Parallel and perpendicular orientation in a thermotropic main-chain liquid-crystalline polymer

The orientational behavior of a thermotropic poly(ether ester), PH31B32, with biphenyl units as mesogens and spacers with methyl substituents, has been analyzed. This polymer is one of the few mesophase-forming systems where the amorphous phase can be easily quenched from the isotropic melt. Moreover, under certain conditions the mesophase of this polymer is able to exhibit an anomalous orientation with the molecular axes perpendicular to the uniaxial deformation direction. Specimens of PH31B32 both in the amorphous state or in the smectic mesophase have been studied by stress-strain and fiber X-ray diffraction experiments. In the first case, a fast strain-induced liquid crystallization was observed. In the second case, several strain rates and deformation temperatures have been tested in order to map out the conditions for obtaining the two different kinds of orientation. From the diffraction results, obtained at a final deformation of 300%, those conditions have been approximately defined, leading to a region (at "intermediate" stretching temperatures and strain rates) of mixed orientations. Exclusively parallel orientation is obtained at low deformation temperatures and high strain rates, while the region with 100% perpendicular orientation will appear at rather low strain rates and high temperatures, outside the detection limit of the dynamometer, and overlapping with the isotropization of the mesophase. A model for the two kinds of orientation, based on the hierarchical structure of liquid-crystalline polymers, is proposed.