Diffusion in Noncrystalline Materials

Noncrystalline materials exist in many different forms. A huge variety of atomic and molecular structures, ranging from liquids to simple monatomic amorphous structures to network glasses to dense long-chain polymers, are often complex and difficult to describe. Diffusion in such materials occurs by a correspondingly wide variety of mechanisms, and is, in general, considerably more difficult to analyze quantitatively than is diffusion in crystals. 

The understanding of diffusion in many noncrystalline materials has lagged behind the understanding of diffusion in crystalline material, and a unified treatment of diffusion in noncrystalline materials is impossible because of its wide range of mechanisms and phenomena. In many cases, basic mechanisms are still controversial or even unknown. We therefore focus on selected cases, although some of the models discussed are still under development and not yet firmly established. 

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Microscopic and mechanistic aspects of diffusion are treated in Chapters 7-10. An expression for the basic jump rate of an atom (or molecule) in a condensed system is obtained and various aspects of the displacements of migrating particles are described (Chapter 7). Discussions are then given of atomistic models for diffusivities and diffusion in bulk crystalline materials (Chapter 8), along line and planar imperfections in crystalline materials (Chapter 9), and in bulk noncrystalline materials (Chapter 10). 

In the absence of molecular dynamics investigations of ion transport in noncrystalline phases of polymers, we describe investigations of the motion of small neutral molecules in amorphous polymers. The success of these studies in providing a description of the motion of (albeit) neutral diffusants encourages their extension to similar investigations of the transport of dopant ions through polymer materials. 

Spherulitic growth is a special case in crystallization. Spherulites form only within a specific temperature range for example, with it-poly(propylene) with a melting point of ITO C, they are first formed below IIS C. With spherulites, the rate of advance of the spherulite boundary is followed. This boundary encloses the crystalline portion of the spherulite. Since spherulites also contain noncrystalline material, however, the spherulite growth rate thus corresponds to the linear crystal growth rate. As the molecular weight increases, the rate of crystallization falls, since the rate of diffusion of segments and molecules decreases. 

Noncrystalline or amorphous materials produce patterns with only a few diffuse maxima, which may be either broad rings or arcs if the amorphous regions are partially oriented [3]. Synthetic polymers, which are branched or cross-linked, are usually amorphous, as are linear polymers with bulky side groups, which are not spaced in a stereoregular manner along the backbone [3]. 

The x-ray scattering from noncrystalline polymers is difruse in nature and is often only used to classify a material as "amorphous." This contribution is concerned with detailing how usefiil stmctural information may be obtained from such diffuse scattering patterns. The techniques for obtaining reliable quantitative intensity data from noncrystalline polymers are outlined with particular emphasis upon the particular problems which have to be faced when dealing with oiganic material. 

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