Preparation of photochromic poly(vinylidene fluoride-co-hexafluoropropylene) fibers by electrospinning

Photochromic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) fibers were prepared by electrospinning from a solution of copolymer and ester-functionalized nitrospiropyran (SPEST) molecules. The surface and internal features of the electrospun (ES) fibers were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray diffraction (WAXD). The strong segregation of fluorine-rich groups on the fiber surface, which occurs during and/or after the electrospinning process, is driven by the lower surface tension for fluorine-rich groups and leads to encapsulation of SPEST predominantly near the core of the fibers, as confirmed by both X-ray photoelectron spectroscopy (XPS) and dynamic water contact angle (CA) measurements. The photochromic behavior of the spiropyran is preserved in the polymeric fibers, as confirmed by steady-state absorption and emission spectroscopy. Both isomeric forms of the photochrome in SP-PVDF-co-HFP were emissive, an effect that is thought to be due to the steric and/or electrostatic restrictions on the ring-opening reaction imposed by the fiber.