Perfluoropolyether-benzophenone as a highly durable, broadband anti-reflection, and anti-contamination coating

Anti-reflection and anti-contamination coatings prepared from fluorinated polymers have widespread and important applications, ranging from protective films for corrosion resistance to high-tech microelectronics and medical devices due to their transparency, low refractive index, stain resistance, and antifouling properties. However, the application of existing coatings is hindered by low surface adhesion to the target substrate and weakness when exposed to mechanical stress or damage, resulting in significant limitations to their practical applications. Herein, we incorporate perfluoropolyether (PFPE) with benzophenone (BP) to develop an efficient coating material (PFPE-BP) possessing broadband anti-reflectivity, anti-contamination properties, excellent abrasion resistance, and stability under elevated temperatures and relative humidity. The presence of BP allows the coating materials to be homogeneously mixed with a commercial hard coating solution to uniformly coat the target substrate. Furthermore, UV light irradiation on the coating surface results in excellent adhesion between BP groups of PFPE-BP and the hard coating matrix.

In this study, we chemically modifies PFPE (i.e., through the covalent incorporation of PFPE with benzophenone to obtain PFPE-BP) to retain the inherent advantages of PFPE, while overcoming its limitations. Thus, we have developed an efficient coating material with broadband anti-reflectivity, anti-contamination behaviour, excellent abrasion resistance, and high resistance to thermal changes and relative humidity. Due to the presence of BP, the coating materials can be mixed homogeneously with a commercial hard coating solution 26 to uniformly coat the target substrate. After coating, UV light irradiation on the coating surface induces excellent adhesion between the BP groups of PFPE-BP and the hard coating matrix. The resulting coating surface offers: (i) excellent dewetting (a static water contact angle of 112.4° and a sliding angle of 2.5°), (ii) broadband anti-reflection (1.2% at 550 nm), (iii) high compatibility with a common hard coating solvent, and (iv) robust stability under the conditions of mechanical stress, elevated temperatures, and high relative humidity.

