Superhydrophilic Antireflective Periodic Mesoporous Organosilica

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Superhydrophilic antireflective periodic mesoporous organosilica coating on flexible polyimide substrate with strong abrasion-resistance Jing Wang, Cong Zhang, Chunming Yang, Ce Zhang, Mengchao Wang, Jing Zhang, and Yao Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14117 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 22, 2017

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ACS Applied Materials & Interfaces

Superhydrophilic antireflective periodic mesoporous organosilica coating on flexible polyimide substrate with strong abrasion-resistance Jing Wang, †, ǁ, § Cong Zhang, †, § Chunming Yang, ⊥ Ce Zhang, †, § Mengchao Wang, †, § Jing Zhang, †, ǁ, § Yao Xu ǁ, * †

Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China

ǁ

State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and

Precision Mechanics, Chinese Academy of Sciences, Xi’an 710119, China

⊥

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

Academy of Sciences, Shanghai 201204, China §

University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT: Superhydrophilic antireflective periodic mesoporous organosilica (PMO) coating was prepared on flexible polyimide substrate via solvent-evaporation-induced self-assembly (SEISA) method, in which tetraethoxysilane (TEOS) and a special bridged silsesquioxane were used as reactants. The bridged silsesquioxane, EG-BSQ, was synthesized through the stoichiometric reaction between 3-glycidoxyporpyltrimethoxysilane (GPTMS) 1

ACS Paragon Plus Environment


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and ethylene diamine (EDA). Under the influence of surfactant, TEOS and EG-BSQ co-condensed and enclosed the ordered mesporous in the coating. The results of grazing-incidence small-angle X-ray scattering (GISAXS) and the transmission electron microscope (TEM) indicated that the mesopores belonged to a Fmmm orthorhombic symmetry structure. With increasing the EG-BSQ concentration, the mesoporous structure in PMO coating become more and more disordered because silica mesopore walls shrunk or collapsed during calcination and consequently the refractive index of PMO coating became larger. The antireflective (AR) PMO coating showed optical transmittance 99.54% on polyimide (PI) much higher than 88.68% of bare PI. The water contact angle of PMO coating was less than 9.0°, which indicated the AR PMO coating was superhydrophilic. Moreover, the PMO coating showed an excellent mechanical property, the transmittance of PMO coating displayed a very low loss of 0.1% after abrasion of 25 cycles by CS-10F wearaser.

KEYWORDS:

antireflective,

PMO

coating,

superhydrophilic,

polyimide,

abrasion-resistance

1. INTRODUCTION

The flexible display is one of the current hot topics in the field of electronic information. Flexible front panel is an essential part of the whole flexible display, has great effect on the performance of the flexible display.1-2 In general, the quality requirement of front panel material in flexible display should be focused on some aspects as follows: (1) higher heat resistance and excellent dimensional stability during hardening, (2) good flexibility, (3) small surface roughness, (4) excellent optical performance.3-6 As a high-performance polymer, 2


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polyimide (PI) has been widely used in microelectronics and optoelectronic field because of its high thermal stability, low dielectric constant, good chemical stability and excellent mechanical property.7-10 As potential front panel on flexible display, PI may play an irreplaceable role in protecting OLED plane. In recent years, many efforts have been made to develop highly transparent and colorless polyimide by incorporation of fluorine, chlorine, and selenophene unit.11-13 These PIs exhibited relatively high refractive index, high thermal stability and good mechanical property combined with transmittance up to 90% in the visible range, which makes them good candidate for flexible display substrate. However, to achieve better visual effect and improve power utility, the visible transmittance of polyimide must be enhanced by antireflective (AR) coating. Conventional AR coatings are single or multi-layer thin films consisting of inorganic low-n materials such as SiO2 (n=1.46) and MgF2 (n=1.38), and most of the conventional AR coatings are applicable only on inorganic substrates. For example, Zhang et al.14 reported a super hydrophilic AR coating based on hollow silica-silica nanocomposites, which showed significant improvement of transmittance on solar glass. However, the calcination temperature was high as 400°C so that this super hydrophilic AR coating cannot be used for flexible organic substrate. On the other hand, seldom AR coating on flexible substrate were reported in recent years because of the difficult bonding of coating material with substrate15-17. A just published article15 introduced an AR coating for polymethylmethacrylate (PMMA) substrate by partially embedding mesoporous silica nanoparticles into the surface of the substrates, from which the transmittance of coated PMMA reached 98.0%. Yildirim and co-workers16 prepared a organically modified 3



