27 No. 2
Rheological Properties and Associated Structural Characteristics of Some Aromatic Polycondensates Including Liquid-Crystalline Polyesters and Cellulose Derivatives
(IUPAC Technical Report)
J.L. White, L. Dong, P. Han, and M. Laun
Pure and Applied Chemistry
Vol. 76, No. 11, pp. 2027–2049 (2004)
Aromatic polycondensates became increasingly important in the 1980s. Characteristic of these polymers are para-linked aromatic rings in their backbones, which tend to make the chains more rigid than aliphatic hydrocarbon (e.g., vinyl) polymers. While such p-linked aromatic polycondensates like poly(carbonate) or poly(ethylene terephthalate) had been known since the 1950s, the full implications were only realized in the 1970s with the discovery that concentrated solutions of poly(p-phenylene terephthalamide), poly(p-benzamide), and similar polymers exhibited rest state birefringence, liquid-crystalline phases, and associated viscosity reductions. This paper describes a comparative experimental study of shear-flow rheological properties of thermotropic polymer liquid crystals by eight different laboratories. The materials involved four different liquid-crystalline polyesters (LCPs), a glass- fiber-filled liquid-crystalline polyester, hydroxypropyl cellulose (HPC), and two non-liquid-crystalline high-temperature polymers, a poly(etheretherketone) (PEEK), and a polyarylate (PAR). Studies were made in both steady shear-flow and dynamic oscillatory experiments. The data from the various laboratories involved were compared. The level of agreement in the data was much less for most liquid-crystalline polymers than for similar isotropic melts. The Cox -Merz rule is valid for PEEK and PAR, but not for the LCPs and HPC. The occurrence of low levels of extrudate swell and high levels of uniaxial orientation in extrudates of the LCPs and HPC is described.
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