Small angle neutron scattering Subject

This state of partial deformation has been determined recently by small angle neutron scattering [194] and has been a favorite subject of investigation in the... 

The conformation of polymer chains in an ultra-thin film has been an attractive subject in the field of polymer physics. The chain conformation has been extensively discussed theoretically and experimentally [6-11] however, the experimental technique to study an ultra-thin film is limited because it is difficult to obtain a signal from a specimen due to the low sample volume. The conformation of polymer chains in an ultra-thin film has been examined by small angle neutron scattering (SANS), and contradictory results have been reported. With decreasing film thickness, the radius of gyration, Rg, parallel to the film plane increases when the thickness is less than the unperturbed chain dimension in the bulk state [12-14]. On the other hand, Jones et al. reported that a polystyrene chain in an ultra-thin film takes a Gaussian conformation with a similar in-plane Rg to that in the bulk state [15, 16]. 

As explained in Sec. 7.1, epoxy networks have been and are still the subject of controversy. This is mainly based on the particular interpretation of results obtained using microscopy techniques. On the contrary, results obtained with small-angle neutron scattering (SANS) proved that typical diepoxy-diamine networks were homogeneous (Wu and Bauer, 1985). 

Fig. 1.17 Small-angle neutron scattering pattern for a sample of per-deuterated lightly branched polyethylene containing 10% of hydrogen containing linear polyethylene. The sample is molten and has just been subjected to deformation in a channel die. The flow axis is horizontal and the effective extension ratio is 3.

Harrison et al. reported the first w/c microemulsion in 1994 (20). A hybrid surfactant, namely F7H7, made of respectively one hydrocarbon and one fluorocarbon chain attached onto the same sulfate head group, was able to stabilize a w/c microemulsion at 35 and 262 bar. For a surfactant concentration of 1.9 wt %, water up to a w = 32 value ([water]/surfactant]) could be dispersed. A spherical micellar structure was confirmed by small-angle neutron scattering (SANS) experiments (21). This surfactant was later the subject of dynamic molecular simulations (22, 23). The calculations were consistent with the SANS data and high diffusivity was predicted, highlighting this important feature of low-density and low-viscosity supercritical fluids (SCF). 

Alternatively, SPBs can be prepared by the dissolution of block copolymers in suitable solvents. " This leads to the formation of micelles with a hydrophobic core and a hydrophilic shell, which show a similar structure to SPB. This technique has been extensively investigated in the literature because the self-assembly of block copolymers is an interesting subject in itself. It should be noted, however, that the micelles have considerable core polydispersity, which can be direaly assessed using small-angle neutron scattering (SANS). The finite breadth of the size distribution renders the analysis of the various properties, for example, the flow behavior, more difScult. Moreover, the interface between the hydrophobic core and the hydrophilic shell cannot be very sharp because the micelles result from self-assembly. 

The outstanding problems central to polymer crystallization is the nature of the chain conformation at the surface of a polymer crystal and melt-crystallized lamellae. This is central to the development of the subject and certainly the use of small-angle neutron scattering (SANS) has gone some way to resolving some of the issues. We look forward to see what the next 2 years will bring in this area. 

Nafion has been the subject of extensive characterization studies [1]. Its microstructure has been exhaustively studied by scattering methods, especially small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) [19-23]. Mechanical properties of Nafion as functions of temperature have been used to identify temperature-induced transitions [2,24—27]. Transport of protons and water has also been the subject of numerous studies over the past 25 years [28-34]. However, there has been limited progress in coimecting the chemical structure of Nafion to mechanical and transport properties, especially how these properties are altered due to environmental conditions. In this chapter, we will review recent studies of mechanical and transport properties of Nafion done under controlled conditions of water activity and temperature. 

The many geometrical and dynamic structural aspects that make surfactant self-assembly such a fascinating subject have been discovered because of the development of sophisticated analytical techniques such as small-angle neutron or X-ray scattering and nuclear magnetic resonance [190]. 

The nature of the bottlenecks for proton conductance in the dry membrane state or on the way to it is, however, still the subject of debates. This wiU only be resolved after more detailed experimental studies (of macroscopic transport parameters such as proton conductance and electro-osmotic coefficients as a function of water content, or gas and liquid permeability before and after operation, and of microscopic structural probes such as small-angle neutron and X-ray scattering) will have discriminated between competing models. By and large, the direction of effects that go with dehydration is obvious enough to be introduced into phenomenological models of overall cell performance. 

Most of the research effort on the ionomers has been devoted to only a small number of materials, notably the ethylenes the styrenes, the rubbers(9)5 and those based on poly(tetra-fluoroethylene), the last of which is the subject of the present volume. As a result of these extensive investigations, it has become clear that the reason for the dramatic effects which are obsverved on ion incorporation is, not unexpectedly, the aggregation of ionic groups in media of low dielectric constant. Small angle X-ray and neutron scattering, backed up by a wide range of other techniques, have demonstrated clearly the existence of ionic... 

Fig. 6. Principle of pinhole SANS a neutron beam entering the instrument from the left is the subject of monochromation (done by the velocity selector), coUimation over a variable distance (achieved by using a set of adaptive system of apertures), scattering on the sample and detection on a two-dimensional position sensitive detector over long distances airning for detecting neutrons at small scattering angles the SANS instruments are very long.

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