Results and discussion
Perfluoropolyether-benzophenone (PFPE-BP) was prepared from perfluoropolyether alcohol (PFPE-CH 2 OH), diisocyanate cyclic trimer (HDI), and 4-hydroxy benzophenone (BP) ( Fig. 1; additional synthetic details can be found in "Materials and methods" section). Addition polymerisation of the isocyanate groups of HDI with the hydroxyl groups of PFPE-CH 2 OH and BP yields PFPE-BP, as confirmed by Fourier-transform infrared (FT-IR) spectroscopy (Fig. 1c). HDI exhibits a prominent peak at 2,270 cm −1 in the FT-IR spectrum (Fig. 1c), indicative of its N=C=O group; this peak is clearly almost non-existent for PFPE-BP, indicating that the -N=C=O moiety www.nature.com/scientificreports/ of HDI was transformed. In addition, the peak at ~ 3,680 cm -1 , attributed to the -OH bonds of both HDI and PFPE-CH 2 OH, is insignificant for PFPE-BP, which indicates that the majority of OH groups of PFPE-CH 2 OH and HDI were transformed. PFPE-BP synthesis are further confirmed by 1 H nuclear magnetic resonance (NMR) spectroscopy ( Supplementary Fig. S1). In the 1 H NMR spectrum, an obvious peak at 6.7 ppm is observed, which is attributed to the N-H resonance of the -NCOH-groups 27 , indicating the successful addition polymerisation of the isocyanate groups of HDI with the hydroxyl groups of PFPE-CH 2 OH and BP. Glass and transparent plastic films inevitably reflect ~ 4% of the incident light from their surfaces 28 . To eliminate disturbance from external light and increase light transmission, thereby enhancing the clarity of display images and the performance of optical components, anti-reflective coatings are typically applied to optical lenses, solar cells, displays, thermochromic windows, eyeglasses, and camera lenses 28 . The destructive interference of light reflected from the interface of layers with different refractive indices is the main working mechanism of successful anti-reflection materials. For instance, to attain a surface with zero reflection, the refractive index of the anti-reflective coating (for single-layer anti-reflection coating) must be equal to the square root of the refractive index of the substrate 29 . Since the refractive indices of glass and most plastics are ~ 1.5, the required refractive index of the coating must therefore be ~ 1.22. However, this theoretical refractive index is lower than that of any known bulk materials appropriate for this purpose. Interestingly, novel PFPE-BP possesses a refractive index of ~ 1.24, as well as excellent transparency due to the small dipole moment arising from the inherent characteristics of PFPE.
To demonstrate that PFPE-BP behaves as an efficient coating material, we first prepared the coating solution by mixing PFPE-BP (1 wt%) with hard coating (49 wt%) and a fluorinated solvent Asahiklin 225 (50 wt%), forming a highly transparent solution. Notably, monomeric PFPE-CH 2 OH forms a cloudy mixture when combined with the hard coating and the fluorinated solvent ( Supplementary Fig. S2). We believe that the excellent compatibility observed with PFPE-BP (and not with PFPE-CH 2 OH) is due to the presence of bulky and relatively high-surface-energy BP moieties on PFPE-BP. After preparation, the coating material was applied to a transparent substrate (polyethylene terephthalate (PET) film) by spin coating, followed by UV light irradiation to promote tight bonding of the coating solution to the substrate (further details have been provided in "Materials and methods" section). The thickness of the coating layer is 160 nm, as confirmed by atomic force microscopy (AFM; Supplementary Fig. S3). Figure 2 displays the performance of the coating surface related to reflectance, with the inset in Fig. 2a showing the resultant coating material solution with high transparency. The reflectance decreases significantly after coating; the coating on both sides of the PET film exhibits only 1.2% reflectance at 550 nm, whereas the bare PET film exhibits 3.4% reflectance at this wavelength (Fig. 2a). Figure 2b further demonstrates the anti-reflection performance of the coatings. The light transparency of the film is observed by shining light at different places on the surface (top, middle, and bottom portions). As shown in Fig. 2b, the PET film coated on both sides provides significant anti-reflection. In addition, these results suggest that the coating thickness remains highly uniform over the entire surface of the film.
We also investigated the anti-contamination performance of PFPE-BP ( . Overall, the surface energies decreased significantly after coating. We also performed a straightforward anticontamination test (Fig. 2d,e), in which the letter 'm' was written on the surface with a permanent marker, and then the lower part of the letter was wiped with a tissue. The ink of permanent marker is composed of a pigment, a glue-like polymer, and isopropanol solvent. Figure 2d shows that the ink is not erased from the bare PET film, and is only smudged into a persistent stain; however, the ink is easily removed from the coated film (Fig. 2e).
Even though only 1 wt% PFPE-BP is used in the coating solution, the anti-reflective and anti-contamination abilities of the coating are significantly enhanced. This is because the PFPE moiety is aligned along the top surface of the coating layer 30 , thus affording the anti-reflective and anti-contamination surface properties of PFPE even at low loading. To confirm this, surface analysis using X-ray photoelectron spectroscopy (XPS) was performed ( Supplementary Fig. S5), which indicated that most of the PFPE-BP is located on the surface of the coating.
To investigate whether the coated substrate could withstand mechanical abrasion, we performed a rubbing test of the coated film as a function of the loading weight using a custom-made rubbing machine (Fig. 3a). The BP moiety in PFPE-BP enables covalent bonding with C-H bonds under UV light irradiation, resulting in tight adhesion to the substrate and the hard coating matrix (Fig. 3b) 31 . UV light irradiation induces a π-π*transition from the benzophenone moiety to the biradicaloid triplet state that abstracts almost any hydrogen atom from neighbouring C-H groups, leading to the formation of two radicals. These two radicals can recombine to establish a covalent bond, thus connecting the two polymer chains 32,33 . We found that our coating surface resists a high normal load of 60 g (Fig. 3c). Notably, commonly used fluorinated coating materials do not generally remain intact even under normal forces < 20 g due to poor adhesion between the fluorinated coating material and the matrix.
The robustness of the anti-contamination coating was next examined by measuring the contact angle of water after every 100 abrasion cycles under a normal load of 10 g (Fig. 3d); the water contact angle remained almost identical to the pre-abrasion value after 1,000 abrasion cycles. The anti-reflective performance of the coating surface after abrasion was also investigated (Fig. 3e). The SEM images of the coating surface after abrasion are presented in Supplementary Fig. S6. The coating surface clearly retains its anti-reflective performance even after harsh mechanical damage after 1,000 abrasion cycles, which also originates from the strong adhesion of Scientific RepoRtS | (2020) 10:15121 | https://doi.org/10.1038/s41598-020-72229-7 www.nature.com/scientificreports/ the PFPE-BP-based coating material with the matrix (hard coating and substrate). Thus, the developed coating material exhibits high mechanical robustness, while retaining highly anti-reflective and anti-contamination coating properties. Stability of the coated film when subjected to thermal changes and humidity was studied using the PET film coated on both sides (Fig. 4). Anti-reflective and anti-contamination coatings generally adsorb moisture under ambient conditions, thus damaging the coated surface and decreasing coating performance. Moreover, such damage is accelerated at elevated temperatures. Therefore, the reflectance at 550 nm was measured as a function of temperature and relative humidity (RH). Notably, the reflectance increases at 50 °C by only 0.1% and 0.3% for RH 70% (500 h) and RH 90% (500 h), respectively (Fig. 4a). These results indicate that the coated film is significantly robust to high relative humidity, likely due to the covalent bonding between the BP moiety and the matrix content (Fig. 3b). However, at high temperature (80 °C) and high RH (90%), the reflectance of the film increases relatively after exposure for 400 h (Fig. 4a). To further investigate this increase in reflectance, we observed the coating surface by scanning electron microscopy (SEM), as shown in Fig. 4b (corresponding to (i)-(iv) in Fig. 4a). The SEM images indicate clear and uniform coating surfaces with RH 90% after 500 h at 50 °C and 300 h at 80 °C; however, detachment and slight agglomeration is observed after 400 h at 80 °C (yellow arrows in Fig. 4b). This damage is likely due to the low glass transition temperature of the PFPE moiety 34 . However, the coated film shows only a slight decrease in performance under severe temperature and RH conditions and after an extended period of time, remaining highly durable under ambient conditions. Therefore, the coated surface is appropriate for practical applications. www.nature.com/scientificreports/

