Liquid-crystal aromatic polyesters

In the last 10-15 years, liquid-crystal aromatic polyesters (TCP) were of special interest, which above all was associated with the discovery of their ability to self-reinforce when melted. This enabled preparation of high accuracy and size stable articles with low coefficients of thermal shrinkage. 

This search has recently been successful in the first commercial liquid crystal aromatic polyester. This is the copolymer of terephthahc acid, p-hydroxybenzoic acid and 4,4 -biphenol (Xydar), and is fabricated at 430 °C. It has an HDT of 355 °C and a high impact strength across the large temperature range — 50 °C to 240 °C. 

A great many aromatic polyesters and copolyesters which show thermotropic behaviour have now been reported, particularly in the patent literature. The copolyesters may contain either combinations of different types of mesogenic units or combinations of mesogenic and non-mesogenic (non-linear) units. The structures of many of the different types of monomers used to form liquid crystal aromatic polyesters and copolyesters are shown in Table 3. ... 

Monomers used in the Synthesis of Liquid Crystal Aromatic Polyesters... 

Linear polyurethanes, 26 Linear step-growth polymerizations, 13 Lipase-catalyzed polyesterifications, 83 Lipases, 82, 84 catalytic site of, 84 Liquefied MDIs, 211, 226-227 Liquid carbon dioxide, 206 Liquid-castable systems, 201 Liquid crystal devices (LCDs), alignment coating for, 269-270 Liquid crystalline aromatic polyesters, 35 Liquid crystalline polyesters, 25, 26, 48-53... 

Liquid crystal polyesters are made by a different route. Because they are phenoHc esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenoHc hydroxyl groups are acylated with a lower aHphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxyHc acid, sometimes in the presence of a catalyst. The phenoHc polyester forms readily as the volatile lower acid distills from the reaction mixture. Many Hquid crystal polymers are derived formally from hydroxyacids (97,98) and thein acetates readily undergo self-condensation in the melt, stoichiometric balance being automatically obtained. 

As with the polysulphones, the deactivated aromatic nature of the polymer leads to a high degree of oxidative stability, with an indicated UL Temperature Index in excess of 250°C for PEEKK. The only other melt-processable polymers in the same league are poly(phenylene sulphides) and certain liquid crystal polyesters (see Chapter 25). 

Highly aromatic thermoplastic polyesters first beeame available in the 1960s but the original materials were somewhat difficult to process. These were followed in the 1970s by somewhat more processable materials, commonly referred to as polyarylates. More recently there has been considerable activity in liquid crystal polyesters, which are in interest as self-reinforeing heat-resisting engineering thermoplastics. 

Liquid crystal polymers (LCP) are a recent arrival on the plastics materials scene. They have outstanding dimensional stability, high strength, stiffness, toughness and chemical resistance all combined with ease of processing. LCPs are based on thermoplastic aromatic polyesters and they have a highly ordered structure even in the molten state. When these materials are subjected to stress the molecular chains slide over one another but the ordered structure is retained. It is the retention of the highly crystalline structure which imparts the exceptional properties to LCPs. 

An increase in rod-like arrangement of the macromolecules can also arise by stretching a polymer either in its solid state, either in the melt or even in solution (for polymers leading to lyotropic liquid crystals such as aromatic polyamides). This is the basis of the development of synthetic fibres including high modulus polyethylene Dyneema , polyamide Nylons and Kevlar , polyester Tergal or Dacron fibres. 

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. 

Wissbrun earlier observed a very long relaxation time and high elasticity for anisotropic melts of aromatic polyesters, as well as several other types of flow anomalies. Unfortunately, in most of these earlier studies, the rheological behavior of liquid crystal melts of polymers could not be directly compared with that of the isotropic phase of the same polymers because of their high clearing temperatures. 

As stated some years ago [2, 4], shape selective catalysis involving bulky molecules continues to be a thrust area in zeolite catalysis. Consequently, test reactions have been developed which are particularly suited to characterize large and super-large pore molecular sieves [34]. In view of possible commercial applications, recent work focussed on the shape selective synthesis of substituted dinuclear aromatics, i.e., 4,4 -diisopropylbiphenyl and 2,6-dialkylnaphthalenes, due to their potential as components in high-temperature resistent polyesters or as liquid crystals. Recent advances in this field are covered in two excellent review articles [35, 36]. 

---