Structural Issues in Materials Chemistry

Historically, artificial polymers such as polypropylene and nylon have been made in a relatively uncontrolled fashion. A solution of monomers is prepared, and then a reaction is initiated which links monomers together to produce an ever growing chain. In such systems, it will always be true that the final material will consist of a range of molecular weights. We need a way to describe this distribution of molecular weights. 

Number Average and Weight Average Molecular Weights—M andM  

Two common representations of the molecular weight of a polymer are the number average molecular weight (M ) and the weight average molecular weight (M ), which are de- 

Why should we be concerned with two different definitions of molecular weight It is easiest to see with an example. Consider a polymer sample that consists of 10 molecules (a small sample ) with molecular weights of 10,000, 11,000, 12,000, 13,000, 14,000, 15,000, 

M is now 8,500, yetM is 17,100. The value of has changed much less than that 

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An incredibly important issue in polymer chemistry is processing, the manner in which the crude polymer product is transformed into the final material used in applications. This is a topic best reserved for a chemical engineering textbook, but it should not be dismissed here, either. As an example, poly(ethylene terephthalate), which is considered the workhorse of the polyester industry, is used as both a fiber and a plaster under the trade names of Dacron and Mylar, respectively (look ahead to Figure 13.15 to see the structures). The processing procedure leads to the different properties and the ultimate applications. 

Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. From both economic and practical viewpoints, the advantages of radical over other forms of polymerization arc many (Chapter 1). However, one of the often-cited "problems" with radical polymerization is a perceived lack of control over the process the inability to precisely control molecular weight and distribution, limited capacity to make complex architectures and the range of undefined defect structures and other forms of "structure irregularity" that may be present in polymers prepared by this mechanism. Much research has been directed at providing answers for problems of this nature. In this, and in the subsequent chapter, we detail the current status of the efforts to redress these issues. In this chapter, wc focus on how to achieve control by appropriate selection of the reaction conditions in conventional radical polymerization. 

A number of reviews can be consulted for an introduction to the fundamentals both theoretical and practical covering XPS. These include Riggs and Parker (2) and the book by Carlson (3). Electron spectroscopy is reviewed in alternate years in the Fundamental Reviews issue of Analytical Chemistry. The last literature review was published in 1980 (4) and this and previous reviews can be consulted for a coverage of all aspects of the literature of XPS. A number of recent symposia have been held on applications of surface analytical methods in various aspects of materials science such as the symposium on characterization of molecular structures of polymers by photon, electron, and ion probes at the March 1980 American Chemical Society meetings in Houston ( 5) and the International Symposium on Physiochemical Aspects of Polymer Surfaces at this meeting as well as the symposium on industrial applications of surface analysis of which this article is a part. Review articles on various applications of XPS in materials science are listed in Table I. 

Subject areas for the Series include solutions of electrolytes, liquid mixtures, chemical equilibria in solution, acid-base equilibria, vapour-liquid equilibria, liquid-liquid equilibria, solid-liquid equilibria, equilibria in analytical chemistry, dissolution of gases in liquids, dissolution and precipitation, solubility in cryogenic solvents, molten salt systems, solubility measurement techniques, solid solutions, reactions within the solid phase, ion transport reactions away from the interface (i.e. in homogeneous, bulk systems), liquid crystalline systems, solutions of macrocyclic compounds (including macrocyclic electrolytes), polymer systems, molecular dynamic simulations, structural chemistry of liquids and solutions, predictive techniques for properties of solutions, complex and multi-component solutions applications, of solution chemistry to materials and metallurgy (oxide solutions, alloys, mattes etc.), medical aspects of solubility, and environmental issues involving solution phenomena and homogeneous component phenomena. 

The purpose of this chapter is to review recent progress in surface structural characterization of select oxide films, with particular emphasis on systems that are sensitive to some of the problems listed above. By thin , we mean a few monolayers up to -1000 A. We take the approach of addressing the issues of oxidation chemistry, either during or after film formation, and lattice and symmetry mismatch with the substrate. We examine representative case studies that illustrate the materials issues listed above. It is assumed that the reader is... 

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