Phase SCLCP

Liquid crystallinity can be attained in polymers of various polymer architectures, allowing the chemist to combine properties of macromolecules with the anisotropic properties of LC-phases. Mesogenic imits can be introduced into a polymer chain in different ways, as outhned in Fig. 1. For thermotropic LC systems, the LC-active units can be connected directly to each other in a condensation-type polymer to form the main chain ( main chain liquid crystalline polymers , MCLCPs) or they can be attached to the main chain as side chains ( side chain liquid crystalline polymers , SCLCPs). Calamitic (rod-Uke) as well as discotic mesogens have successfully been incorporated into polymers. Lyotropic LC-systems can also be formed by macromolecides. Amphiphihc block copolymers show this behavior when they have well-defined block structures with narrow molecular weight distributions. 

One of the most interesting features of SCLCPs is related to the fact that a liquid crystalline phase can be orientated and frozen by cooling it to below the glass transition temperature. It is therefore necessary to drive SCLCP systems from microscopic self-organized mesophases to macroscopic order. 

Block copolymers with well-defined segments often show microphase-separated morphologies (such as lamellar layers, hexagonal ordered cylinders, and micelle formation). If we use SCLCP blocks together with non-liquid crystalline segments, the mesophases are formed within one of the separated microdomains. If the non-SCLCP block has a higher Tg than the phase transition temperature of the mesophase, the amorphous block should physically support the SCLCP microdomains, forming a self-supported SCLCP system. 

Block copolymers consisting of a smectic SCLCP-block and a partially crystalline apolar block were synthesized via ROMP of IV-n with cyclooctene and initiator 1 or 2 [63]. The block copolymers also formed smectic liquid crystalline mesophases and showed lamellar phase-separation. 

The homopolymer showed an enantiotropic nematic mesophase, whereas the diblock copolymer generated microphase-separated lamellae, in which the SCLCP block possessed a nematic-isotropization transition similar to the homopolymer (Table 17). Upon heating, the nematic microphase decreased continuously in the nematic phase from 38.5 nm to 27 nm and showed a constant value of about 26 nm after the nematic-isotropization transition. Therefore, materials in which these block copolymers are macroscopically aligned are expected to show reversible contraction in one dimension, making this polymer system an interesting candidates for an artificial muscle or actuator. 

X-ray characterization revealed non-conventional packing in the smectic phase. It represented a novel class of SCLCPs in which the main chain was not confined in-between the smectic layers but rather penetrated them. 

Increasing the spacer length has much the same effect on the thermotropic behavior of SCLCPs as increasing the length of the flexible substituent has on that of low molar mass liquid crystals. That is, it destabilizes some phases and stabilizes others. For example, just as increasing the length of the flexible substituent depresses the melting... 

Chemical modification of the polymer structure allows the obtention of nematic and smectic phases [4, 5]. If the side group and/or the chain are chiral, then cholesteric or chiral smectic C (SmC) phases can be obtained. These can also be obtained by mixing a chiral compound with the SCLCP. SmC SCLCPs are of particular interest and their behavior is described in Sec. 2 of this Chapter. 

The backbones that have been most commonly employed are those of the acrylate [177, 190, 194, 196], methacrylate [152, 171,196,198-200], and siloxane [152,177, 197] types. Polyethers [207-209], polyesters [182,191, 192], and polystyrenes [177, 189, 195] have also been reported. Typical spacer groups consist of between 3 and 12 methylene units. The phase transitions of a number of SCLCPs containing NLO meso-genic groups are collected in Table 19. Unfortunately, the molecular masses of many of these polymers have not been determined, and the influence of the polymer structure on the phase transitions can not therefore be quantitatively discussed. However, the general points to emerge from these data are as follows ... 

In order to achieve amorphous polar solids, some research groups have explored ferroelectric SCLCPs, and have shown that the basic rules governing the relationships between molecular structure and macroscopic ferroelectric LC properties are the same [225-228]. The main difference between low molar mass and polymer ferroelectric LCs, however, is the existence of stable glassy phases in most of the latter. 

The techniques used to obtain the untwisted SmC phase structure in low molar mass LCs and SCLCPs are limited to thin layers. In contrast to this, LC elastomers can be macroscopically uniformly oriented by mechanical deformations [230], and this orientation process is not limited to thin samples or suitable dielectric anisotropy of the material. Furthermore, for LC elastomers the oriented structure can be chemically locked in by crosslinking, resulting in the so-called liquid single crystal elastomers [231],... 

Clearly the mesogenic unit will have a great influence on the liquid crystal phases generated and the transition temperatures. Just as was seen in Chapter 3, there is an enormous number of mesogenic units and each can easily be adopted as a side chain mesogenic unit for an SCLCP. Figure 5.6 shows a typical template for some possible mesogenic units commonly employed in SCLCPs (m and n are usually one or two). 

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