[
    {
        "id": 1,
        "chunk": "# A facile approach to UV-curable super-hydrophilic polyacrylate coating film grafted on glass substrate  \n\nTao Liang, Hongqiang Li, Xuejun Lai, Xiaojing Su, Lin Zhang, Xingrong Zeng  \n\n$\\circleddash$ American Coatings Association 2016  \n\nAbstract The super-hydrophilic polymer coating film can easily be be peeled off from a substrate with the existence of water, which is a fatal drawback in practical applications. Herein, a facile approach is proposed to prepare UV-curable super-hydrophilic polyacrylate coating film that is chemically grafted on the surface of $\\gamma$ -methacryloxypropyltrimethoxysilanemodified glass substrate. Fourier transform infrared spectroscopy and scanning electron microscopy confirmed that the polyacrylate coating films were successfully grafted onto the glass substrate and exhibited rough micro-groove structure. The obtained polyacrylate coating film possessed super-hydrophilicity with the water contact angle close to nearly zero as well as good transmittance and antifogging property.  \n\nKeywords UV-curable, Polyacrylate, Glass substrate, Super-hydrophilicity, Antifogging",
        "category": " Abstract"
    },
    {
        "id": 2,
        "chunk": "# Introduction  \n\nIn recent years, super-hydrophilic materials have been paid more and more attention, due to their potential applications in biological medicine,1 water harvesting,2 self-cleaning,3 antifogging,4 electroactive material,5 and microfluidic devices.6 When a water droplet falls onto textured and/or structured materials (rough and/ or porous), it will spread completely and quickly to form a thin liquid film, and the contact angle is nearly zero, and these materials can be thought to be superhydrophilic.7  \n\nCurrently, there are three important strategies to prepare super-hydrophilic coating films. The first one, including chemical vapor deposition8 and sol–gel method,9,10 is to use the photo-chemically active materials such as $\\mathrm{TiO}_{2}$ and $\\mathrm{znO}$ that can become super-hydrophilic under ultraviolet (UV) light exposure.11–13 However, when the film is placed in a dark environment, it will lose the super-hydrophilicity within a few hours. The second one, including layerby-layer assembly14 and electrostatic spinning,15 is to fabricate or modify the surface chemical and geometric microstructure into a texture surface or porous films, which can absorb the water on solid surface and promote the residual water droplet to spread out on the interface of solid and water. Unfortunately, the above two methods require harsh reagents16 and multistep processes,17 and are hard for end-use. The third one is to directly prepare super-hydrophilic polymer coating films using hydrophilic monomers such as acrylic acid,18 poly(ethylene glycol) monomethacrylate,19 and 2-(methacryloyloxy) ethyl phosphorycholine20 by UV-initiated polymerization. Due to the low cost,21 simple process,22 high efficiency, and lack of pollution,23 this method is considered as a very promising one to prepare the super-hydrophilic coating films. However, it is difficult to obtain superhydrophilicity only with the hydrophilic monomers. Furthermore, the obtained super-hydrophilic coating films are easy to be peeled off from the substrate with the existence of water, which is a fatal drawback in practical application.  \n\nIn this article, we first used a piranha solution to treat glass substrate so as to increase the number of hydroxyl groups, and then the substrate was further modified by $\\gamma$ -methacryloxypropyltrimethoxysilane (MPS) to introduce $C{=}\\dot{C}$ double bond. The superhydrophilic polyacrylate coating films chemically grafted onto glass substrate was prepared with 2- methacrylatoethyl trimethyl ammonium chloride (DMC) and trimethylolpropane triacrylate (TMPTA) as hydrophilic monomer and crosslinker, respectively, by UV-initiated polymerization. The chemical structure and micromorphology of the film were characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM); the effect of the contact time and monomer ratio on the contact angle were studied; the transmittance and antifouling property were also investigated.",
