On the Importance of Noncrystalline Phases in Semicrystalline Electrospun Nanofibers

Tailoring the properties of electrospun fibers requires a detailed understanding and control of their microstructure. We investigate the structure/property relationships in fabrics of randomly aligned fibers of polylactide, a prevalent biopolymer, either as-spun or after annealing and solvent-induced crystallization. In-depth characterization by field-emission scanning electron microscope (FESEM), wide-angle X-ray diffraction (WAXD), attenuated total reflection Fourier transform infrared (ATR-FTIR), and modulated temperature differential scanning calorimetry (MT-DSC) reveals that the as-spun fibers comprise crystalline and mesomorphic phases, as well as oriented but mobile amorphous chain segments. These chains are mostly responsible for the low-temperature cold crystallization and for the recovery endotherm around the glass transition, while the mesophase transforms into crystals with nearly zero enthalpy. Such behaviors are attributed to high molecular orientation, which is further evidenced by unveiling a fibrillar superstructure in nanofibers. The thermodynamics and structural evolution under different conditions are described from an energy landscape perspective. Finally, we propose a micromechanism, based on a modified supramolecular model, which helps to elucidate the fibers’ molecular dynamics.