Crystal-to-Crystal Transition and the Structure Development of Electrospun Poly(ethylene 2,6 naphthalate) (PEN) Nanofibers from Solution

Poly(ethylene naphthalate) (PEN) nanofiber membranes were prepared using hexafluoroisopropanol (HFIP) as a solvent. Crystallization and conformation of PEN nanofibers were characterized for the first time by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. SEM micrographs revealed that the size and uniformity of nanofibers depended on the PEN concentration in HFIP solution. DSC results showed that cold crystallization temperature (T_cc) decreased and the crystallinity of PEN nanofibers increased with increasing the concentration of spinning PEN/HFIP solution. FTIR and Raman spectroscopies showed that PEN nanofibers formed metastable β-crystals during electrospinning due to the fast evaporation of a solvent, while the concentration of spinning solution did not affect the crystal form of as-prepared PEN nanofibers. Upon annealing of PEN nanofiber membranes above 140 °C, metastable β-crystals were transformed into the thermodynamically most stable α-crystals. Our polarized FTIR spectroscopic studies revealed that no molecular orientation was developed when fibers were collected randomly.