A study of processing methods for producing biaxial orientation in thermotropic liquid crystalline polymers

Four methods of biaxial deformation were considered in an attempt to obtain biaxial orientation and properties in two thermotropic copolyesters: hydroxybenzoic acid-co-polyethylene terephthalate (HBA/PET) and hydroxybenzoic acid/2, 6 hydroxynaphthoic acid (HBA/HNA). They consisted of solid phase forming, extrusion blow molding, biaxial stretching, and film blowing. Some measure of biaxial properties and orientation was obtainable in all four processes, from the layered structure apparent in the blow molded bottles to the uniplanar orientation in the stretched film. Tensile and flexural properties were exceptional in the blown film in comparison to the other processes considered: tensile strengths of 300 to 400 MPa and moduli of 13 GPa were observed in the axial direction, for example. This result is believed due to the absence of a skin-core texture in which the core often remains relatively unoriented.

A second objective of this work was to determine the feasibility of using Doi's theory for anisotropic solutions of rigid rods to predict the structure developed during the deformation processes for these thermotropic systems. Doi's theory of nematics, although limited to monodomains and containing numerous mathematical approximations in order to obtain a closed form for the kinetic equation, was used to predict the orientation development in the blown film process. Although qualitative agreement with experiment was found, quantitative agreement could not be expected.

Since Doi's theory was developed by examining an isolated rigid rod in solution and modifying the resultant equations to account for interactions of neighbors, transient predictions on a lyotropic solution were also made and compared to experimental results. In particular, stress growth on the inception of a steady simple shear flow was examined for the system of poly(p-phenylene terephthalamide)/ 100% sulfuric acid. Comparisons to Doi's theory were made for both isotropic and anisotropic solutions. Quantitatively the theory was found to be unacceptable, particularly at higher shear rates in which the equilibrium stress was at least an order of magnitude lower than experimental results showed. It did, however, predict an overshoot in the shear stress, which increases with deformation rate.