Crystal chain disorder

Figure 2.46 Model of packing of conformationally disordered chains of sPP in disordered modifications of form II presenting kink bands.189,190 Chains drawn with thick and thin fines are at 0 and 0.5 along b. In the ordered portion of crystal chains are in ordered twofold (T2G2)B helical conformation, whereas in the defective portion (delimited by dashed lines) chains are in T2G2T6 conformation.

The occurrence of disorder-to-order transitions involving segments of a protein, the remainder of which has a well-defined conformation, has been documented in crystal structures of a variety of systems. The segments involved may be at the N terminal or the C terminal end (disordered arm ) or in the middle of the polypeptide chain (disordered loop ). The length of the disordered segment varies from five or six amino acids to a sizable portion of the... 

In the process of identification of condis crystals it was observed that conformational mobility alone is not sufficient to prove the presence of a condis phase. Large amplitude molecular jump motion may be possible already in crystals without disorder if the symmetry is identical before and after the jump. The frequencies of these jumps can be surprisingly large and the moving parts of the molecules substantial. In the condis phase quick reptation can lead to extension of folded chain crystals, and is possibly also involved in rearrangements on mechanical deformation and membrane functions. 

When polyurethanes are stretched about 150%, the nearly-straight, short, soft segments crystallise. This increases the tensile strength and abrasion resistance of polyurethane rubbers. A similar strain-crystallisation phenomenon, which occurs in natural rubber at about 500% strain, limits the extension of rubber bands. Both the polyurethane soft segments and natural rubber have crystal melting points in the region 25-60 °C. In the unstretched state, the chain disorder prevents crystallisation. 

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. 

Reaction of [Mn(CO)jX] (X = Cl, Br, I) with 9S3 in DMF gives the facially-coordinated [Mn(CO)3(9S3)]X as air- and water-stable yellow crystals (Eq. la) [95]. The complex crystallizes with three crystallographically independent cations per unit cell each cation has mirror symmetry. Carbon atoms in the 9S3 chain disorder to accommodate the mirror symmetry. Kinetic studies show that displacement of CO is zeroth-order in 9S3, consistent with a limiting dissociative mechanism in which loss of CO cis to X is the rate determining step. Upon treatment with 5 MHCl [Mn(CO)3(9S3)]+ expels one CO to give [Mn(CO)2(9S3)Cl] (Eq. lb), while treatment with NOBF4 oxidatively removes one CO to afford [Mn(CO)2(OH2)(9S3)] (Eq. Ic). 

The presence of point defects such as vacancies or interstitial atoms or ions is well-established in atomic and ionic crystals. The situation is somewhat different in macromolecular crystals where the types of point defects are restricted by the long-chain nature of the polymer molecules. It is relatively easy to envisage the types of defects that may occur. They could include chain ends, short branches, folds or copolymer units. There is accumulated evidence that the majority of this chain disorder is excluded from the crystals and incorporated in non-crystalline regions. However, it is also clear that at least some of it must be present in the crystalline areas. 

Snetivy D and Vancso G J 1994 Atomic force microscopy of polymer crystals 7. Chain packing, disorder and imaging of methyl groups in oriented isotactic polypropylene Po/yme/ 35 461... 

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