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.

Martin used HREM to image the deformation and disorder in extended chain polymer fibers, such as PBZT and PBO [509] showing that deformation occurs by strain localization into kink bands. The ultrastructure of the fibers as a function of process conditions showed structural... 

Fig. 5.114 Peeled-back Vectran fibers are shown in a top down view in the STM of a region 500 run in the x and y axes and a maximum z axis range of 50 ran. The fibrillar texture and local disorder is observed. A kink band is observed as are clear discontinuities across individual microfibrils and damaged microfibrils within the kink band. (From Sawyer et al [435] reproduced with permission.)...

A common feature of highly oriented fibers is poor compressive properties, demonstrated by kink bands that can be seen in the OM and the SEM. Kink bands have been studied [677,679-682] and a mechanism for their formation, consistent with tensile loss, was proposed. Martin [693, 705, 706] addressed this issue by use of HREM, imaging the deformation and disorder in extended chain polymer fibers [705], showing that deformation occurs by strain localization into kink bands. High resolution EM images revealed the crystalhte size, shape, orientation, and internal perfection. The nature of the disorder within a kink band was imaged, modeled, and compared with diffraction data. Kinks were observed [706] by TEM investigation of PBZT... 

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.

As indicated, epitaxial crystallization implies contact planes of the substrate and deposit that are crystallographic. In polymers, only hkO planes should be considered since they are parallel to the chain axis. By contrast, planes of type hkl cut the chain axis, which does not generate an exposed, free contact face, since the chains continue on both sides of the plane, possibly with a change in orientation, as in kink-bands. However, exposed planes of type hkl exist in polymer crystals, as they are associated with chain folding. This lamellar folds surface is by necessity disordered or, at least, not crystallographically ordered. A priori, the folds surface would not be thought of as a contact plane in any epitaxial crystallization. However, a few examples of epitaxies that involve the end, fold surface of polymer crystals have been reported. 

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