Broadband Antireflection Coatings Based on Low Surface Energy

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Broadband Antireflection Coatings Based on Low-Surface Energy/ Refractive Index Silica/Fluorinated Polymer Nanocomposites Ting-Xuan Lin, Kuan-Ju Chen, Po-Yen Chen, and Jeng-Shiung Jan ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00208 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 26, 2018

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ACS Applied Nano Materials

Broadband Antireflection Coatings Based on LowSurface Energy/Refractive Index Silica/Fluorinated Polymer Nanocomposites

Ting-Xuan Lin, Kuan-Ju Chen, Po-Yen Chen, and Jeng-Shiung Jan*

Department of Chemical Engineering, National Cheng Kung University No 1, University Rd., Tainan 70101, Taiwan

[email protected]*

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Abstract We have demonstrated the fabrication of broadband antireflection coatings (ARCs) comprising low-surface energy/refractive index (RI) silica/polymer nanocomposites by silica mineralization

of

layer-by-layer

(LbL)

methacrylate)-block-poly(2,2,3,3-tetrafluoropropyl

assembled

poly(2-(dimethylamino)ethyl

methacrylate)/poly(L-glutamic

acid)

(PDMA-b-PTFP/PGA) multilayer films without any post-treatments. The introduction of the fluorinated polymer (PTFP segments) effectively lowered not only the RI of the as-fabricated coatings but also the surface energy of the constituted pore surface, which rendered the ARCs with high transmittance and durable AR performance by preventing the absorption and capillary condensation of moisture at ambient conditions. Moreover, the formation of nanosized PDMA-b-PTFP vesicles can render the ARCs exhibiting small pore size, which can improve their light transmittance. The coated substrate with an average transmittance over 97.0% was obtained at the visible wavelength region. The combination of LbL assembly and silica mineralization can warrant the preparation of conformal, intact coatings with good mechanical properties. This study demonstrated a novel concept on introducing low-surface energy/RI materials for fabricating broadband, moisture-repellent ARCs.

KEYWORD: antireflection, silica/polymer composites, fluorinated polymer, low surface energy, low refractive index, layer-by-layer assembly, mineralization


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Introduction The improvement of light transimission for many optical devices including lenses, solar cells, and flat panel displays can be achieved by deposition of antireflection

coatings

(ARCs).1-9 Based on the antireflective mechanism, the control of film refractive index and coating thickness is critical for the fabrication of a good AR coating,10-12 which can render the destructive interference of light at the interfaces.13-14 An ideal single-layer ARC should satisfy two well-known principles: (1) its required thickness should be one quarter of the wavelength of the incident light (λ/4), and (2) its effective refractive index should equal to the square-root of that of the substrate. The key point to induce AR characteristics is to form lowrefractive index (low-RI) coatings with optimal thickness on the substrate surface. The conventional way is to introduce low-n materials such as magnesium fluoride (MgF2, n = 1.38).15-16 Because of the limited choice of low-RI materials, many groups turned to develop broadband ARCs by fabricating mesoporous inorganic and polymeric coatings exhibiting optimal refractive indices via various fabrication methods. An alternative approach is to fabricate the nanostructured topography exhibiting continuous RI gradient at the air/substrate interface,3, 6, 17-19 which is inspired by the corneal lenses in moth eyes.20 However, many fabrication processes are only applicable to flat surfaces and require some complicated and/or expensive equipments. Solution-based processes including sol-to-gel process and layer-by-layer (LbL) assembly are relatively easy and cost-effective as compared to other processes. The fabrication of ARCs via LbL assembly has been shown to be feasible and an average transmittance about 97.0% or above at the the visible wavelength range (400−800 nm) on substrates can be acheived,5, 21-25 whereas many of the approaches required high temperature treatment, which can be only restricted to inorganic substrates. Polymeric substrates such as poly(methyl methacrylate) (PMMA) and polyethylene terephthalate (PET) exhibited low heat resistance and low adhesion of inorganic materials. Several groups have reported the fabrication of the ARCs onto polymeric substrates via the LbL assembly of the 3 ACS Paragon Plus Environment


as-prepared mesoporous or hollow silica nanoparticels without high temperature treatment.7, 22-23

