Thermotropic Main Chain Liquid Crystal Polymers

Thermotropic Main-Chain Liquid Crystal Polymers... 

THERMOTROPIC MAIN-CHAIN LIQUID CRYSTAL POLYMERS... 

The first examples of thermotropic main-chain liquid crystal polymers were produced by Roviello and Sirigu in 1975 when they reacted alkyl acylchlorides with p, p dihydroxy-a,-(x -dimethylbenzalazine, giving the stmcture (I), which exhibited anisotropic fluid phases after melting. 

In some polymer liquid crystals, several mesophases can be identified. In main-chain liquid crystal polymers there is usually a transition from the crystal to a mesophase, whereas in more amorphous systems when a glass transition is present, the mesophase may appear after this transition has occurred. In multiple transition thermotropic systems, the increase in temperature leads to changes from the most-ordered to the least-ordered states, i.e., crystal (k) smectic (S) nematic (N) isotropic (i). 

Rg. 12.18 Some examples of main-chain liquid-crystal polymers (a) poly(p-phenylene benzobisoxazole) (PBO), lyotropic (b) poly(4-carboxybenzene-propionic acid), thermotropic and (c) a thermotropic polymer with a long flexible decamethylene spacer. 

This chapter is concerned with the synthesis of main chain, liquid crystal polymers, but only with the reactions used to prepare polyesters and polyamides. At the time of this review, polyesters have been of interest only as thermotropic polymers and polyamides for lyotropic behaviour, but this situation will, likely, soon change. There are undoubtedly aromatic polyesters which show liquid crystallinity in solution, and a thermotropic polyurethane has recently been reported. ... 

Because of the structural similarity between semi-rigid thermotropic polymers and LCPs, i.e. the presence of rigid moieties separated by flexible spacers, all polymers of this kind are commonly referred to as the main chain liquid crystal polymers (mcLCP). 

Low-molecular-weight liquid-crystalline compounds have been known for about 100 years. However, main chain liquid crystal polymers (MCLCPs) have attained prominence only in the last 15 years. In 1956 Flory predicted lyotropic behaviour and this theoretical prediction was well demonstrated in the synthetic polymer area with the discovery by Kwolek of the aramids, e.g. poly(/ -phenyleneterephthamide). This led to interest in thermotropic main chain LCPs and, although aromatic polyesters which are thermotropic were described during this period in patents issued to and Carborundum Co., their liquid-crystalline... 

Both thermotropic and lyotropic liquid crystal polymers exhibit characteristic features with regard to their microstructureJ Anisometrical monomers such as rods or discs are connected to chains in an appropiate manner. These anisometrical monomers are considered to be the mesogens and may be part of main chain LCP, side chain LCP, or of both types together (Fig. 6). Between the mesogens are located flexible spacers of non-mesogenic character. Sufficient flexibility is a prerequisite for liquid crystal formation, with an increase in either temperature or solvent concentration. 

TABLE 5.3 Chemical Constitution Typical of Thermotropic Main-Chain-Polymer Liquid Crystals, i.e., [4]... 

Group Arrangements Typical of Thermotropic Main-Chain Polymer Liquid Crystals... 

ELASTIC MODULI OF THERMOTROPIC MAIN CHAIN POLYMER LIQUID CRYSTALS... 

SINCE the discovery of liquid crystalline phenomenon for low molecular weight liquid crystals (LMWLCs) more than 100 years ago, anisotropic ordering behaviors of liquid crystals (LCs) have been of considerable interest to academe [1-8], In the 1950s, Hory postulated the lattice model for various problems in LC systems and theoretically predicted the liquid crystallinity for certain polymers [1-3], As predicted by the Hory theory, DuPont scientists synthesized lyotropic LCPs made of rigid wholly aromatic polyamide. Later, Amoco, Eastman-Kodak, and Celanese commercialized a series of thermotropic main-chain LCPs [2]. Thermotropic LCPs have a unique combination of properties from both liquid crystalline and conventional thermoplastic states, such as melt processibility, high mechanical properties, low moisture take-up, and excellent thermal and chemical resistance. Aromatic main-chain LCPs are the most important class of thermotropic LCPs developed for structural applications [2,4-7]. Because they have wide applications in high value-added electronics and composites, both academia and industry have carried out comprehensive research and development. 

The procedures which employ the intrachain scattering to evaluate the molecular orientation parameters have been applied successfully to main-chain and side-chain liquid crystal polymers and to non-crystalline polymers in general. - They have recently been reviewed by Windle. We shall explore their use through the example of a semi-rigid thermotropic co-polyester The X-ray... 

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