Microstructures/microstructured materials Spectroscopic methods

NMR is not, of course, the only analytical technique used to establish the composition and microstructure of polymeric materials. Others include >66 ultraviolet-visible spectroscopy (UV-Vis), Raman spectroscopy, and infrared (IR) spectroscopy. IR and Raman spectroscopy are particularly useful, when by virtue of cross-linking (see. e.g. Chapter 9), or the presence of rigid aromatic units (see Chapter 4). the material neither melts nor dissolves in any solvent suitable for NMR. The development of microscopy based on these spectroscopic methods now makes such analysis relatively simple (see below). Space precludes a detailed account of these and many other techniques familiar to the organic chemist. Instead we focus for the remainder of the chapter on some of the techniques used to characterize the physical properties of polymeric materials. 

Molecular weight distribution information obtained by size-exclusion chromatography on its own is insufficient to characterize the properties of complex polymers, such as copolymers and block and graft polymers [23,514,524]. For these polymers the chemical composition and functionality type distributions are equally important. A major obstacle to the characterization of these materials is that their molecular properties are present as joint distributions. Unlike the mass distribution the composition and functionality distributions can only be determined by separation methods that employ interactions with the stationary phase. To fully characterize a complex polymer it is not unusual to use manual or automated tandem techniques where the sample is fractionated according to its chemical or end group composition for subsequent further separation by size-exclusion chromatography to establish their mass distribution. Chromatographic methods may also be combined with spectroscopic methods to determine microstructural information. 

Although much has been learned about the structures of polysilylenes, a tremendous amount of work remains before a full understanding of these materials is developed. The microstructure of the polymers can be studied directly by solution NMR spectroscopic techniques. The determination of the chain conformation in solution is diflScult, particularly at low temperature. Light-scattering techniques may be able to establish the solution dimensions of the polysilylenes through the low-temperature thermochromic transition. The chain conformation in the solid state can be established by X-ray and electron difiraction methods. Solid-state Si NMR spectroscopy can become... 

This review of physical, bulk chemical, surface chemical, and spectroscopic techniques for the characterization of powders shows that no one or two techniques can provide all the necessary details regarding powder properties. In fact, each method reveals a distinctly different characteristic of the powder. Often, a choice must be made in terms of the selection and measurement of relevant properties of a powder. When one is faced with this question of which specific properties to measure, the relationships between powder properties and their effect on the final microstructure and properties of the ceramic should be explored. Currently, the quality and reproducibility of data are significantly affected by the unavailability of standard methods and standard reference materials. Efforts are underway in different organizations around the world to alleviate this problem. 

Fluoropolymers can take on an amazing variety of characteristics depending on the details of their molecular structures. Modem methods of polymer synthesis have been adapted to provide tremendous flexibility in designing fluoropolymer structures so that materials can be prepared for a variety of applications. Understanding the microstructures of these polymers is essential to probe their structure-property relationships and to improve the overall performance of fluoropolymer materials. Modern spectroscopic tools are sorely needed to keep up with the requirements to characterize fluoropolymers so that proof of preparation of the desired structures is obtained and the quantities of the desired stmctural elements can be measured. 

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