Adopting the concept of the LbL assembly of hollow nanoparticels, our group recently

demonstrated the feasibility of fabricating ARCs comprising vesicular nanostructures via silica mineralization of LbL assembled films composed of complex vesicles without posttreatment.26 This approach can warrant the preparation of conformal coatings with interpenetrated organic and inorganic networks in the nanometer scale. Despite the great advances in the development of ARCs, one issue worth of addressing is that these reported approaches for ARCs are susceptible to absorb water vapors and pollutants due to the presence of open pores and/or interstitial void space between silica nanoparticles, resulting in the deterioration of the ARCs durability. The most common route to address this problem is to introduce superhydrophobicity in ARCs via different approaches including silanization of the nanostructured coatings and postdeposition of fluorinated polymers,27, 28-31 which tended to compromise their AR performance. Recently, several groups proposed alternative approach to by preparing ARCs with closed pore structures.32-34 However, the preparation of ARCs with closed pore structures required sacrificial templates and the removal of the templates using high temperature treatment, which is not suitable for the fabrication of moisture-repellent ARCs coated onto polymeric substrates. We reason that the introduction of low-surface energy materials in the coatings could not only prevent the absorption of water vapors but also facilitate the fabrication of ARCs on polymeric substrates without high temperature treatment. It is well known that fluorinated polymers are lowsurface energy materials. To demonstrate that this is a proof-of-concept, poly(2,2,3,3tetrafluoropropyl methacrylate) (PTFP) was selected since it exhibits not only low surface energy but also low RI (1.417). Poly(2-(dimethylamino)ethyl methacrylate)-blockpoly(2,2,3,3-tetrafluoropropyl methacrylate) (PDMA-b-PTFP) block copolymers were synthesized via atom transfer radical polymerization (ATRP) using CuBr asthea catalyst. Upon quaternization, these quaternized PDMA-b-PTFP block copolymers self-assembled to 4 ACS Paragon Plus Environment

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form vesicles with tunable size in aqueous solution by controlling their block ratio and chain length. The silica/polymer composite coating onto substrates with AR characteristics were prepared by LbL assemble of quaternized PDMA-b-PTFP vesicles and poly(L-glutamic acid) (PGA), followed by silica mineralization. It is expected that AR coatings with a high average transmittance and durable AR performance can be prepared due to the presence of lowsurface energy/low-RI PTFP segments, which can prevent the absorption and capillary condensation of moisture. On the basis our previous studies,25,

26, 35, 36

the preparation of

nanostructured materials using the combined biomineralization and LbL assembly technique render the preparation of materials and coatings with tunable porosity and nanostructures. It is expected that PDMA-b-PTFP vesicles with sizes smaller than 100 nm can be prepared due to their relatively flaxible chains. Moreover, both the fission and dissociation of the vesicles would possibly occur during the LbL assembly and mineraliztion processes due to the relatively weak electrostatic interactions between the tertiary amine and carboxyl group. These are beneficial for the fabrication of AR coatings with low surface roughness and small pore size, which can improve their light transmittance by minimizing the reflection and scattering of light.24, 37

Experimental Section Synthesis

of

poly(2-(dimethylamino)ethyl

methacrylate)-block-poly(2,2,3,3-

tetrafluoropropyl methacrylate) (PDMA-b-PTFP) block copolymers and poly(Lglutamic acid) (PGA). PDMA-b-PTFP block copolymers were synthesized by sequential polymerizing 2-(dimethylamino)ethyl methacrylate (DMA, 98%, Sigma-Aldrich)

and

2,2,3,3-tetrafluoropropyl methacrylate (TFP, 98%, TCI) ATRP using CuBr asthea catalyst.38, 39 1

