Excellent UV-absorbing cotton fabric with high durability and hydrophobicity based on surface-initiated ATRP of polymerizable organic UV-absorber

Ultraviolet (UV) protective cotton fabric is a convenient and reliable way to protect human body from sunlight. Herein, a polymerizable UV-absorber, 2-hydroxy-4-(3-methacryloxy-2-hydroxylpropoxy) benzophenone (BPMA) was prepared from the ring-opening addition reaction of 2,4-dihydroxybenzophenone (UV-0) with glycidyl methacrylate. The initiators tethered cotton fabric (CF-Br) was prepared by the immobilization of α-bromoisobutyryl groups. PBPMA grafted cotton fabric (CF-g-PBPMA) was fabricated via surface-initiated activators generated by electron transfer for atom transfer radical polymerization (SI-AGET ATRP). The results indicated that the structure of targeted polymerizable BPMA was defined, and PBPMA chains were successfully grafted from CF-Br to obtain CF-g-PBPMA. Compared with the UV protection factor (UPF) of the original cotton fabric (3.5), the UPF of CF-g-PBPMA achieved 79,630.2 at PBPMA polymerization degree of 404 due to the incorporation of 2-hydroxy benzophenone in repeating units of PBPMA. The as-prepared CF-g-PBPMA can be labeled as "Excellent UV Protection" according to the ASTM D 6603 with UPF values of above 40. Compared with the original cotton fabric, the CF-g-PBPMA displayed excellent superhydrophobicity with the water contact angle (WCA) increasing from 46° to 154° due to the increased surface roughness of cotton fabric induced by the accumulated PBPMA. After 200 washing cycles, the chemically bonded PBPMA enabled the CF-g-PBPMA outstanding durability with the WCA and UPF achieving 135° and 730.5, respectively.


Introduction
Nowadays, the level of ultraviolet (UV) radiation reaching the Earth's surface has increased due to the ozone depletion and other environmental related issues. UV protective cotton fabric is a convenient and reliable way to protect human body from sunlight (Gorjanc et al. 2014). According to the ASTM D 6603, the label UPF is used to determine the protection performance of cotton fabric against UV radiation. When the UPF values fall in the ranges of 15-24, 25-39, and 40 or greater, the phrases "Good UV Protection", "Very Good UV Protection", and "Excellent UV Protection" can be used to quantitively describe the UV protective performance, respectively.
White or light color dyed cotton fabric is very popular with consumers in summer. The UPF value of such cotton fabric is very poor (Guo et al. 2017). UV light absorbers (UVA), which can convert the absorbed radiation energy into less harmful thermal modified cotton fabric maintained durable UV protective performance, the UV protective performance still need further enhancement.
It provides a feasibility to further improve the UV protection performance by tethering polymer brushes or chains containing massive the orthohydroxyl groups on the surface of cotton fabric via surfaceinitiated activators generated by electron transfer for atom transfer radical polymerization (SI-AGET ATRP) . To the best of our knowledge, long-term UV protective cotton fabric has ever been prepared by polymer grafting of the orthohydroxyl groups.
Synthesis of 2-hydroxy-4-(3-methacryloxy-2-hydroxylpropoxy) benzophenone (BPMA) As described in an earlier literature (Dong et al. 2018a, b), the procedure for preparing BPMA was illustrated in Fig. S1. A mixture of NaOH (0.155 g, 3.88 mmol) and UV-0 (10.7 g, 0.05 mol) were added into a 250 mL three-necked round bottom flask with a ventilation pipe, a stirring paddle and a refluxing condensation pipe. The reactor was deaired by vacuum pump and was aerated with nitrogen gas to exclude the oxygen gas. Then, GMA (7.81 g, 0.055 mol) was introduced into the reactor by syringe. The reaction was carried out at 80 °C for about 5 h. After the reaction was completed, a coarse product of yellow viscous liquid was obtained, which was dissolved in EA and washed with 1 wt% dilute sulfuric acid. The EA was removed under reduced pressure. Then, the pure BPMA was collected by chromatography on silica gel using the mixed solvent of PE and EA (8:1, v/v) as eluent. After complete removal of solvents under vacuum, 13.7 g of BPMA were obtained with the yield of 74%. The 1 H-NMR spectrum of BPMA ( Fig. S2) and the FT-IR spectra of UV-0 and BPMA ( Fig. S3) are provided in supporting information.

