SolâGel Based Hydrophobic Antireflective Coatings on Organic

Article pubs.acs.org/cm

Sol−Gel Based Hydrophobic Antireflective Coatings on Organic Substrates: A Detailed Investigation of Ammonia Vapor Treatment (AVT) Mickael Boudot,†,‡ Vincent Gaud,‡ Mélanie Louarn,‡ Mohamed Selmane,† and David Grosso*,† †

Sorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de le Matière Condensée de Paris, F-75005 Paris, France ‡ Polyrise SAS, F-33607 Pessac, France S Supporting Information *

ABSTRACT: We report a method to prepare hydrophobic, antireflective mesoporous silica-based films on polymer substrates from sol−gel approaches combined with an ammonia vapor treatment (AVT) to avoid any thermal curing. Strategies involving the combination of direct co-condensation of pure and methylated-hybrid silica precursors with further post-functionalization with methyl groups were used. Coatings with the best reflectance (transmittances up to 99.6% in the visible range), full water repellence, and good resistance to abrasion (failures occurred at the substrate interfaces) were obtained by optimizing both sol−gel and AVT conditions. Using in situ, time-resolved, spectroscopic ellipsometry, we demonstrate that the structure of the film can be significantly and rapidly modified from molecular to mesoscales, under the action of H2O and NH3 vapors. The identified mechanism follows a local dissolution/condensation associated to Ostwald ripening that can easily be controlled by adjusting the applied conditions. These structural modifications were much less intense for co-condensed methylated mesoporous matrices due to the stabilizing effect of the organic pendant groups. These conclusions are supported by complementary characterizations obtained with environmental ellipsometry porosimetry, GI-SAXS, SEM-FEG, UV−visible transmittance, crockmeter, and FTIR.

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INTRODUCTION Since the first anti-reflective (AR) coatings produced by Fraunhofer in 1817, when he noticed that reflection on a glass surface subjected to an atmosphere of sulfuric and nitric acid vapors was reduced, many different systems have been developed for many different applications (i.e., buildings, transportation, entertainment, energy, camera, etc.).1 Despite the various specifications already addressed to answer the associated requirements, efforts are still needed to improve mechanical and chemical resistance to enhance durability and efficiency. Among the industrial-viable methods to produce these coatings, sol−gel chemistry is one of the most interesting because it is versatile, cost-effective, and easy to combine with liquid deposition techniques that permit an accurate control of the coating thickness.2−4 AR coatings are mainly composed of silica due to its low intrinsic refractive index. They are generally produced by the sol−gel process because it offers the possibility to incorporate porosity to reduce the refractive index.5−7 Additional properties such as hydrophobicity,8,9 self-cleaning,10−12 antifogging, or even a combination of all of them13 have been combined with antireflectivity to improve their optical property efficiency and long-term stability. Nevertheless, these sol−gel coatings are obtained through classical routes that generally involve thermal treatments at temperatures that may © 2014 American Chemical Society

not be sustained by all types of substrate, such as thermally sensitive polymers. Optical coatings on polymer substrates is a fast-growing field due to the increasing number of applications such as plastic eyeglass lenses, optical lenses, components for photographic equipment, flexible electronics, optoelectronics, and lighting, to name a few. Among optical functions, antireflectivity is one of the most demanded and currently two different main strategies are used to decrease reflection on polymer substrate surfaces. On one hand, soft materials such as polymers could be shaped at the surface of the substrate using selective reactive etching, self-assembly, phase separation, or template imprinting processes to obtain porous or moth eye structures.14−18 Even if they show excellent AR properties, their poor wear resistance is their main drawback. On the other hand, ceramic-based AR films are more interesting because they are intrinsically more wear resistant. They can be processed as moth eye structure, single layer, or multilayer films19−22 by dry deposition techniques, template imprinting, and/or reactive ion etching with excellent optical properties. However, these methods Received: November 14, 2013 Revised: February 4, 2014 Published: February 4, 2014 1822

dx.doi.org/10.1021/cm403787v | Chem. Mater. 2014, 26, 1822−1833

Chemistry of Materials

Article

Figure 1. Scheme describing the various sequences of processing and treatments applied to prepare the different studied AR coatings. Films are named S10-T and S5M5-T when they are thermally cured. Films cured by the selected ammonia vapor treatment(a) are named S10-N and S5M5-N. Post-hydrophobized films are named S10-NH and S5M5-NH. (a)The selected ammonia treatment takes place at room temperature with NH3 and H2O vapors, autogenously generated, from a saturated aqueous 28 wt % ammonium hydroxide solution (see Results and Discussion).

