High pressure mesomorphism in polymers

Date of Completion

January 1987


Chemistry, Polymer|Plastics Technology




The aim of the research reported herein was to discover whether liquid crystallinity could be induced in polymers via the application of high pressure. In addition to exploring such pressure-induced mesomorphism, we aimed to develop techniques for characterizing pressure-induced mesophases in polymer melts. These aims necessitated the development of a straightforward multifunctional pressure apparatus, based on a modified Instron capillary rheometer, capable of measuring the pressure-volume-temperature relationship, differential thermal analysis, rheological properties and thermal diffusivity of polymers at elevated pressures. The purpose of including the rheological experiment was to identify pressure-induced mesophases at elevated pressures. In the resulting apparatus, the maximum operating pressure is 6000 bars and the maximum temperature is approximately 380$\sp\circ$C.^ During this study, three polymeric systems were examined: a semi-crystalline high density polyethylene, an amorphous polysulfone and a liquid crystalline polymer "HIQ-20" copolyester. The first two systems were chosen to demonstrate the pressure dependencies of the first order and second order transition temperatures. The third system, HIQ-20, was chosen to demonstrate pressure-induced mesomorphism. "HIQ-20" is a liquid crystalline copolyester with compositions of 20% hydroxybenzoic acid, 40% isophthalic acid and 40% hydroquinone. Its liquid crystallinity has been confirmed by differential scanning calorimetry (DSC) and thermal optical analysis techniques, and its shows a melting temperature of 324$\sp\circ$C and a clearing temperature of 342$\sp\circ$C. In HIQ-20, two unusual pressure effects were observed: a new pressure-induced crystal habit and a pressure-induced mesophase. The pressure-induced crystal habit was confirmed by the x-ray diffraction and high pressure DTA techniques. A minimum pressure of 300 bars was required to induce a new crystal habit from an ordinary crystal habit. The high pressure crystal form exhibited superior physical properties over the low pressure crystal form, which implied that the pressure dependent morphological changes in thermotropic copolyesters could be very significant.^ The pressure-induced mesophase was observed by DSC and the high pressure DTA techniques. This phase appeared at high temperatures after application of pressure. This finding confirmed the possibility of broadening the range of liquid crystalline polymers via the use of pressure. ^