We report evolving pore formation in both surface and interior structures of polystyrene (PS) fibers
electrospun from solutions of dimethylformamide (DMF), as low, and tetrahydrofuran (THF), as high
volatile solvents. The environmental conditions, i.e., temperature and relative humidity of electrospinning
process were systematically changed using a high accuracy conditioning box to provide
a comprehensive investigation of morphology evolution within electrospun fibers. Three-angle phase
diagrams of H2O/DMF/PS and H2O/THF/PS systems were constructed at temperature of 20, 40, and 60
C
by measuring the cloud points as well as calculating the free energy of mixing, in order to analyze the
thermodynamic instability of polymer solution jet undergoing solvent evaporation and water penetration.
According to the results, pores form within the fiber if DMF has low volatility and develop onto the
fiber surface if THF high volatile solvent is employed. In the former case vapor-induced phase separation
is involved while in latter case mechanism of breath figure formation is dominant. Three types of
morphology for fiber surface, i.e., non-porous, porous and wrinkled are recognized as relative humidity
and temperature of electrospinning process vary. Morphology transition of surface fiber is taken place at
certain relative humidity while it shifts by varying the temperature. Superimposing the relative humidity
and temperature not only describes the suppression of bead formation but also controls the mean
diameter of electrospun fiber.