were compared with homologous ones that were thermally treated. Compatibility with scaling-up is briefly discussed in terms of preparation time, cost-effectiveness, feasibility, etc. In addition, a fair part of the present work was dedicated to the detailed understanding of the structural and chemical transformations of the films upon ammonia vapor treatment, as revealed by a set of combinatory ex situ and in situ timeresolved techniques such as spectroscopic ellipsometry, environmental ellipsometric porosimetry (EEP), GI-SAXS, SEM-FEG, FTIR, UV−visible transmittance, and crockmeter. The results clearly evidenced that a local Ostwald-ripening driven mechanism takes place within the films after coabsorption of water and ammonia. We showed that the latter leads to an efficient mechanical consolidation of the silica framework accompanied by a dramatic modification of the structure only for pure SiO2 films, if conditions are not accurately controlled. A more modest structural modification was observed for hybrid SiO2 films. After optimization of the AVT conditions, we obtained fairly stable water repellent mesoporous AR coatings on PMMA with an increase in transmittance as high as 6.5% compared to the bare substrate.

remain fairly complicated and too costly to be scaled up. Another new promising approach consists in transferring thermally cured ceramic films from a thermally resistant substrate to the final plastic susbtrate.23 Even if it is attractive, this approach still needs further development and requires some technical adjustments that may be avoided if a direct treatment could be applied. In the present work, we explore the possibility to prepare AR films by curing mesostructured templated silica-based layers by ammonia vapors to avoid high temperature treatment. Ammonia is the ideal candidate to be used because of its high volatility (vapor pressure of NH3 is 8.9 × 105 Pa at 21 °C). Belleville and Floch24 first described such base-catalyzed treatment, leading to surface silanol condensation for silica colloidal AR coatings. The stiffening of mesoporous silica film by ammonia vapors was mentioned in the literature and was only used as an intermediate treatment that was systematically completed by a final curing at high temperature (T > 400 °C).25−27 As a result, ammonia vapor treatment has never been fully explored and further investigations are still required to determine its full potentiality, especially for low temperature condensation of sol−gel silica films on polymers. In the case of organically templated layers, such as mesoporous AR coatings, ammonia vapor treatment cannot eliminate the organic phase and has to be combined with an extraction step, which would be a selective dissolution in ethanol and/or a decomposition by UV−O3. In the present work, the investigation concerns mesoporous methylated films, exhibiting both AR and hydrophobicity, on poly(methyl methacrylate) (PMMA) substrates, which is widely used in optic due to its high transparency. Two parallel routes were investigated. In the first one, methylation occurs as a post-grafting treatment, while in the second one, a co-condensation of TEOS and MTEOS is used. Both approaches were optimized and led to hydrophobic AR mesoporous films. The optical and mechanical properties (efficiency and durability) of these ammonia vapor treated films

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EXPERIMENTAL DETAILS

Sample Annotation. The first term indicates the precursor ratio and the second term identifies the treatment sequence as described in Figure 1. Chemical Solutions. Absolute ethanol (EtOH) was purchased from Normapur while TEOS (tetraethylorthosilicate), MTEOS (methyltriethoxysilane), hydrochloride acid (2 M HCl), 28−30 wt % ammonium hydroxide solution, F127 Pluronic (EO106PO70EO106), and ClTMS (chlorotrimethylsilane) were purchased from Aldrich. All products were used as received. Mesoporous methyl-functionalized silica thin films (S5M5) were prepared from solutions composed of TEOS/MTEOS/F127/HCl/H2O/EtOH with a respective molar ratio of 0.5:0.5:0.002:0.005:5:41. Mesoporous pure silica films (S10) were prepared from solutions composed of TEOS/F127/HCl/H2O/EtOH with a respective molar ratio of 1:0.002:0.005:5:41. TEOS and 1823



