Defects and Disorder in Polymer Crystals

Moreover, even in the case of regular constitution, configuration, and conformation, disorder may be present in the crystals because of the presence of defects in the mode of packing. Disorder in the packing may occur while some structural feature, for instance, some atoms or the axes of helical macromolecules, maintain periodic positions [137], The degree of disorder in the 

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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... 

The normal vibration calculations based on a correct structure and correct potential field permit a good correlation to be made between predicted and observed ateorption bands in the FIR spectra of high-crystalline polymers in spite of the disordered regions existing in polymer crystals. The size and defects of these ciystals influence band shape and position because of finite boundary conditions. It also may give rise to additional al orption bands not predicted by the calculation (because the selection rule cannot be applied in this case). The additional bands are observed, indeed, in tlK FIR spectra at the frequencies corresponding to tlK maxima in tte spectrum density of phonon states in the low-frequency regon [19, 23]. 

It is well known (66) that the a-relaxation process of crystalline polymers consists of at least two processes, referred to as ai and U2 in the order of lower temperature, respectively. The ai-process (67-77) is pronounced in melt crystallized samples and is associated with the relaxation of grain boundaries, such as dislocation of lamellae with a frictional resistance related to disordered interface layers. The magnitude of the ai-process increases with the increase in the crystal defects. The o 2-process (71,73,78-83) is pronounced in single crystal mats and is ascribed to incoherent oscillations of the chains about their equilibrium positions in the crystal lattice in which intermolecular potential suffers smearing out. The magnitude of the Q 2-process increases with the increase in the lamellar thickness and/or the degree of crystallization (39). 

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. 

The importance of understanding the microscopic motion of matter has been expressed in the introduction of this article. The ramifications of dynamic disorder on the processes of polymer physics and chemistry are, indeed, very broad. In this regard, many defect types have in the past been proposed for polymer crystals (see the Appendix, Sect. 6). Often they were based on more or less extensive molecular mechanics calculations. Most of these defects were thought to explain some piece of the information needed for developing an understanding of the deformation of polymer crystals, but there were always some missing parts. 

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