neutron scattering is an invaluable tool for studying the important area of ions binding to aqueous polymer solutions. Neutron difference methods allow direct evidence polymer-ion binding to be observed. Further developments in neutron scattering techniques and instmmentation will lead to increased precision in the future. Quantitative neutron quasielastic measurements provide an additional useful tool allowing studies of the modifications of the water dynamics to be studied. 

NHT The barriers to rotation for the NH4 ion in the ordered and disordered phases of NH4C1 and NH4Br have been calculated from their torsional frequencies and the results compared with the activation energies determined from n.m.r. results.46 Neutron quasielastic scattering has been used to probe the reorientations of NH4 ion in a single crystal of ammonium bromide.47 A Raman study of the phase transitions in NH4Br has been undertaken over a range of temperature and pressure. It was found that the ordered phase On is cubic, and that in this phase the NHJ ions are ordered.48... 

Gamlen, P.H., Thomas, R.K., Trewern, T.D., et al. (1979). Structure and dynamics of ammonia adsorbed on graphitized carbon black. Part 3. Neutron quasielastic and inelastic spectra. J. Chem. Soc. Faraday Trans. 1, 75, 1553-69. 

In previous papers we have reported on some detailed studies on the behaviour of the order parameter C, (T) which was assimilated to the correlation length for critical fluctuations accessible by means of neutron small-angle scattering as well some measurements on the molecular dynamics across the two phase transitions as explored by neutron quasielastic scattering as well as computer simulations. 

Figure 13, Examples of neutron quasielastic spectra of acid Nafion membrane for...

Figure 14. Broadened component of neutron quasielastic spectra, experimental points and best fit Lorentzian line, a Nafion membrane containing 15 /o (/, (J — 0.59 A l. b Idem, but Q = 1.05 A-1, c Bulk pure Ht0 at 28° C, Q = 0.59 A . The small sharp component in Figure 14c comes from the quartz sample holder. Instrument IN5, incident wavelength 10 A. Temperature 250 C. Note the systematic deviation from the Lorentzian shape in Figures 14a and 14b.

Figure 15. Half width at half maximum of the broadened component of the neutron quasielastic spectra obtained with an acid Nafion membrane containing 15% H20 by weight. The points are the widths of the best fit Lorentzian lines from spectra obtained with an incident wavelength of 10 A (Q), 11 A and 13 A (A). The full line is the theoretical width predicted by the model with diffusion in a sphere (with D = 1.8 X 10 5 cm2/s and a = 4.25 A). The two theoretical asymptotes for Q O and Q 00 are also shown (compare with Figure 2 of Reference 2 for more details). Half width at half maximum of the best fit Lorentzian lines to the spectra obtained with bulk water at 28° C (incident wavelength 10 A) is denoted by +. The straight line passing through the points (+) is the theoretical width predicted by the simple self diffusion model with Dt = 2.5 X 10-5 err /s. Note the different vertical scales for the Nafion sample and the bulk water sample.

In conclusion, we mention that comparisons between pSR relaxation rates and neutron quasielastic linewidths have also been made (though less elaborate) for CeRu2Si2 and CePt2Sn2 as discussed earlier. 

The ordering of the transverse molecular axes, which occurs in certain low-temperature smectic phases, has been studied by NMR and NQR methods [7.40]. These measurements show that the uniaxial reorientation of the molecular cores around their long axes are strongly biased. It is generally assumed that in nematic and smectic A phases, the uniaxial rotation (7-motion) is not biased. However, recent neutron quasielastic scattering experiments [7.41] in the nematic phase of MBBA seem to support the notion that the rigid benzylideneaniline core is restricted to a uniaxial rotational diffusion of finite angular excursion. Restricted libration within 7 =z 00/2 for internal motions in macromolecules has been considered by London and Avitabile [7.42], and Wittebort and Szabo [7.43]. 

J. A. Janik, J. M. Janik, K. Otnes, K. Roscis-zewski, S. Wrobel, Estimation of rotational correlation times for PAA and MBBA by the dielectric relaxation and the neutron quasielastic scattering methods, Proc. Int. Conf Bangalore, Liq. Cryst. 1973, 253. 

