Page 220 - Biodegradable Polyesters

198 8 Systematic Development of Electrospun PLA/PCL Fiber Hybrid Mats

structures. Successful electrospinning resulting in generated fibrous structures
partially relies on sufficient solution viscosity. It can be seen directly from
Figure 8.2 that the use of 15% LMW PCL within PLA/PCL solution offers the
lowest solution viscosity at 20 cP as opposed to that of 15% HMW PCL at the
highest level of 476 cP. As a result, the former solution can easily prevent the
elongation of solution droplets into fibers, thus having more or less “slurry”
structures. When the effect of solution concentration alone is considered, the
generation of smaller microfibers with diameter of 689 ± 15 nm for 9 wt%/v
HMW PCL within PLA/PCL solution can be ascribed to the relatively low
solution viscosity in comparison with 15% HMW PCL. The enhanced solution
viscosity is associated with an increase in the amount of molecular entanglements
per polymer chain and a lower quantity of solvents used because of the increase
of solution concentration, which induces the enhancement of surface tension to
overcome steady jet configuration, and thus the reduced extent of elongation in
solution droplets with relatively large fiber diameters [62–66].

8.3.2
Effect of Blend Ratio

The use of pristine PLA dissolved in DCM/DMF has been shown to produce
homogeneous PLA nanofibers with average diameter of 450 ± 15 nm, indicated
in Figures 8.3a and 8.4. Using PLA/15% HMW PCL at a blend ratio of 3/1
demonstrates that the quality of electrospun fibers deteriorates to a certain extent
with a larger average diameter at 553 ± 15 nm (Figures 8.3b and 8.5). The further
increase of PCL amount at PLA/PCL blend ratio of 1/1 induces nonuniform
morphology with curved coarse fibers and increased average fiber diameter up
to 610 ± 20 nm. At the blend ratio of 1/3 for PLA/PCL, uniform fiber formation
takes place again despite even larger fiber diameter at 744 ± 15 nm. Nonetheless,
the pristine PCL (i.e., blend ratio: 0/1) produces the poorest fiber morphology
among all aforementioned material structures mainly because considerably high
solution viscosity of HMW PCL prevents the solution droplets from being drawn
out into continuous fibers, Figure 8.4. A steady jet in electrospinning stems from
the appropriate development of a large number of entanglements per polymer
chains associated with the solution viscosity [64, 67].
On the other hand, PLA/15 % LMW PCL at the blend ratio of 3/1 results in an
average fiber diameter of 490 ± 15 nm, as observed from Figure 8.4. It reveals that
increasing the blend ratio from 1/0 to 3/1 has minor effect on the fiber diameter.
In addition, beads and fiber inhomogeneity become obvious at the blend ratio of
1/1 with decreased fiber diameters to 250 ± 25 nm (largest diameter: 425 nm and
smallest diameter: 130 nm). Notwithstanding a wide range of fiber inhomogeneity,
fiber diameter tends to be reduced by lowering the solution viscosity (Figure 8.4).
The lower solution viscosity facilitates the rapid fragmentation of charged polymer
jets at short distances, which might be the reason for the existence of beads as
critical defects in electrospinning. With increasing PCL amount above the blend
ratio of 1/3, there is little sign of produced fiber structures.