2.2 Characterization of the As-spun Fibers2.2.1 BirefringenceThe refractive indices parallel and perpendicular to the fiber axis were measured using an interference microscope (Carl Zeiss Jena) fitted with a polarizing filter. The birefringence was calculated as the difference of two refractive indices.Cross-sectional distribution of birefringence was also ana-lyzed according to a method described in a previous paper[11].2.2.2 DensityThe density of the samples was measured at 25°C using a density gradient column prepared from n-heptane and car- bon tetrachloride.