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MTEOS were first dissolved in EtOH, HCl (2 M), and H2O before addition of F127. Solutions were stirred at least for 24 h at room temperature and then stored at −18 °C for 6 months maximum before use. Film Processing. Films were prepared by dip coating in the previous solution substrates (silicon wafer (100), PMMA) at room temperature and at relative humidity of 20% up to the disappearance of the drying line. The withdrawal speed was adjusted at 3 mm·s−1 to fix the final thickness at around 160 ± 10 nm.28 As-prepared films were then aged at room temperature for 5 min in a chamber where the relative humidity was maintained at 75% to promote the formation of cubic mesostructure.29 Previous to deposition, the surface of PMMA substrates was activated with 5 min of UV−O3 treatment (UV lamp: 254 nm, BMT 802x ozone generator). The following successive steps involved in the stabilization treatment are reported in Figure 1 for all types of studied samples. Static ammonia treatments were conducted by exposing films to a saturated ammonia vapor for 10 min, 30 min, 1 h, 2 h, 5 h, 10 h, or 20 h at room temperature. This was done by allowing the films to stand 4 cm above the surface of an aqueous ammonium hydroxide solution (28%) in a sealed chamber. Extraction of the copolymer (F127) was performed on ammonia-strengthened films by immersion in ethanol for 1 min followed by rinsing with ethanol and drying in air flow. After AVT, film names were completed with a -N to give S10-N and S5M5-N. A UV−O3 treatment for 5 min and 1 h respectively for S10-N and S5M5-N was performed before post-grafting. Post-grafting of methyl groups on the pore surface was conducted by exposing porous films to ClTMS saturated vapors. To do so, 3 mL of ClTMS were introduced into a sealed chamber of 100 mL, in which the samples were allowed to stay for 10 min at 70 °C before being washed with ethanol and dried in air flow. Hydrophobic samples are then annotated with (-NH) to give S5M5-NH and S10NH. Characterization. Spectroscopic ellipsometry and environmental ellipsometric porosimetry (EEP) analyses were performed on a UV− visible variable angle spectroscopic ellipsometer (M2000 Woolam) equipped with a controlled atmosphere cell in which relative humidity was controlled by mass flow controllers (the setup and analysis protocol were previously described30). The data analyses were performed with Wase 32 software using a Cauchy model. For in situ time-resolved AVT investigations, the aim of this analysis is to record in situ the evolution of thickness and refractive index using ellipsometry analysis during ammonia treatment so as to determine the optimal conditions. To do so, the cell shown in Figure 2 was built

aqueous solution to generate NH3/H2O vapors or directly pure NH3 from a cylinder (Air Liquid). Fourier transform infrared (FTIR) measurements were performed on a Bruker Vertex 70 V spectrophotometer in transmission mode. For grazing incident small-angle X-ray scattering (GI-SAXS), a Rigaku S-max 3000 apparatus equipped with a microfocus source λ = 0.154 nm and an image plate detector placed at 1433 mm from the sample were used. The angle of incidence was 0.2°. Diffraction patterns were interpreted using Igor software. Microscope images showing the film structure were collected using a field-emission gun scanning electron microscopy (FEG-SEM - Hitachi SU7000 instrument). Total reflectance (diffuse + specular) spectra were collected with a Cary Series UV−Vis−NIR spectrophotometer between 400 and 800 nm. Mechanical properties were estimated in terms of resistance to abrasion using the crockmeter test that consists in a rubbing of the top of the films with a pad. The latter pad is covered with a calibrated fabric (ISO Test Method 105 − F09). It is loaded with a constant 5 N charge and is moved back and forth in a straight line. A single cycle corresponds to a pad back and forth motion. The silicon wafer is not damaged by the rubbing action. The number of cycles reported in the graph corresponds to the selected end point for which 50% of the surface of the substrate has been cleared from the film, which corresponds to full rubbing of 50% of the sample. For each sample, ellipsometry investigations revealed that optical properties and the thickness of the remaining 50% film were not significantly modified by the rubbing test.

