Rheological characterizaton, and the development of molecular orientation and texture during flow for a liquid crystalline copolymer of para- hydroxybenzoic acid and polyethylene terephthalate

It is generally agreed that the high physical properties arising in as-processed liquid crystalline materials are due to the high degree of molecular orientation which develops during the processing step. In order to more fully understand and predict such behavior, a constitutive equation describing the flow behavior of these materials would be useful. It has been suggested that in order to describe the rheology of liquid crystalline fluids such a constitutive equation would need to include molecular orientation effects. The purpose of part of this study has been to examine the usefulness of several constitutive equations for describing the steady and transient behavior of several liquid crystalline polymers. These include a copolyester of para-hydroxybenzoic acid and polyethylene terephthalate, and an anisotropic solution of 12 weight% Kevlar in 100% sulfuric acid. It was found that in the case of the copolyester system, the steady shear and dynamic viscosities were equal over certain temperature ranges. For this reason, the constitutive equation of Zaremba, Fromm, and DeWhitt (ZFD model) was used to predict the steady state behavior of the system studied. From knowledge of either the steady shear or dynamic viscosity it was possible to predict both the steady state normal stresses (N1) and the storage modulus (G'). The model could not, however, predict the transient behavior of the systems studied. Ericksen’s anisotropic fluid theory has been investigated in detail as it takes molecular orientation effects into account. Ericksen’s theory can partially explain the transient behavior of the systems studied in terms of molecular orientation which develops during shear flow. However, wide angle x-ray scattering (WAXS) and scanning electron microscope (SEM) studies reveal that shear flow has little effect on the development of molecular orientation during flow. In addition, any orientation produced during flow may be lost within thirty seconds at the melt temperature. It appears that a disruption of texture is occurring during flow which may need to be incorporated into the theory of Ericksen.