Substituted Hydroquinone Terephthalates

Many researchers have reported the structure-thermal property correlations in LCPs from substituted hydroquinones (HQs) and dicarboxylic acids. Lenz and co-workers have investigated the liquid crystallinity of the polyarylates obtained from substituted HQs and terephthalic acid (TA) [6-10], substituted HQs and l,10-bis(phenoxy)decane-4,4/-dicarboxylic acid [8], and substituted HQs and a,oo-bis(phenoxy)alkane-4,4/-dicarboxylic acid [11], Kricherdorf and Schwarz [12] and Osman [13] reported the liquid crystallinity of the polyarylates obtained from substituted HQs and 1,4-cyclohexanedicarboxylic acid, while Krigbaum el al. [14], Heitz and co-workers [15] and Kricherdorf and Engelhardt [16] investigated the liquid crystallinity of the polyarylates synthesized from substituted HQs and substituted TAs. In addition, Jackson reported the liquid crystallinity and the moduli of fibers and injection molded specimens of the polyarylates... 

Hydroquinone(HQ) or a naphthalenediol in the above reactions can be replaced with other diols such as biphenols and bisphenols. In the same manner, other dicarboxylic acid such as cyclohexane dicarboxylic acid, isophthalic acid, naphthalene dicarboxylic acids can be used in place of terephthalic acid. Of course, these substitutes should not destroy the linearity of the polymer chains if one is to obtain thermotropic compositions. 6-Hydroxy-2-naphthoic acid is a substitute for 4-hydroxybenzoic acid in Reactions 18 and 19. 

P8 M is not the only polymer forming the isotropic smectic phase. To date, we have observed formation of that phase for a half-dozen chiral polymethacrylates and polysiloxanes. Table 5.1 summarizes the chemical structure and phase behavior of synthesized side-chain homopolymers, which carry chirally substituted side chains derived from asymmetric esters of terephthalic acid and hydroquinone. Such a structure with alternating orientation of carboxylic link groups seems to favour the formation of the IsoSm phase, whereas isomeric derivatives of p-hydroxybenzoic acid, where all carboxylic links have the same orientation, form only conventional Sm A and Sm C phases. Molar mass of all the synthesized homo- and copoly(meth)acrylates is within the range of 1 to 2-10 g mol the poiysilox-anes have the average degree of polymerization, p 35. 

A different type of flexible substituent are phenylalkyl substituents [19]. Monosubsti-tuted polyesters with phenylalkyl substituents with different lengths of the alkyl linkage are characterized by high melting temperatures (Table 1). The thermal transitions can be lowered signifieantly if the terephthalic moieties and the phenylene moieties are substituted by phenylalkyl moieties. The replacement of the phenylalkyl substituent at the hydroquinone moiety by f-butylhy-droquinone results in an increase of the glass transition temperature. The polycondensa-... 

A selection of monosubstituted and disub-stituted poly-(/ -phenylene-terephthalate)s is compared in Table 2. Poly(p-phenylene-terephthalate)s with methyl, methoxy, chlo-ro, or bromo substituents on either the hy-droquinone or the terephthalic acid moiety exhibit melting temperatures of 350 °C or higher. Thermotropic liquid crystalline behavior is observed in these samples, although it is in the range of thermal decomposition. A comparison of the mono- and diphenyl substituted polyesters reveals an important trend. The monosubstituted poly-(p-phenylene-terephthalate) with the phenyl substituent in the hydroquinone moiety melts at 346 °C, also forming a nematic melt up to a clearing temperature of... 

The effect of bulky side groups is exemplified by the large melting point lowering observed when phenylhydroquinone is substituted for hydroquinone in terephthalic polyesters, as shown in Figure 4. 

The next development in liquid crystal polyesters was the preparation by polycondensation based on terephthalic acid (TPA) and hydroquinone (HQ) or p-hydroxybenzoic acid (HBA). The polyesters are insoluble with very high melting temperatures of 600 °C for poly (TPA/HQ) and 610 °C for poly (HBA), which are by far too high to obtain stable liquid crystalline phases for melt processing. In 1972, Economy and coworkers patented several copolyester compositions, and one of these are the copolymerization of poly (4-hydroxybenzoic acid) (PHB) with 4,4 -dihydroxybiphenyl (BP) and terephthalic acid (TPA) due to the need for lower melting, melt-processable polymers. Considerable synthetic efforts have been attempted in order to decrease the melting temperatures of aromatic LC polyesters while retaining LC properties. The copolyester structure was tailored by partial substitution of TPA with isophthalic acid to produce a melt-spinnable material. 

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