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RESULTS AND DISCUSSION Because organic polymer substrates have a refractive index comprised between 1.45 and 1.60,31 close to glass, an ideal AR film should be hydrophobic and should have a refractive index around 1.20 and 1.26 at λ = 700 nm, which corresponds to a porosity that is close to 40% of the volume if the matrix is silica. For instance, a mean value of n = 1.24 is susceptible to provide AR coatings with a minimal reflectance of 0% at a specific chosen wavelength on substrate with a refractive index of 1.49 according to Fresnel (r = {[nairns − n2]/[nairns + n2]}2, where ns = refractive index of the substrate, nair = refractive index of air, and n = refractive index of the film). In sol−gel silica films, it is well-known that the pore volume and, therefore, the final refractive index depends on the proportion of organic template to inorganic precursor that is combined in the initial solution. Here, F127 copolymer was used to generate the mesoporosity because it leads to well-ordered 3D structures composed of pores and walls in the range of 5−10 nm13,32 which ensures higher matrix stability and mechanical resistance than in the case of smaller pores generated with ionic surfactants,33 for instance. 1. Selection of the S5M5 Hybrid Matrix. The use of methyl silane as coprecursor is known to confer hydrophobicity to the final material, as long as the temperature remains below 500 °C.13 In order to select the optimal systems to undergo the AVT, films that were prepared with the proper ratio of F127, but with various ratios of MTEOS/TEOS, and cured at 450 °C for 10 min were analyzed by EEP. All final films had thicknesses between 100 and 140 nm, were porous, and had a refractive index of 1.22 ± 0.02 in the visible range. Typical water adsorption/desorption isotherms are reported in Figure 3b while the maximal water uptake capacity at 100% RH is reported in Figure 3a. It is clear that the quantity of water that can condense within the film porosity progressively decreases with the ratio of Me. Sample S5M5-T, prepared with 50 mol % of MTEOS and cured at 450 °C, is fully water repellent contrary to sample S10-T prepared with no MTEOS and cured at 450 °C. Indeed, the latter can be loaded with more than 50%

Figure 2. Representation of the ellipsometric sealed cell used for the dynamic in situ investigation of the optical properties during ammonia vapor treatment. so that the sample can stand above 6 mL of a 28% aqueous ammonia solution, in the naturally generated saturated vapor pressure, while the ellipsometer incident and reflected beams were allowed to pass through two nonbirefringent windows at an angle of 90°. The cell was also equipped with a thermoregulation system to heat the whole cell up to 50 °C. Ψ and Δ were collected at an incident angle of 70° every 20 s and fitted to extract the corresponding thickness and refractive index using a Cauchy model characteristic of a dielectric material. In a second experiment, the treatment was performed by replacing the static solution with a flux (1 L·min−1) of gas containing ammonia. The flowing gas was either a dry and filtrated air bubbling in 28% ammonia 1824



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Figure 3. (a) Water up-take of 450 °C sintered methyl functionalized silica thin film versus MTEOS molar ratio. (b) Environmental ellipsometric porosimetry (EEP) of pure TEOS film (S10-T) and 50 mol % MTEOS/50 mol % TEOS films (S5M5-T) that were sintered at 450 °C for 10 min.

