X-ray and neutron reflectivity

The reflectivity is defined as the ratio of the intensities of the reflected and incident beams and should be differentiated from the reflectance which is the ratio of the amplitudes of the incident and reflected waves. The reflectance in general is a complex number because there is usually a change in phase of a wave on reflection whereas reflectivity is a real number varying from zero to unity. The specular reflection can provide information on the composition distribution normal to the surface. The reflectivity is a function of both the angle of incidence of the beam to the surface and the refractive index changes of the substrate. The reflectivity is a function of the length scale of interactions of 

At the interface between two dissimilar materials the grazing angle of incidence, is related to the angle of reflection, 0i, by 

When n is less than 1 there will exist a critical angle above which the angle of reflection will be real for all ineident angles. At this critical angle of incidence. 

In the ease of neutron reflectivity the cos 0c term will be small allowing simplification of eqn (9.24) to give 

For a smooth surface the components of the incident beam s wave vector normal to the surface are which is equal to (27i/2)sin 0q in air and in the polymer is which can also be written as (A — where k  

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The inner structure of polyelectrolyte multilayer films has been studied by neutron and X-ray reflectivity experiments by intercalating deuterated PSS into a nondeut-erated PSS/PAH assembly [94, 99]. An important lesson from these experiments is that polyelectrolytes in PEMs do not present well-defined layers but are rather interpenetrated or fussy systems. As a consequence, polyelectrolyte chains deposited in an adsorption step are intertwined with those deposited in the three or four previous adsorption cycles. When polyelectrolyte mobility is increased by immersion in NaCl 0.8 M, the interpenetration increases with time as the system evolves towards a fully mixed state in order to maximize its entropy ]100]. From the point of view of redox PEMs, polyelectrolyte interpenetration is advantageous in the sense that two layers of a redox polyelectrolyte can be in electrochemical contact even if they are separated by one or more layers of an electroinactive poly ion. For example, electrical connectivity between a layer of a redox polymer and the electrode is maintained even when separated by up to 2.5 insulating bUayers [67, 101-103]. 

Neutron and X-ray reflectivity studies of adsorbed block copolymers... 

Numerous techniques have been employed to examine the monolayer structure of phospholipids at the air/water interface including surface tension, fluorescence, neutron and X-ray reflection, and IR and Raman spectroscopy. In contrast, very few techniques are suitable to examine monolayers at the oil/water interface. Surface tension and fluorescence microscopy [46-48] have shed some light on these buried monolayers, but most other surface techniques are hampered because of effects from the bulk liquids. Since VSFS is insensitive to the bulk, it is an excellent technique for probing these monolayers. 

The neutron and x-ray reflectivity analyses are resonably consistent and indicate that the chain is fully extended and perpendicular to the surface at... 

J. Schmitt, T. Grunewald, G. Decher, P.S. Pershan, J. Kjar and M. Losche, Internal structure of layer-by-layer adsorbed polyelectrolyte films a neutron and X-ray reflectivity study, Macromolecules, 1993, 26, 7058 D.M. Kaschak, J.T. Lean, C.C. Waraksa, G.B. Saupe, H. Usami and T.E. Mallouk, Photoinduced energy and electron transfer reactions in lamellar polyanion/polycation thin films, J. Am. Chem. 

There have not been many systematic studies of the degree to which the surface of a polymer is diffuse on this scale, although neutron and x-ray reflectivity measurements are sensitive enough to provide this information. What observations exist are consistent with an interface whose width is about 5... 

Reflection of waves from interfaces 3.1.1 Neutron and x-ray reflectivity... 

Schmitt, J., et al.. Internal structure of layer-by-layer adsorbed polyelectrolyte films a neutron and X-ray reflectivity study. Macromolecules, 1993. 26 p. 7058-7063. 

Lodge [102] and Norman [103] reviewed the use of scattering techniques in the characterization of polymers, particularly block copolymers. Static and dynamic light scattering, small-angle neutron and X-ray reflectivity were analyzed and emphasis was placed on their similarities and differences. 

The benefits of neutron and X-ray reflectivity studies for polymer films were summarized in the early 1990s. At that time, it was already evident that the surface and interfaces of polymer layers can be tailored to obtain funaional surfaces and interfaces and to obtain composites with functional fillers with desired properties. Creating a funaional surface implies the formation of new surface properties and/or of new internal interfaces. So, for example, the incorporation of a filler of nanopartides into a polymer multilayer imports magnetic properties, creates new interfaces, and modifies the chararter-istics of the polymer-polymer interfaces. 

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