Photochromic microcapsules anchored on cotton fabric by layer-by-layer self-assembly method with erasable property

https://doi.org/10.1016/j.reactfunctpolym.2020.104762Get rights and content

Highlights

  • A novel electrostatic LBL self-assembly method was used to prepare the photochromic fabric.

  • The existence of chitosan can improve the fatigue resistance of photochromic property.

  • The prepared fabric has potential applications in the field of optical data storage and UV-protective textiles.

Abstract

Photochromic materials have great potential applications in many fields due to their reversible color change under light irradiation. Herein, a novel photochromic cotton fabric with high fatigue resistance was creatively fabricated via a facile electrostatic layer-by-layer self-assembly technology of photochromic microcapsules and sodium alginate. Chitosan was used to encapsulate photochromic dyes to form microcapsules to improve fatigue resistance, meanwhile, as the wall material of microcapsules in weak acid environment can provide the positive charge needed for electrostatic layer-by-layer self-assembly process. After the anionic modification of cotton fabric to achieve negative charge, electrostatic layer-by-layer self-assembly was taken place between -NH3+ on the wall material of microcapsules and -COO in sodium alginate under the condition of pH = 5. For clear characterization of the regular charge changes of fabric surface during the assembly process, the methylene blue (a kind of cationic dye) dyeing of cotton fabrics was carried out, the K/S and Lab value of the dyed fabric showed a regular “odd-even” oscillation. The assembled five-layer microcapsules fabric can change from colorless to purple rapidly within 12 s under UV light, and reversibly return to the original state under the irradiation of green light quickly in 90 s. It can maintain excellent reusability photochromic performance (94.65%) after more than 20 reversible color-changing cycles, implying the good reusability in erasable optical data storage. Furthermore, since the photochromic dyes can absorb UV light, the as-prepared photochromic cotton fabric has excellent UV protection properties.

Introduction

Smart textiles are fabrics able to respond and adapt behavior in an intelligent way under specific external conditions [[1], [2], [3]]. Among them, photochromic textiles due to the ability to block UV radiation and sense environmental changes, have been widely used in sensing environmental changes, brand protection, and military camouflage [4,5]. As a common organic photochromic material, spiropyrane has been widely used in this field because of its remarkable discoloration and low background fluorescence [6]. The photochromic mechanism of spiropyrane lies in the cleavage of Csingle bondO bond on the spiral ring under UV irradiation, leading to the change from a colorless closed-loop system to a colored open-loop system, on the contrary, under the irradiation of visible light or heat, the open-loop system can be reversibly transformed into a closed-loop system and restored to its original state [[7], [8], [9], [11], [10]]. Although there are a large number of unsaturated bonds in spiropyrane molecules, they are easy to be affected by oxygen or pH changes when exposed to the external environment directly, which influence its long-time service in the practical applications. Therefore, we propose the effective way to overcome poor fatigue resistance is to encapsulate the photochromic compounds with microcapsule technology [[12], [13], [14]].

The conventional preparation methods of photochromic fabrics mainly include direct dyeing and surface coating. Since most photochromic dyes are disperse dyes, problems such as photochromic properties, fatigue resistance, and poor washing fastness will occur when they are directly applied to natural textile substrates [15,16]. And the surface coating method may have some problems such as poor hand feeling and air permeability. By comparison, layer-by-layer assembly (LBL) is a simple, economical, and universal technique [17], the substrate can be functionalized and the interaction can be electrostatic attraction or other driving forces [18]. For the electrostatic LBL method, which involves the sequential adsorption of polycations and polyanions with opposite charges, the sequential deposition process can be achieved by charging the substrate and forming a multi-layer structure as a result [17,19,20]. As is widely used in LBL technology, chitosan has a pKa of 6.2, the amino group becomes highly positively charged due to protonation in weakly acidic environments [21,22]. Besides, chitosan can be also used as wall material to prepare microcapsules, according to previous reports in the literature [12,13,23]. However, it is seldom employed in the field of photochromic fabrics. Herein, the use of chitosan can improve photochromic fatigue resistance, meanwhile, as the wall material of microcapsules in a weak acid environment can provide the positive charge needed for the electrostatic LBL self-assembly process.