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nanoporous silica coating on cellulose acetate (CA) and polyetherimide (PEI), the transmittance of coated substrate increased by 8% on CA and 10% on PEI. Jiang et al.17 prepared an AR film by spin-coating the PMMA/polystyrene (PS) mixed lattices, followed by selectively removing PS particles and the minimum reflection of coated PMMA decreased to 0.02%. However, all of the preparation method was very complex and the adhesion was not satisfactory. Similar to other polymer substrates, AR coatings for PI substrates are restricted by low adhesion of inorganic materials such as silica-based coating and by poor durability of polymer-based coating.18-19 Accordingly, periodic mesoporous organosilica (PMO) coating, containing lots of organic groups inside the SiO2 channel walls, may be a good choice to resolve these two problems together. In the past decades, many PMO materials have been synthesized, which provided excellent performance including efficient fluorescence emission, light-harvesting antenna property, organic chromophores, and low-dielectric-constant.20-25 However, there have been no reports on the synthesis of super hydrophilic antireflective PMO coating on flexible PI substrate. We report here a simple procedure to prepare antireflective PMO coating with superhydrophilic

property

by

dip-coating

process

via

solvent-evaporation-induced

self-assembly (SEISA) process. This process was schematically shown in Scheme 1. TEOS and EG-BSQ monomer were used as reactants.26 These compounds were hydrolyzed and co-condensed in methanol solvent with acetic acid as catalyst and F127 as surfactant to prepare TEG (TEOS/EG) sol for deposition of PMO coating, followed by calcination at low-temperature. Surprisingly, the obtained PMO coating exhibit not only extremely high transmittance on PI substrate, but also superhydrophilicity and excellent mechanical property. 4


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Scheme 1. Illustration for the preparation of periodic mesoporous organosilica antireflective thin coating. 2. EXPERIMENTAL SECTION

2.1 Materials. Tetraethyl orthosilicate (TEOS, 99%) and 3-Glycidoxypropyltrimethoxysilane (GPTMS, 99%) was purchased from Acros Organics. Ethylene diamine (EDA, 99%), Methanol (99.5%) and Ammonia (25-28%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Acetic acid (CH3COOH, 99.5%) was purchased from Tianjin Beichenfangzheng Chemical Reagents Factory and Pluronic F127 was purchased from Sigma Aldrich. Polyimide substrates were obtained from Soken Chemical and Engineering Co., Ltd.

2.2 Preparation of TEG sol. The TEG solution was prepared by the following process. At first, the precursor EG-BSQ was prepared according to our previously reported procedure shown by Figure 1.26

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Figure 1. The reaction routine from GPTMS and EDA to EG-BSQ.

The TEG sol used for the PMO coating was prepared by two types of silane precursor, TEOS and EG-BSQ. Methanol (MeOH) was used as solvent and acetic acid as catalyst. At first, the EG-BSQ and TEOS were mixed with acetic acid and MeOH, the mixture was stirred for 1.5 h at 60 °C. A second solution was prepared by mixing F127, acetic acid, methanol and deionized water, and slowly added into the former solution. The final molar ratio was Si (total amount of TEOS and EG-BSQ): MeOH: CH3COOH: H2O: F127=1: 25: 0.02: 7: 0.05. Five sols were synthesized with different TEOS/EG molar ratios of 9, 4, 2, 1, 0.5， were accordingly named as TEG9-S, STEG4-S, TEG2-S, TEG1-S and TEG0.5-S. The sols were stirred for 24 h and then aged for more than 7 days at room temperature.

2.3 Preparation of the PMO coatings. Highly transparent PI substrates were cleaned by ultrasonication in ethanol for 10 minutes before use. AR PMO coating was deposited on PI substrate by dip-coating followed by annealing. PI substrate was immersed in the TEG sol for 60s, and then withdrew at speed of 100 mm/min. Finally, the PMO thin film was annealed at 250 °C for 30 minutes to remove the surfactant template. Here, it should be noted that the PMO coatings prepared with different TEG sols were marked as TEG9-F, TEG4-F, TEG2-F, TEG1-F and TEG0.5-F. 6


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2.4 Preparation of the TEG xerogels. The xerogels were obtained from the corresponding TEG sols by drying the solvent at 100 °C and then annealed at 250 °C for 30 minutes to remove the surfactant template. The xerogels were accordingly named as TEG9-X, STEG4-X, TEG2-X, TEG1-X and TEG0.5-X.

