Poly(ethylene terephthalate-co-4,4'-bibenzoate) (PETBB) fibers containing 5, 15, 35, 45, 55, and 65 mol% bibenzoate (BB) were melt spun. Fiber structure has been determined using wide angle X-ray diffraction, birefringence, and FTIR spectroscopy. When drawn to their respective maximum draw ratios, the structures and properties of high BB containing fibers (PETBB45, 55 and 65) are significantly different than those of PET and low BB containing fibers (PETBB5, 15, and 35). For example, 90% of the ethylene glycol units in high BB containing fibers are in the trans conformation, while only 80% of these units are in trans conformation in PET and low BB containing fibers. Overall orientation of the high BB containing fibers is higher (orientation factor f > 0.85) than those of PET and low BB containing fibers (f < 0.6). Orientation of the crystalline regions is quite high (f(cr) similar to 0.95) for both groups of fibers, while orientation of the amorphous regions (f(am)) of high BB containing fibers is higher (similar to 0.8) than those of the PET and low BB containing fibers (similar to 0.4). High BB containing fibers exhibit much higher storage modulus and modulus retention with temperature than low BB containing fibers. Glass transition temperature determined from the dynamic loss tangent peak decreased with increasing BB content, while this transition completely disappeared in the high BB containing fibers. The magnitude of the secondary transition, observed at about -50 degrees C, decreased with increasing BB content. Another secondary transition, not observed in PET, was observed at about 70 degrees C in high BB containing fibers. These dynamic mechanical results have been rationalized in terms of the observed structural parameters. (c) 2007 Elsevier Ltd. All rights reserved.