Page 49 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

ELECTROSPINNING POLY(Ε-CAPROLACTONE) UNDER CONTROLLED ENVIRONMENTAL CONDITIONS
Table 2.2 Physical properties of the solvents used for electrospinning.43
Boiling point at 1 bar
[°C]
Chloroform 61 Tetrahydrofuran 65
Vapor pressure at 20°C
[kPa]
Dielectric Solubility in constant water
at 20°C [%]
21.1 4.81 0.81 2 19.3 7.58 100
fibers are spun from THF rich solvents. It seems that pore formation during our experiments more probably happened by mean of a VIPS rather than a TIPS/evaporative cooling mechanism, since the latter is more likely related to solvent volatility than to water miscibility. Physical properties of the used solvents are summarized in Table 2.2.
Many authors investigated the mechanism behind featured surfaces on electrospun fibers, most of them by studying the e ect of solvent volatility on the nanoporous morphology of polystyrene (PS) fibers in the THF/DMF mixtures.7,9,12–14,44 All of them reported that pores disappeared with increasing DMF ratio (low vapor pressure solvent), replaced by wrinkled or smooth fiber surfaces, as expected for confirming the hypothesis of a strong role of solvent volatility. On the contrary, our PCL fibers electrospun from THF are almost smooth or slightly wrinkled until very high RHs (craters are formed only at 90%), while those spun from a solution in CHCl3 are highly porous already from relative humidity of 50%.
Nevertheless, water solubility of the solvent alone can’t explain our results. In fact, surface morphologies
have been reported for a wide variety of hydrophilic and hydrophobic polymeric systems with very di erent solvents, which cannot be explained simply by means of the properties mentioned, making it very di icult to deduce a general rule. For example PS, polysulfone (PSU) and polyacrilonitrile (PAN) were electrospun from the water soluble DMF, resulting in highly porous fibers at high RHs, instead of being mainly smooth or wrinkled.9,10,12–14 Featured surfaces depend on both solvent and polymer properties, and are a result of a complex interaction between these properties and water vapor, resulting in a VIPS in which the water vapor plays di erent roles since it can be a non-solvent for the solvent, the polymer or both. The thermodynamic interaction between the studied polymer-solvent-non-solvent systems was already assessed for di erent polymers,11,13,16 but also the kinetics of the phase behavior of the ternary system can’t be neglected. The role of the kinetics of phase separation is corroborated by the study of Pai et al.,11 who demonstrated a competition between the phase separation kinetics, and two other characteristic times, which are the buckling
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