H NMR (TFA-d1, δ, ppm): 1.10-1.30 (4H, -CH2CH3C(COOCH2-)-), 2.05-2.25 (6H, -



CH2CH3C(COOCH2-)-), 3.22 (6H, -CH2N(CH3)2), 3.74 (2H, -OCH2CH2N(CH3)2), 4.50-4.70 (2H, -OCH2CH2N(CH3)2; 2H,-COOCH2CF2CHF2), and 5.81~6.06 (1H, -COOCH2CF2CHF2). The quaternized PDMA-b-PTFP block copolymers were prepared by using methyl iodide (CH3I, >99%, Sigma-Aldrich).40 Specifically, methyl iodide (1 mL) was added to the PDMAb-PTFP block copolymers (2.5 g) dissolved in THF and the resulting mixture was stirred at room temperature for 1 day. Then the solution was dialyzed against DI water for 3 days and lyophilized to yield the polymer. PGA polypeptide was synthesized by following a previously reported procedure.41, 42 Preparation of quaternized PDMA-b-PTFP assemblies. The polymeric assemblies were prepared by nanoprecipitation, followed by dialyzing against DI water. The quaternized PDMA-b-PTFP block copolymers (20 mg) were dissolved in DMF (10 mL) and the polymer solution was injected into DI water (36 mL) at 0.1 mL min−1 using a syringe pump under a stirring speed of 200 rpm. Then the resulting mixture was dialyzed against DI water using a dialysis tube (MWCO 1,000 g/mL, Sigma-Aldrich). The water was changed every 1 h for 10 h and three times in the following 6 days. Then the polymer solution was passed through a nylon syringe filter (1 µm). Preparation of polymer/silica films. The silica/polymer films were prepared by sequential LbL assembly of the quaternized PDMA-b-PTFP vesicles and negatively charged PGA140 on glass or PMMA substrates, followed by silica mineralization. Glass substrates (3 in. × 1 in., REX Glass & Mirrors Co.) were cleaned by using RCA process. Poly(methyl methacrylate) (PMMA) substrates (Agiltron) were sequentially treated with ethanol and oxygen plasma (50 W) for 30 sec at a gas flow rate of 4.95 cc/min and gas pressure of 2.4×10-5 Torr. The quaternized PDMA-b-PTFP vesicular solution (0.5 mg/mL) and PGA solution (0.5 mg/mL) were prepared using DI water. The LbL assembled polymer films were coated onto glass or PMMA substrates by sequential adsorption of quaternized PDMA-b-PTFP vesicles and 6 ACS Paragon Plus Environment

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negatively charged PGA140. The adsorption time for each polymer solution was 10 min and the coated substrates were washed twice by NaCl aqueous solution (0.1 N) for 1 min. The coated glass or PMMA substrates were then immersed in an orthosilicic acid solution (0.35 M) for 5 hr. The coated substrates were washed with DI water for at least three times and placed in an oven (60 oC) overnight. Instrumentation and Characterization. PDMA homopolymers were analyzed by a Viscotek gel permeation chromatography-light scattering (GPC-LS) system. The eluent flow rate was 1 mL/min at 55 °C and the mobile phase was DMF with 0.05 M LiBr. PDMA and PDMA-b-PTFP polymers dissolving in TFA-d1 were characterized by a Mercury 300 Varian spectrometer. Dynamic light scattering (DLS) and zeta potential analysis of the quaternized PDMA-b-PTFP assemblies were carried out on an Otsuka ELSZ-1000 light scattering system equipped with the cumulant method and CONTIN algorithms for data fitting. Transmission electron microscopy (TEM) analysis of the quaternized PDMA-b-PTFP assemblies and composite materials were carried out on a Hitachi-700 microscope. Field-Emission Scanning Electron Microscopy (FE-SEM), TEM, and atomic force microscopy (AFM) analyses of the AR coatings were carried out on a Hitachi SU8010 microscope, Hitachi-700 microscope, and DI NS3a-3/MMAFM (tapping mode in air), respectively. The coated films were dried in an oven overnight before characterization. The cross-sectional view of the AR coatings was characterized by a FE-SEM (JOEL, JSM-7001F) and the specimens were prepared by using a FEI Nova-200 dual beam focused ion beam (DB-FIB, NanoLab). The coated films were analyzed by a MP100-ME thin film material analysis system (Mission Peak Optics Inc.). Energy-dispersive X-ray (EDX) analysis of the AR coatings and composite materials were performed on Hitachi SU8010 FE-SEM and JOEL JEM-2010 TEM, respectively.

Results and Discussion



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The aim of this study is to prepare composite AR films with additional functionality by incorporating fluorinated polymers. Scheme 1a illustrated the fabrication process of silica/fluorinated polymer composite AR films exhibiting porous structures. The quaternized PDMA-b-PTFP block copolymers were prepared based on the synthesis pathway shown in Scheme

1b.

