Liquid-crystal polyesters

In Chapter 3 it was pointed out that certain rod-like polymers showed many of the attributes of liquid crystals in the melt. In particular, these molecules were oriented in shear to such an extent that interchain entanglement was small and the melts had a low viscosity. On cooling of the melt these rod-like molecules remained oriented, effectively self-reinforcing the polymer in the direction of flow. The essential differences in the properties of liquid crystal polymers 

Figiire 25.25. Typical sequences found in intractable liquid crystal polymers with 400°C 

However, momomers based on these units alone have extremely high melting points and are intractable. This has led to the use of a variety of techniques to produce more tractable materials which may be processed without degradation whilst retaining many of the features of liquid crystalline materials. These techniques include  

Polymers based on these structures became available as Victrex SRP (ICI), Vectra (Hoechst) and Xydar (Amoco). It is reported that the Vectra materials are based on p-hydroxybenzoic acid and hydroxynaphthoic acid monomers, whilst Xydar is based on terephthalic acid, p-hydroxybenzoic acid and pp -dihydrox-ybiphenyl. Whilst properties vary between grades, in general they possess the following outstanding properties  

High continuous use temperatures with a UL thermal index around 215°C. 

High heat distortion temperatures (over a range 170-350°C) for unfilled materials with some further increase with some types when filled with glass 

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Polyester sheet products may be produced from amorphous poly(ethylene terephalate) (PET) or partiaHy crystallized PET. Acid-modified (PETA) and glycol modified (PETG) resins are used to make ultraclear sheet for packaging. Poly(butylene terephthalate) (PBT) has also been used in sheet form. Liquid-crystal polyester resins are recent entries into the market for specialty sheet. They exhibit great strength, dimensional stabHity, and inertness at temperatures above 250°C (see Polyesters,thermoplastic). 

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. 

The homopolymers of p-hydroxybenzoic acid have such a high value for the that they are somewhat intractable. Useful materials may, however, be made by copolymerising with a view to introducing some molecular flexibility or reducing chain packing or introducing some non-linear links. Commercially important liquid crystal polyesters are discussed in Chapter 25. 

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. 

For the liquid crystal polyesters the basic structural units are derived from such materials as 5-hydroxybenzoic acid, terephthalic acid and hydroquinone. Some basic sequences from such materials are shown in Figure 25.25. 

Figure 25.26. Techniques for reducing of liquid crystal polyesters...

Liquid crystal polyesters Polyester Thermoplastic Elastomers Poly(pivalolactone)... 

Liquid crystalline polymers (LCPs), 10 374, 517-518 13 370-372. See also Liquid-crystal polyesters (LCPs) Liquid-crystal polymers (LCPs) thermotropic, 13 381-382 Liquid-crystalline thermotropic polyesters, 20 34... 

High-Performance Liquid Crystal Polyesters with Controlled Molecular Structure... 

HIGH-PERFORMANCE LIQUID CRYSTAL POLYESTERS 4.2 MODULI OF INJECTION MOLDED SPECIMENS... 

Other polyesters of commercial importance are polycarbonates, liquid crystal polyesters, unsaturated polyesters, and copolymers (Secs. 2-8e, 2-14g, 2-12, 2-13). 

More recent developments involve the preparation of liquid crystalline polyarylates and liquid crystal polyesters in general. 

The field of liquid crystal polyesters (LCP) is expanding very rapidly and has been reviewed [22], The area of chiral liquid-crystalline polyesters has also been researched [32]. Several companies have started to exploit their use on a commercial scale. The high-melting LCP s are expected to find use as replacements for metals, ceramics, composites, etc., in such areas as telecommunications equipment, aerospace, automotive parts, computers, and electrical/electronic components. 

The thermotropic liquid-crystal polyester Vectra A (Hoechst-Celanese) is reported to have the repeating units (3) and (4) and Xydar (Dartco) the repeating units (5) and (3). 

Bashir et al. (1998), in a study of phase transitions in a monotropic liquid-crystal polyester, found that PC and HF instruments performed differently in cooling mode. Better resolution of thermal events (i.e., better separation of peaks on the thermogram) was obtained with PC instruments. These authors pointed out that the performance of any DSC instrument might not be the same during heating as during cooling. 

Bashir, Z., Khan, N., Price, D.M. 1998. Transition-temperature differences on cooling for a monotropic liquid-crystal polyester observed in DSCs of different design. Thermochim. Acta 319, 47-53. 

Strzelecki and coworkersfound that the use of (+) 3-methyladipic acid in the synthesis of liquid crystal polyesters led to a cholesteric polymer. They prepared a homopolymer and a series of copolyesters of the following structures ... 

A liquid crystal polyester having azoxybenzene mesogenic units and decamethylene spacers was subjected to a proton NMR study after the polymer was aligned in a magnetic field of 1 Tesla Below the melting point of the polymer, the NMR spectrum... 

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