2.5 Control experiments.

To highlight the super hydrophilicity of our PMO coating, four control experiments were carried out.

The first control experiment was to prepare a silica coating without template F127 using TEOS, MeOH, and acetic acid as reactants. The final molar ratio was Si: MeOH: CH3COOH: H2O =1: 25: 0.02: 7. The sol was stirred for 24 h and aged for more than 7 days at room temperature. The silica coating without F127 was marked as A-silica coating.

The second control experiment was to prepare an EG-BSQ coating using only EG-BSQ sol without TEOS. The EG-BSQ coating was prepared without any thermal treatment.

The third control experiment was to prepare a mesoporous silica coating without EG-BSQ. The final molar ratio was Si: MeOH: CH3COOH: H2O: F127 =1: 25: 0.02: 7: 0.05. The sols were stirred for 24 h and aged for more than 7 days at room temperature. The mesoporous silica coating was calcined at 250°C for 30 minutes.

The forth control experiment was to prepare a typical inorganic AR coating using silica colloidal suspension without additive. This inorganic AR coating was dip-coated on clean PI substrate and was marked as B-silica coating. 7



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2.6 Characterizations. The chemical structure of TEG xerogel was characterized using nuclear magnetic resonance (NMR). 13C liquid NMR spectra of TEOS, EG-BSQ and TEG9 precursors were measured on a Bruker AVANCE IIITM 400 MHz spectrometer with CDCl3 as solvent. 29Si cross-polarization (CP) MAS NMR was carried on a Bruker AVANCE IIITM 600 MHz spectrometer using a 7 mm probe. The coatings were measured by a vacuum fourier transform-infrared (FT-IR) spectrometer (Bruker, VERTEX 70v) in the range 500-4000 cm-1. Thermal Gravity Analysis was performed on a thermogravimeter (TG-DSC, STA-409PG, NETZSCH) in air atmosphere. The temperature ranged from 25 to 600 °C, with a temperature rate of 10 °C⋅min-1 and gas flow of 500 mL⋅min-1 on samples of about 10 mg.

The experimental transmittance of substrates and coatings were investigated by a UV-VIS-NIR spectrometer (U-4100, Hitachi), and the wavelength range is from 400 to 1600 nm. The cross-section image of AR PMO coating was taken on a scanning electrons microscope (SEM JSM-5900LV). The refractive index and thickness of PMO coatings were measured by a spectroscopic ellipsometer (SC620, Sanco) at 60 ° incidence. The surface morphology and roughness of PMO coatings were analyzed by an atomic-force microscopy (AFM, XE-100, PSIA). The pore structure of PMO coating was measured using a JEOL-2100 High-resolution transmission electron microscopy (TEM). The ordered structure of PMO coating was studied by 2D grazing-incidence small-angle X-ray scattering (GISAXS), which was measured at BL16B1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) in China. The incident X-ray wavelength λ was 0.124 nm and the camera length was 1810 mm. N2 ad/desorption experiment was done on BELSORP-max gas adsorption analyzer and the pore size 8


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distribution was analyzed using the Barrett–Joyner–Halenda (BJH) method. The water contact angles of coatings were received from an optical contact angle measuring instrument (SL200B, Kino, America). The mechanical property of PMO coating was measured by a Taber 5700 Linear Abraser. The coatings were rubbed for 25 cycles by CS-10F Wearasers™ with speed of 15 cycles per minute.

3. RESULTS AND DISCUSSION

3.1 NMR and TG characterizations of antireflective PMO coating.

Antireflective periodic mesoporous organosilica thin coatings with different TEOS/EG molar ratio were prepared by dip-coating PI substrates with TEG sols and subsequent calcination treatment. As illustrated in Scheme 1, the whole preparation process was separated into third steps: The first step involved the hydrolysis of TEOS and EG-BSQ in methanol and the following

copolycondensation;

The

second

step

was

solvent-evaporation-induced

self-assembly (SEISA) process to prepare template-containing coating on PI substrate; Finally, the template-containing coating was annealed at 250 °C to remove surfactant.