Then

the

quaternized

PDMA-b-PTFP

vesicles

were

prepared

via

nanoprecipitation

Scheme 1. (a) Schematic illustration of the preparation of silica/fluorinated polymer composite AR films by silica mineralization of LbL assembled PDMA-b-PTFP/PGA multilayer films. (b) The synthesis pathway of the quaternized PDMA-b-PTFP block copolymers.

and deposited onto glass and PMMA substrates along with the negatively charged PGA via LbL assembly. The LbL assembled multilayer films comprised of PDMA-b-PTFP/PGA complex vesicles and/or bilayers were finally silicified to afford the fabrication of the composite AR films with the low-surface energy/RI PTFP polymer. The AR performance of 8 ACS Paragon Plus Environment

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the composite films would be improved due to not only the presence of the vesicular cavity and void space created by the silicified vesicles but also the incorporation of the low RI PTFP polymer. The reflection and scattering of the incident light in the composite films can be minimized and, consequently, their AR performance can be improved by decreasing the vesicular cavity and the difference in the RI values between the air/solid interfaces. Moreover, the PTFP polymer resided in the porous films would prevent water molecules from infiltrating into the films so that their AR performance can be retained at ambient environment.

Synthesis and self-assembly of PDMA-b-PTFP block copolymers. The PDMA-b-PTFP block copolymers with different degrees of polymerization (DPs) and block ratios were prepared via ATRP using CuBr asthea catalyst. 1H NMR analysis confirmed the successful synthesis of the PDMA-b-PTFP block copolymers, evidenced by the assignment of all the chemical shifts to the protons on the block copolymers (Figure S1 and S2). The block ratio was determined by the methylene protons (-CH2CH3C(COOCH2-)-) on the main chain of both blocks and the methylene protons (-OCH2CH2N(CH3)2) on the side-chain of the PDMA block (Table S1). The DP of the PDMA block was calculated based on GPC analysis (Table S1). The quaternized PDMA-b-PTFP block copolymers were prepared by using methyl iodide according to the previously reported procedure.40 The quaternized PDMA-b-PTFP block copolymers would aggregate to form assemblies in an aqueous solution due to hydrophobic interactions exerted by PTFP block. DLS analysis revealed that these quaternized PDMA-b-PTFP can aggregate to form assemblies with hydrodynamic diameters (sizes) between 50 and 350 nm (Table S2), depending on their DP and block ratio. Notably, the quaternized PDMA20-b-PTFP40 block copolymer can form assemblies with sizes smaller than 100 nm (~54 nm), which would be appropriate for the fabrication of AR films. On the basis of the contour length and hydrophilic fraction of PDMA lower than 50%,43-45 the quaternized PDMA20-b-PTFP40 block copolymer self-assembled to 9 ACS Paragon Plus Environment


form vesicles. TEM analysis confirmed the formation of vesicular assemblies because of the visible ring structures (Figure S3). The quaternization of the PDMA block resulted that these assemblies carried positive charge, which was confirmed by zeta potential analysis (Table S2). The as-prepared assemblies exhibited excellent colloidal stability for several months probably due to their high zeta potential values (> 30 mV).

Fabrication of polymeric and silica/polymer composite films. In order to minimizing the scattering of the light in the AR films, the vesicles assembled by the quaternized PDMA20-bPTFP40 with the smallest size and the highest fraction of PTFP was selected for fabrication of composite films onto glass and PMMA substrates. Our previous study showed that the ARCs prepared by the silica mineralization of LbL assembled polystyrene-block-poly(Llysine)/poly(L-glutamic acid) (PS-b-PLL/PGA) films with the 3 - 5 of bilayer number exhibited the film thickness between 80 and 160 nm.26 The sizes of the PS-b-PLL vesicles were ranged between 100 and 160 nm. Moreover, Cohen and co-workers reported that the preparation of the LbL assembled films containing silica hollow nanoparticles with average size of 75 or 100 nm at pH 3.0 and the film thickness can be tuned between 90 and 150 nm by varying the bilayer number between 3 and 6.22 Based on these results, we reason that the LbL assembled PDMA20-b-PTFP40/PLGA140 multilayer films with bilayer number higher than 6 bilayers is required for the preparation of the composite films with thickness higher than 100 nm since the size of the PDMA20-b-PTFP40 vesicles (~ 54 nm) is smaller than that of the PSb-PLL ones and their vesicular membranes would not be as rigid as those of PS-b-PLL ones. That said, the quaternized PDMA20-b-PTFP40 vesicles and negatively charged PLGA140 with 6, 8, 10, and 12 bilayers were prepared in DI water via LbL assembly. To ensure the conformal coating on substrates, the glass substrates were cleaned by using RCA process and the PMMA ones were cleaned by ethanol, followed by plasma treatment. The film thickness of the dry PDMA20-b-PTFP40/PLGA140 multilayer films, which ranged between 25 and 60 nm, 10 ACS Paragon Plus Environment

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increased linearly with the increment of the deposited bilayer number (Table 1). Moreover, the refractive indices of the multilayer films was found to smaller than 1.5 and gradually decreased with the increment of the bilayer number (Table 1), suggesting the incorporation of the low-RI PTFP. SEM analysis showed the conformal coatings of the multilayer films and, qualitatively speaking, the increase of the surface roughness upon the increase of the bilayer number (Figure 1 and S4).

