Deuterium, neutron scattering

Two of the most important functions in the application of neutron scattering are the use of deuterium labelling for the study of molecular confomiation in the bulk state and the use of deuterium solvent in polymer solutions. In the following, we will consider several different applications of die general fomuda to deuteration. 

Two physically reasonable but quite different models have been used to describe the internal motions of lipid molecules observed by neutron scattering. In the first the protons are assumed to undergo diffusion in a sphere [63]. The radius of the sphere is allowed to be different for different protons. Although the results do not seem to be sensitive to the details of the variation in the sphere radii, it is necessary to have a range of sphere volumes, with the largest volume for methylene groups near the ends of the hydrocarbon chains in the middle of the bilayer and the smallest for the methylenes at the tops of the chains, closest to the bilayer surface. This is consistent with the behavior of the carbon-deuterium order parameters,. S cd, measured by deuterium NMR ... 

The best insight into the relaxation behavior of star polymers in dilute solution can be expected if, in addition to the whole star system, different parts of the star are considered separately. This can be achieved easily by neutron scattering techniques on systems where not only the entity of arms, but also single arms, the core or shell parts are labelled by proton deuterium exchange. With respect to the core-shell labelling it is convenient to build up the arms as diblock copolymers of A-B type with protonated or deuterated but otherwise chemically identical A and B blocks. 

The size and shape of polymer chains joined in a crosslinked matrix can be measured in a small angle neutron scattering (SANS) experiment. This is a-chieved by labelling a small fraction of the prepolymer with deuterium to contrast strongly with the ordinary hydrogenous substance. The deformation of the polymer chains upon swelling or stretching of the network can also be determined and the results compared with predictions from the theory of rubber elasticity. 

The fact that neutron scattering factors are similar for all elements means that light atoms scatter neutrons as effectively as heavy atoms and can therefore be located in the crystal structure for example the X-ray scattering factors for deuterium and tungsten are 1 and 74, respectively, whereas the equivalent neutron values are 0.667 and 0.486. This... 

The opposite signs for the neutron scattering power of hydrogen and deuterium (—0.38 and +0.65) offers the possibility for investigating (slow) self-diffusion between different water sites and/or localization of water molecules with different mobility if diffraction experiments are carried out for a sample where D20 is exchanged in steps vs. H20. 

For a given network the intercrosslink distance d should be proportional to G1/3. This is exactly what was found by neutron scattering on networks with deuterium-labelled nodules. In particular dc is the distance between neighbor crosslinks in the nascent network, the swelling ratio being Qc (Fig. 2). 

Neutron scattering is also useful for the study of adsorbed material on hydrosol particles. The neutron scattering length densities of hydrogen and deuterium differ considerably. By preparing a hydrosol in an appropriate H20/D20 mixture, it is possible to match the neutron scattering length densities of the dispersion medium and the core particles. The neutron beam thus sees only the adsorbed layer, the thickness of which can be estimated. Alternatively, the dispersion medium can be matched to the adsorbed layer to permit estimation of the core-particle size. 

Fig. 4 Paths to integer folded forms in long alkanes. While solution crystallization gives integer forms (E, F2,...) directly, for melt crystallization this is true only for the E form. Below the melting point of F2, the transient non-integer form (NIF) appears and subsequently transforms isothermally either to the once-folded F2 form or the mixed-integer folded-extended (FE) form, based on layer triplets. Deuterium- labelled chain ends, prepared for neutron scattering experiments, are indicated with circles...

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