In this study, five types of PLLA fibers were prepared via melt
electrospinning at 200 °C with or without hot airflow; and the hot
airflow could be set at 80 or 120 °C with slow or fast speed. In
general, all of the melt electrospun PLLA fibers were morphologically
similar without significant discrepancies; nevertheless, the
PLLA fibers electrospun under the condition of 120 °C hot airflow
with fast speed appeared to be slightly thinner. Additionally, no
beads and/or beaded fibers could be identified microscopically
[22]. The SEM image in Fig. 2A shows the representative morphology
of PLLA fibers prepared in this study. The fibers had diameters
of several microns, and such diameters are common for
melt electrospun polymer fibers; note that for the fibers electrospun
from polymer solutions, the diameters are typically thinner
than 1 mm. This is primarily because the viscoelastic force associated
with a polymer melt is usually orders of magnitude larger
than that associated with a polymer solution, while the solidifi-
cation rate of jets/filaments during melt electrospinning may be
considerably faster than that during solution electrospinning. The
fast solidification rate would also lead to the formation of polymer
fibers with low crystallinity (i.e., the quenching effect) [43], particularly
for PLLA that has relatively slow crystallization speed. It is
interesting to note that some fine PLLA fibers with diameters of
500 nm (Fig. 2B) were frequently identified, whereas the formation
mechanism of these fine PLLA fibers is still under investigations.