Copolyester structure, liquid-crystalline

One may now ask whether natural systems have the necessary structural evolution needed to incorporate high-performance properties. An attempt is made here to compare the structure of some of the advanced polymers with a few natural polymers. Figure 1 gives the cross-sectional microstructure of a liquid crystalline (LC) copolyester, an advanced polymer with high-performance applications [33]. A hierarchically ordered arrangement of fibrils can be seen. This is compared with the microstructure of a tendon [5] (Fig. 2). The complexity and higher order of molecular arrangement of natural materi-... 

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. 

The effect of incorporating p-hydroxybenzoic acid (I) into the structures of various unsaturated polyesters synthesised from polyethylene terephthalate (PET) waste depolymerised by glycolysis at three different diethylene glycol (DEG) ratios with Mn acetate as transesterification catalyst, was studied. Copolyesters of PET modified using various I mole ratios showed excellent mechanical and chemical properties because of their liquid crystalline behaviour. The oligoesters obtained from the twelve modified unsaturated polyesters (MUP) were reacted with I and maleic anhydride, with variation of the I ratio with a view to determining the effect on mechanical... 

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. 

To date, reports of investigations on the gas transport properties of main chain liquid crystalline polymers appear to have been limited to the work conducted in our laboratory. Chiou and Paul (4.) have briefly described the transport parameters of an extruded film of an LCP having a similar structure to the commercial product Vectra. This copolyester belongs to the family of napthylene thermotropic polymers (NTP s) commercialized by Hoechst-Celanese Corp. whose synthesis and properties have been described previously (iLS.). Transient permeation experiments were conducted with a series of gases. The effective... 

Blackwell J, Biswas A, Gutierrez GA, Chivers RA (1985) X-ray analysis of the structure of liquid-crystalline copolyesters. Faraday Disc. Chem. Soc. 79 73 see also Chivers RA, Blackwell J, Gutierrez GA (1984) Polymer, 25 435... 

An equally important observation for the above copolyester LCPs is that the ordered arrangement of polymeric mesophases in the melt is retained upon cooling, which is manifested in greatly improved mechanical properties (see Figure 5.5b). The liquid crystalline behavior is therefore advantageous from the standpoint of both processing and properties. Thermotropic liquid crystal copolyesters of structures similar to (I) are now available commercially. 

Preparation of Main Chain Liquid Crystalline Polymers (MCLCPs) Table 8. Representation of some typical structures of polyesters and copolyesters. 

Mosophase-forming ability of a copolyester also strongly depends on its comonomer sequence. For instance, the ordered sequence copolyester whose structure shown below is not thermotropic, while the random copolyester having the same overall composition is liquid crystalline[1,61. Moreover, the former is semicrystalline, while the latter is amorphous. 

Teoh, M. M., Liu, S. L., Chung, T. S. (2005). Effect of Pyridazine Structure on Thin-Fihn Polymerization and Phase Behavior of Thermotropic Liquid Crystalline Copolyesters. J. Polym. Sci. B, 43(16), 2230-2242. 

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. 

Finally, results of structural investigations on random copolymers were recently reported.X-ray methods were used to investigate the physical structure of high strength melt-spun liquid crystalline aromatic copolyester fibers prepared from p-hydroxybenzoic acid (B), 2,6-dihydroxynaphthalene (N) and terephthalic acid (P)... 

Our intent, as mentioned earlier, is not to review all the studies concerned with liquid crystalline fluids but to compare their properties with flexible chain polymers, interpret their properties in terms of the domain structure, and look for correlations between flow characteristics and processing conditions. We first examine the behavior of liquid crystalline copolyesters in steady shear flow and in small strain dynamic oscillatory flow. 

PRELIMINARY THERMAL AND STRUCTURAL STUDIES OF BLENDS BASED ON A THERMOTROPIC LIQUID CRYSTALLINE COPOLYESTER AND POLY(ETHYLENE) TEREPHTHALATE... 

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. 

Guezala et al. [420] studied the viscoelastic behaviour of solutions of the thermotropic main chain liquid crystalline X7G (see Fig. 160), Le. a copolyester (M 20 kg/mol) of poly(hydroxy benzoic acid) (60%) and poly(ethylene tere-phthalate) (40%) in m-cresoi the material used is non-random with respect to chain structure and domains of poly(hydroxy benzoic acid) have been reported. Solutions were studied, varying in concentration from 2.5 to 45 wt%. Special care was taken to avoid water absorption, because in its presence degradation and/or trans terification might occur. 

Skin/core morphologies are common in blends of LCP s and thermoplastic polymers and they play a significant role in defining the properties of both extruded and injection molded samples. Usually, LCP s in the skin have a higher degree of orientation than in the core when the blends are extruded or injection molded (Husman et al. 1980 Hedmark et al. 1989 Lee 1988). Baird et al. (Baird and Mehta 1989 Baird and Sukhadia 1993) observed a skin/core morphology in blends of PA 66 with HBA/HNA and 40 PET/60 PHB and 20 PET/80 HBA copolyesters. More LCP fibers were present in the skin than in the core for both systems. Isayev and Swaninathan (1994) also reported shell-core structure in the fracture surfaces of injection molded blends of HNA/HBA liquid crystalline copolyesters and poly (etherimide). 

Pazzagli Federico, Pad Massimo, Magagnini Pierluigi, Pedretti Ugo, Como Carlo, Bertolini Guglielmo, Veracini Carlo (2000). A. Effect of polymerization conditions on the micro structure of a liquid crystalline copolyester,./. Annl. Polvm. Sci.. 77(1), 141-150. 

Li Xin-Gui. (1999). Structure of liquid crystalline copolyesters from two acetoxyben-zoic acids and polvethvleneterephthalate. J. AddI. Pohm. Sci.. 73(14), 2921-2925. 

Considerable literature exists on the structural organisation of thermotropic liquid crystalline aromatic copolyesters. Majority of these copolyesters are synthesised via the copolymerisation route, which helps in tailoring the characteristic properties to the predecided values. Another major advantage of copolymerisation is that it helps in conferring specific chemical properties to the major component present in the system. 

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