Melt spinning of thermotropic liquid‐crystal polyesters to form ultrahigh‐modulus filaments

The properties and structure of ultrahigh‐modulus filaments were investigated for wholly aromatic copolyesters (WACPs) containing 60 and 70 mol% p‐oxybenzoate, based on p‐hydroxybenzoic acid, p,p′‐biphenol, terephthalic acid, and isophthalic acid and for poly(ethylene terephthalate co‐p‐oxybenzoate) containing 60 mol% p‐oxybenzoate. As‐spun filaments with varying degrees of molecular orientation were spun from melts by taking the spin‐extension ratio as a variable at given temperatures. The as‐spun filaments were further subjected to thermal annealing. Changes in the structural ordering with the extension ratio were monitored by wide‐angle x‐ray scattering, scanning electron microscopy, viscoelastic properties, and measurements of the thermal expansion coefficient. The increase in modulus is correlated well with the crystallite orientation at a relatively low extension ratio. However, for extension ratios above 10, the modulus of as‐spun filaments is almost independent of the crystallite orientation. Modulus values as high as 100 GPa are obtained for WACP filaments spun at extension ratios above 500. It is suggested that ultrahigh‐modulus values can be reached in highly oriented noncrystalline chains. Furthermore, the results for annealed filaments indicated that the relaxation of molecular orientation occurs partially in the oriented noncrystalline regions during the stage of long‐time annealing above Tg, leading to depression of the modulus.