Influence of precursor solvent and confined environment on the polymorphic transition in electrospun Poly(l-lactide) fibers

Abstract

The present study examined the thermal and crystallization properties of poly(l-lactide) (PLLA) chains confined in electrospun fibers. The heat of fusion and crystallite sizes decreased with a decrease in fiber diameter because of the confinement effect. The crystal structure of PLLA fibers exhibited α′ and α forms, which comprised loosely and closely packed chains, respectively. As-electrospun PLLA fibers exhibited an unstable α′ phase because of the rapid solidification of polymer chains during electrospinning. Subsequent thermal annealing at a temperature higher than 140 °C induced a polymorphic transition from the α′ phase to the stable α phase. This finding differed from that observed in bulk PLLA, which exhibited α crystals regardless of annealing temperature. Furthermore, the polymorphic transition of α′ crystals to α crystals in fibers fabricated with 95:5 chloroform/trifluoroethanol solvent occurred with lower annealing temperatures or shorter annealing times relative to those fabricated with 60:40 chloroform/trifluoroethanol solvent. This behavior was attributed to the low entanglement content in the PLLA chains in fibers fabricated with 95:5 chloroform/trifluoroethanol solvent; the low entanglement content increased chain mobility, thereby accelerating polymorphic transition.