Fiber spinning, structure, and properties of poly(ethylene terephthalate-co-4,4′-bibenzoate) copolyesters

Thermal, rheological, and fiber spinning behavior of poly(ethylene terephthalate-co-4,4'-bibenzoate) random copolymers containing 5-65 mol % bibenzoate (BB) units have been studied. Copolymer randomness was determined by C-13 solution NMR. Copolymers with >40 mol % BB units spin like thermotropic liquid crystals and have mechanical proper-ties approaching those of liquid crystals; however, no evidence of liquid crystallinity was observed in these fibers based on thermal, optical, and rheological studies. Thus, we believe these polymers are frustrated liquid crystalline polymers. The alpha-relaxation attributed to the glass transition temperature is completely suppressed in the fully drawn and heat-treated PETBB55 fibers. Structure development during fiber spinning as well as fiber mechanical properties have been studied. Fibers containing more than 40 mol % bibenzoate show the development of a new crystal structure and exhibit much higher tensile modulus than the copolymers containing a lower bibenzoate percentage. Copolymers containing more than 40 mol % bibenzoate also exhibit high melt relaxation times compared to the lower bibenzoate copolymers.