Co-continuous nanoscale assembly of Nafion–polyacrylonitrile blends within nanofibers: a facile route to fabrication of porous nanofibers

Abstract

A facile route to fabrication of porous high surface area nanofibers is demonstrated via electrospinning of a polymer blend followed by selective removal of one of the components. First, electrospinnability of Nafion–polyacrylonitrile (PAN) solutions in N,N-dimethylformamide is studied over a wide range of blend compositions. Pure Nafion solutions are not electrospinnable by themselves owing to the presence of aggregates in solution due to ionic interactions, thereby resulting in low solution viscosities. Addition of only 5 wt% PAN in Nafion allows us to successfully electrospin nanofibers, possibly due to the enhancement of entanglements within the solution. An extensional rheology study is conducted to quantify and understand non-electrospinnability. In the second step, the nanoscale internal morphology of Nafion–PAN nanofibers is studied using transmission electron microscopy of microtomed nanofiber sections. The electrospinning process allows us to develop the co-continuous morphology of PAN and Nafion within the nanofibers. To further corroborate our findings and develop high surface area nanofibers, we selectively remove Nafion or PAN to form pure mesoporous carbon (after calcination) and mesoporous Nafion nanofibers respectively. Interestingly, even with an initial composition of 80 : 20 (wt/wt) Nafion : PAN, the nanofibers retain their overall shape after selective Nafion removal, indicating the presence of a continuous PAN phase. The resultant porous carbon nanofibers exhibit a specific surface area of >1500 m² g⁻¹. While mesoporous carbon nanofibers can have a wide range of applications such as in supercapacitors, high surface area porous Nafion nanofibers will facilitate the development of super-sensitive gas sensors.