9



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Figure 2. 13C NMR spectra of (a) TEOS, (b) EG-BSQ monomer and (c) TEG9 precursor. (b) was reproduced from the article published by RSC advances, 2015, 5, 56998-57005, http://pubs.rsc.org/en/content/articlelanding/2015/ra/c5ra07731a/unauth#!divAbstract.

Figure 3. 29Si MAS NMR spectrum of the typical TEG9-X.

The formation mechanism of PMO coating was investigated by MAS NMR and TG analysis. Figure 2 shows the

13

13

C liquid NMR,

23

Si

C liquid NMR spectra of TEOS,

EG-BSQ26 and TEG9 precursor without surfactant. There are ten

13

C NMR peaks in TEG9 10


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precursor, which were assigned to two carbon atoms of TEOS and eight carbon atoms of EG-BSQ, suggesting that TEOS reacted with EG-BSQ successfully.

29

Si MAS NMR

spectrum was recorded to obtain of the relative number of different kinds of silicon sites in the PMO coating. Figure 3 shows the 29Si MAS NMR spectrum of the corresponding xerogel TEG9-X. Because the relaxation time for the silicon atoms is very long, the delay time of NMR experiment was set to 100 s. As can be seen from Figure 3, three Qn (n=1-4 in theory) peaks were observed at –110.21 ppm (Q4, (SiO)4Si), -101.80 ppm (Q3, (SiO)3SiOH), -91.75 ppm (Q2, (SiO)2Si(OH)2). In addition to those Qn peaks, two Tm (m=1-3 in theory) peaks derived from EG-BSQ appeared at -67.32 ppm, (T3, (SiO)3SiCH3) and -63.94 ppm (T2, (SiO)2SiCH2(OH)), confirming that the Si-C bonds are stable and retained in the final mesoporous framework.

Figure 4. TG curves of samples in air atmosphere: EG-BSQ precursor, TEG2-X gel and F127. Figure 4 shows the thermal gravity curves of the EG-BSQ precursor, TEG2-X gel and F127. Thermal decomposition of F127 began at temperature around 150 °C and showed a weight loss of 97.1% when temperature reached 250 °C in air atmosphere. The TG curve of 11



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EG-BSQ precursor shows the loss weight rate of 3.2% at temperature of 250 °C, which can be attributed to the loss of adsorbed water. Therefore, the suitable calcination temperature was 250 °C to remove surfactant. From the thermogravimetric analysis of TEG2, the weight loss in the temperature range 25-250 °C was 56.3%, which mostly can be mainly ascribed to the decomposition of F127 template.27 The TG results indicated that the F127 could be removed by annealing at 250 °C, and the PMO framework were preserved below 250 °C. On the basis of above-mentioned 13C liquid NMR, 29Si MAS NMR and TG results, the formation mechanism of PMO coatings can be determined evidently.

3.2 Pore structure of antireflective PMO coating.

Figure 5. (a) Nitrogen adsorption-desorption isotherms and (b) pore size distributions from the adsorption branches through the BJH method of the PMO coatings. To investigate the pore structure of the antireflective PMO coating, the coating fragments were scratched off the substrate and tested by nitrogen adsoption-desorption analysis. Figure 5 shows the nitrogen adsorption-desorption isotherms and the pore size distributions of the annealed coatings. The type-IV isotherms are shown in Figure 5a, typical of mesoporous structure. Type-H2 hysteresis loops appear between the relative pressure of 0.4 and 0.7 for the coatings TEG9-F, TEG4-F, TEG2-F and TEG1-F, showing the existence of cage-like 12


