Abstract:

Objective Photoresponsive luminescent fibers represent a promising new type of optical functional material for applications in optics, sensing, and biomedicine. Many efforts have been focused on photochromic fibers and fluorescent fibers to develop photoresponsive fibers. However, long-afterglow luminescent fibers as a type of photoresponsive fibers are still rare, partially related to the process complexity, dependance on advanced technologies, specialized equipment, and high preparation costs.

Method In this work, spirooxazine (SPO) with photochromic properties and long-afterglow material isophthalic acid (IPA) were introduced into the polyurethane spinning solution through doping. A stimuli responsive luminescent polyurethane fiber was prepared using wet spinning. The effect of doping amount of IPA on the long-afterglow luminescence performance of fibers was investigated. The dynamic multi-color long-afterglow luminescence properties of polyurethane fibers were characterized by the phosphorescence spectra, long-lived phosphorescence lifetime and the afterglow luminescent images.

Results The fluorescence emission positions of polyurethane fibers with different mass fractions of isophthalic acid (IPA) were found almost the same, with a significant fluorescence emission characteristic peak at 404 nm. However, the fluorescence emission band of the fibers was relatively broad when the doping mass fraction of IPA was below 10%, and it became noticeably narrower when the content exceeded 10%. The phosphorescence emission intensity of polyurethane fibers gradually increased as the mass fractions of IPA increased. However, there was no significant improvement in the phosphorescence intensity, the afterglow duration and the luminance of the polyurethane fibers when the mass fraction of IPA increased from 12.5% to 15.0%, indicating an optimal doping IPA mass concentration was 12.5%. The afterglow luminescent polyurethane fibers displayed a bright green afterglow lasting over 7 s after the UV light was removed. In addition, the long-lived phosphorescence lifetime of polyurethane fibers with different mass fractions of (IPA) was also investigated. IPA emitted at 500 nm with a long-lived phosphorescence lifetime of 1 667 ms when excited at 315 nm. It was found that the long-lived phosphorescence lifetime of polyurethane fibers containing different mass fractions of IPA were almost the same as that of IPA, indicating that the long-lived phosphorescence lifetime of polyurethane fibers doped with IPA was not significantly changed. Additionally, the long-afterglow luminescence color of polyurethane fiber was regulated utilizing the photochromic properties of spirooxazine (SPO) in order to obtain the polyurethane fibers with dynamic multi-color long-afterglow luminescent over time. There was a significant fluorescence emission characteristic peak at 404 nm for the polyurethane fibers co-doped with IPA and SPO (IPA/SPO/polyurethane fibers). While the phosphorescence emission characteristic peak of IPA/SPO/polyurethane fiber had a blue shift, moving from 500 nm to 435 nm, because of the light response characteristic of SPO. IPA/SPO/polyurethane fibers emitted blue fluorescence under 365 nm UV irradiation. After turning off the UV irradiation, the polyurethane fibers exhibited a dynamic process of rapid recovery from blue to white under daylight. In the dark, polyurethane fibers quickly displayed a blue afterglow lasting about 1 s. It turned cyan after 3 s and finally turned green. The luminescence intensity of polyurethane fibers gradually decreased and disappeared after 7 s.

Conclusion Isophthalic acid (IPA) is proven to be an excellent energy donor for the molecular doping systems. It can be doped into polyurethane spinning to endow polyurethane fiber with long-afterglow luminescence properties. The polyurethane fiber exhibited the best long-afterglow luminescent performance when the addition amount of IPA was 12.5% of the mass concentration of the polyurethane spinning solution. The long-afterglow luminescence color of polyurethane fiber was regulated utilizing the photochromic properties of spirooxazine (SPO). The polyurethane fibers co-doped with IPA and SPO (IPA/SPO/polyurethane fibers) exhibited excellent photochromic and long afterglow luminescence properties. The color of polyurethane fiber quickly changed from colorless to blue upon UV irradiation. After the UV light was turned off, it exhibited a dynamic long afterglow luminescence gradually changing from blue to green, with a luminescence duration of about 7 s. The polyurethane fibers not only provide visual photochromism but also regulate its dynamic multi-color long afterglow performance over time.

Key words: polyurethane, photochromic, spirooxazine, isophthalic acid, wet spinning, luminous fiber