Table 1. The film thickness and refractive index (RI) of LbL assembled (PDMA20-bPTFP40/PLGA140)n multilayer films with different numbers of bilayer (n). Number of Bilayer (n)

Thickness (nm)

Reflective index (RI)

6

29.7±0.3

1.41

8

35.2±1.9

1.40

10

46.4±3.4

1.36

12

55.8±1.6

1.33



Figure 1. SEM images of the top view of the dry (PDMA20-b-PTFP40/PLGA140)n multilayer films coated onto glass substrate with the bilayer number (n) of (a) 8 and (b) 10.

It worth noting that the film thickness of these multilayer films ranged between 30 and 60 nm. The results verified that our reasoning on the requirement of the higher bilayer number for the multilayer films. In addition, the multilayer films were assembled in DI water, which would result in the less amount of polymers absorbed on the substrates due to the charge repulsion.35, 46, 47

Consequently, it required more deposition cycle to achieve the target film thickness.

Characterization of silica/PS-b-PLL/PGA composite films. The silica/PDMA20-bPTFP40/PLGA140 composite films with the bilayer number (n) of 8, 10, and 12, denoted as silica/(PDMA-b-PTFP/PLGA)n, were prepared by silica mineralization of the corresponding polymeric multilayer films. SEM analysis showed that the composite films composed of connective nanoparticles were conformal and intact (Figure 2, S5, and S6). At a given bilayer 12 ACS Paragon Plus Environment

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number, the composite films coated onto PMMA substrate exhibited higher degree of particle aggregation than those coated onto glass one. Unlike the composite films coated onto PMMA substrate, the ones coated onto glass substrate did not exhibit apparent particle aggregation upon increasing the bilayer number. As can be seen, the composite films exhibited connective nanoparticles with sizes much smaller than 100 nm (Figure 2c and 2d). The average particle sizes based on the selected ones (50 particles) were determined to be 46.1 ± 6.4 and 49.0± 7.7 for the coated glass and PMMA substrates, respectively (Figure S7). EDX analysis of the composite films showed the presence of carbon, oxygen, silicon, and fluoride elements well distributed on the coated substrates (Figure 3), suggesting the incorporation of fluorinated polymer and deposition of silica in the multilayer films. TEM analysis showed that the silica/(PDMA-b-PTFP/PLGA)8 composite films were comprised of vesicular aggregates and the cavity size was much smaller than 50 nm (Figure 4), which was smaller than the size of the PDMA20-b-PTFP40 vesicles. It can be seen these vesicular aggregates did not exhibit welldefined morphology as compared to those in the silica/(PS-b-PLL/PGA) composite ones in our previous study.26 The results suggested that the degree of deformation/fission for PDMA20-b-PTFP40 vesicles was higher than that for PS-b-PLL vesicles upon polyionic complexation and mineralization. Additionally, the silica/PDMA20-b-PTFP40/PGA complex bilayers might be also present in the composite films possibly due to the dissociation of PDMA20-b-PTFP40 vesicles during the rinsing process. EDX analysis was performed on the silica/(PDMA20-b-PTFP40/PLGA140)8 porous, composite materials scraped off from the substrates (Figure S8). It showed that carbon, oxygen, silicon, and fluoride elements were evenly distributed on the porous materials (Figure S8), suggesting the silica and low-surface energy PTFP were both on the surface of the pores.



Figure 2. SEM images of the top view of the silica/(PDMA20-b-PTFP40/PLGA140)8 composite films coated onto (a, c) glass and (b, d) PMMA substrates.


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Figure 3. EDX analysis of silica/(PDMA20-b-PTFP40/PLGA140)8 composite films coated onto PMMA substrate.

Figure 4. TEM image of the silica/(PDMA20-b-PTFP40/PLGA140)8 composite materials coated onto glass substrate. The composite materials were scraped off from the substrates.