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mesopores in the coatings.28 With increasing the EG-BSQ concentration, the hysteresis loops turned narrower and almost disappeared when the molar ratio of TEOS/EG reaches 0.5. The pore size distribution of TEG9-F, TEG4-F and TEG2-F showed a sharp peak at 5.4 nm, this suggests that the sizes of micelles in these sols are same (Figure 5b). On the other hand, the pore dimeter was 4.5 nm of TEG1-F, which is 0.9 nm smaller than that of coatings with molar ratio of TEOS/EG>1.0. The pore size mainly depends on the size of F127 micelles in TEG1-S was smaller than those in TEG9-S, TEG4-S and TEG2-S. Moreover, the pore size distribution of TEG1-F was relatively wide, suggesting most of silica mesopore thin walls shrunk or collapsed during calcination. When the molar ratio of TEOS/EG reaches 0.5, the pore size distribution of TEG0.5-F showed a broad peak around 22.2 nm. This should be attributed to the lack of TEOS in TEG0.5-S. Seen from Table 1, the surface area and pore volume all decrease from 624.6 m2/g and 0.5078 cm3/g of TEG9-F to 284.7 m2/g and 0.2398 cm3/g of TEG1-F, because silica framework of mesopores collapsed during calcination with the decreased concentration of TEOS.

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Table 1. Structure parameters of PMO coatings with various TEOS/EG molar ratios Sample

Surface area/(m2/g)

Pore volume/(cm3/g)

Pore size/(nm)

Porosity/(%)

TEG9-F

624.6

0.5078

5.4

46.69

TEG4-F

623.1

0.5010

5.4

46.69

TEG2-F

581.7

0.4785

5.4

42.35

TEG1-F

284.7

0.2398

4.5

34.88

TEG0.5-F

156.3

0.3832

22.2

30.06

Figure 6. 2D GISAXS patterns of (a) TEG9-F, (b) TEG4-F, (c) TEG2-F, (d) TEG1-F, (e) TEG0.5-F. The hollow circles and squares identify transmitted and reflected Bragg peaks, respectively. To further confirm the pore structure of PMO coating, 2D GISAXS experiment was performed. The 2D GISAXS patterns of the annealed antireflective PMO coatings are shown in Figure 6. After calculation, the patterns clearly show the face-centered orthorhombic Fmmm structure for the F127 templated coating. For the TEG9-F and TEG4-F, seven diffraction spots are clearly observed and indexed to (020), (131), (111), (220), (002), (113) and (313) reflections of Fmmm space group.29 However, the number of spots decreased with the increase of EG-BSQ concentration. There was no diffraction spot in the image of 14


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TEG0.5-F which means poor periodicity of mesopores in coating. After calcination, F127 template decomposition and highly ordered inorganic-organic mesopores formed in the coating, however, a mesoporous structure tends to collapse or not formed with less of TEOS. Generally speaking, periodic mesoporous structure can be synthesized with molar ratio of TEOS/EG>0.5 due to the enough rigid inorganic silicon chains, which derived from inorganic precursor, and supported pore structure in coating.

Scheme 2. Schematic illustration of the formation process of the mesoporous structure with different TEOS/EG molar ratio. Scheme 2 shows the schematic illustration of the mesoporous structure of PMO coatings with different TEOS/EG molar ratio. During dip coating, the evaporation of methanol and water at an air/film interface induced a rapid increase in concentrations of F127 and silica species. Then by the SEISA process, inorganic and organic siliceous species began to surround the micelles. Finally, after calcination to remove surfactant, the organic chains were intertwined with the inorganic network together, and then constitute a hybrid inorganic-organic network. The chemical bond between the organic component and the 15



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inorganic component resulted in mechanically robust and thermally stable coatings. As the EG-BSQ concentration increased, the inorganic parts are gradually become less and less, the soft organic chains did not hold up the pore structure of coating, and the pore wall become thinner and thinner. When the molar ratio of TEOS/EG reached 0.5, nearly no regular micelles formed during SEISA process. Scheme 2 also shows the TEM images of the PMO coatings scratched off the substrates. From the TEM images, the film structure is highly ordered in films TEG9-F, TEG4-F and TEG2-F with pore size of ～6.5 nm. When the concentration of TEOS was equal to EG-BSQ, there are fewer periodic mesostucture in coating because the inorganic component is too low to surround completely the micelles. In addition, it was found that the framework of TEG0.5-F became fluffy and the pore structure became wormlike, which formed by organic silica chains intertwined with each other, and the F127 template shows a small effect in the process. All of the nitrogen adsorption/desorption analysis, GISAXS and TEM results were mutual complementary and offered a very detailed analysis of PMO coating structure. 3.3 Optical property of antireflective PMO coating.