        "category": " Introduction"
    },
    {
        "id": 3,
        "chunk": "# Experimental",
        "category": " Materials and methods"
    },
    {
        "id": 4,
        "chunk": "# Materials  \n\n2-Methacrylatoethyl trimethyl ammonium chloride (DMC, $72\\mathrm{\\mt{\\%}}$ in water solution) was purchased from General Electric Co., Ltd (USA). TMPTA, MPS, and 2-hydroxy-2-methylpropiophenone (Darocur 1173) were provided by Aladdin Reagent Co., Ltd (China). Sulfuric acid $\\mathrm{(H}_{2}\\mathrm{SO}_{4}$ , $98\\%$ ) and ethanol were purchased from Guangzhou Chemical Reagent Factory (China). Hydrogen peroxide $\\left(\\mathrm{H}_{2}\\mathrm{O}_{2}\\right)$ , $30\\%$ ) was supplied by Chinasu Specialty Products $\\mathbf{\\boldsymbol{C}}\\mathbf{\\boldsymbol{o}}$ , Ltd (China). Glass slide was purchased from Chinasu Sail Brand Products $\\mathrm{Co}$ ., Ltd (China); the length, width, and thickness were 76.2, $25.4~\\mathrm{mm}$ , and $1{-}1.2~\\mathrm{mm}$ , respectively. All materials were used as received without further purification.",
        "category": " Materials and methods"
    },
    {
        "id": 5,
        "chunk": "# Modification of glass substrate  \n\nGlass slides were first treated by submerging in a freshly prepared piranha solution containing $70\\%$ $\\mathrm{H}_{2}\\mathrm{SO}_{4}$ and $\\hat{30\\%}$ $\\bar{\\mathbf{H}_{2}\\mathbf{O}_{2}}$ at $90^{\\circ}\\mathrm{C}$ for $^{\\textrm{1h}}$ , and then rinsed with copious amounts of deionized water, and the hydroxylated glass substrates were obtained.24 Then, $\\boldsymbol{10}\\ \\mathrm{g}$ MPS was added into $190~\\mathrm{g}$ ethanol solution $(V_{\\mathrm{ethanol}}/V_{\\mathrm{water}}=2)$ in a $500~\\mathrm{mL}$ beaker and stirred for $^{1\\mathrm{~h~}}$ at room temperature. Subsequently, the solution was heated to $70^{\\circ}\\mathrm{C}$ , the hydroxylated glass substrates were put into and kept for $45~\\mathrm{{min}}$ , then taken out and placed in a vacuum oven at $110^{\\circ}\\mathrm{C}$ for $^\\textrm{\\scriptsize1h}$ for further reaction. At last, the glass substrates were thoroughly rinsed with ethanol by ultrasonic for three times at room temperature, then dried in vacuum oven at $40^{\\circ}\\mathrm{C}$ for $24\\mathrm{~h~}$ , and the MPS-modified glass substrates were prepared.25",
        "category": " Materials and methods"
    },
    {
        "id": 6,
        "chunk": "# Preparation of super-hydrophilic polyacrylate coating films chemically grafted on glass substrate  \n\nAppropriate amount of DMC solution, TMPTA, Darocur 1173, and ethanol were uniformly mixed, and the obtained solution was dripped and scraped onto the MPS-modified glass substrate by flat-plate knife coater, and then exposed at UV light (INTELLIRAY 400, Uvitron International, Inc., USA) for $400\\ \\mathrm{s}$ ； the distance between the samples and the center of the UV light lamp was $15\\ \\mathrm{cm}$ . With ethanol evaporating and the occurrence of UV-curing reaction, the films with the average thickness at $13\\pm1~{\\upmu\\mathrm{m}}$ were obtained. The formulations for UV-curable super-hydrophilic polyacrylate coating films are listed in Table 1, and the preparation process for the superhydrophilic coating films chemically grafted on glass substrate is shown in Fig. 1.",
        "category": " Materials and methods"
    },
    {
        "id": 7,