To visualize the cross-sectional view of the silica/(PDMA-b-PTFP/PLGA)8 composite films, the samples were etched by the focus ion beam (FIB) and characterized by SEM. The SEM images revealed the composite films exhibited porous structures (Figure S9), consistent with the previous results. The composite films coated onto glass and PMMA substrates with 8 and 10 bilayers were also characterized by AFM analysis. The AFM topography of these composite films was consistent with those from SEM analysis, showing that they exhibited connective nanoparticles (Figure 5). The root-mean-square (rms) roughness of these composite films was quantified by analyzing the AFM data and ranged between 15 and 45 nm (Table 2). The composite films coated onto PMMA substrate with 8 bilayers exhibited the lowest rms roughness (~ 17 nm) among all analyzed samples. The water contact angle of the coated glass substrates was slightly higher than that of the bare one (water contact angle: 25.9 ± 1.0), whereas the water contact angle of the coated PMMA substrates was slightly lower than the bare one (water contact angle: 45.9 ± 0.9) (Table 2). The coated PMMA substrates 15 ACS Paragon Plus Environment


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exhibited lower water contact angle than the coated glass ones. Moreover, PDMA20-b-PTFP40 thin film coated on a glass substrate was prepared by dispensing the polymer dissolved in DMF onto a glass substrate, followed by removing the solvent using vacuum oven at 100 oC. Upon dispensing water, the water cannot wet the coated substrate, evidenced by the formation of water droplets (Figure S10). And the water contact angle was measured to be 106.5 ± 2.7, revealing that the fluorinated polymer exhibited low surface energy. It is possibly due to the creation of patchy surface comprised of silica and PTFP both in and on the composite films, resulting in the repellence of water molecules.

Figure 5. AFM images of the top view of the silica/(PDMA20-b-PTFP40/PLGA140)n composite films coated onto (a, b) glass and (c, d) PMMA substrates with the bilayer number (n) of (a, c) 8 and (b, d) 10.

Table 2. Film thickness, root-mean-square (rms) roughness, contact angle of the silica/(PDMA20-b-PTFP40/PLGA140)n composite films coated onto glass and PMMA substrates by AFM and thin film spectrum analyses. Substrate

Number of Bilayer

Thickness (nm)

RMS roughness (nm)

Contact angle

Glass

8

129.3±4.1

43.5±0.7

48.7±4.5


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10

142.9±3.8

27.7±0.5

43.9±4.6

12

156.7±3.5

-

-

8

130.8±0.9

17.2±0.6

27.9±2.8

10

140.5±0.9

36.5±4.0

20.2±4.2

PMMA

AR properties of the composite films. The AR characteristics of the coated glass and PMMA substrates were analyzed by the thin film spectrum system. The equipped software algorithms were utilized to determine their film thickness, light transmission, and RI (Table 2, Figure 6a and 7a). The composite films coated onto glass and PMMA substrates clearly exhibited excellent AR performance, evidenced by the improvement of the transmission as compared to the bare substrate over the visible wavelength region. The photographs of the glass and PMMA substrates coated with the silica/(PDMA-b-PTFP/PLGA)8 composite films apparently showed the suppression of light reflection as compared to the bare ones, indicating the enhancement in light transmission (Figure 6b, 7b, and S11). It can be seen that the optimal wavelength for the coated substrates shifted to the higher wavelength upon the increment of the film thickness (Figure 6a and 6b). The silica/(PDMA-b-PTFP/PLGA)8 composite films with the film thickness of ~130 nm exhibited the best AR performance than those with higher bilayers (i.e., 10 and 12 bilayers). For the coatings on glass substrates, the silica/(PDMA-bPTFP/PLGA)8 composite films exhibited a maximum transmittance of 97.5% at the wavelength of ~540 nm and the transmittance was higher than 94.5% at the visible wavelength range (Figure 6a).



Figure 6. Transmission spectra of (a) the silica/(PDMA20-b-PTFP40/PLGA140)n composite films with different bilayers (n) coated onto glass substrate. (b) A photograph image of the glass substrate with the left portion coated with an AR silica/(PDMA20-b-PTFP40/PLGA140)8 composite film, showing the suppression of reflection.