Preparation of initiator immobilized cotton fabric by esterification
The original cotton fabric was thoroughly washed with deionized water, ethanol and acetone in turn to remove dust, and then dried in an oven at 50 ℃ for 8 h. Next, 2 g of dried cotton fabric were added into a 150 ml three-necked round bottom flask containing triethylamine (2 ml) and DMF (50 ml), and then 2 ml of BIBB were added dropwise under mild magnetic stirring. After the reaction was carried out for 48 h at 20 °C, the modified cotton fabric was rinsed thoroughly with ethanol until the washed liquid became clear. The initiator immobilized cotton fabric was obtained by drying in vacuum oven at 40 °C for 16 h and was defined as CF-Br.
Fabrication of PBPMA grafted cotton fabric by SI-AGET ATRP 2 g of CF-Br was added to Schlenk flask containing methanol (40 mL) and EBiB (0.012 g, as the sacrificial initiator) solution. Under gentle magnetic stirring, BPMA (1-3 g), CuBr 2 (2.6 mg), and bpy (7.2 mg) were sequentially added to Schlenk flask. After bubbling with N 2 for 0.5 h under mild magnetic stirring, AscA (4.8 mg) was added to the reactor and the polymerization was carried out at 60 ℃ for 12 h. The obtained cotton fabric was collected and washed with methanol to remove the free PBPMA and BPMA. The PBPMA grafted cotton fabric was then dried in an oven at 40 °C for 24 h, which was defined as CF-g-PBPMA, in which 1 g and 3 g of BPMA were noted as CF-g-PBPMA-1 and CF-g-PBPMA-2, respectively. The free PBPMA homopolymer was obtained by precipitation into cyclohexane for GPC characterization. The molecular weight of the grafted polymer can be well-estimated by the addition of EBiB as a sacrificial initiator (Table S1). After testing the mechanical properties of cotton fabric samples, the results were showed in Table S2.

Characterization
Using dimethyl sulfoxide-d6 as solvent, high resolution proton nuclear magnetic resonance ( 1 H-NMR) spectrum was recorded on a Varian INO-VA-400 spectrometer of 400 MHz. The surface compositions of cotton fabric samples were characterized by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) on a Perkin-Elmer Spectrum 2000 Fourier transform infrared spectrometer (Specac Ltd). XPS measurements of original cotton fabric, CF-Br, and CF-g-PBPMA were performed by an ESCALAB 250Xi instrument (USA). The surface morphologies of modified membranes were by a field-emitting scanning electron microscope (FESEM, XL 30S-FEG, Holland) at an acceleration voltage of 10 kV. All the samples were coated with gold. The molecular weight and molecular weight distribution (Đ) of the free PBPMA homopolymer were determined by gel permeation chromatograph (GPC; HLC-8320), and the measurements were conducted with differential refractometer using columns (TSK gel super HZM-M and TSK gel SuperHZ3000 in series) with THF as the eluent (35 °C, flow rate of 1 mL/min) with diphenyl ether used as internal references. Linear polystyrene standard was used for calibration. The water contact angle (WCA) tests were carried out with contact angle measuring instrument (DSAIOMKZ, China), and each sample was tested at more than three different locations. UV-visible absorption spectra of samples were measured were measured by using a UV-visible UH5300 spectrophotometer (Hitachi, Tokyo, Japan). The mechanical properties of the original cotton fabric, CF-Br, and CF-g-PBPMA were conducted on the electronic universal testing machine (Shenzhen SUNS Technology Stock Co., Ltd.) according to GB/T 3923.1-2013. The BET surface area of cotton fabric samples was determined by analyzing the nitrogen adsorption and desorption characteristics using a surface area analyzer (ASAP2020M + C). The samples were degassed for at 12 h at 105 ℃ under vacuum prior to the measurements. According to the AATCC TM183-2020, each specimen of the treated cotton fabric samples was cut at least 50 × 50 mm and placed the specimen flush against the sample transmission port opening in the sphere, and were examined for their UV protective performance by Cary 5000 UV-visible Spectrophotometer (Agilent, America). The UPF was calculated for each measurement according to the following formula: In this formula, E λ = relative erythemal spectral effectiveness, S λ = solar spectral irradiance, T λ = spectral transmittance (measured), Δλ = measured wavelength interval (nm).
Laundering durability was performed to evaluate the stability of surface hydrophobicity and UV absorption performance of the cotton fabric samples in analogy to the ISO 105-C02:1989(E) method. Samples of cotton fabric were immersed into an aqueous solution containing 5 g/L AEO nonionic detergent. The bath was thermostatically adjusted to 50 ℃. The test was run for 45 min at 750 rpm. The samples of cotton fabric were then removed, rinsed in cold distilled water for 10 min and finally dried at room temperature for 12 h. Samples of cotton fabric were washed for 1, 5, 100 and 200 washing cycles.

