There are odd spherical beads in my PVdF nanofibers that look spiky or porous under an SEM (picture below). As you can see, there are two features that stand out from the surrounding nanofibers--the lower item appears to be a bead in the solution, but I don't know what the larger item is. I've already tried tinkering with the voltage and the distance between needle tip and collector, and I added a heat lamp above the setup to make the solvent evaporate faster. I also kept my samples covered to make sure dust didn't get on them, so I don't think that's what the spiky balls are.
The PVdF formula I'm using is 1.47 g PVdF powder, 1.5 mL acetone, and 5 mL DMF. (This is a 1:1:4 ratio by mass.) I'm collecting on an inclined gap between two brass plates with a voltage of about 10-12 kV.
The formation of beads in elctrospun fibers is a well-known problems. Here are some main reasons of it:
Low molecular weight/concentration/viscosit;
High surface tension;
Low charge density
-Addition of salt to the solution enhances the charge carrying capacity of the solution and this generally favors the production of beads-free fibers;
-Increase the polymer solution and make sure the conductor plate is making continuous contact with the needle ( and that the grounded wire brush is making continuous contact with the wheel if you are preparing oriented fibers).
We have published a paper on electrospun PVDF scaffolds using various voltage. The solution we used is 20% PVDF in 1:1 (DMAC:Acetone). There maybe some beads formation, but I never see a sphere like that. What is the size of the sphere? You may want to check the chemical reagents you used, make sure it is pure and clean. Also clean the needles, eletrospinning box etc. Just clean everything before you spin.
Have you solved it? Is it because, at RT, the polymer solution gelates?
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