End-on side-chain polymers

Fig. 4 Molecular model of the local chain conformation of LC end-on side chain polymers depending on the length of the flexible spacer oblate conformation for an even number of spacer atoms (a) and prolate chain conformation for odd numbered spacers (b)...

Thermoelastic measurements on such samples reveal a spontaneous elongation along n at the transition to the smectic phase, indicating a prolate polymer backbone conformation in the smectic elastomer [137]. On another hand, SANS results for end-on side-chain polymers in the smectic phase indicate an oblate chain conformation, with the backbone preferentially confined in the plane of the layers (Sect. 2.2). Thus, the chain distribution and macroscopic shape of the smectic elastomer change their sign if crosslinking is made under uniaxial mechanical stress in the isotropic and/or nematic phase. This result is remarkable and indicates that the oblate chain conformation of a smectic end-on polymer can be easily turned into prolate by a low uniaxial extension during solvent evaporation. 

Chemistry on soluble polymer matrices has recently emerged as a viable alternative to solid-phase organic synthesis (SPOS) involving insoluble cross-linked polymer supports. Separation of the functionalized matrix is achieved by solvent or heat precipitation, membrane filtration, or size-exclusion chromatography. Suitable soluble polymers for liquid phase synthesis should be crystalline at room temperature, with functional groups on terminal ends or side chains, but must not be not cross-linked they are therefore soluble in several organic solvents. 

Most of the known photochemical procedures for the synthesis of block and graft copolymers are based on the modification of already existing polymers with photolabile groups incorporated at defined positions, i.e. at the chain end, at side chains, or in the main chain (see Chart 11.13) [84]. 

As mentioned earlier, for end-on main-chain smectic polymers the polymer chains connect the smectic layers. As a result, polymer defects are expected to be directly translated into layer distortions (Fig. 4b-d). This probably offsets any possible influence of damping of the layer fluctuations (potentially leading to increased order) because of the connectivity of the layer structure via the chains. As mentioned already in Sect. 2.3, main-chain polymers and elastomers have little tendency to form a smectic phase. They have been less thoroughly investigated than their side-chain counterparts. X-ray structural information of several main-chain elastomers with about 10% of approximately the same cyclic multifunctional crosslink have been compared with their homopolymer counterparts by De Jeu et al. [155]. As no results are available for other crosslink concentrations, little can be said about the specific contribution of the crosslinks to disorder. 

End-functionalized mesogenic molecules are used to form the second type of supramolecular side-chain polymers (Figure 2(a) [ii], Figure 23) [97-108], Polymeric complex 28 has been prepared based on poly(4-vinylpyridine) [97], The hydrogen-bonding formation between imidazole and carboxylic acid moieties yields supramolecular side-chain polymers of 29 [103], which exhibit smectic A phases. The interactions of carboxylic acid/dialkylamine [104-106], phenol/amine [107], and hydroxyl/pyridine [108] were used for the preparation of the side-chain mesogenic complexes. 

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