Figure 4. Plots of the relative thickness measured by in situ ellipsometry versus AVT time for samples S5M5 (a) and S10 (e).

investigated. First, in the case of S5M5-N, one observes that the higher contraction of 11% was achieved for the static NH3/ H2O atmosphere at 20 °C AVT. This is due to the higher quantity of water and ammonia absorbed in the film in these conditions. The fact that the use of NH3 without H2O led to only 5% of contraction confirms the critical role of water in this treatment. Raising the temperature up to 50 °C is accompanied by a decrease of absorption, as testified by the thickness increase (see Supporting Information SI1(a)). However, the associated reduction of absorbed NH3 is not compensated by the thermal effect, suggesting that a moderate temperature is not beneficial. However, the related condensation is still lower than the 26% obtained with the thermal treatment at 450 °C (S5M5-T), as expected. Very interestingly, treating methyl-free S10 films at 450 °C led to a higher contraction of 36%, and the reversed tendency was observed for AVT. Indeed, S10-N films showed an expansion of 16% of the initial thickness compared to the 11% contraction of the S5M5-N films for the same conditions. This singular behavior suggests that critical modifications of the silica backbone occur during AVT, which also depends on the presence of methyl moieties. This point will be addressed later in the article. Figure 4 shows selected typical plots of the variation of thickness versus time for S10 and S5M5 films, during AVT performed in static NH3/H2O atmosphere at 20 °C only, identified as the optimal conditions. For clarity purposes, thickness and refractive index variation plots, obtained in all conditions, are reported in the Supporting Information (SI 1). The introduction of the active vapor into the chamber was directly followed by a strong swelling of the films, attributed to the absorption of the NH3/H2O mixture. No distinction between both species could be made. The main difference between S10-N and S5M5-N lays in the extended swelling of the hybrid film, reaching almost twice the initial thickness before both systems stabilize at around 25% swelling. This difference is likely attributed to the higher elasticity of the hybrid matrix. Note that the hydrophobic characteristic of the methyl groups may not have a strong influence due to the presence of the highly hygroscopic F127 agent, which is the main driving force of the vapor absorption. The fact that the S5M5-N thickness shrinks back to 25% after 20 min is attributed to the silica cross-linking triggered by the high quantity of absorbed NH3. In the case of the S10-N, this springback effect is more moderate due to the more stiffened initial matrix and to the associated lower NH3 absorption. In summary, the optimal AVT conditions were selected to be a static NH3/H2O atmosphere at 20 °C, based on efficiency, speed, cost, and ease of implementation considerations. This AVT will be used from now on for the rest of the study. It is

of its volume with water and presents the characteristic of a mesoporous film with narrow pore size distribution (data not shown). It is important to notice here that S5M5 films submitted to treatment below 350 °C for 10 min are not hydrophobic, suggesting that the thermal treatment plays a critical role in the reorganization of the pendant alkyl groups at the surface of the porosity. This point will be addressed in detail later in the article. 2. Optimization of AVT Conditions. In the following part, only the previously selected initial films (S5M5 and S10) will be considered. The in situ time-resolved ellipsometric analysis during AVT has been done in order to indirectly follow the contraction ratio of the film as a quantitative indicator to evaluate the condensation of silica in a first approximation.34 The condensation is known to be faster in the presence of NH3 but is also very dependent on the presence of H2O due to the protolysis equilibria between both species. Because the fraction of adsorbed species is governed by dynamic equilibrium with atmosphere, it is important to evaluate the influences of both relative pressures and also of the convection. Table 1 lists the Table 1. Film Composition and Treatment Conditions Used for the in Situ Ellipsometry Measurements AR films sample

films

a b c d e

S5M5-N S5M5-N S5M5-N S5M5-N S10-N S5M5-T S10-T

NH3 treatment conditions gas

process

NH3/H2O static NH3/H2O flow NH3/H2O flow flow NH3 NH3/H2O static 450 °C, 10 min 450 °C, 10 min

temp. (°C) 20 20 50 20 20

thickness contraction %a 11 9 8 5 −16 26 36

a

Contraction ratios have been measured on 1 h treated samples after drying out of the treatment cell.

thickness contractions measured for the different applied conditions (static vs flux, 20 °C vs 50 °C, and pure NH3 vs NH3/H2O). They were calculated from thicknesses measured initially and after 1 h of different AVT (see Figure 4). For the sake of comparison, both S5M5 and S10 films were immersed in a 0.1 M aqueous ammonia solution. They partially or fully dissolved in a few seconds, suggesting that this liquid phase treatment was not appropriated and was then not further 1825



