Polyesters liquid crystalline type

Unusual properties of fully aromatic polyesters are observed if they have at least partially a rigid planar chain structure. In particular, they can form thermotropic liquid crystalline states (see Example 4-5). As already discussed in Sect. 1.2.4 an important structural prerequisit for LCPs of Type A in order to attain the liquid crystalline state of aromatic polyesters (and aromatic polyamides, see Example 4-14), is a rigid main chain according to the following construction principle ... 

Thermotropic liquid crystalline polymers, like polyesters containing mesogenic units on the main chain, may not be described by the wormlike chain model (cf. Sect. 1.2). The present article does not consider this type of polymers. 

Iimura, K., Koide, N., Ohta, R. and Takeda, M. Synthesis of thermotropic liquid crystalline polymers, 1, azoxy and azo-type polyesters. Makromol. Chemie 182, 2563 (1981)... 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

Ringsdorf s research group have prepared novel types of rigid-rod polyesters and polyamides 56 with a disc-like mesogen in tte mainchain [75]. Most polymers with six lateral substituents appear to be thermotropic liquid crystals. Polyamides with Z = H and having four substituents on the diamine component are not liquid crystalline. The two substituents on the diacid component seem to contribute to decrease further the intermolecular hydrogen bonding. 

LlOl Luperox 101 (2,5-di-(t-butylperoxy)-2,5-dimethylhexane) L130 Luperox 130 (2,5-dimethyl-2,5-di-(t-butylperoxy) hexyne-3) LCP Liquid crystalline polymer(s) (polyester-type unless noted)... 

A lot of molecular structures give rise to thermotropic liquid crystallinity, but only the aromatic ester type polymers and copolymers are successfully prepared as structural materials. Aromatic polyesters can be classified into three types based on their molecular composition and thermomechanical property [74]. 

Type - 3 polyesters were synthesized by copolymerization of 4-acetoxybenzoic acid (HBA) and PET. The properties of PET/HBA copolymers with varied composition are tabulated in Table 8.6. It was found that the reaction mixture become turbid for HBA composition higher than 35%, this was due to liquid crystalline nature of copolymer. It can be observed from the table that non-liquid crystalline PET / 30-HBA is stronger than that of compositions with liquid crystalline nature. Low production cost of PET/HBA with good mechanical properties and processability made it very attractive. However, because of the tendency toward homopolymerization of the acyl ester of 4-hydroxybenzoic acid to form poly(4-oxybenzoyl)... 

---