Side-chain induced mesomorphism

B. Side Chain-Induced Mesomorphism in Flexible Polymers... 

The development of H-bonded complexes is now considered. An influence of UV irradiation on optical properties of low-molecular-weight azobenzene-con-taining material (Fig. 2.4b) has been investigated (Aoki et ah, 2000) on the basis of such interactions. The first observation of photoinduced optical anisotropy in H-bonded complexes of azobenzene dyes and copolymers (Fig. 2.4b) has been recently demonstrated (Medvedev et ah, 2005). In this case, the induced anisotropy was stable, and the maximum dichroic ratio of 2 has been observed. A kinetics of the induction of birefringence (maximum value of ca. 0.01) in one of these complexes is shown in Fig. 2.5. An influence of H-bonding on the mesomorphic and photoorientation properties was recently demonstrated (Cui and Zhao, 2004). In this approach, the amorphous azopyridine side-chain polymer was converted into liquid crystalline polymers through self-assembly with a series of commercially available, aliphatic, and aromatic carboxylic acids (Fig. 2.4d). 

Several tetraphenylporphyrins substituted in the four meso positions by various groups have also been investigated, and the induction and stability of mesomorphic properties were found to be dependent on the nature of the side chains. Simple alkyl chains directly connected in the four meso positions of metalloporphyrins ((23) M = Zn, Co, Cu, Ni, Pd) were not sufficient to induce mesophase. 

Barbera et al. prepared a series of thallium(I) complexes, in which the position and number of side chains were changed systematically ((58) M = Tl). A minimum of four chains was required to induce mesomorphism in these systems (Table 37), and on the basis of X-ray diffraction, the monotropic mesophases were identified as Colii. The formation of the columns was due to the association of these half-disk compounds into dimers, and to their subsequent stacking. The dimeric nature of these thallium(I) complexes was confirmed by the analysis of the single-crystal structure of the non-mesomorphic dimethoxy analog. In the crystalline phase, the co-planar... 

Amphiphilic behavior is related to conducting polymers and to mesomorphic polymers for several reasons. First, doping of PANi with surfactant molecules has been found to induce liquid crystallinity. Second, alkylation of semiflexible conducting polymers (PT and PANi) favors the formation of layered structures. Third, both rigid and flexible macromolecules with alkyl side chains may be considered as diphilic, in which case lyotropic behavior is to be expected. In the next section examples of mesomorphism and structural organization for macromolecules with flexible side groups will be given. 

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