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using FTIR were performed to better understand the efficiency of template extraction. FTIR spectra plots (see Supporting Information SI3) indicate the evolution of the organic phases (methyl pendant groups and F127) during the first 1 h of treatment and after washing. The spectrum of S5M5-T, for which F127 is fully decomposed, is plotted for comparison. The Si-CH3 stretching band, located at 2980 cm−1 with a negligible contribution of the CH3 (stretching) from F127, is always present which suggests that both applied treatments do not eliminate the methyl group as expected. CH3 groups that are pendant at the surface are responsible for the water repellent properties. However, no significant band shift has been observed, meaning that surface and internal Si-CH3 could not be differentiated. On the other hand, the characteristic bands of the CH and CH2 (stretching) corresponding to F127 (see circled area in Supporting Information SI3) in the region comprised between 2950 cm−1 and 2850 cm−1 disappear during the thermal treatment but also after at least 1 h of AVT followed by washing. Above 1 h of AVT treatment, the F127 must be accessible to the solvent to be extracted. As a result, the AVT induces structural inorganic rearrangement that induces a progressive percolation of the F127 elongated micelles, necessary to enable full extraction. No collapsing of the revealed porosity due to capillary stresses was observed by EEP investigation whatever the time of AVT treatment. Table 2 lists the refractive indices and the accessible pore volume probed on S5M5-N coatings, using environmental ellipsometry porosimetry (EEP) during AVT. One remarks that 1 h of AVT is necessary to obtain n = 1.24 at λ = 700 nm, the required value for AR properties on selected substrates. This characteristic does not evolve with longer treatments, confirming the stabilization deduced from GI-SAXS. The EEP analyses showed that water condenses in the porosity of S5M5-N films contrary to what was observed for S5M5-T films in Figure 3b. It is important to discuss this effect before carrying on the structural investigation. The porosity being open in both samples, the reason why S5M5-T is water repellent, whereas S5M5-N is not, is certainly due to a difference of hydrophobicity. Along the whole treatment process and assuming that no by-product organic moieties remain from F127 extraction, the surface of the accessible porosity must contain the Si−Me group, promoting hydrophobicity, together with Si−OH and Si− (OH2+) groups, promoting hydrophilicity. We know that the thermal treatment leads to higher condensation degree than the AVT from the calculated contractions (26% vs 11%; see Table 1), and we know that water repellence is not obtained for thermal treatment below 350 °C. Consequently, the degree of condensation of the silica network is to be directly linked to the density of methyl groups pending at the surface of the pore walls. A scheme illustrating the local rearrangement of the silica-based units at the interface of the pore walls is proposed in Figure 6. In the case of a thermal curing, the higher degree of cross-linking in the SiO2 framework is accompanied by a conformation change of the siloxane units at the surface, characterized by the relocation of the methyl groups from inside the oxide network toward the surface of the oxide pores, which enriches the interfaces with hydrophobic groups. The lower condensation degree associated to the AVT results in the presence of a higher proportion of hydroxyl groups leading to a higher hydrophilicity. This effect is further enhanced in the case of AVT because hydroxyl groups may be stabilized by their interaction with the F127 PEO chains at the pore interface. For S5M5-T, it is not the case because F127 is fully decomposed

important to notice from Figure 4 that the systems seem to be stable after 20 min (no significant thickness evolution was recorded above 1 h for all samples), suggesting that a treatment of at least 20 min would be required to stabilize the network. We will see later that several hours of AVT are necessary to obtain the best properties. The stability of these -N films was revealed when the copolymer template was extracted in solution to lead to porous films with low refractive index. 3. Influence of AVT Time on Porosity of S5M5-N. Evolution of film structure is a good indication of the chemical transformation taking place during the ammonia vapor treatment. Figure 5a shows a typical GI-SAXS pattern of a

Figure 5. (a) Typical GI-SAXS pattern of the S5M5-N mesoporous film treated for 10 h in AVT. (b) SEM-FEG image of the corresponding S5M5-N film.

