Long-range ordered structure 414 Subject

Compared to crystalline materials, the production and handling of amorphous substances are subject to serious complexities. Whereas the formation of crystalline materials can be described in terms of the phase rule, and solid-solid transformations (polymorphism) are well characterised in terms of pressure and temperature, this is not the case for glassy preparations that, in terms of phase behaviour, are classified as unstable . Their apparent stability derives from their very slow relaxations towards equilibrium states. Furthermore, where crystal structures are described by atomic or ionic coordinates in space, that which is not possible for amorphous materials, by definition, lack long-range order. Structurally, therefore, positions and orientations of molecules in a glass can only be described in terms of atomic or molecular distribution functions, which change over time the rates of such changes are defined by time correlation functions (relaxation times). 

Stimulated by a variety of commercial applications in fields such as xerography, solar energy conversion, thin-film active devices, and so forth, international interest in this subject area has increased dramatically since these early reports. The absence of long-range order invalidates the use of simplifying concepts such as the Bloch theorem, the counterpart of which has proved elusive for disordered systems. After more than a decade of concentrated research, there remains no example of an amorphous solid for the energy band structure, and the mode of electronic transport is still a subject for continued controversy. 

Diffuse crystal/crystal interfaces often appear in systems subject to incipient chemical or structural instabilities associated with phase separation, long-range ordering, or displacive phase transformations [2], Examples of interfaces associated with the first two types are shown in Fig. 18.7. 

This book is all about materials that are soft in nature, but the way in which the constituent molecules in many soft materials are arranged is not random. Instead, they have some structure that can be measured, even if they do not exhibit long-range order like a crystalline solid. The molecules or particles that make up a material are subject to attractive and repulsive forces from each other and any solvent molecules in the... 

A few definitions are in order. Depending on temperature and structure, amorphous polymers exhibit widely different physical and mechanical behavior patterns. At low temperatures, amorphous polymers are glassy, hard, and brittle. As the temperature is raised, they go through the glass-rubber transition. The glass transition temperature (Tg) is defined as the temperature at which the polymer softens because of the onset of long-range coordinated molecular motion. This is the subject of Chapter 8. 

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