Jogs, defects

In general, however, identification of the crystal cell is only part of the problem of characterizing the structure of crystalline polymers. Crystals are never perfect and the units cells do not infinitely duplicate through space even when they are grown very carefully from dilute solution using low molecular mass materials. As with the organic crystals considered in Chapter 3, a variety of defects can be observed and are associated with chain ends, kinks in the chain and jogs (defects where the chains do not lie exactly parallel). The presence of molecular (point) defects in polymer crystals is indicated by an expansion of the unit cell as has been shown by comparison of branched and linear chain polyethylene. The c parameter remains constant, but the a and b directions are expanded for the branched polymer crystals. Both methyl and... 

Miyashita et al. [102] have proposed a possible model for the hexagonal-orthorhombic phase transition (Fig. 10). They proposed that since the hexagonal crystal contains many defects along the chain axis, such as kinks and jogs, on its phase transition to the orthorhombic phase, these defects are excluded from the crystal... 

Figure 10 Model for the phase transition of PE from the hexagonal to the orthorhombic phase, (a) In the hexagonal phase, there are many defects along C-axis (kinks, jogs), (b) On the phase transition, these defects are excluded to form boundaries between bands. (From Ref. 102.)...

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Typical crystalline macromolecular substance lattice defects result from end groups, kinks, jogs, Reneker defects, and chain displacements. Distortion of the whole crystal lattice can be conceived in terms of the paracrystal. The defects can be classified in terms of point, line, and network defects. 

Figure 5-12. Some lattice defects in poly (ethylene). From left to right aW-trans conformation (defect-free), Reneker defect, kink, and jog.

Kinks Jogs, and Reneker defects are conformational defects (see Figure 5-12). With kinks and j ogs, a part of the chain is displaced perpendicular to the long axis by false conformations. This kind of defect is called a kink when the displacement is smaller than the interchain distance (example ... 

TTTTG TG TTTT. . . ). If, on the other hand, the displacement is larger than the interchain distance (e.g.,.. . TTTTG TTTTG TTTT. . . ), then the defect is called a jog. Kinks and jogs shorten planar chains and twist helices. 

Dislocations glide by the movement of kinks and climb by the movement of jogs. Since climb requires changing the number of point defects (reacting or absorbing them), we call it nonconservative motion. 

FIGURE 12.16 Schematic showing local charge on defects in rocksalt. (a) A comer is always charged, (b and c) possibilities for jog on an edge dislocation. 

Climb is nonconservative motion because vacancies and/or interstitials must be absorbed or emitted (their number is not conserved). When a jog moves, it can either glide or climb. The special point to remember is that the glide plane of a jog is not the same as the glide plane of the dislocation on which is sits. If we force a dislocation jog to move on a plane, which is not its glide plane, it must adsorb or emit point defects, but it can glide. Since it is charged, it carries a current when it glides. 

We saw in chapter 12 that dislocations move by the movement of jogs or kinks that are, themselves, defects on the dislocation. When we consider the movement of surfaces at an atomic scale, the surfaces (like dislocations) actually move by the translation of defects along them. The principal surface defects are line defects known as steps, or ledges if they are higher than a few lattice planes, and these, in turn, translate by kinks moving along them. 

A jog is a section of dislocation, which does not lie in the slip plane. The jog cannot move without generation of point defects, that is, vacancies. The jogged dislocation can slip if a steady diffusion of vacancies occurs from it. The nonconservative slipping of jogged dislocations is dependent on the material redistribuhon. The shorter the distance between jogs, the lower the dislocation velocity. Hence, it is the diffusion process that controls the velocity of the deforming dislocations. It is the generation and diffusion of the lattice vacancies that determine the strain rate. 

In addition, conformational disorder in polymer crystals may give rise to point and line defects which are tolerated in the crystal lattice at a low cost of free energy as kinks [104,105], jogs [106,107] and dislocations [108,109]. Such crystallographic defects arise whenever portions of chain adopt conformations different from the conformation assiuned by the chains in the crystal state [99], and have been widely discussed in the literature, in the case of polyethylene [108,109] and some aliphatic polyamides [99,106]. Point and... 

Zero-dimensional defects or point defects conclude the list of defect types with Fig. 5.87. Interstitial electrons, electron holes, and excitons (hole-electron combinations of increased energy) are involved in the electrical conduction mechanisms of materials, including conducting polymers. Vacancies and interstitial motifs, of major importance for the explanation of diffusivity and chemical reactivity in ionic crystals, can also be found in copolymers and on co-crystallization with small molecules. Of special importance for the crystal of linear macromolecules is, however, the chain disorder listed in Fig. 5.86 (compare also with Fig. 2.98). The ideal chain packing (a) is only rarely continued along the whole molecule (fuUy extended-chain crystals, see the example of Fig. 5.78). A most common defect is the chain fold (b). Often collected into fold surfaces, but also possible as a larger defect in the crystal interior. Twists, jogs, kinks, and ends are other polymer point defects of interest. 

Jogs are formed by climb. They are favorable sites for the absorption and emission of point defects. In thermal equilibrium, the atoms at jog sites are in dynamic equilibrium and arrive and leave the jog at equal rates. If there is an increase in vacancies, for example, in the vicinity of a dislocation line above the thermal equilibrium value, the probability of atomic exchange at a jog with a vacancy increases, climb occurs and the extra plane (defining the dislocation line) shrinks. Therefore, excess vacancies promote the process of climb. Similarly, an excess of interstitial atoms adds atoms to the existing jog, which causes it to grow. In summary, when atoms are removed from an extra plane, the crystal collapses... 

The formation of jogs by climb may be considered to be a thermally activated process, since point defects are involved. It is proper to talk about nucleation and the motion of jogs. An activation energy exists, expressed by ... 

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