16


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Figure 7. Transmittance spectra of PMO coatings with different molar ratio of TEOS/EG on highly transparent and colorless polyimide substrates. In order to investigate the optical property of antireflective periodic mesoporous organosilica thin coating, highly transparent PI substrates were coated by dipping them into the TEG sols with the withdraw rate of 100 mm/min. The transmittance spectra of antireflective PMO coatings on PI substrates are shown in Figure 7, the optical property of blank PI is also shown for comparison. The corresponding maximum transmittance (Tmax) is listed in Table 2. The maximum transmittance of the coated PI increased from 98.15% to 99.67% with increasing TEOS concentration, much higher than 88.68% of the blank PI in the wavelength range of 550-1500 nm, which have never been reported. As seen from Figure 7, the TEG9-F, TEG4-F and TEG2-F have obviously high transmittance above 99.50%. In addition, the transmittance was nearly equal between TEG9-F, TEG4-F and TEG2-F, so that the molar ratio of TEOS to EG-BSQ can be chosen in a wide range to obtain high transmittance on PI substrate.

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Table 2. Optical parameters of PMO coatings with various TEOS/EG molar ratios Sample

Molar ratio of TEOS/EG

nf

Thickness/(nm)

Tmax /(%)

Rq/(nm)

TEG9-F

9

1.235

274

99.67

2.26

TEG4-F

4

1.235

227

99.66

2.20

TEG2-F

2

1.250

176

99.62

2.22

TEG1-F

1

1.285

130

99.45

2.02

TEG0.5-F

0.5

1.308

95

98.15

2.28

nf is the refractive index of coating; Tmax is the maximum transmittance in spectrum; Rq is the root-mean-square roughness value of coating.

The refractive index and thickness of the PMO coatings were calculated using a spectroscopic ellipsometer and also shown in Table 2. With the TEOS/EG molar ratio decreased from 9.0 to 0.5, the refractive index of the PMO coating increases from 1.235 to 1.308, and, the coating thickness decreased from 274 nm to 95 nm. In the SEISA process, the physical thickness of coating is mainly controlled by the solution viscosity and the dip-coating speed.30, 31 Seen from Figure S1, the viscosity of TEG sol showed a very small change from 3.5 to 4.5 mPa⋅s according to different TEOS/EG molar ratio and there was no big difference in the viscosities of TEG sols measured a shear rate of 300s-1, 500s-1 and 800s-1, Therefore the decrease of coating thickness must be due to the mesopore collapse or no mesopore formed with the same withdraw speed, which agreed with the pore structural analysis. Figure S2 shows the cross-sectional SEM images of the PMO coatings. It could be seen that the homogenous and uniform structure of coating. The coating thickness decreases obviously with the increase of EG-BSQ concentration and which coincided with the spectroscopic ellipsometry results. The surface morphology of PMO coatings was confirmed by AFM. Figure S3 showed 18


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the AFM images and line-scan height profiles of blank PI and PMO coatings. The surface roughness directly affected the surface light scattering. The higher the roughness is, the lower the transmittance should be. As can been seen from Figure S3, there are many scratches on the surface of blank PI substrate, and the root-mean-square roughness (Rq) value is high as 9.73 nm, which can induce serious light scattering. With PMO coating, the surface became smoother and no scratches were seen. The Rq decreased to around 2.2 nm, because TEG sol filled into the scratches on PI surface. Thus, the PMO coatings were all smooth enough to avoid surface scattering.32 On the basis of the above analysis, antireflective PMO coating can effectively reduce the reflection and avoid light scattering on the surface of PI substrate. It suggested the potential application of the antireflective PMO coating on PI substrate in optical devices. 3.4 Superhydrophilicity of antireflective PMO coating.

Figure 8. Water contact angle of PMO coating deposited on the glass followed by calcination (250°C for 30 minutes) according to the TEOS/EG molar ratio. The inset is the digital image of PMO coating on glass to show the antifogging property. The hydrophilicity of AR coating would prevent fog distort the image and reduce light transmittance. Generally, superhydrophilic coating showed excellent antifogging property 19



