Structural defect concept

Further on, the rightfulness of application of the structural defect concept to polymers yielding process description will be considered. As a rule, previously assumed concepts of defects in polymers were primarily used for the description of this process or even exclusively for this purpose [4—11]. Theoretical shear strength of crystals was first calculated by Frenkel, basing on a simple model of two atoms series, displaced in relation to one another by the shear stress (Fig. 4.1a) [3]. According to this model, critical shear stress Tg is expressed as follows [3] ... 

Chemical reactions in boundary lubrication are different from static reactions even if the reactive substances involved are the same. The temperature to activate a chemical reaction on rubbing surfaces is usually lower than that required in the static chemical process. Some believe this is because of the naked surfaces and structural defects created by the friction/wear process, which are chemically more active. Kajdas proposed a new concept that accumulations of stress and strain in friction contacts could cause emission of low-... 

In the previous sections composition variation has been attributed more or less to point defects and extensions of the point defect concept. In this section structures that can be considered to be built from slabs of one or more parent structures are described. They are frequently found in mineral specimens, and the piecemeal way in which early examples were discovered has led to a number of more or less synonymic terms for their description, including intergrowth phases, composite structures, polysynthetic twinned phases, polysomatic phases, and tropochemical cell-twinned phases. In general, they are all considered to be modular structures. 

We shall proceed from a concept which in a certain sense is contrary to that of the two-dimensional gas. We shall treat the chemisorbed particles as impurities of the crystal surface, in other words, as structural defects disturbing the strictly periodic structure of the surface. In such an approach, which we first developed in 1948 (I), the chemisorbed particles and the lattice of the adsorbent are treated as a single quantum-mechanical system, and the chemisorbed particles are automatically included in the electronic system of the lattice. We observe that this by no means denotes that the adsorbed particles are rigidly localized they retain to a greater or lesser degree the ability to move ( creep ) over the surface. 

The essential difference between treatments of chemical processes in the solid state and those in the fluid state is (aside from periodicity and anisotropy) the influence of the unique mechanical properties of a solid (such as elasticity, plasticity, creep, and fracture) on the process kinetics. The key to the understanding of most of these properties is the concept of the dislocation which is defined and extensively discussed in Chapter 3. In addition, other important structural defects such as grain boundaries, which are of still higher dimension, exist and are unknown in the fluid state. 

Some experimental studies point out that the diffusion rate of pure hydrocarbons decreases with the coke content in the zeolite [6-7]. Theoretical approaches by the percolation theory simulate the accessibility of active sites, and the deactivation as a function of time on stream [8], or coke content [9], for different pore networks. The percolation concepts allow one to take into account the change in the zeolite porous structure by coke. Nevertheless, the kinetics of coke deposition and a good representation of the pore network are required for the development of these models. The knowledge of zeolite structure is not easily acquired for an equilibrium catalyst which contains impurity and structural defects. 

This section summarises some properties common to non-polar stoichiometric oxide surfaces and presents theoretical arguments to explain them. Their specificities come from the local environment of the surface sites, which have a lower coordination number than in the bulk. From this point of view, a close parallel with unsupported clusters or ultra-thin films can be established [3]. We will not explicitely consider here the properties associated to structural defects, such as steps or kinks, for the reason of space limitation. However, most of the time, the same concepts as those akin to terrace sites apply, but with an even larger strength since the local environment is more reduced. We will successively analyse structural characteristics, energetics, electron distribution, one-particle and two-particle excitations. 

The difficulties of the second, stepwise, synthesis of ladder polymers involve, in particular, the final step, namely the polymer-analogous formation of the double-stranded structure. Here, an optimal design of the open-chain precursor polymer is mandatory to arrive at a quantitative conversion with hi chemo-and regioselectivity in order to minimize structural defects. These problems have raised serious doubts in the literature [3] about the feasibility of this concept. 

To account for this phenomenon of spinless conductivity, physicists have introduced the concept of transport via structural defects in the polymer chain. In a conventional semiconductor, an electron can be removed from the valence band and placed in the conduction band, and the structure can be assumed to remain rigid. In contrast, an electronic excitation in polymeric materials is accompanied by a distortion or relaxation of the lattice around the excitation, which minimizes the local lattice strain energy. The combined... 

As is seen, the interdependence of two values is rather weak. The modification action ( disturbance ) of the graphite surface (increasing A) leads to facilitating fracture of an interfacial layers, and, as a consequence, to increasing the size of critical structural defect, a fit [46, 47]. Figure 12.10 shows that the correlation of a jt to (A) meets the above concept. 

For bulk materials, all techniques based on structure-insensitive properties, as described in this section and elsewhere, yield closely similar data. The crystallinity model is thus a valid defect concept to describe structure-insensitive properties of semicrystalline polymers. It breaks down for three-phase systems, consisting, for example, of a crystalline phase, a mobile amorphous phase, and a rigid-amorphous fraction (see Chap. 6). In addition, one does not expect valid answers for structure-sensitive properties. 

The mechanisms by which these polymers conduct electricity have been a source of controversy ever since conducting polymers were hrst discovered. At first, doping was assumed to remove electrons from the top of the valence band, a form of oxidation, or to add electrons to the bottom of the conduction band, a form of reduction. This model associates charge carriers with free spins, unpaired electrons. This results in theoretical calculations of conduction that are much too small (59). To account for spinless conductivity, the concept of transport via structural defects in the polymer chain was introduced. From a chemical viewpoint, defects of this nature include a radical cation for oxidation effects, or radical anion for the case of reduction. This is referred to as a polaron. Further oxidation or reduction results in the formation of a bipo-laron. This can take place by the reaction of two polarons on the same chain to produce the bipolaron, a reaction calculated to be exothermic see Figure 14.17 (55). In the bulk doped polymer, both intrachain and intrachain electronic transport are important. 

Zhu, Y, 1995, Structural defects in YBajCujO, /, siqterconductors, in High-Temperature Superconducting Materials Science and Engineeritig. New concepts and Technology, ed. D. Shi (Peigamon Press, Oxford) pp. 199-258. 

A cluster model of pol5miers amorphous state structure allows introducing principally new treatment of structure defect (in the full sense of this term) for the indicated state [1,2], As it is known [3], real solids structure contains a considerable number of defects. The given concept is the basis of dislocations theory, widely applied for crystalline solids behavior description. Achieved in this field successes predetermine the attempts of authors number [4-11] to use the indicated concept in reference to amorphous polymers. Additionally used for crystalline lattices notions are often transposed to the structure of amorphous pol mers. As a rule, the basis for this transposition serves formal resemblance of stress - strain (a - ) curves for crystalline and amorphous solids. 

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