Nowadays, the reversible and erasable/rewritable properties of photochromic materials are attractively applied as optical memory [24]. For example, Wang et al. [25] applied organic photochromic materials to substrates such as thin films and paper to endow erasable and rewritable ability. Garai et al. [26] used photochromic metal-organic frameworks (MOFs) and tested their capability as inkless and erasable printing media. Yang et al. [27] explored a rapidly responsive photochromic hydrogel through the introduction of ammonium molybdate into gelatin and hydrophobic associative polyacrylamide, which exhibits a good erasable and rewritable performance. However, there seems to be no report on optical data storage based on fabric. By alternating UV light and visible light/heat treatment on photochromic fabrics, various patterns can be recorded repeatedly, indicating potential application prospects in erasable optical data storage. Furthermore, photochromic materials can absorb UV light, which implies them can be used as functional UV-protective textiles [28]. Compared to those prepared by metal salts [[29], [30], [31]], metal-organic frameworks (MOFs) [32], nanoparticles [[33], [34], [35]], the photochromic textiles have a comprehensive protection effect with low-cost and easy fabrication.

In this paper, we creatively used the electrostatic LBL self-assembly method to prepare a kind of photochromic cotton fabric with high fatigue resistance, the schematic diagram of the preparation route is shown in Fig. 1b. Firstly, the photochromic microcapsules were prepared with chitosan as shell and spiropyrane as the core, then the cotton fabric is anionized to achieve the purpose of charging the substrate, finally, LBL self-assembly of anionic cotton fabric was carried out by using -NH3+ in microcapsule wall material and -COO in sodium alginate molecule in weak acid environment. It is worth noting that the use of chitosan can not only improve the fatigue resistance of spiropyrane, but also provide the positive charge needed in the electrostatic LBL self-assembly process. To the best of our knowledge, this is the first time to use electrostatic LBL self-assembly method to anchor photochromic microcapsules on the fabric. The relationship between the microcapsule loading, photochromic properties, and the number of assembly layers on the cotton fabric was investigated. The color-changing rate, fading rate, fatigue resistance, UV protection property, and durability of the photochromic fabric were also determined. Interestingly, the prepared photochromic fabric with erasable property can absorb UV light and record various patterns repeatedly, making it a promising candidate with potential reuse in many fields include UV protective textiles and erasable optical data storage.

Section snippets

Materials

Cotton fabric (140 g/m2) was purchased from Saintyear Holding Group Co., Ltd., China. Chitosan (deacetyl degree 80–95, BR), Tween 80 (CP), Glutaraldehyde solution 25% (BR), acetic acid (AR), sodium hydroxide (AR), petroleum ether (AR), Sodium alginate (CP), Ethanol (AR, 95%), Sodium chloride were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium chloroacetate (AR, 98%) were purchased from Aladdin and used without any further purification. The photochromic compound

Characteristic of photochromic microcapsules

To observe the surface morphology of the microcapsules, SEM image was shown in Fig. 2a,the result indicates that the prepared microcapsules have a near-spherical shape and a relatively smooth outer wall. The surface morphology shows good compactness, which can effectively avoid the leakage of the core material and the diffusion of the surrounding medium into the microcapsule. The particle size of the microcapsule was also tested by the laser particle analyzer and the result is shown in Fig. 2b,

Conclusion

In this paper, photochromic cotton fabric was prepared by electrostatic LBL self-assembly of the photochromic microcapsule and sodium alginate on anionized cotton fabric. The prepared photochromic microcapsules have a compact shell and a particle size of 0.71 μm. SEM images demonstrate that the load of microcapsules on the fabric increased with the increase of the assembled layers and FTIR proved the successful loading of microcapsules on cotton fabric. Besides, the regular and observable

Funding

This research was supported by “The Fundamental Research Funds for the Central Universities No. 2232020G-04”.

Author contributions

Zhichao He conceived and designed the study, performed the experiments, analyzed the data, and wrote the paper.

Wei Wang provided resources and financial support.

Ji Fan, Bingwei Bao, Xintong Qin helped to perform the experiments.

Dan Yu provided resources, financial support, writing-review & editing.

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time due to technical and time limitations. Data will be made available on request.

Declaration of Competing Interest

We declare that we have no conflict of interest with any person or institution.

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