Hairpin Defects

Li MH, Brulet A, Davidson P, Keller P, Cotton JP (1993) Observation of hairpin defects in a nematic main-chain polyester. Phys Rev Lett 70 2297... 

FIGURE 12.52 The combination of basic structural units into microdomains within a carbon fiber (a) skin region (b) core region (c) a hairpin defect and (d) a wedge disclination. (From Bennet, S.C. and Johnson, D.J. in Society of Chemical Industry, London, p. 377, 1978.)... 

Small angle neutron scattering (SANS) experiments illustrate the drastic change in conformation occuring at the I-N transition and provide a measure of anisotropy of the radius of gyration and its increase with decreasing temperature across the nematic phase [26, 27]. The appearance of hairpin defects (abrupt changes in chain direction... 

Figure 5.23 A schemalic microstructure of PAN based carbon fiber depicting combination of basic structural units into microdomains. A, Skin region B, Core region C, A hairpin defect D, A wedge disdination. Source Reprinted with permission from Bennett SC, Johnson DJ, Strength structure relationships in PAN-based carbon fibres, London International Carbon and Graphite Conference, Soc Chem Ind, Lend, 377,1978. Copyright 1978, The Society of Chemical Industry.

Fig. 3 Side-chain LC polymer with (a) prolate nematic conformation and (b) oblate smectic conformation, (c) Representation of two hairpin defects confined to a long, thin cylinder (aftm-...

Similar measurements on main chain liquid crystal polymers have been used to determine the persistence length of the chains in the nematic phase. Again, there are theoretical predictions for the temperature behavior. It has been suggested that as the temperature increases the number of hairpin bends of the chain will increase and the conformation of the chain will become more isotropic [26]. This has been confirmed qualitatively, but since most of the materials have been polyesters and have had nematic phases at quite high temperatures the effects of transesterification have interfered [27-30]. However, similar results have been found in a polyether [31] where the chains were found to contain one hairpin defect on average. 

The main molecular defects displayed by the nematic phase of main-chain polymers were predicted long ago by DeGennes and called hairpins [20]. These defects appear when a main-chain polymer suddenly performs a 180° rotation (Fig. 6). 

Apart from hairpins, other types of defect can be present in main-chain polymers (see Fig. 4). First, we note that chain ends represent a source of the local distortion of the director field [39]. Furthermore, a certain number of hairpins could become entangled. In contrast to standard hairpins, these kinds of defect cannot be removed by applying mechanical stress. Such entangled hairpins can easily suppress chain reptation and thus represent a source of (physical) crosslinking in the polymer matrix. Although not being quenched, as crosslinks in elastomer networks they introduce local sources of random orientational disorder in the director field. 

Fig. 4 Cartoons of a mam-chain smectic polymer (a) standard picture, (b) end defect, (c) hairpin, and (d) entangled hairpin...

In the case of polymer single crystals, the situation is not very different from that of regular crystals these entities undergo fusion or melting at a rather well-defined temperature because the dimensions of the crystalline domains are relatively large. The only structural defects are those corresponding to chain folding, hairpin turns, loops, and chain ends. 

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