Conclusion
Herein we have developed an efficient coating material boasting broadband anti-reflectivity, anti-contamination, excellent abrasion resistance, and stability under elevated temperatures and relative humidity. The key to this successful coating is the combination of perfluoropolyether (PFPE) with benzophenone (BP), which was used www.nature.com/scientificreports/ as a coating additive. Due to the presence of BP, the coating materials were homogeneously mixed with the hard coating solution to uniformly coat the target PET film substrates. After coating, UV light irradiation induced better adhesion between the BP group of PFPE-BP and the substrate matrix. The resulting coating surface exhibited (i) excellent dewetting, as indicated by a static contact angle > 112.4° and a sliding angle < 2.5°, (ii) broadband anti-reflection (1.2% at 550 nm) due to the relatively low refractive index of the coating material, (iii) high compatibility with a common hard coating solvent, resulting in high adhesion and hardness of the coatings, and (iii) robust stability to mechanical stress, elevated temperature, and high relative humidity due to the strong covalent bonding between the BP moieties of PFPE-BP and the substrate. The resultant coating surface offers excellent broadband anti-reflectivity, outstanding anti-contamination performance, robust mechanical stability, and good stability under conditions of high relative humidity. We believe that our coating material can be easily extended to various applications, such as solar cell panels, optical devices, architectural and automotive glass, droplet manipulators, and fluid control mechanisms, as well as the analyses of wettability and self-cleaning coatings. For future applications, ultra-high-strength and solventresistant coating materials should be developed to further promote their practical applications, along with simple and cost-effective methods for large-scale production.

Preparation of perfluoropolyether-benzophenone (PFPE-BP). PFPE-BP was synthesised by addi-
tion polymerisation of the isocyanate groups in diisocyanate cyclic trimer (HDI) with the polyol groups in perfluoropolyether alcohol (PFPE-CH 2 OH) and hydroxyl of 4-hydroxy benzophenone (BP). BP (0.43 g, 2.2 mmol) was first dissolved in Asahiklin 225 (10 mL), then PFPE-CH 2 OH (1 g, 1.1 mmol) and HDI (0.55 g, 1.1 mmol) were added. The mixing procedure was performed in a 50-mL round bottom flask. After stirring at room temperature for 24 h, the product was precipitated and collected via filtration at room temperature. Purification of the product was performed following two repetitions of a dissolution-precipitation process. PFPE-BP was obtained as a transparent liquid. PFPE-BP (1 wt%) was used in the coating solution by mixing with the hard coating (49 wt%) and the fluorinated solvent Asahiklin 225 (50 wt%).
Coating procedure. The substrates used for coating were cleaned with excess ethanol and acetone (10 s for each solvent). All the coating processes in this study were performed via conventional spin coating of the cleaned substrates. The substrates were vacuum-locked during spin-coating. A uniform coating with a thickness of ~ 160 nm was prepared at an acceleration rate of 1,000 rpm/s and a spinning rate of 2,000 rpm for 30 s. After spinning, the film was dried in air at 40 °C for 3 min then at 70 °C for 5 min.
Characterization. SEM observations were performed using a Hitachi S-4800 field-emission scanning electron microscope operated at 5 kV. The thin films were first coated with a layer of Pt by ion sputtering. The mor-