Liquid crystals Neutron scattering

Hamley I W, Garnett S, Luckhurst G R, Roskilly S J, Pedersen J S, Richardson R M and Seddon J M 1996 Orientational ordering in the nematic phase of a thermotropic liquid crystal A small angle neutron scattering study J. Chem. Phys. 104 10 046-54... 

Liquids are difficult to model because, on the one hand, many-body interactions are complicated on the other hand, liquids lack the symmetry of crystals which makes many-body systems tractable [364, 376, 94]. No rigorous solutions currently exist for the many-body problem of the liquid state. Yet the molecular properties of liquids are important for example, most chemistry involves solutions of one kind or another. Significant advances have recently been made through the use of spectroscopy (i.e., infrared, Raman, neutron scattering, nuclear magnetic resonance, dielectric relaxation, etc.) and associated time correlation functions of molecular properties. 

We have been particularly interested in the study of the plastic states of organic compounds,41 which are characterized by high values of AS of formation from the crystalline state, the AS of fusion (plastic-liquid transition) being much smaller. We find that the AH as well as the AS of the crystal-plastic transition generally decrease as the temperature range of stability of the plastic phase increases the AH and the AS of the plastic-liquid transition, on the other hand, increase as the temperature range of stability of the plastic phase increases.42 Neutron scattering, NMR spectroscopy, and several other techniques have been employed to study molecular reorientation in the plastic state.41 We... 

Some indirect experimental evidence exists for the liquid-liquid critical point hypothesis from the changing slope of the melting curves, which was observed for different ice polymorphs (30, 31). A more direct route to the deeply supercooled region, by confining water in nanopores to avoid crystallization, has been used more recently by experimentalists. These researchers applied neutron-scattering, dielectric, and NMR-relaxation measurements (32-35). These studies focus on the dynamic properties and will be discussed later. They indicate a continuous transition from the high to the low-density liquid at ambient pressure. The absence of a discontinuity in this case could be explained by a shift of the second critical point to positive pressures in the confinement. This finding correlated with simulations, which yield such a shift when water is confined in a hydrophilic nanopore (36). 

A variety of experimental techniques have been used to investigate IL structures. Neutron diffraction, X-ray scattering, extended X-ray absorption fine structure (EXAFS) [97-105], and theoretical methods [64, 106-111] are probably the most powerful approaches to quantify IL structure and interactions. For example, X-ray diffraction and absorption studies along with computer simulations have shown that ILs are by no means molecular liquids [64, 89,94,112,113], Unlike a conventional solvent like hexane or chloroform, ILs contain a large number of internal interfaces and exhibit different degrees of order [89, 114], ILs can form liquid crystals, extended hydrogen-bonded networks, inclusion compounds, or microphase separated structures, where polar and non-polar regions are separated by a complex interface [86, 89, 114],... 

With one component systems like plastic crystals [14] or liquid crystals [f4, 15] the experimental situation is rather simple since only the nuclei of interest contribute to the spectra With a composite system like water absorbed in a membrane, it is important to be able to separate the contribution to the scattered intensity from the water, and from the polymer. A useful indication on the relative intensities of these contributions may be obtained by comparison between the neutron diffraction patterns of the wet and dry membranes. This is because the intensity of the diffraction curve at a given Q value is approximately the integral over the energy of the NQES spectrum at the same Q. This is why it is often useful to make a neutron diffraction experiment before performing the quasielastic study. 

In the latter two phases backbones have the spindle-like conformation, i.e., the prolate shape with (R%) > R p), the characteristic of main chain liquid crystalline polymers. Important means of investigating the conformations of side chain liquid crystalline polymers include small angle neutron scattering from deuterium-labeled chains (Kirst Ohm, 1985), or small angle X-ray scattering on side chain liquid crystalline polymers in a small molecular mass liquid crystal solvent (Mattossi et al., 1986), deuterium nuclear resonance (Boeffel et al., 1986), the stress- or electro-optical measurements on crosslinked side chain liquid crystalline polymers (Mitchell et al., 1992), etc. Actually, the nematic (or smectic modifications) phases of the side chain liquid crystalline polymers have been substantially observed by experiments. 

Richardson R M 1998 Neutron scattering Handbook of Liquid Crystals Vol 1. Fundamentals ed D Demus, J Goodby, G W Gray, H-W Spiess and V Vill (New York Wiley-VCH)... 

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