The silica/(PDMA-b-PTFP/PLGA)8 composite film coated onto PMMA substrate exhibited better AR performance than that coated onto glass substrate, evidenced by its transmittance higher than 97.0% at the visible wavelength range (Figure 7a). The value is close to the ideal single-layered ARCs (98.09%).34 It can be partly attributed to its relatively low rms roughness (~ 17 nm), which would decrease the reflection and scattering of incident light. SEM and AFM analyses showed that these composite films exhibited individual aggregates on the surface and, rather, more particle connecting/aggregating with each other can be seen in the films. It suggested that these composite films exhibited denser packing of silica/polymer materials on the bottom than on the surface, resulting in the composite films exhibiting RI gradient. It might be due to this reason that these composite films exhibited broadband AC 18 ACS Paragon Plus Environment

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characteristics. The RI values for the silica/(PDMA-b-PTFP/PLGA)8 composite films coated onto the glass (RI: 1.45) and PMMA (RI: 1.50) substrates were measured to be 1.27 and 1.33, respectively. On the basis of the equation in ref 12, the corresponding porosities were calculated to be 45.6% and 40.6% for the ones onto the glass and PMMA substrates, respectively. It worth noting that the silica/(PDMA-b-PTFP/PLGA)10 composite film coated onto PMMA substrate exhibited transmittance higher than 97.0% at the visible wavelength between 550 and 800 nm. The gradual decrease of transmittance upon decreasing the wavelength can be possibly attributed to the relatively high rms roughness (~ 36 nm) and slightly large in film thickness. Consistent with our previous study,26 the coated PMMA substrate exhibited denser packing of silica/polymer materials than the coated glass substrate, evidenced by the lower porosity for the silica/(PDMA-b-PTFP/PLGA)8 composite film onto PMMA one. Hence, the drastically decrease in transmittance for the coated PMMA substrate can be attributed to the denser packing of silica/polymer materials and the increase of film thickness. The adhesion test of the AR silica/(PDMA-b-PTFP/PLGA)8 composite film coated onto glass substrate was evaluated by using a Scotch tape. Upon removing the tape from the coated substrate, the composite film exhibited little change in the light transmission (Figure S12), suggesting that the composite film was not peeled off from the substrate. Moreover, the composite film was mechanically stable to withstand the paper/ethanol wipe test, evidenced by the little change in the AR performance (data not shown).



Figure 7. Transmission spectra of (a) the silica/(PDMA20-b-PTFP40/PLGA140)n composite films with different bilayers (n) coated onto PMMA substrate. (b) A photograph image of the PMMA substrate with the left portion coated with an AR silica/(PDMA20-bPTFP40/PLGA140)8 composite film, showing the suppression of reflection.

On the basis of the above results, the feasibility of preparing silica/polymer composite films containing porous nanostructures and the low-RI PTFP via silica mineralization of LbL assembled (PDMA-b-PTFP/PLGA) composite films comprising of nano-sized vesicles (~ 54 nm) was demonstrated and their AR performance was found to better than the previously reported silica/(PS-b-PLL/PGA) composite films. Cohen and co-workers reported the preparation of LbL assembled films containing silica hollow nanoparticles (average size: 76 nm) onto PMMA substrate, which can achieve the transmittance higher than 97.0% at the visible wavelength between 550 and 800 nm.22 Our previous study reported the preparation of silica/(PS-b-PLL/PGA) composite film coated onto PMMA substrate and the coated one with 20 ACS Paragon Plus Environment

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4 bilayer exhibited transmittance higher than 97.0% at the visible wavelength between 600 and 800 nm.26 The gradual decrease of transmittance upon decreasing the wavelength was observed in both cases. In this study, the synergy of using the nano-sized vesicles and the incorporation of the low-RI polymer significantly improved the AR performance of the asprepared composite films. Because the porosity was dictated by both the cavity size and void space created by the particle packing, the occurrence of the fission/dissociation for the vesicles upon polyionic complexation and mineralization is advantageous for the formation of pores with dimensions smaller than the vesicular size. Moreover, this can facilitate the preparation of the continuous, intact films with low rms roughness, small pore size, and fine control on the film thickness. In turn, the preparation of composite films can be finely controlled to obtain the one with excellent AR performance.

