Thermotropic liquid crystalline melt processing

Materials and Processing. Copolyesters of poly(ethylene terephtha-late) (PET) and para-oxybenzoic acid (FOB) were supplied by the Tennessee Eastman Corporation. Past work Indicates the copolyesters form thermotropic liquid crystalline phases at compositions containing more than 30 mole% POB (26,27,28). The composition of the copolyester studied here contains 60 mole% POB. Quiescent liquid crystalline films were made by compression molding the copolyester at 210, 230, 255, and 285 C, and followed by a quench Into Ice water, ambient air, or cooled In the press with the power off. Film thicknesses ranged between 0.05-0.15 mm. Another sa(q>le of the 40/60 PET/POB copolyester was melted at 270 °C In a Mettler hot stage, manually sheared between glass slides, and then ambient air cooled. 

Thermotropic liquid crystalline polymers have recently been a topic of scientific investigation (1-31. Because of their thermotropic liquid crystalline character from rigid structural moieties incorporated in the polymer backbone, these polymers possess outstanding melt processability and excellent mechanical properties. 

Jackson and Kuhfuss reported a thermotropic polymers system comprised of polyethylene terephthalate) modified with HBA (4). They demonstrated not only the thermotropic liquid crystalline behavior but also the excellent melt processability of these materials to yield specimens with high modulus values. However these copolymers had very low use temperatures as reflected by low heat deflection temperatures. 

The purpose of this research is to synthesize a series of thermotropic liquid crystalline PECs consisting of HBA, DHBP, and carbonyl units with good melt processability and high heat deflection temperatures. The carbonate linkage is expected to reduce LCTT without reduction of the chain rigidity and heat resistance. 

Ternary blends from a thermotropic liquid crystalline polymer, PEN, and PET were prepared by melt blending and melt spinning to fibers. The mechanical properties of ternary blend fibers could be significantly improved by annealing at 180°C for 2 h. This is attributed to the development of more ordered crystallites and to the formation of more perfect crystalline structures. The interfacial adhesion between PEN and liquid crystalline polymer phases is enhanced when the blends are processed with dibutyl-tindilaurate as a reactive catalyst to promote transesterification. 

From practical considerations, two properties are of prime interest The effect of liquid crystalline behavior on viscosity and the ability of the polymer to retain the ordered arrangement in the solid state. Liquid crystalline behavior during the melt results in lower viscosity, because the rigid polymeric mesophases align themselves in the direction of the flow. As a result, the polymer is easier to process. Also, retention of the arrangement upon cooling yields a material with greatly improved mechanical properties. Several thermotropic liquid crystalline copolyesters are now available commercially. 

Thermotropic liquid crystalline polyesters can be prepared by the use of meta-oriented modifiers to reduce the melting points of para-substituted aromatic polyesters to the processible range. Resorcinol, isophthalic acid and m-hydroxybenzoic acid are commonly used. Figure 14 shows the effect of copolymerized m-hydroxybenzoic acid on the physical properties of melt processable, modified polyesters based on terephthalic acid and methylhydro-quinone. As the m-hydroxybenzoic acid (MHB) content increases... 

K. G. Blizard and R. R. Haghighat, Processing and properties of polyimide melt blends containing a thermotropic liquid crystalline polymer, Polym. Engr. and Sci., vol. 33, No. 2, 1-9. 

It has been well recognized that melt blending of a thermotropic liquid crystalline polymer (LCP) and an isotropic polymer produces a composite in which fibrous LCP domains dispersed within the blend act as a reinforcement il ). The so-called insitu composite possesses several advantages in comparison with the inorganic reinforced thermoplastic composites. Firstly, LCP lowers the blend viscosity in the actual fabrication temperature range (3-5), Hence, the enhanced processability endows moldability for fine and complex shaped products. 

Liquid crystalline polymers [326,329-336] have found application as high modulus fibers and films with unique properties due to the formation of ordered lyotropic solutions or thermotropic melts which transform easily into highly oriented, extended chain structures in the soUd state. Thermotropic polymers are melt process-able and thick extrudates and molded parts are formed with high strength characteristics as in fiber reinforced thermoplastics. 

The first melt-processable (later categorized as thermotropic liquid-crystalline) polymer, based on p-hydroxybenzoic acid and biphenol tereph-thalate, was reported by Steven Cottis in 1972. This polymer is now available on the market as Xydar . In 1973, the first well-characterized thermotropic polymer, a copolyester of p-hydroxybenzoic acid and ethylene terephthalate, was patented by Herbert Kuhfuss and W. Jerome Jackson (Eastman-Kodak Co., USA). They reported the discovery of liquid-crystalline behaviour in this polymer in 1976. At the beginning of the 1980s, the Celanese Company developed a family of processable thermotropic liquid crystalline polymers based on hydroxybenzoic acid and hydroxynaphthoic acid, later named Vectra . 

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