        "chunk": "# Characterization  \n\nTo confirm the chemical structure of the modified glass and the glass with super-hydrophilic polyacrylate coating film, FTIR was carried on a Tensor 27 (Bruker, Germany) spectrometer. The spectra were acquired over the range $4000{\\mathrm{-}}600~{\\mathrm{cm}}^{-1}$ , the scanning was performed with a resolution of $4~\\mathrm{cm}^{-1}$ , and all the spectra were collected 48 times to ensure accuracy.  \n\nAccording to reference 26, sessile drop contact angle measurement was performed with deionized water using DSA100 contact angle analyzer (KRUSS, Germany) equipped with a video capture. A total of $1~\\upmu\\mathrm{L}$ of deionized water was dropped onto a dry coating film with a micro-syringe at room temperature. The obtained water contact angles were the mean values measured from five different places on the surface. All water contact angle measurements were performed in room temperature with the humidity of $20{-}40\\%$ .  \n\nThe morphology of the coating film was observed with Phenom TM scanning electron microscope (FEI, Holland) at an accelerated voltage of $5\\mathrm{kV}$ . The chemical composition of the prepared coating films was measured by energy dispersive $\\mathbf{X}$ -ray spectroscopy (EDS) performed in SEM. All the samples were sputtering coated with Au prior to observation.  \n\nTable 1: Formulations for UV-curable super-hydrophilic polyacrylate coating films   \n\n\n<html><body><table><tr><td>Samples</td><td>Mass (DMC)/Mass (TMPTA)</td><td>DMC solution (g)</td><td>TMPTA (g)</td><td>Darocur 1173 (g)</td><td>Ethanol (g)</td></tr><tr><td>S1</td><td>9/1</td><td>4.0</td><td>0.32</td><td>0.16</td><td>20.0</td></tr><tr><td>S2</td><td>8/2</td><td>4.0</td><td>0.72</td><td>0.16</td><td>20.0</td></tr><tr><td>S3</td><td>7/3</td><td>4.0</td><td>1.23</td><td>0.16</td><td>20.0</td></tr><tr><td>S4</td><td>6/4</td><td>4.0</td><td>1.92</td><td>0.16</td><td>20.0</td></tr><tr><td>S5</td><td>5/5</td><td>4.0</td><td>2.88</td><td>0.16</td><td>20.0</td></tr></table></body></html>  \n\n![](images/11b0beb6d565b9035e540bfd9d361ceeefd8e110ffa3007ef9f70f766bdbda9e.jpg)  \nFig. 1: Schematic for UV-curable super-hydrophilic polyacrylate coating film chemically grafted on glass substrate  \n\n![](images/39d202665c472648ff2c62e9b8b651359c4310fb11b863992f0721608dca8e8d.jpg)  \nFig. 2: FTIR spectra of (a) pure glass, (b) MPS-modified glass, and (c) UV-curable polyacrylate coating film (S2) chemically grafted on glass substrate  \n\nUV-Vis spectra were recorded using Lambda 950 UV-Vis–NIR spectrometer (PE, America), with air as reference. The transmission of wavelength ranged from 300 to $800\\ \\mathrm{nm}$ , which was taken to evaluate the film transparency.  \n\nIn order to measure the antifogging property, the pure glass and the glass grafted with super-hydrophilic coating film were cooled at $-18^{\\circ}\\mathrm{C}$ in a refrigerator for  \n\n$30~\\mathrm{{min}}$ and then moved to atmospheric environment with the humidity of $20{-}40\\%$ . After $90~\\mathrm{s}$ the images were taken using Nex-5T camera (Sony, Japan).",
        "category": " Materials and methods"
    },
    {
        "id": 8,
        "chunk": "# Results and discussion",
        "category": " Results and discussion"
    },
    {
        "id": 9,
