Degree of liquid crystallinity

The existence of long T2 component and the T2 increase with time suggest the degree of liquid crystallinity in LCP. The same kind of behavior was also observed for the other copolymer type main chain LCPs. The detailed analysis will be also given in the future. 

If one assumes that a fully developed N phase (the degree of liquid crystallinity is unity) is formed on cooling, a process which is facilitated by the large drop in viscosity at the I-N transition, the smaller value of Ah, may be attributed to an incomplete isotropization on heating. Perhaps, as a direct result of molecular segregation across the biphase. 

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... 

The high melting points of the polyterephthalates of substituted hydroquinones can also be reduced to a melt-processable level by modification of the polyesters with 2,6-naphthalenedicarboxylic acid. Since this is a symmetrical aromatic dicarboxylic acid, the polyester melts have a high degree of liquid crystallinity and, therefore, plastics and fibers with high tensile properties can be produced from them. 

To be able to achieve maximum tensile and stiffness properties in melt-processed polyesters, the molten polyesters must have both a high degree of liquid crystallinity, which enables high orientation to be attained on injection molding or melt spinning, and sufficiently long relaxation times so that this orientation is maintained as the melt cools. Kinked components in the polymer chains reduce the amount of polymer orientation that can be achieved, and flexible components reduce the relaxation time. 

The disiloxanyl (n = 1) polymer also exhibits a somewhat elusive liquid-liquid thermal transition and may suggest some degree of liquid crystalline order. We are currently pursuing the synthesis and polymerization of a homologous series of polysiloxane monomers (Figure 5) from commercially available bis-chlorosiloxanes in order to systematically reveal the physical properties and potential liquid crystallinity of linear PFCB silicones. 

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