Light driven PVDF fibers based on photochromic nanosilica@naphthopyran fabricated by wet spinning

Abstract Photoresponsive polyvinylidene fluoride (PVDF) fibers doped with naphthopyran-functionalized silica nanoparticles (SiO 2 @NPT) were successfully prepared by dry–jet wet spinning. The incorporated photochromic nanomaterials, SiO 2 @ S1 and SiO 2 @ S2 , were previously prepared by covalent post -grafting of the silylated NPTs – 2 H -naphtho[1,2- b ]pyran derivatives, S1 and S2 – onto nano-sized SiO 2 (15 ± 3 nm). The morphological and chemical characterization of the resulting doped fibers PVDF@SiO 2 @ S1 and PVDF@SiO 2 @ S2 was evaluated by scanning electron microscopy with energy dispersive spectroscopy and Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR). Both PVDF@SiO 2 @ S1 and PVDF@SiO 2 @ S2 fibers presented average diameters of 133 ± 8 μm and 98 ± 7 μm, respectively, and a porous outer surface. The presence of the SiO 2 @NPT within the fibers was confirmed through the observation of dense clusters embedded within the polymeric matrix. Furthermore, the FTIR-ATR spectra of the fibers revealed that the PVDF matrix was composed of α and β crystalline and amorphous phases. The PVDF@SiO 2 @ S1 and PVDF@SiO 2 @ S2 fibers showed photoresponsive properties under UV and sunlight irradiation, exhibiting fast coloration kinetics and good optical contrast (Δ E * ab  = 7.2 and 15.1, respectively), and changing from a pale orange and an off-white color to a more intense yellow-orange and purple coloration, respectively, in less than 1 min. Moreover, they showed an initial fast bleaching, losing half of their color in less than 30 min ( t 1/2  = 28 and 20 min for PVDF@SiO 2 @ S1 and PVDF@SiO 2 @ S2 fibers, respectively), but displaying a residual coloration that took 1 and 2 h to return to the initial uncolored state ( t 3/4  = 95 and 49 min, respectively). The PVDF@SiO 2 @ S2 fibers presented the best photochromic performance.