Side main chain mesogen linked

Polymers with Side-Chain Mesogens Linked at the Main Chain Spacer... 

As an alternative concept to combined LC polymers, polymers with the side-chain mesogens linked at the main-chain mesogen were prepared [24] (see Figure 5, type B). 

Table 2. Structure, molecular weight and phase behaviour of the combined polymers in which the side-chain mesogen is linked to the main-chain mesogen.

Considerations by V. Shibaev and N. Plate (see this issue) led to a similar conclusion. Investigations on comb like polymers26), where each monomer unit of the macromolecule carried a non-branched alkyl chain of m methylene groups, have shown that for m > 8 side chain crystallization takes place independently to the main chain conformation. Consequently, if mesogenic molecules are linked to the side chains, they should occupy a l.c. order without influence of the backbone conformation. Following these considerations, alkyl chain lengths m > 8 are necessary for the formation of the l.c. order. As shown later, however, nearly only smectic polymers are possible under these conditions (see Chap. 2.3.3.). 

With these three different examples it has been demonstrated that the systematics observed for the polymorphism of m-l.c. s is also valid for the side chain polymers, provided that a flexible spacer connects the rigid mesogenic moieties to the polymer main chain. Deviations from this behavior are observed, when the mesogenic moieties are directly linked to the backbone. Under these conditions, normally no liquid crystalline behavior is to be expected, according to the model considerations mentioned in Chap. 2.1. Some examples, however, proved l.c. properties for such systems, which are characterized by two striking properties Very high glass transition temperatures and only smectic structures even in case of short substituents... 

Thus, the remoteness of mesogenic groups from the backbone provided by a polymethylene spacer secures them sufficient autonomy from the main chain. On the other hand, the fact that mesogenic groups are chemically linked with the main chain of the macromolecule assists their cooperative interaction. This is why comblike polymers have come to be accepted as convenient matrices for constructing LC polymers. Already a few hundred liquid-crystalline polymers with various mesogenic side groups have been synthesized. 

All Liquid Crystal Polymers are characterised by the fact that they contain stiff meso-genic groups, often inserted in flexible chain systems (so called "spacers") and connected to them by linking functional groups the mesogenic unit is inserted either in the main chain or in the side chains or (in exceptional cases) in both. We shall discuss MCLCPs and SCLCPs. A schematic representation of common structures of LCPs is displayed in Fig. 6.14 (Jansen, 1996). An example of a SCLCP with disc-like mesogens is displayed in Fig. 6.15 (Franse et al., 2002, 2004). 

Here, we have designed and synthesized multifunctional poly(bithienylene-phenylene)s with either racemic (Poly-9) (M — 14,000) or chiral moieties ((/ )-/ (S)-Poly-lO (Mn = 10,000, 8,000, respectively), which exhibit fluorescence, liquid crystallinity, and photoresponsive properties (Fig. 11.20). The polymers are composed of a 7t-conjugated main chain, poly(bithienylene-phenylene), which acts as a fluorescence moiety and mesogen core, and photochromic DE moieties [71, 72] are linked with racemic or chiral alkyl groups in the side chains. The DE photoresponsive moiety isomerizes between its closed and open forms upon irradiation of UV and visible light, respectively. 

Figure 1. Structure of macromolecules with side meso-genic groups, (a) Mesogenic groups are directly linked to the main chain (b) meso genic groups are linked to the branches of comb-like polymers.

Polymer (la) in principle has the same chemical constitution as polymer (lb), however, the rigid mesogenic benzoic acid phenyl-ester group is linked to the backbone by the short flexible spacer of only two methylene units. Owing to their chemical constitution strong interactions of the main chain and the mesogenic side groups are to be expected. Actually this polymer exhibits unusual optical properties No typical nematic... 

Finkelmann reported synthesis of a novel cross-linked smectic-C main-chain liquid-crystalline elastomer that was formed by polycondensation of vinyloxy-terminated mesogens, tetramethyldi-siloxane, and pentamethyl-pentaoxapentasilicane. The introduction of the functional vinyloxy group allows the synthesis of well-defined networks with good mechanical properties due to ehmination of side reactions as in the case of vinyl groups [60]. 

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