Structural Studies with Small Angle Scattering

STRUCTURAL STUDIES WITH SMALL ANGLE SCATTERING 3-1. Basic Principles... 

Therefore, ionomer solutions cannot be treated as a whole and cannot even be divided into only two different problems the ionomer (or associating) behavior in nonpolar solvents and the polyelectrolyte behavior in polar solvents. A new structural problem will probably arise with most of the ionomers even if some of the solution properties appear to be similar. A lot of structural studies including small angle scattering and electron microscopy experiments will be necessary to understand the structure of ionomer solutions and such studies will be very helpful to understand the structure-properties relationships of ionic polymers. 

The chemistry of hydrolysis and condensation of silicon alkoxides is now understood in considerable detail, as indicated in the chapter by Coltrain and Kelts (Eastman Kodak Co.). Extensive use of nuclear magnetic resonance has revealed the influence of factors such as pH on the kinetics of the competing reactions. With this information it is possible to rationalize the structures of the aggregates, as revealed by studies of small-angle scattering of X-rays and neutrons. This level of understanding opens the possibility for deliberate control of gel structure and properties. Nonsilicate systems have received less... 

High resolution multidimensional NMR experiments can provide the dendrimer chemist with a wealth of additional information extending far beyond the determination of the molecular structure. In the interpretation of (2D)-NOESY (NOESY=nuclear Overhauser enhancement spectroscopy) spectra, a knowledge of the spatial interrelationships between protons in different parts of the dendrimer scaffold can be acquired from proton-proton NOE interactions. At the same time, the prevailing conformation of the dendritic branches in the solvent used can be deduced from this information. Furthermore, studies of dendrimer/sol-vent interactions and the influence of solvent on the spatial structure of the dendrimer are also possible [22]. Thus the information content of such NMR experiments resembles that of small-angle scattering experiments on dissolved dendrimers (see Section 7.6). 

In the present work the meso- and macro-structural characteristics of the mesoporous adsorbent MCM-41 have been estimated with the help of various techniques. The structure is found to comprise four different length scales that of the mesopores, the crystaUites, the grains and of the particles. It was found that the surface area estimated by the use of small angle scattering techniques is higher, while that estimated by mercury porosimetry is much lower, than that obtained from gas adsorption methods. Based on the macropore characterization by mercury porosimetry, and the considerable macropore area determined, it is seen that the actual mesopore area of MCM-41 may be significantly lower than the BET area. TEM studies indicated that MCM-41 does not have an ideal mesopore structure however, it may still be treated as a model mesoporous material for gas adsorption studies because of the large radius of curvature of the channels. 

A model of structure has been developped from electron microscopy and X-ray studies in Nafions (6). No basic difference exists between X-ray and neutron techniques. Nuclear interactions of neutrons with matter are characterized by the coherent scattering length and the corresponding values for H and D are very different Because of the different origins of the contrast, X-ray and neutron small angle scattering techniques are complementary. 

Fig. la-d. Small-angle scattering from a dilute, random dispersion of membranes (vesicles), a corrected intensities or thickness factor obtained from the experimental intensity distribution 1(h) by multiplication with h. b Structure factor (amplitude function) with arbitrarily chosen signs (-k,—, +, —,).c Autocorrelation function of the electron density q(x) profile across the membrane obtained by cosine transformation of I,(h) (Eq. 5a) the insert shows the profile obtained by de-convolution. d Centrosymmetric electron density profile obtained by cosine transformation (Eq. 5b) of F,(h). From a study on lipoprotein X, an assembly of unilamellar vesicles (Ref. 84, with permission)... 

The periodic systems that are subjected to small-angle scattering studies are mostly of lamellar morphology, as in folded-chain lamellar crystals, membrane structures, and block copolymers with lamellar ordering. We will therefore confine our discussion to lamellar systems only and discuss them in more detail in the rest of this section. 

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