Volino, F., Pineri, M., Dianoux, A.J., De Geyer, A. (1982) Water mobility in a water-soaked Nafion membrane a high-resolution neutron quasielastic study. Journal of Polymer Science Polymer Physics Edition, 20, 481-496. 

Direct experiment-simulation quasielastic neutron scattering comparisons have been perfonned for a variety of small molecule and polymeric systems, as described in detail in Refs. 4 and 18-21. The combination of simulation and neutron scattering in the analysis of internal motions in globular proteins was reviewed in 1991 [3] and 1997 [4]. 

A dynamic transition in the internal motions of proteins is seen with increasing temperamre [22]. The basic elements of this transition are reproduced by MD simulation [23]. As the temperature is increased, a transition from harmonic to anharmonic motion is seen, evidenced by a rapid increase in the atomic mean-square displacements. Comparison of simulation with quasielastic neutron scattering experiment has led to an interpretation of the dynamics involved in terms of rigid-body motions of the side chain atoms, in a way analogous to that shown above for the X-ray diffuse scattering [24]. 

M Bee. Quasielastic Neutron Scattering Principles and Applications m Solid State Chemistry, Biology and Materials Science. Philadelphia Adam Hilger, 1988. 

The Q and ft) dependence of neutron scattering structure factors contains infonnation on the geometry, amplitudes, and time scales of all the motions in which the scatterers participate that are resolved by the instrument. Motions that are slow relative to the time scale of the measurement give rise to a 8-function elastic peak at ft) = 0, whereas diffusive motions lead to quasielastic broadening of the central peak and vibrational motions attenuate the intensity of the spectrum. It is useful to express the structure factors in a form that permits the contributions from vibrational and diffusive motions to be isolated. Assuming that vibrational and diffusive motions are decoupled, we can write the measured structure factor as... 

Specification of. S SkCG, CO) requires models for the diffusive motions. Neutron scattering experiments on lipid bilayers and other disordered, condensed phase systems are often interpreted in terms of diffusive motions that give rise to an elastic line with a Q-dependent amplitude and a series of Lorentzian quasielastic lines with Q-dependent amplitudes and widths, i.e.. 

Figure 10 Elastic incoherent structure factors for lipid H atoms obtained from an MD simulation of a fully hydrated DPPC bilayer, and quasielastic neutron scattering experiments on DPPC bilayers at two hydration levels for (a) motion in the plane of the bilayer and (b) motion m the direction of the bilayer normal.

The magnetic interaction of neutrons with protons or deuterons of the CH3 (CD3) group leads to the appearance of peaks of quasielastic and inelastic scattering, corresponding to AE and EaEb... 

When the various results obtained by combined elastic and quasielastic neutron scattering measurements on star shaped polymers in dilute solutions... 

Springer T (1972) Quasielastic neutron scattering for the investigation of diffusive motions in solids and liquids. Springer, Berlin Heidelberg New York... 

Bee M (1988) Quasielastic neutron scattering. Adam Hilger, Bnstol... 

Polybutadiene in the Melt. A Quasielastic Neutron-Scattering Study. 

Phys. Condens. Matter, 15, S1127 (2003). Self-Motion and the a-Relaxation in Glass-Forming Polymers. Molecular Dynamic Simulation and Quasielastic Neutron Scattering Results in Polyisoprene. 

Macromolecules, 35, 7110 (2002). Segmental Dynamics of Atactic Polypropylene as Revealed by Molecular Simulations and Quasielastic Neutron Scattering. 

Lateral diffusion of phospholipids in model membranes at ambient pressure has been studied over the years by a variety of techniques including fluorescence recovery after photobleaching (FRAP), spin-label ESR, pulse field gradient NMR (PFG-NMR), quasielastic neutron scattering (QENS), excimer fluorescence and others.In general, the values reported for the lateral diffusion coefficient (D) range from 10 to 10 cm /s in the... 

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