Capillary breakup and electrospinning of PA6 solutions containing FeCl3: experimental findings and correlations

In several applications, ranging from electronic to chemical sensing, great interest has grown for the exploitation of conducting polymer nanofibers, whose processing is, however, not straightforward, due to polymer low solubility and presence of rigid backbones. An interesting method to overcome this issue consists in the electrospinning of a spinnable polymer to obtain a template for the successive in situ polymerization of the conducting polymer monomers. Considering PANI nanofibers, a suitable template can be electrospun from PA6 solutions in formic acid containing FeCl3. In this system, the ionic salt may perturb or prevent H-bonds formation between amide groups of PA6 backbones: this could modify solution viscoelasticity, and thus affect fibres morphology. The aim of the present work is to identify the effect of FeCl₃ on the solution rheological behaviour and to correlate it to electrospun fibres morphology. To this aim, solutions at several salt content underwent electrospinning and were characterized both in shear, by rotational rheometry, and extension, by capillary breakup rheometry, while fibres morphology and crystallinity were evaluated through SEM and DSC. The rheological analysis enlightens that a critical FeCl₃ content exists above which the viscous component of the viscoelastic response becomes predominant. At the same concentration, the SEM observations of the electrospun fibres show the formation of severely inhomogeneous structures. A correlation between these results is proposed through the adimensional analysis of competing viscoelastic stabilization and surface tension-driven instability phenomena. Besides the aforementioned effects, the FeCl3 content affects also fibre crystallinity, as above a critical concentration fibres turn out to be completely amorphous. Interestingly, this concentration coincides with the one at which a transition is observed in the rheological behaviour.