Scattering of Light, X-Rays, and Neutrons

Static and dynamic light scattering have been used to investigate PDMS in both liquid and supercritical carhon dioxide. The solvent quality of the COj was found to be adjustable by independently varying temperature or density. The results give the theta temperature and strength of excluded volume interactions.  

Small-angle scattering techniques have been applied to polysiloxane materials. One important example is the characterization of fillers introduced into polysiloxane elastomers, or the reverse, the incorporation of such elastomers into ceramic matrices (in both cases to improve mechanical properties). - Another example is characterization of the anisotropy induced by strain in silica-PDMS composites. Chapter 9 describes some of this work. Elastic neutron scattering can be illustrated by the characterization of polysiloxane blends, and quasielastic neutron scattering by studies of the dynamics of PDMS. There have also been Monte Carlo calculations of PDMS particle scattering functions, including how they varied with chain length, chain structure, and temperature.  

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All scattering phenomena (light, x-rays and neutrons) can be interpreted in tenns of this equation (B1.9.5). 

All scattering phenomena (light, x-rays and neutrons) can be interpreted in terms of this equation (B 1.9.5). These techniques differ mainly in the structural entities that contribute to the Kj term. For light, the refractive index or polarizability is the principal contributor for x-rays, the electron density is the contributor and for neutrons, the nature of the scattering nucleus is the contributor. Equation (B1.9.5) thus represents a starting point for the discussion of the interference problem presented below. 

All three types of radiation scattering techniques light. X-ray and neutrons, have been used to measure the interaction parameters and study the phase equilibria. Using the relations derived for polymer solutions (see Eqs 2.31-2.36), the methods can be used to measure M, < s and Aj, thus Xj2 ... 

Light-scattering including SAXS and SANS" Intensity of scattered visible light, X-rays and neutrons 1 X lO""... 

While rheological measurements are wonderfully quantitative, they are usually poor qualitative probes of fluid structure. This is because in rheological experiments, the structural changes responsible for the measured relaxation behavior remain hidden. Thus, rheometry is often most useful when supplemented by other experimental methods that characterize fluid structure and flow-induced structural changes. Some of the most useful methods are microscopy, light, x-ray, and neutron scattering, and polarimetry. 

Scattering methods are among the most powerful and widely used in the study of complex fluids. Light, x-rays, and neutrons can be scattered (Lindner and Zemb 1991). For light scattering, length scales of 2000 A to 100 p,m are probed, while both x-rays and neutrons can reach smaller length scales, from 10 to 1000 A for x rays and from 10 to 200 A for neutrons. 

On a molecular scale liquid surfaces are not flat, but subject to Jluctuations. These irregularities have a stochastic nature, meaning that no external force is needed to create them, that they cannot be used to perform work and are devoid of order. Their properties can only be described by statistical means as explained in sec. 1.3.7. Surface fluctuations are also known as thermal ripples, or thermal waves, in distinction to mechanically created waves that will be discussed in detail in sec. 3.6. Except near the critical point, the amplitudes of these fluctuations are small, in the order of 1 nm, but they can, in principle, be measured by the scattering of optical light. X-ray and neutron beams. From the scattered intensity the root mean square amplitude can be derived and this quantity can, in turn, be related to the surface tension because this tension opposes the fluctuations ). 

Scattering techniques, such as light, x-ray, and neutron scattering, are extensively used to investigate various structural features of polymeric systems, including crystalline order and crystallinity, conformation, local structure, domain sizes, etc. 

Scattering Techniques. The thickness of adsorbed layers can be determined using light. X-ray, and neutron scattering techniques (3). Quasi-elastic light scattering measurements on dispersion of particles measure the diffusion coefficients of particles in the dispersion. The size of these particles is related to the diffusion coefficient by the Stokes-Einstein equation. This measurement is performed on the dispersion with and without the adsorbed polymer. The difference in the radii in the two measurements is the thickness of the adsorbed layer. In these measurements it should be ensured that the particles in the dispersion are monodisperse and that there is no flocculation. In X-ray and neutron... 

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