3.2 Mechanical Properties of the LIB FibersFigs. 6 and 7 show the tenacity and initial Young's modulus of standard and LIB fibers, respectively, as a function of take-up speed. As a consequence of the enhanced orienta- tion and crystallinity development in LIB spun fibers, the tenacity and Young's modulus of these fibers are signfi- cantly higher than those of the standard high-speed spun fibers. The difference is very significant for the fibers spun at a speed above 2 km/min. The effect of enhanced molecu- lar orientation development on the Young's modulus ofthese fibers is more clearly elucidated in Fig. 8, whereYoung's moduli of standard and LIB fibers are plottedagainst the birefringence of the corresponding fibers. As canbe seen in the figure, the data fall onto two lines: the one