Polymer fibers with hierarchically porous structure: combination of high temperature electrospinning and thermally induced phase separation

Isotactic polypropylene (iPP) fibers with hierarchically porous structure were successfully prepared by electrospinning at 200 °C combined with thermally induced phase separation (TIPS). Dioctyl phthalate (DOP) and dibutyl phthalate (DBP) are typical diluents for iPP in TIPS and are used as solvents for electrospinning. An ionic liquid was added to increase the solution conductivity, facilitate the electrospinning process, and maintain a stable cone-jet electrospinning mode. Theoretical calculation demonstrates that the jet cools rapidly, and phase separation takes place in the jet during its travelling path, as the system traverses across the phase diagram from the single phase region to the metastable region. For the iPP/DOP system, the surface morphology of fibers changes from aligned microvoids bridged by fibrils to a wrinkled structure with the addition of ionic liquid, as the ionic liquid inhibits iPP crystallization. The pore morphology can also be modulated by varying co-diluent composition. Open pores appear on the fiber surface and the cross-section varies from closed cellular pores to a bi-continuous structure with the increase of DBP content in the co-diluent, which clearly demonstrates the phase separation mechanism changes from solid–liquid to liquid–liquid phase separation. The as-spun porous fibers show more than a 100-fold increase in specific surface area compared with the non-porous ones. The main advantages of this method are the pore formation process has a precise mechanism and the pore morphology is well correlated with the phase diagram. Furthermore, it is readily extended to other polymers with TIPS. Highly porous poly(vinylidene fluoride) (PVDF) fibers can be easily prepared from PVDF/DBP solution with a 157-fold increase in specific surface area.