The Preparation And Properties Of Thermotropic Liquid Crystal Polyarylate Fiber

Thermotropic liquid crystal polymers (TLCPs) are high performance materials. Thermotropic liquid crystal polyarylates are one kind of TLCPs, developed in the 70's of the 20th century. Liquid crystal polyarylates are of high commercial interest not only because of their high modulus and high strength, but also because of their good heat resistance, chemical resistance, inherent flame retardancy, low creep and good properties for processing. At present, only a few companies in USA and Japan grasp the key technology to manufacture the thermotrpoic liquid crystal polyarylate fibers. Many efforts have been made on the research and development of TLCPs for many years in our country, and its commercialization has been started in recent years, but rarely realizing commercialization due to the problems of spinning process control and equipments. Therefore, it is still necessary for scientists and engineers make great efforts to solve these problems.In this thesis, the rheology behavior of thermotropic liquid crystal polyarylate was analysised,the influence of processing condition on fiber performance was discussed. The spinningequipments were improved by adding the slow cooling equipment. The influences of heattreatment on the properties of thermotropic liquid crystal polyarylate fibers were also discussed.(1)The melting point of this thermotropic liquid crystal polyarylate was 289.43°C, and thedegradation temperature was above 515°C. The thermotropic liquid crystal polyarylateexhibited nematic liquid crystal properties between 290 and 585°C. It exhibited nematic liquidcrystal schlieren texture at 290°C, and began to degraded at 590°C. The phenomenon oftransition from anisotropy to isotropy phase can not be observed, indicating a very highisotropic temperature and above the thermal decomposition temperature.(2) Liquid crystal polymer melt is an anisotropic non-Newtonian fluids, and the rheological properties are significantly different from those isotropic materials/fluids. The results of steady-state rheological tests showed that the apparent viscosity and the structural viscosity index (△η) of the TLCP decreased significantly and the non-Newtonian index (n) increased as the temperature increased. Under a certain temperature, the TLCP exhibited typical shear thinning behavior. The viscous activation energy of the TLCP melt was higher than those of common polymer melts. The results of dynamic rheological tests showed that the shear storage modulus (G') and shear loss modulus (G") decreased as the temperature increased. The characteristic modulus and characteristic frequency were 2800GPa and 32.5 rad·s^-1 respectively.(3) In this research, the thermotropic liquid crystal polyarylate fibers were prepared by melt-spinning, the spinning temperatures were 295-320°C, the rotational speed of metering bump was 20rpm, the speed of winding was 50-150m/min. The breaking strength and Young's modulus of as-spun fiber were about 0.93GPa and 43.51Gpa respectively. The breaking elongation was about 1.39% and the maximum weight loss temperature of as-spun fiber was 513.2°C. The thermotropic liquid crystal polyarylate fibers showed skin-core structure, the orientation of the fibrocortex was higher than that of the fiber-core. A multiple-fibril-structure of thermotropic liquid crystal polyarylate fibers was observed.(4) After heat-treatment, the thermal and mechanical properties of thermotropic liquid crystal polyarylate fibers were improved significantly. With the temperature of heat-treatment improved and the time of heat-treatment prolonged, the melting point of fibers was increased, the heat resistance of the fibers was enhanced. The degree of orientation, crystallinity, breaking strength, Young's modulus and breaking elongation of as-spun fibers were improved significantly. The suitable heat-treatment condition was under 260°C for 48h and the comprehensive properties of thermotropic liquid crystal polyarylate fibers were better: the melting point was 332.6°C, the breaking strength, Young's modulus and breaking elongation were 3.18Gpa, 123.92Gpa and 2.25% respectively.