Results and discussion
Fabrication of CF-g-PBPMA Because the surface of the original cotton fabric contains massive active hydroxyl groups, it is possible to modify the original cotton fabric (Scheme 1a). After treatment with BiBB, α-bromoisobutyryl groups bonded cotton fabric (CF-Br) was obtained with initiating sites for ATRP (Scheme 1b). Then, PBPMA was grafted from the surfaces of CF-Br fibers to obtain the robust and durable CF-g-PBPMA via SI-AGET ATRP for excellent UV protection (Scheme 1c).

Performance characterization of BPMA
The UV absorption performance of BPMA and UV-0 was evaluated by UV absorption spectra. As shown in Fig. 1, the peak at 240 nm in the UV absorption spectrum of BPMA was the π → π* transition which was similar to UV-0. In the UV spectrum of BPMA, a hypsochomic shift from 292 to 287 nm was the π → π*transition due to the newly appearance of an ether bond Ar-O-C, which was formed by the ring-opening addition reaction between 4-position hydroxyl of UV-0 and GMA. The UV absorption band of BPMA at 322 nm corresponded to the n → π* transition of carbonyl, which was also similar to that of UV-0. The three peaks at 240, 287, and 322 nm in the UV absorption spectrum of BPMA confirmed the successful ring-opening addition reaction between UV-0 and GMA, which would retain the excellent UV absorption performance of UV-0.

Microstructure, surface hydrophobicity and chemical compositions of CF-g-PBPMA
To further confirm the initiator tethering and subsequent PBPMA grafting, the pristine cotton fabric, Fig. 1 The UV-visible absorption spectra of BPMA and UV-0 (1 × 10 -7 mol/ml in methanol) CF-Br, and CF-g-PBPMA were characterized by FT-IR to identify the chemical variation of cotton fabric during the whole synthetic process, respectively, (Fig. 2). Compared with the spectrum of original cotton fabric (Fig. 2a), a new peak at 1717 cm −1 corresponding to the C = O stretching vibration verified that the active hydroxyl groups on the surface of the original fabric were successfully esterified with BIBB. Therefore, the introduced α-bromoisobutyryl groups on the surface of cotton fabric were acted as ATRP initiating sites (Fig. 2b). As shown in Fig. 2c, a new absorption band located at 1254 cm −1 in the spectrum of CF-g-PBPMA pertained to the C-O-C after the polymer was grafted. The significantly enhanced peaks at 1724 and 1625 cm −1 corresponded ester carbonyl and ketone carbonyl. In addition, the peaks located at 1502 and 704 cm −1 were attributed to the aromatic skeletal vibration of benzene ring and C-H out-of-plane bending vibration of monosubstituted benzene rings, respectively.
The surface chemical composition of original cotton fabric, CF-Br, and CF-g-PBPMA were further identified by XPS analysis (Fig. 3). As shown in Fig. 3a, the binding energy of 533 and 286 eV in the original cotton fabric belonged to O1s and C1s, respectively. After modification with BiBB, a new peak of Br3d at 68 eV appeared (Br element proportion: 0.16 at%) confirmed the successful tethering of α-bromoisobutyryl groups on the cotton fibers. After the polymer was grafted, the peak intensity and position of CF-g-PBPMA were similar to those of CF-Br due to the similar chemical compositions between CF-g-PBPMA and CF-Br. In addition, the C1s core spectra of CF-Br (Fig. 3b) and CF-g-PBPMA ( Fig. 3c) can be curve-fitted into four components with binding energies at 288.8, 287.3, 286.4 and 284.6 eV corresponding to the C = O, C-O, CH 2 and C-H, respectively. After the SI-AGET ATRP process, the areas of C = O, C-O and C-H spectra of CF-g-PBPMA were obviously higher than those of CF-Br, which confirmed the successful polymer grafting.
The surface structures of original cotton fabric and modified cotton fabric were illustrated via SEM analyses (Fig. 4). The original cotton fabric presented a typically smooth surface ( Fig. 4a and b). After being treated with BiBB, there was no significant difference between the surface smoothness of CF-Br ( Fig. 4c and d) and that of original cotton fabric. Compared with relatively smooth surface of CF-Br, the irregular and rough folds were observed on the surface of CFg-PBPMA ( Fig. 4e and f). The highly irregular wrinkles and a greater surface roughness were originated from the accumulation of grafted PBPMA chains and thus confirmed the successful polymer grafting.
In order to evaluate the surface hydrophobicity of cotton fabric, the water contact angles (WCAs) were examined and illustrated in Fig. 5. When the water droplet approached the original cotton fabric, a WCA of about 46° could be observed (Fig. 5a). However, the water droplet was adsorbed completely within 10 s owing to original cotton fabric bearing abundant hydroxyl groups on the surface. After initiator tethering, the immobilized α-bromoisobutyryl groups enabled the hydrophilic cotton fibers to exhibit hydrophobicity with a WCA of about 121° (Fig. 5b). After PBPMA grafting, the WCAs of CF-g-PBPMA-1 and CF-g-PBPMA-2 achieved 140° (Fig. 5c) and 154° (Fig. 5d) due to UV protective performance of CF-g-PBPMA The UV protective performance of the cotton fabric was evaluated via the UPF value and illustrated in Fig. 6. The UPF values of original cotton fabric and CF-Br were 3.5 and 2.7, respectively, displaying not reaching enough UV protective performance between 280 and 400 nm. After PBPMA grafting, the UV transmission of CF-g-PBPMA-1 decreased obviously and the UPF increased to 241.5 at PBPMA polymerization degree of 26 owing to the incorporation of 2-hydroxyl benzophenone in repeating units of PBPMA. In addition, the UV transmission of CF-g-PBPMA-2 further decreased and the UPF increased to 79,630.2 at PBPMA polymerization degree of 404 due to the incorporation of 2-hydroxyl benzophenone in repeating units of PBPMA. The as-prepared CF-g-PBPMA can be labeled as "Excellent UV Protection"

