Polyesters with Noncoplanar Aromatic Moieties

Noncoplanar aromatic monomers, such as 2,2 -disubstituted biphenylene derivatives and 4,4 -functionalized 1,1 -binaphthyl derivatives (Fig. 14) have been used as comonomers in para-linked aromatic polyesters with remarkable effects on the phase transition temperatures, the crystallinity, and the solubility. The incorporation of these noncoplanar monomers will not initially reduce the chain stiffness. The phenyl rings are forced by the 2,2 -substitution into a noncoplanar conformation which strongly decreases the intermolecular inter- 

Another example of such a type of polyester is shown in Fig. 4. The polyester po-ly(2,2 -dimethyl-4,4 -biphenylene-phenyl-terephthalate) derived from 2,2 -dimethyl-biphenyl-4,4 -diol (BDMDH) and phenyl-terephthalic acid forms a nematic melt. Crystallization from the melt is completely 

The formation of anisotropic thermorever-sible gels was observed with this polyester, for example, in 3-phenoxytoluene as the solvent. These mixtures also retained their anisotropy above the gel melting point at a concentration of 35% or more [29]. In highly aligned fibers spun from the nematic melt and post-drawn, a tensile modulus of 40-45 GPa and a tensile strength of up to 550-650 MPa was reached [28c]. 

Even higher glass transition temperatures are achievable if4,4 -functionalized 1,1 -binaphthyl monomers are used [28 b, 30]. For example, the homopolyester derived from 4,4 -dihydrox-1,1 -binaphthyl (BDP) and phenylterephthalic acid with an inherent viscosity of 1.90 dl/g has a Tg of 183 °C. The polyester is nematic up to its decomposition temperature and forms a nematic glass below Tg. In a similar homopolyester based on 4,4 -dicarboxy-1,1 -binaphthyl and t-butyl-hydroquinone the Tg was found to be 189 °C. In both cases, good solubility in common chlorinated solvents is maintained. This type of monomers was, for example, also used to modify wholly aromatic copolyesters based on 6-hydroxy-2-naphthoic acid (HNA) and 2,6-naphthalene dicarbox-ylic acid (NDA) [25 a]. 

In conclusion, the selected examples of this chapter clearly demonstrate that it is possible with the discussed structural mod-ifiations to modify in a systematic manner the melting behavior over a wide temperature range, the degree of crystallization from highly crystalline to amorphous, and the so- 

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