        "chunk": "# FTIR analysis  \n\nFTIR spectra of pure glass, MPS-modified glass, and UV-curable polyacrylate coating film (S2) chemically grafted on glass substrate are shown in Fig. 2. Compared to the spectrum of pure glass, two new absorption peaks at 1727 and $164\\dot{0}~\\mathrm{cm}^{-1}$ were observed in the spectrum of MPS-modified glass, which were attributed to the carboxyl group and $\\scriptstyle\\mathbf{C}=\\mathbf{C}$ bonds, respectively. The peaks at 938 and $1481~\\mathrm{cm}^{-1}$ were ascribed to the stretching vibration of $\\mathrm{\\mathbf{Si}\\mathrm{-}\\mathbf{OH}}$ and deformation vibration of $\\mathrm{CH}_{2}$ , and the peak at $1155~\\mathrm{cm}^{-1}$ was probably assigned to the asymmetric stretching vibration of $\\mathrm{Si-}$ $\\mathrm{\\Gamma}_{\\mathrm{{O-}\\bar{\\mathrm{{Si}}}}}$ from the reaction of functional groups between MPS and hydroxylated glass.26 In the spectrum of S2, the absorption peak at $1092~\\mathrm{cm}^{-1}$ was assigned to quaternary ammonium in DMC, and the characteristic peaks between 2850 and $2925~\\mathrm{{cm}^{-1}}$ were ascribed to $\\mathrm{\\bar{C}\\mathrm{-}H}$ stretching vibration from $\\mathrm{CH}_{3}$ and $\\mathrm{CH}_{2}$ in DMC and TMPTA. Furthermore, the strong absorption peaks at 1725 and $1300~\\mathrm{cm}^{-1}$ , which were attributed to the stretching vibration of $\\scriptstyle\\mathbf{C=O}$ and $\\scriptstyle{\\mathrm{C-O}}$ in DMC and TMPTA, were also observed, indicating that DMC and TMPTA had been successfully grafted onto the modified glass.  \n\n![](images/709b07fa9f48300c987b521845d4da9601ed37a4d898b09fe63608aa06060d0e.jpg)  \nFig. 3: SEM images of super-hydrophilic polyacrylate coating film (S2) acquired at (a) ${\\pmb5000}\\times$ magnification, EDS analyses of the film on the electron image with mapping of (b) carbon, (c) oxygen, and (d) chloride elements  \n\n![](images/73b9b21e6e7541f63e636a3e6a755e241c4cfd6f2362240080c3078176a20ecc.jpg)  \nFig. 4: Images for water contact angle of (a) pure glass and (b) MPS-modified glass; Images for water contact angle of the glass coated with S2 at (c) 0 s and (d) 2.5 s",
        "category": " Results and discussion"
    },
    {
        "id": 10,
        "chunk": "# Surface morphology  \n\nFigure 3 shows SEM images of super-hydrophilic polyacrylate coating film (S2), and EDS analyses of the film on the electron image with mapping of (b) carbon, (c) oxygen, and (d) chloride elements. As seen in Fig. 3a, the super-hydrophilic polyacrylate coating film exhibited obvious rough micro-groove structure. It might be related to the volatilization of ethanol and the formation of the crosslinking structure during the UVcuring process. It is known that increasing roughness is beneficial for the improvement of the hydrophilicity of hydrophilic solid surface.27 For example, in order to improve hydrophilicity, Dong et al. incorporated silica nanoparticles into the polymer coating film to increase roughness.28 Undoubtedly, to fabricate roughness is one of the most robust and efficient methods to improve the hydrophilicity of the coating films. From EDS mapping images shown in Figs. 3b–d, it demonstrate that $67.10~\\mathrm{wt\\%}$ of carbon, $21.25~\\mathrm{wt\\%}$ of oxygen, and $11.65~\\mathrm{wt\\%}$ of chloride elements are uniformly distributed on the surface of coating film. Furthermore, Si element is not detected, indicating that the modified glass substrate is covered completely by the coating film.",
        "category": " Results and discussion"
    },
    {
        "id": 11,
        "chunk": "# Super-hydrophilicity  \n\nThe super-hydrophilicity of pure glass, MPS-modified glass, and the UV-curable super-hydrophilic polyacrylate coating film was investigated by a contact angle analyzer. The images for water contact angle of pure glass and MPS-modified glass are presented in Figs. 4a and b. The water contact angle of the pure glass and MPS-modified glass are at almost $3\\bar{1}^{\\circ}$ and $74.5^{\\circ}$ , respectively. Obviously, the MPS-modified glass is more hydrophobic than pure glass, which is due to the hydrophobic MPS. This result also indicates that  \n\nMPS was successfully grafted onto the glass surface. Figures $_{4\\mathrm{c}}$ and d show the images for water contact angle of the glass coated with S2 at 0 and $2.5\\mathrm{~s~}$ . When the contact time increased from 0 to $2.5\\mathrm{~s~}$ , the water contact angle rapidly decreased from almost $180^{\\circ}$ to less than $10^{\\circ}$ , which verified the super-hydrophilicity of the film S2. It may be due to the following reasons: the first one was the roughness of the coating film (see Fig. 3), which plays an important role for superhydrophilicity.29,30 The second one was the crosslinking network structure formed during the UV-curing process. In addition, the existence of the hydrophilic groups, such as cationic monomer DMC and trimethyl ammonium ions, also plays a role for the superhydrophilicity of the coating film.  \n\nIn order to further investigate the super-hydrophilicity of the UV-curable polyacrylate coating film, the evolution of contact angle and base diameter of water drops with contact time on the coating film were investigated, as shown in Fig. 5. It is easy to observe the occurrence of water contact angle hysteresis caused by capillary phenomena.31 Before $\\mathrm{10~\\dot{s}}{}_{\\mathrm{;}}$ , with contact time increasing, the water contact angle decreased considerably to nearly zero and the base diameter of the water droplet increased rapidly. With contact time further increasing, the water contact angle was almost unchangeable, while the base diameter still kept an increasing tendency, though the increasing rate had a little decrease. This phenomenon can be explained as follows: when water droplet contacted the film, the hydrophilic groups introduced by DMC quickly adsorbed the water, and then transferred water to its surrounding, which led to the rapid decrease of the water contact angle and the increase of the base diameter of the water droplet. With contact time increasing, the water would flow into and fill the near micro-grooves and crosslinking network,32 and then the rest of the water would further spread to the larger area. At this stage, the water contact angle was almost unchanged, while the base diameter of the water droplet still increased.  \n\n![](images/629a61d7c3ab782878bfd4458dc4fe93d861f500f0d0961090eb23fc61ebe2d7.jpg)  \nFig. 5: Evolution of contact angle and base diameter of water drops with contact time on UV-curable polyacrylate coating film (S2)  \n\nFigure 6 shows the effect of mass(DMC)/- mass(TMPTA) on water contact angle of the UVcurable polyacrylate coating films. With mass(DMC)/- mass(TMPTA) decreasing from $9/1$ to 5/5, the water contact angle decreased first and increased later. When the mass ratio was at $8/2$ , the water contact angle reached the lowest value of near zero. Under the irradiation of UV light and the initiation role of Darocur 1173, DMC not only reacts with TMPTA and MPS grafted on glass substrate, but also reacts with itself, which will have a large effect on the formation of the micro-groove structure and the crosslinking degree of the coating film. In particular, low crosslinking degree is beneficial for the water penetrating into the crosslinking structure of the film, while high crosslinking degree restrains the water into the crosslinking structure. Therefore, with mass(DMC)/mass(TMPTA) increasing from $9/1$ to 8/2, the crosslinking degree increased and the dense network was formed, which resulted in the decrease of the contact angle.33 However, with the further increase of mass(DMC)/- mass(TMPTA), the contact angle increased instead.",
        "category": " Results and discussion"
    },
    {
        "id": 12,
