Page 48 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 2
A common evidence coming from these pictures is that fiber surface turns from smooth to porous, or crater-like, with increasing relative humidity, which is consistent with other authors’ evidences.8–10,12–14 A temperature increase delays the occurrence of pores and roughness on the fibers at higher level of relative humidity. Fibers spun from solutions in which the non-water soluble solvent (CHCl3) is prevalent, exhibit small and circular pores. On the other hand, those from solutions with a higher ratio of the water soluble solvent (THF) result in some crater-like surface features. In the lower temperature range explored (20° and 30°C) and for solutions with higher content of CHCl3, the surface of the resulting fibers changes from smooth to porous above 50% of RH, while for solutions with a major content of THF the fibers surface exhibits only slight wrinkles at 50% of relative humidity, and then has deep and wide craters at 90% of relative humidity. At 40°C the surface is featureless for all the solutions investigated. This is probably due to the enhanced solubility of PCL in the solvent with increasing temperature, which causes a delayed solidification, avoiding that the phase- separated morphology is locked into the solidified polymer matrix.
The two most common explanations of the phenomenon of featured surface of the fibers are:
- Pores as imprints le by water drops, which condensed from the humid environment because of the evaporative
cooling of the solvent. It explains the occurrence of surface features at higher humidity, since in a more saturated environment it would be more probable to reach the dew point of water.
- Phase separation occurs between a polymer-rich phase and a polymer-lean one. The former results in the polymer matrix while the latter forms the pores. The most common causes of phase separation during electrospinning are
a temperature drop caused by fast evaporation of the solvent (thermally induced phase separation or TIPS, close to the evaporative cooling hypothesis), or the entrance of a third component, as water vapor, in the polymer-solvent system to act as a non-solvent for the polymer, the solvent, or both (vapor induced phase separation, VIPS).
Our evidences are in contrast with the hypothesis of a thermally induced phase separation. Indeed, both CHCl3 and THF have a high vapor pressure, which would cool the surface of the fiber and then lead to TIPS and water condensation almost identically, and weakly considering their relative volatility with respect to water. Nevertheless, we observe a high porosity in fibers spun with CHCl3 rich solvent systems, while it is gradually absent if
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