Aromatic liquid-crystalline copolyesters

Bharadwaj Rishikesh, Boyd Richard H. (1999). Chain Dynamics in the Nematic Melt of an Aromatic Liquid Crystalline Copolyester A Molecular Dynamics Simulation Study. J. Chem. Phys, 1(20), 10203-10211. 

Morton et al. [ 125] studied the isothermal crystallization kinetics of polyethylene terephthalate (PET) in blends with fully aromatic liquid crystalline copolyester (Vectra A). As Vectra fractions increase in the blend, both the PET crystallization rates and the percentage of PET that is crystalline decrease. 

Aromatic liquid crystalline (LC) copolyesters of commercial importance are made by processes which produce compositionally heterogeneous chemical sequences along the polymer chain. Such chemical heterogeneity is responsible for a number of interesting properties of thermotropic LC polymers. These properties include a small amount of 3Current address General Electric Plastics, Mount Vernon, IN 47620... 

Yerlikaya Zekeriya, Aksoy Serpil, Bayramli Erdal. (2001). Synthesis and Characterization of Fully Aromatic Thermotropic Liquid-Crystalline Copolyesters Containing m-hydroxybenzoic Acid Units J. Polym. Sci. A, 39(19), 3263-3277. 

Bi Shuguang, Zhang, Yi., Bu Haishan, Luise Robert, R., Bu Jenny, Z. (1999). Thermal Transition of a Wholly Aromatic Thermotropic Liquid Crystalline Copolyester. J. Polym. Sci. A, 37(20), 3763-3769. 

The term hierarchical structure was first described for aramid by Dobb et al. [4] and for injection molded thermotropic liquid crystalline copolyester by Weng et al. [5]. It has been found that no matter which LC polyester is concerned, wholly aromatic or semi-aromatic ones. 

A type of aromatic-aliphatic liquid crystalline copolyesters has been synthesized from 2-chlorohydroquinone, 1,4-CHDM and terephthaloyl chloride (Figure 6.14). The CHDM units act in these copolyesters as flexible spacers providing the necessary chain mobility to be thermotropic. The structure and thermal behavior of these copolyesters were characterized by hot-stage polarized light microscopy, SEM, FTIR, NMR, DSC and TGA (57). It has been reported that they are insoluble in common solvents and may form mesophases at temperatures near the melting temperature of PET (58). 

In a study of the effects of temperature on the melt viscosities of copolyesters of terephthalic acid, i sophthalic acid, and methylhydroquinone, McFarlane and Davis observed that minima in the melt viscosity versus temperature curves occurred at about 340 to 360°C in compositions containing 40 to 60 mol % isophthallc acid (total diacids equal 100 mol %). The increase in the melt viscosities with increasing temperature after the minima presumably is due to the Increase in the isotropic content of the polymers and the decrease in the degree of liquid crystallinity. We did not observe this phenomenon in the rigid-rod, all-aromatic, liquid crystalline polyesters that did not contain any meta component because of the high temperatures involved (above the decomposition temperatures of the polyesters). A poly(terephthalate-Isophthalate) of methylhydroquinone containing 70 mol % isophthallc acid was not liquid crystalline and, therefore, did not exhibit a minimum in a plot of melt viscosity versus temperature... 

White JL, Dong L, Han P, Laun HM (2004) Rheological properties and associated structural characteristics of some aromatic polycondensates including liquid-crystalline polyesters and cellulose derivatives. Int Union Pure Appl Chem 76(ll) 2027-2049 Wiberg G, Hillborg H, Gedde UW (1998) Assessment of development and relaxation of orientation in a sheared thermotropic liquid crystalline copolyester. Polym Eng Sci 38 1278-1285 Wilson TS, Baird DG (1992) Transient elongational flow behavior of thermotropic liquid crystalline polymers. J Non-Newt Fluid 44 85-112... 

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. 

However, interchange reactions appear to be rather slow at temperatures below Tm(15). We(50) observed that thermal treatment of some of liquid crystalline, aromatic copolyesters at the temperatures substantially lower than Tm did not lead to any changes in the comonomer sequence even after a prolonged period of time. A copolyester especially of 4-hydroxybenzoic acid, however, can undergo a special type of sequence changes below Tm, which is called the crystallization induced reaction(51). 

Although studies concerning main chain liquid crystalline polymers were originated by Onsager (la) and Ishihara (lb) in the late 1940 s, extensive work in this field did not really begin until the early 1970 s. Jackson and Kuhfuss (2) reported the first thermotropic polyester by modifying polyethylene terephthalate with various amounts of p-hydroxybenzoic acid (HBA). They found that the copolyester with HBA content of at least 35 mole % have opaque melts. Subsequent studies in the area of aromatic polyesters by various authors resulted in a large number of patents and publication. (3.) These polymers were all derived from unsubstituted and... 

A comprehensive interpretation of the microstructure of the liquid crystalline aromatic copolyesters is presented. The role of the synthetic route and of high temperature processing on the microstructure are clearly defined. As a result of this study a predictive model now exists which permits interpretation of the very subtle chemical processes which can occur at elevated temperatures leading to either randomization or ordering of the microstructure. 

Materials. The amorphous matrix polymer was polyetherimide (Ultem 1000) sold by the General Electric Company. The semicrystalline matrices were PEEK and a high molecular weight polyphenylenesulfide (PPS) provided by ICI and Phillips Petroleum Co. (Barthesville, OK.), respectively. The reinforcing phase was one of several LCPs a liquid crystalline aromatic copolyester consisting of 73 %... 

Fig. 3.5 Structural hierarchy in liquid-crystalline fibers. The mechanical performance of highly oriented polymers can approach the ultimate theoretical properties at high degrees of elongation. Anisotropic, rod-like macromolecules, like aromatic copolyesters composed of 2,6-naphthyl and 1,4 phenyl units, often form oriented structures, which can exhibit liquid crystallinity. Extensive structural studies of fibers of these oriented copolyesters showed a hierarchical structure like the one depicted in this Figure. In aramids (Kevlar or Twaron) similar structures may exist. Adopted with permission from [17]...

The unique molecular packing of rod-like chains in liquid crystalline polymers (LCP) closely resembles the extended chain structure of highly oriented flexible chain polymers, suggesting that these materials are good candidates for barrier applications. Thermotropic LCP s, first developed in the early 1970 s, have been the object of much interest because of their excellent mechanical properties and ease of product fabrication. Preliminary observations have shown that a commercially available wholly aromatic thermotropic copolyester has gas permeability coefficients that are lower than those of polyacrylonitrile (4.). These results raise some fundamental questions as to the nature of the mechanism for transport of small molecules through a matrix of ordered rigid rod-like chains. 

Takahashi Toshisada, Shoji Hirotoshi, Tsuji Masaharu, Sakurai Kensuke, Sano Hiro-fumi, Xiao Changfa. (2000). The Structure and StretchabUity in Axial Direction of Fibers from Mixes of Liquid-Crystalline all-Aromatic Copolyesters with Polyethyleneterephthalate. Fiber, 56(3), 135-144. 

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