Durability of the composite films at different temperatures. One question regarding the application of these AR coatings in the real world is the durability of the AR composite film upon weather change, which have never been demonstrated before. To test that, two coated glass substrates were stored in a fridge (-4 oC) and an oven (100 oC) for 7 days, respectively. The substrates were taken out from the fridge and oven every day and placed at ambient environment. Once the temperature of the substrates reached room temperature (26~30 oC) and the condensed water was evaporated from the substrates, the AR performance of the coated glass substrates was measured. It can be seen that the coated glass substrate stored in a fridge (-4 oC) still retained its AR performance, evidenced by the little change in the light transmission (Figure 8a). Rather, the one stored in an oven (100 oC) exhibited slight decrease in the light transmission at the wavelength lower than 550 nm over time (Figure 8b), suggesting the change in the nanostructures of the composite film. SEM analysis confirmed the change in the film nanostructures, evidenced by the presence of large particles on the surface (Figure S13). The elemental analysis showed these large particles mainly composed of 21 ACS Paragon Plus Environment


sodium chloride and polymer, evidenced by the presence of sodium, chloride, carbon, nitrogen, oxygen, and fluoride elements (Figure S14). It can be attributed that the polymers were melted and leached out from the film at high temperature and the ions, which are sodium and chloride, were also leached out with the polymers. Upon cooling down to the room temperature, the salt and polymers aggregated to form large particles on the surface, resulting in the increase in the surface roughness and consequently the decrease in the light transmission at the low visible wavelength region. One observation that should be noted is that the water condensed on the substrates would not infiltrate into the porous films. It is also worth noting that, before the tests, these samples have been stored at ambient environment (humidity: 50~70%) for at least two months and they still retained their AR performance (data not shown), suggesting no water resided in the films. The presence of the water in the films would result in the deterioration of their AR performance. Rather, our previously reported porous silica coatings onto glass substrate templated by alkyl chain-modified PLL/PGA LbL assembled films showed the deterioration of their AR performance over time upon placing at ambient environment, whereas their AR performance can be resumed by removing the water resided in the films via heating treatment (60 oC).25 On the basis of the results, it is highly possible that the silica/(PDMA-b-PTFP/PLGA) composite films exhibited the patchy surface comprised of silica and low-surface energy PTFP both on the surface of the pores, which would consequently prevent water molecules from infiltrating into the porous films. The coated substrates exhibited low water contact angles, suggesting that the composite films was covered with more siliceous materials on the surface than inside the films. The quantification of water contact angle is an indication of surface wettability in a macroscale. However, the wettability of the pore surface cannot be quantified using the water contact angle measurements because of the difference in the length scale. The elemental analysis showed that carbon, oxygen, silicon, and fluoride elements were evenly distributed on the porous materials (Figure S8), indicating that the composite films possibly exhibited the patchy pore 22 ACS Paragon Plus Environment

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surface comprised of silica and low-surface energy PTFP. This may result in the low wettability of the pore surface. Therefore, these composite films can retain their AR performance at ambient environment (humidity: 50~70%) due to the low wettability of the pore surface.

Figure 8. Transmission spectra of the silica/(PDMA20-b-PTFP40/PLGA140)8 composite films coated onto glass substrate at day 0, 1, 3, 5, and 7. The samples were stored in (a) a fridge (-4 o C) and (b) an oven (100 oC).

Conclusion



We successfully demonstrated the concept of introducing low-surface energy/RI fluorinated polymers for fabricating broadband, moisture-repellent ARCs through the combined LbL assembly and silica mineralization. Because of the presence of the low-surface energy/RI PTFP segment, the as-fabricated ARCs exhibited not only high transmittance due to the presence of the small pore size and low-RI PTFP, but also durable AR performance due to the introduction of low-surface energy PTFP, resulting in the prevention of the absorption and capillary condensation of moisture at ambient conditions. It can be seen that the silica/(PDMA20-b-PTFP40/PGA140)8 composite coatings on glass and PMMA substrates exhibited optimal film thickness (~130 nm) and transmission spectrum. For the coated PMMA substrate, the transmission higher than 96.5% at the visible wavelength between 400 and 800 nm can be achieved. Moreover, the formation of vesicles with small sizes (< 100 nm) can render the ARCs exhibiting with small pore size, which can improve their light transmittance. The deposition of the nano-sized PDMA-b-PTFP vesicles and introduction of the low-surface energy/RI material are the key success for this approach.

Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional research data supporting this publication, including GPC characterization of polymers, 1H NMR spectra of block copolymers, DLS and zeta potential analyses of polymeric assemblies, TEM analysis of polymeric assemblies, SEM analysis of composite films, and photograph images of the coated glass substrates.

Notes The authors declare no competing ﬁnancial interest.

Acknowledgments 24 ACS Paragon Plus Environment

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J.-S. J. acknowledges funding support from the Ministry of Science and Technology Taiwan grants MOST106-2221-E-006-206 and MOST105-2221-E-006-248.

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Broadband Antireflection Coatings Based on Low Surface Energy

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