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Sheath-core type bicomponent melt-spun fibers were produced by extruding the melts of low-isotacticity polypropylene (LPP) as the core component and the blend of LPP and high-isotacticity PP (IPP) as the sheath component. IPP content in the sheath was changed from 8 wt% to 40 wt% while sheath/core composition was varied from 50/50 to 10/90. Accordingly overall IPP content was kept constant at 4 wt%. Even though the overall IPP content was intact, bicomponent fibers with lower contraction ratio after spinning, higher elastic recovery and slightly higher modulus and strength were obtained by increasing the IPP content in the sheath and decreasing the sheath layer composition, i.e., localizing the IPP to the region near the surface in the fiber cross-section. Structure analysis of the as-spun fibers suggested the suppression of crystallization of LPP in the sheath by blending IPP. By contrast, enhancement of molecular orientation and crystallization of the sheath component were found to occur by localizing the IPP to the region near the fiber surface. It was speculated that this behavior was caused by the kinematic mutual interaction of the sheath and core components in the melt spinning process.

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