Thermotropic liquid crystalline copolyesters with non-coplanar biphenylene units tailored for blend fiber processing with PET

Via melt polycondensation two series of liquid crystalline thermotropic polyesters were synthesized for in situ reinforcement components in PET blend fibers. The copolymerization of a monomer bearing a bulky side group (tert-butyl hydrochinone) or a crankshaft monomer (6-acetoxy-2-naphthoic acid) with the non-coplanar 2,2′-dimethyl biphenol and terephthalic acid yielded two fully aromatic thermotropic polyesters. By varying and adjusting the individual monomer compositions, the melting behavior can be tailored to match the PET processing window. Solution viscosities up to ηinh=6.57 dl/g were achieved. By thermal characterization with DSC in combination with polarization microscopy up to 310°C neither a crystal melting point nor an isotropic transition was detected. Hence, the nematic polyesters solidify below the glass transition temperature into a glassy solid without crystallization. The polyesters are thermally stable above 400°C and form fluid melts. Fibers were spun from both series. Optimal processing was encountered at polyesters containing 60 mol% HNA, which could be spun continuously into high quality fibers. The fibers were characterized by tensile test, WAXS, and SEM. Fiber orientation was improved at a higher draw-down ratio and the Youngʹs modulus was determined to 49 GPa at a tenacity of 550 MPa.