Durability analysis
In order to evaluate the durability of CF-g-PBPMA, the UV protective performance and surface hydrophobicity of CF-g-PBPMA were tested after 1, 5, 100 and 200 washing cycles.
After 1, 5, 100, and 200 washing cycles, the WCAs of CF-g-PBPMA-1 and CF-g-PBPMA-2 were shown in Fig. 7. The surface hydrophobicity of CF-g-PBPMA-1 and CF-g-PBPMA-2 with increasing numbers of washing cycles declined gradually. However, even after 200 washing cycles, the WCA of CF-g-PBPMA-1 and CF-g-PBPMA-2 still achieved 123° (Fig. 7e) and 135° (Fig. 7j) due to the chemically bonded PBPMA, which further demonstrated excellent durability of surface hydrophobicity. Figures 8 and 9 displayed the UV protective performance of CF-g-PBPMA-1 and CF-g-PBPMA-2 after 1, 5, 100 and 200 washing cycles. The UV-visible transmittance of CF-g-PBPMA-1 and CF-g-PBPMA-2 with increasing numbers of washing cycles marginally increased in the range of 280-400 nm. In addition, even after 200 washing cycles, the UPF value of CF-g-PBPMA-1 and CFg-PBPMA-2 still reached 58.7 (Fig. 8e) and 730.5 (Fig. 9e) due to the chemically bonded PBPMA, which further demonstrated excellent durability of UV protective performance. Table 1 illustrated the UPF values of optimal CF-g-PBPMA and other reported cotton fabric with UV protective performance. Among all the UV protective fabrics, the CF-g-PBPMA possessed the highest UPF value (79,630.2) at PBPMA polymerization degree of 404 due to the incorporation of 2-hydroxyl benzophenone in repeating units of PBPMA. The CF-g-PBPMA exhibited excellent performance of UV protection with outstanding Fig. 9 The UV-visible transmittance spectra of CF-g-PBPMA-2 (a), CF-g-PBPMA-2 after one cycle of washing (b), CF-g-PBPMA-2 after five cycles of washing (c), CF-g-PBPMA-2 after one hundred cycles of washing (d) and CF-g-PBPMA-2 after two hundred cycles of washing (e). (Note: The 280-500 nm region was selected for clarity.)  Xu et al. (2021) durability and superhydrophobicity, which provides a novel and efficient strategy for the preparation of multifunctional fabrics.

Conclusions
In this study, CF-g-PBPMA was successfully prepared by SI-AGET ATRP method with synthetic BPMA as functional monomer. Compared with the UPF of the original cotton fabric (3.5), the UPF of CF-g-PBPMA achieved 79,630.2 at PBPMA polymerization degree of 404 due to the incorporation of 2-hydroxyl benzophenone in repeating units of PBPMA. The as-prepared CF-g-PBPMA can be labeled as "Excellent UV Protection" according to the ASTM D 6603 with UPF values of above 40. Compared with the original cotton fabric, the CF-g-PBPMA displayed excellent superhydrophobicity with the WCA increasing from 46° to 154° due to the increased surface roughness of cotton fabric induced by the accumulated PBPMA. After 200 washing cycles, the chemically bonded PBPMA enabled the CF-g-PBPMA outstanding durability with the WCA and UPF achieving 135° and 730.5, respectively. The multifunctional CF-g-PBPMA has potential applications in various areas, such as medical, military, biological, and optoelectronic industrial fields.