Stereocomplex Crystal Formation in Sheath/Core and Sea/Island Poly(L-lactic acid)/Poly(D-lactic acid) Fibers Prepared through Laser-Heated Melt Electrospinning and Subsequent Annealing Processes

Abstract

Sheath/core (S/C) and sea/island (S/I) bicomponent fibers consisting of poly(L-lactic acid) (PLLA) as the sheath or sea component and poly(D-lactic acid) (PDLA) as the core or island component were fabricated using a bicomponent melt-spinning process. The obtained fibers were then subjected to laser-heated melt electrospinning (LES) using a rotating collector. The molecular orientation of the LES fibers increased as the fiber diameter decreased because of the increase in take-up velocity. Wide-angle X-ray diffraction revealed that the as-spun fibers were in an amorphous state. The differential scanning calorimetry (DSC) measurements showed a cold crystallization peak, followed by melting peaks of the homocrystal (HC) and stereocomplex crystal (SC), with the SC peak being more prominent for the S/I fibers. It is speculated that the distance required for the interdiffusion of PLLA and PDLA molecular chains in the fiber cross section substantially affected the transition from HC to SC during DSC measurements. The S/C and S/I fibers annealed at 120 °C after the LES were composed of HC. However, after annealing at 190 °C, which is above the melting temperature of HC, the S/C fibers fused, whereas the S/I fibers remained intact, resulting in well-separated ultrafine fibers composed of highly oriented SC.