Atmospheric pressure plasma jet treatment of PCL polymer solutions to improve electrospinning.

An atmospheric pressure plasma jet (APPJ) specifically designed for liquid treatment has been used in this work to improve the electrospinnability of a 5 w/v% solution of poly-ε-caprolactone (PCL) in a mixture of chloroform and N,N-dimethylformamide. Untreated PCL solutions were found to result in non-uniform fibers containing a large number of beads, while plasma-treated solutions (exposure time of 2 to 5 minutes) enabled the generation of beadless, uniform nanofibers with an average diameter of 450 nm. This enhanced electrospinnability was found to be mainly due to the highly-increased conductivity of the plasma-modified PCL solutions. Consequently, more stretching of the polymer jet occurred during electrospinning leading to the generation of bead-free fibers. Plasma treatment also results in an increased viscosity and decreased pH values. To explain these observed changes, optical emission spectroscopy (OES) has been used to examine the excited species present in the APPJ in contact with the PCL solution. This study revealed peaks attributed to H, CH, CH2 and C2 species which could be responsible of the degradation of solvent molecules and/or PCL structure during the plasma treatment. Size exclusion chromatography (SEC) and X-ray photoelectron spectroscopy (XPS) results showed that the molecular weight and the chemical composition of PCL were not significantly affected by the APPJ treatment. Plasma exposure mainly results in degradation of the solvent molecules instead of modifying the PCL macromolecules, preserving as much as possible the original polymer. A hypothesis for the observed macroscopic changes in viscosity and pH could be the generation of new chemical species such as for example HCl and/or HNO3. These species are characterized by high conductivity, low pH value, strong polarity and could enhance the solvent quality for PCL leading to expansion of the polymer coil, which could in turn explain the observed enhanced viscosity after plasma modification.