Disordered conformations conformational kink-band 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.

Figure 2.47 Models of packing in limit-ordered forms (a) II and (c) IV of sPP and (b) model of conformationally disordered modification, presenting kink bands, intermediate between limit-ordered models of form II and form IV. In defective region of model (b), delimited by dashed lines, chains are packed as in form IV, whereas in ordered regions chains are packed as in the form II.

Conformational disorder and kink-bands structures have recently been found also in random copolymers of syndiotactic polypropylene with small amounts of ethylene.192 193 The ethylene units are included in the crystalline regions193 and induce the crystallization of the metastable form II of sPP with conformationally disordered chains characterized by kink bands. Portions of chains containing the ethylene units tend, indeed, to assume a trans planar conformation, producing the kink-band defects in chains in the prevailing twofold helical conformation.192193... 

This implies a modification of the chain direction and, finally, a generation of a helix with a larger cross section than that of the trans planar chain.218 Such phase transitions are often associated with the onset of some conformational disorder characterized by the occurrence of kink bands (Figure 2.48b), similar to those found in sPP samples quench crystallized from solutions189,190 (Figures 2.46 and 2.47) or in polyethylene.219 It is apparent from Figure 2.48... 

Figure 2.48 Possible cooperative mechanism of crystal-crystal transition from (a) trans planar form III of sPP into (c) isochiral helical form II. Transition occurs through formation of (b) intermediate conformationally disordered modifications containing kink bands, characterized by helical sequences having same chirality (R = right-handed helix). Formation of helical sequences of opposite chirality (right- and left-handed) produces (if) steric interactions between neighboring chains.

Figure 2.29 Limit-ordered models of packing of forms II [31] (a) and IV [67] (c) of sPP and model of a conformationally disordered modification, presenting kink-bands (b), intermediate between forms II and IV. A kink-band in the form II of sPP, with (T2G2)n helices, is characterized by a defective region with (T2G2T6G2)n conformational sequence, as in form IV [154, 155]. Reprinted from Reference [143] with permission from American Chemical Society, Copyright 2006.

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