Liquid Crystalline Aromatic Polyesters

Aromatic polyesters that do not contain any flexible structural units are often nonmeltable or extremely high melting polymers that cannot be processed. Copolymerization is a way to obtain processable wholly aromatic polyesters The Tm versus copolyester composition curve is a U-shaped curve exhibiting a minimum that is generally well below the Tm of corresponding homopolymers. Liquid crystalline aromatic polyesters, for instance, are usually copolymers.72 An example is Ticona s Vectra, a random copolyester containing 4-oxybenzoyl and 6-oxy-2-naphthoyl units in ca. 70 30 mol ratio. This copolymer melts at ca. 

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 crystalline aromatic polyesters are a class of thermoplastic polymers that exhibit a highly ordered structure in both the melt and solid states. They can be used to replace such materials as metals, ceramics, composites and other plastics... 

Sueoka, K., Nagata, M., Ohtani, H., Nagai, N., and Tsuge, S., Thermal Degradation Mechanisms of Liquid Crystalline Aromatic Polyester Studied by Pyrolysis-Gas Chromatography/Mass Spectrometry, /. Polym. Sci Part A, 29,1903,1991. 

N. Khan, Z. Bashir, and D. M. Price, Liquid crystalline aromatic polyesters formed with terephthalic acid, phenyl hydroquinone, and naphthalene or anthracene diols. Journal ( Applied Polymer Science, 58,1509 (1995). 

Polyarylate polymers are aromatic polyesters derived from aromatic dicarboxylic acids and diphenols. In contrast to liquid crystalline aromatic polyester (derived from dicarboxylic hydroxy acids), polyarylates exhibit amorphous character on molding Tg is ca 198°C. Polyarylates present a competitive cost(performance profile in the context of amorphous engineering thermoplastics, delivering an excellent balance of mechanical properties, particularly practical heat resistance which substantially exceeds that of polycarbonates (qv). 

In an earlier paper in this series, an overview was given of the effect of structure on the melting points and polymer properties of thermotropic liquid crystalline aromatic polyesters. The basic structure in these... 

As indicated above, when the films were heated for extended times in air, the films became brittle (broke on creasing). In liquid crystalline aromatic polyesters containing substituted hydroqulnones, the effect of the substituent on stability in air is as follows (decreasing stability, left to right, based on time required for 10-mil thick films suspended in a forced-air oven at 150 C to become brittle) ... 

A methyl or chloro substituent attached to a hydroquinone moiety in a liquid crystalline aromatic polyester decreases the thermal-oxidative stability of the polyester, but the substituent does permit greater chemical resistance of a shaped object to be obtained by heat treatment in air at 300°C. The stability of substituted hydroquinone polyesters in air at 150 C decreases with various substituents in the order of phenyl, tert-butyl, chloro, methyl. 

Liquid crystalline aromatic polyesters are well known and their structure-property relationships are chronicled in detail in the literature [204-208], Their generic structure is ... 

Jackson, W. J., Jr, Liquid Crystalline Aromatic Polyesters An Overview, J. Appl. Polym. ScL, Appl Polym. Symp., 41, 25-33, 1985. 

Liquid crystalline aromatic polyamides were the first chemical class to be commercialised. The best known example is Kevlar fibre which is spun from liquid crystalline solution to obtain the benefit of the high orientation present in the nematic phase. Subsequently, melt-processable main-chain polyesters were developed and brought to the market (Amoco with Xydar, Hoechst-Celanese with Vectra). 

Liquid crystal polymers (LCP) are highly ordered crystalline aromatic polyesters with high mechanical strength. They are exceptionally inert, having very high thermal and chemical stability, are highly resistant to UV radiation and are inherently flame retardant and antistatic. 

Early studies of the thermal degradation of so-called polyarylates were covered by Neiman [100] and Ehlers and co-workers [101]. Since then, several highly aromatic and specifically liquid crystalline (mesogenic) polyesters have been examined in terms of their anaerobic thermal degradation characteristics. These inclnde homopolymers of hydroxybenzoic acids [102-105] copolymers of hydroxybenzoic acid with hydroxynaphthoic acid [105-108] polymers which are essentially copolymers of hydroxybenzoic acid and alkyene terephthalates [107-118] copolymers of hydroxybenzoic acid with other aromatic polyesters [119-122] phenolic and bisphenolic terephthalates [123-127] poly(oxynaphthoate)s [128] and liquid crystal polyesters (LCP) containing unsaturated acids as part of a copolyester chain [129-131]. 

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. 

Aromatic polyesters constitute an important class of main-chain liquid-crystalline polymers, but present the inconvenience of their reduced solubility and very high transition temperatures (sometimes not detected before the degradation of the sample). Their processability can be improved in several ways [2,3], e.g., reduction of the rigidity of the mesogen, lengthening of the spacer, or introduction of lateral substituents. 

Wholly aromatic polyesters, in which both R1 and R2 are aromatic, are either high-7 amorphous polymers or veiy high melting semicrystalline polymers that often exhibit liquid crystalline properties. 

Amorphous bisphenol-A polyarylates are soluble in dioxane and in chlorinated solvents such as CH2C12, 1,2-dichlororethane, 1,1,2-trichloroethane, and 1,1,2,2-tetrachloroethane while semicrystalline and liquid crystalline wholly aromatic polyesters are only sparingly soluble in solvents such as tetrachloroethane-phenol mixtures or pentafluorophenol, which is often used for inherent viscosity determinations. 

A procedure similar to that given below for wholly aromatic liquid crystalline polyesters can be used (see Section 2.4.4.2). The inherent viscosity of the resulting polymers were ca. 0.6 dL/g (0.5 g/dL in phenol-o-dichlorobenzene at... 

Weathering tests, 245 Wholly aromatic liquid crystalline polyesters, degradation of, 38 Wholly aromatic polyamides, 136-137, 139 synthesis of, 184-189 Wholly aromatic polyesters, 25-26, 32 copolymerization and, 35 synthesis of, 71-72... 

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 ... 

Preparation of a Liquid Crystalline (LC), Aromatic Main-Chain Polyester by Polycondensation in the Melt... 

Further calculations on aromatic polyesters, polyamides and liquid crystalline systems ... 

---