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because of the rapid spreading of condensed water droplets on the surface. To intuitively show the antifogging ability of PMO coating, glass partly coated by PMO coating, was cooled at -5 °C for 24h in a refrigerator, and then exposed to humid room temperature environment. Seen from the photograph in Figure 8, the uncoated part was covered with tiny water droplets and the words below were blurred by strong light scattering. In contract, the coated part remained highly transparent; the word below can be seen clearly. To illustrate the hydrophilic nature of the PMO coatings, the water contact angle of the coating was measured and shown in Figure 8. The water contact angles on PMO coatings are all less than 9°. In Figure S5, the three control experiments showed that the A-silica coating without F127, EG-BSQ coating without TEOS, and mesoporous silica coating without EG-BSQ possessed water contact angles 41.56°, 60.05° and 21.65° respectively, larger than that of all the PMO coatings, which indicate the super hydrophilic nature of PMO coating.33 This super hydrophilicity was attributed to the presence of abundant hydroxyl groups and amino groups of EG fragments in PMO coating (shown in Figure 9a). From the vacuum fourier transform infrared spectroscopy (FT-IR) of the PMO coating (seen Figure S4), the spectrum of TEG2-F shows a broad absorption band centered at 3400 cm-1 attributed to hydroxyl group. The peak at 961 cm-1 was assigned to the silanol groups, and the peak at 1383 cm-1 was assigned to the C-N bond in tertiary amine groups. Thus, the water molecules can be rapidly absorbed into the hydrophilic segments of the coating by hydrogen bonding effect.34, 35

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Figure 9. Schematic representations of (a) the superhydrophilic mechanism of PMO coating, (b) the abrasion-resistance mechanism of PMO coating. 3.5 Mechanical property of antireflective PMO coating.

Figure 10. Transmittance spectra of TEG2-F and B-silica coating (control experiment) on polyimide before and after abrasion of 25 cycles with speed of 15 cycles per minute. The used abrasion was Wearasers CS-10F, and the instrument was Taber 5700. Mechanical property is a key issue of antireflective coating for practical application.36 The abrasion resistance of PMO coating was measured by a Table Linear Abraser. The coated PI substrate was rubbed 25 cycles with speed of 15 cycles per minute using the CS-10F wearaser. The transmittance spectra of B-silica coating and TEG2-F before and after abrasion are shown in Figure 10, from which the transmittance of TEG2-F displayed only 0.1%

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decrease in the region of 600-1600 nm after rubbing, indicating a strong adhesion between PMO coating and PI substrate. However, the transmittance of B-silica coating decreased more than 4% after rubbing, indicating that the B-silica coating has a very poor adhesion with PI substrate. As can be seen from Figure S6, only a few small scratches appeared on the surface of abraded PMO coating, indicating that the PMO coating has excellent abrasion resistance. As shown in Figure 9b, the hydroxyl groups in TEG sol hydrogen-bonded with carbonyl group in PI substrate, which resulted in the high mechanical durability. This result suggested that the antireflective PMO coating we obtained can maintain a high transmittance in practical application.

4. CONCLUSIONS

Using TEOS and the special EG-BSQ precursor as reactants, we successfully obtained superhydrophobic antireflective periodic mesoporous organosilica coating on flexible polyimide substrate with low refractive index in the range 1.30-1.23. The excellent antireflection was successfully realized on the PMO-PI coating system and the maximum transmittance of PMO coated polyimide increased to 99.67% from 88.68% of the blank polyimide. Benefitted from the superhydrophilicity, PMO coating showed obvious antifrogging property. Meanwhile, the strong hydrogen-bonding interaction between PMO coating and PI substrate resulted in the high mechanical durability of coating. This work represented a breakthrough to the traditional fabrication of antireflective coating on plastic substrate. The excellent transmittance performance of PMO coating on PI substrate may promote the potential application in fields of flexible display in the future.

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Supporting Information

Supplementary information about the viscosity of TEG sol; cross-sectional SEM images of PMO coatings; AFM images of blank polyimide substrate and PMO coatings; vaccum IR spectrum of TEG2 xerogel; Water contact angles of A-silica coating without F127, EG-BSQ coating without TEOS, and mesoporous silica coating without EG-BSQ; Optical microphotographs of TEG2-F coating and blank polyimide before and after abrasion.

Corresponding Authors

*E-mail: [email protected]

Notes

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the National Key Native Science Foundation of China (No. U1530148). Additionally, the authors gratefully acknowledge the assistance of Prof. Hugh W. Hillhouse and Dr Steve Gaik of University of Washington during the analysis of GISAXS patterns.

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Table of Contents Graphic and Synopsis.

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Superhydrophilic Antireflective Periodic Mesoporous Organosilica

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