Nano-Crystalline Sandwich Formed in Polylactic Acid Fibers

Fibers have traditionally been made through melt or solution processes from macromolecules. Most of these fibers have crystalline domains where the segregation of different crystalline features is extremely difficult due to the statistical nature of the formation and growth of these domains. A fibrous nano-crystalline sandwich is reported where distinctly different crystalline regions are formed in layers along the continuous fiber direction during the spinning process and locked in place. This approach employs side-by-side bicomponent nanofiber electrospinning where the components are the enantiomeric pair of poly(l-lactic acid) and poly(d-lactic acid). The formation of the poly(lactic acid) (PLA) stereo-complexes at the junction interphase of the two components is demonstrated through diffusion, which subsequently crystallize into continuous sandwich domains. The stereo-complex crystalline core in the fiber possesses a melting point 50 °C higher than, and properties substantially different from, the regular PLAs at the fringe areas of the fiber. This nano-crystalline sandwich fiber structure can be scaled to the micrometers in a commercial bicomponent process.