Electrospun isotactic polypropylene fibers: Self-similar morphology and microstructure

Macroscopically aligned isotactic polypropylene (iPP) fibers with different diameters ranging from 0.7 to 13.2 μm were successfully fabricated by high-temperature solution electrospinning using an infrared (IR) heating system. Polarized optical microscope (POM), scanning electron microscope (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and polarized Fourier transform infrared (FTIR) were employed to characterize the surface morphology and microstructure of the electrospun fibers. From SEM images, three macroscopically structural models have been found: (1) cylindrical fibers, (2) twisted fibers and (3) beaded fibers. Interestingly, regardless of fiber's shape and size, they (especially for cylindrical fibers) exhibit well-defined self-similar morphology. That is, the tertiary, secondary and primary morphology are similar with each other. Thermally induced phase separation (TIPS) is the most pertinent origin to explain the formation of self-similar morphology. WAXD and DSC results show that α-form crystal prevails in the electrospun iPP fibers. For comparison, the solution-cast film was also prepared under the same conditions as those adopted in electrospinning except that the voltage was 0 kV. Compared with the solution-cast film, lower crystallinity, smaller average crystallite size and higher melting point are achieved in the electrospun iPP fibers. The results of polarized FTIR spectra demonstrate that the macroscopically aligned electrospun fibers are highly oriented along the fiber's longitudinal axis at molecular level. Furthermore, when the electrospun iPP fibers were embed in iPP matrix, surface-induced crystallization is significantly enhanced and finally develops into transcrystalline layer which can be even revived after recrystallization. As a result, it is logical that long-lived thermodynamically stable structure has been formed in the electrospun iPP fiber. Combined with FTIR result, the α-form crystal formed in the fiber is conjectured to be a kind of oriented structure with thermodynamically long-lived stability which is similar to "shish-kebab" structure.