3.4 Mechanism of Fiber Structure De

3.4 Mechanism of Fiber Structure DevelopmentThe change of structural and physical characteristics ofas-spun fibers brought by the introduction of LIB can nowbe analyzed on the basis of the results obtained from thesimulation. As can be seen from the Fig. 16, the solidifica-tion stress changes drastically inside the LIB depending onthe position at which the crystallization is assumed tooccur. The solidification stress increases from a value of7 MPa at the upper surface of the LIB to a value of 30 MPaat the outlet of the LIB. High level of solidification stressand the consequent increase in the molecular orientationcan be realized only when the crystallization is assumed tooccur at a position close to the outlet of the LIB. At thisposition, the solidification stress is significantly higher thanthat of the standard high-speed spun fibers and such highlevels of solidification stress is necessary to account for thesignificant enhancement of molecular orientation in the LIBfibers. This consideration agrees with the experimental re-sults in which birefringence showed a steep increase in thetake-up speed range of 1 to 2 km/min where crystallizationstarted to occur (Figs. 2 and 4).Support for the postulate that the crystallization is tak-ing place inside the LIB comes from the density data also(Fig. 4). As can be seen, the density values of LIB fibersremain unchanged above a take-up speed of 2 km/minindicating that the crystallization has occurred at a constanttemperature viz. at the temperature of LIB. Whereas, thestandard high-speed spun fibers show higher densities attake-up speeds of 6 and 7 km/min and above despite thefact that these fibers have lower level of spinline stress andorientation development as compared to the LIB fibers.