        "chunk": "# Transmittance  \n\nTransmittance is one of the important factors for the practical applications of the super-hydrophilic coating films, especially in the fields with high requirements on safety and appearance, such as auto windshields and glass curtain wall. Figure 7 shows UV-Vis spectra and optical photographs of pure glass and the glass grafted with super-hydrophilic polyacrylate coating films. From UV-Vis spectra shown in Fig. 7, the transmittance of the glass grafted with super-hydrophilic polyacrylate coating films were a little lower than that of pure glass between 300 and $400~\\mathrm{nm}$ . However, when the wavelength was above $400~\\mathrm{nm}$ , the transmittance was almost same. The slightly low transmittance of super-hydrophilic coating films between 300 and $400~\\mathrm{nm}$ should be due to the roughness of the film, which resulted in the higher light scattering. However, the refractive index of the polymer was lower than that of pure glass, which was beneficial for the improvement of the transmittance. In addition, from the inserted optical photographs, it can be seen that there was no difference between the two photographs, and the glass grafted with the super-hydrophilic polyacrylate coating films exhibited the same good transmittance as pure glass.  \n\n![](images/a1af2e66b8626621174d6402be39c19fa5294e7d65768aa24faa65940100c651.jpg)  \nFig. 6: Effect of mass(DMC)/mass(TMPTA) on water contact angle of UV-curable polyacrylate coating films  \n\n![](images/d97f87b0ff7c84e20fc049283840d5e38914690fe6805da8a5592062f7adc1cc.jpg)  \nFig. 7: UV-Vis spectra of pure glass, glass grafted with S1, S2, and S3, and the insert shows an optical photographs of pure glass (left) and the glass grafted with S2 (right)",
        "category": " Results and discussion"
    },
    {
        "id": 13,
        "chunk": "# Antifogging property  \n\nAntifogging property is important for the surfaces that need to be free of fog to reduce safety hazards. Figure 8 shows antifogging images for pure glass and the glass chemically grafted with UV-curable superhydrophilic polyacrylate coating film. It is obvious that pure glass was easy to be fogged and became illegible, whereas the glass grafted with UV-curable superhydrophilic polyacrylate coating film still remained clear and exhibited excellent antifogging property, which was because the condensed water droplets could rapidly spread out on the super-hydrophilic coating film and the probable light scatting phenomenon was eliminated. More importantly, the films are chemically grafted onto the surface of glass substrate, so it is durable in practical applications, even if being used at humid environment or with the existence of water.",
        "category": " Results and discussion"
    },
    {
        "id": 14,
        "chunk": "# Conclusion  \n\nA UV-curable super-hydrophilic polyacrylate coating film chemically grafted onto the MPS-modified glass substrate was successfully prepared using DMC and TMPTA as hydrophilic monomer and crosslinker, respectively. The UV-curable super-hydrophilic polyacrylate coating film appeared as a rough micro-groove structure and possessed super-hydrophilicity. With mass(DMC)/mass(TMPTA) decreasing, the water contact angle of the coating films decreased first and increased latterly. When the mass ratio was $8/2$ , the water contact angle reached the lowest value at nearly zero. The UV-curable super-hydrophilic polyacrylate coating film also exhibited excellent transmittance and antifogging property. This approach to prepare the UV-curable super-hydrophilic polymer coating film has the characteristic of high efficiency and simplicity, which are needed in practical applications. Besides glass, the UV-curable polymer coating film can also be chemically grafted onto other inorganic or metal substrates modified by silane coupling agent.  \n\n![](images/628175e4f29b290c079a06bbdf7fff25a736d47421cb828d84b051e4b00e50d6.jpg)  \nFig. 8: Antifogging images of (a) pure glass and (b) glass grafted with S2",
        "category": " Conclusions"
    },
    {
        "id": 15,
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        "category": " References"
    }
]