Copolyesters isotropization temperature

Copolymerization is one of the most efficient synthetic techniques to decrease the crystallinity and melting temperature of a given polymer. The melting-point depression occurring in copolyesters (Vectra) based on p-hydroxybenzoic acid and 2,6-hydroxynaphthoic acid (HNA) is in fact relatively moderate in comparison with that of other copolymers (Fig. 6.35). The minimum in melting point occurs at about 40 mol% HNA. The decrease in isotropization temperature in the copolyesters is moderate, leading to the desired expansion of the temperature region of a nematic phase. 

Fig. 5. Phase diagram of copolyester HTH-HIH/PG samples , melting temperature (TJ , isotropization temperature T. ...

Characterization439 Inherent viscosity before and after solid-sate polymerization is 0.46 and 3.20 dL/g, respectively (0.5 g/dL in pentafluorophenol at 25°C). DSC Tg = 135°C, Tm = 317°C. A copolyester of similar composition440 exhibited a liquid crystalline behavior with crystal-nematic and nematic-isotropic transition temperatures at 307 and 410°C, respectively (measured by DSC and hot-stage polarizing microscopy). The high-resolution solid-state 13C NMR study of a copolyester with a composition corresponding to z2/zi = 1-35 has been reported.441... 

PHB/PET copolyester are presented in Figure 11. Data is presented for several temperatures and we see that die swell increases with increasing temperature which is contrary to flexible chain isotropic polymers. Values of die swell based on a theory proposed by Tanner (15) and calculated from measured rheo . ggical properties have been compared with measured values of D./D. The... 

At temperatures above the melting temperature of the crystalline phases, the system becomes single-phase again and is transformed into oriented nematic LC melt. The copolyester is isotropic (or melts) at temperatures of about 400°C, at which temperatures its active chemical destmction takes place. 

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... 

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