Abstract:

Objective Spiropyrans are one of the most widely studied photochromic compounds, but they are prone to deactivation due to external environmental factors such as the concentration and polarity of solution, pH value, and temperature. Additionally, photochromic materials have the disadvantages of poor affinity with fabrics, complex preparation processes, and low washfastness, which severely restrict the development of photochromic textiles. Therefore, a strategy for preparing robust photochromic cotton fabrics is proposed.
Method This study constructed robust photochromic cotton fabrics through thiol-ene click chemistry. Vinylated spiropyran (SP—CH＝CH₂) and thiolated spiropyran (SP—SH) were chemically bonded with with a 3-mercaptopropyltriethoxy silane (MPTES) modified cotton fabric and a [3-(methacryloyloxy)propyl] trimethoxy silane (KH570) modified cotton fabric, denoted as F1 and F2, respectively. The surface elemental content and chemical composition of the photochromic cotton fabrics were characterized by scanning electron microscopy coupled with energy dispersive spectroscopy and Fourier-transform infrared spectroscopy (FT-IR). Thermal stability and surface hydrophobicity of photochromic cotton fabrics were evaluated using thermogravimetric analysis and contact angle measurement. The reversible coloration properties, light fatigue resistance, and wash fastness of photochromic cotton fabrics were assessed based on visual appearance, color parameters, and the number of reversible coloration cycles.
Results FT-IR results confirmed that click chemistry reactions occurred between thiol groups and vinyl groups on the two photochromic cotton fabrics (F1 and F2). Compared with the spectrum of F1, the C＝C stretching vibration peak at 1 638 cm^-1 disappeared in F2, indicating that SP—SH completely reacted with the double bonds on the KH570-modified cotton fabric. The maximum thermal decomposition temperatures of the two photochromic cotton fabrics were slightly lower than those of the original cotton fabric. However, owing to the introduction of silane coupling agents during the modification process of photochromic cotton fabrics, which contain high-energy Si—O bonds, the residual mass of F1 and F2 were higher compared to the original cotton fabric. The fabric appearance images clearly showed that the original cotton fabric was white, while F1 and F2 were yellow before UV light exposure and turned purple or purplish-red after UV light exposure, with F2 having higher L^*, a^*, and b^* values than F1. EDS results showed that the atomic contents of Si and S on F1 were 0.03% and 0.02%, respectively, while they increased to 0.26% and 0.44% on F2, respectively, indicating a higher grafting rate of SP—SH. After washing, the percentage of Si and S atoms on the surface of F2 decreased less, proving that the click chemistry reaction between SP—SH and KH570-modified cotton fabric can yield highly durable photochromic cotton fabrics. For unwashed F1, with an increase in cycles up to 6, some spiropyran could not reversibly fade from the fabric surface. After washing, the K/S values of the photochromic cotton fabrics significantly decreased, showing poor wash fastness. The resulting F2 achieved over 20 reversible color cycles. Even after 1 and 5 soap washing cycles, it retained the ability to undergo more than 15 reversible color changes with slight decreases in K/S values. Additionally, F2 displayed hydrophobic properties, with water contact angles of 96.8° under UV light exposure and 125.8° under visible light exposure, respectively.
Conclusion By utilizing thiol-ene click chemistry, thiolated spiropyran was chemically reacted with vinyl-modified cotton fabric to successfully prepare a robust photochromic cotton fabric (F2) with excellent stability and uniform distribution. Compared to SP—CH＝CH₂, SP—SH exhibited a higher grafting rate on cotton fabric, enabling F2 to achieve over 20 reversible color changes. Even after one and five washing cycles, F2 retained more than 15 reversible color transitions, demonstrating superior washing resistance and light fatigue resistance. The high-fastness photochromic cotton fabric displayed contact angles of 96.8° and 125.8° under UV and visible light exposure, respectively, indicating hydrophobic properties. Additionally, this approach endowed the fabric surface with enhanced water-repellent functionality.

Key words: click chemistry, spiropyran, photochromic, cotton fabric, thiol-ene, smart textiles