S5M5-N film after 10 h of AVT. Independently of the time of treatment, all S5M5-N samples exhibited scattered signal patterns characteristic of wormlike mesostructures,35 further confirmed by the SEM-FEG image (Figure 5b). The present wormlike structure differs from what is usually reported in the literature with pure SiO2/F127 films prepared in similar conditions. This has already been observed and was attributed to the lowering polarity induced by the methyl groups.36,37 The intensity profiles of the GI-SAXS scattered signals integrated on the area indicated in Figure 5a and recorded for various durations of AVT are given in Supporting Information (see SI2). The intensity and the position of the peak accounting for the correlation distance only slightly vary with AVT time, suggesting that the structure is globally maintained. The corresponding structural periodicity averaged at 12 ± 0.4 nm, as listed in Table 2. Only the as-prepared S5M5 shows a periodicity centered at 16.2 nm, confirming the global shrinkage of the nanostructure upon AVT. Chemical analyses Table 2. Values of Refractive Index, Thickness (Ellipsometry), Porous Volume (EEP), and Correlation Distance (GI-SAXS) Obtained for S5M5-N Films with Different Periods of Treatment, Followed by Ethanol Washing To Extract F127

S5M5-N

time of AVT

refractive index (RH = 0%, 700 nm)

porous volume (%)

d-spacing GI-SAXS (nm)

thickness (nm)

as-prepared 10 min 30 min 1h 2h 5h 10 h 20 h

1.34 1.29 1.25 1.24 1.23 1.23 1.25

11 22 33 33 31 32 32

16.2 11.7 11.6 12.2 12.4 11.9 12.5 11.8

160 136 140 148 142 144 145 148 1826



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Figure 6. Scheme illustrating the Me-Si(O)33− conformation change at the inorganic interfaces leading to water repellence for S5M5-T against partial hydrophilicity for S5M5-N.

between 250 and 350 °C.38 Coming back to the structural investigation based on the EEP study displayed in Figure 7a,

porosity was mainly composed of pores with diameter of 3 nm. At 1 h of treatment, the pore size distribution was centered at 4.5 nm with a mean diameter between 2 and 9 nm, as expected for disordered materials templated with F127. The structure underwent no significant changes for longer treatment. These observations are in agreement with interpretation of the previous GI-SAXS and FTIR data. We can conclude that 1 h of AVT is sufficient to stabilize the siliceous network and to allow the full extraction of the organic template, releasing thus the entire porosity. 4. Influence of AVT Time on Porosity of S10-N. In the following part, the same investigations were conducted on the pure silica mesoporous film system, as an alternative to the previous S5M5, and also to clarify the role of the methyl group. As previously intuited, a significant mesostructure transformation of the S10-N films upon AVT was evidenced. Indeed, the GI-SAXS pattern of the as-prepared S10 coating shown in Figure 8a reveals the characteristic diffraction points of a body cubic centered mesostructure (Im3m space group).39 Patterns recorded after 30 min, 1 h, and 5 h of AVT are radically different and let us know that the mesostructure rapidly turned into a wormlike organization after 30 min of treatment, before transforming into a 2D hexagonal mesostructure (p6m space group)29 and finally into a disordered collapsed structure over 5 h. Ghosts signals are indicated in red circles. They are shifted from the “real” diffraction points because they originate from the trapping of part of the diffracted signals into the coating by evanescence before departing the film at the Brewster angle (critical angle).35 The periodic distances in the direction parallel to the surface, corresponding to diffraction reinforcement along the integration zone in Figure 8, were measured to be 14.0 nm, 13 nm, 13.5 nm, 15.2 nm, and 16.5 nm for as-prepared, 10 min, 30 min, 1 h, and 2 h of AVT, respectively. Because of the sequence of structural transformation, one cannot extract critical information from this evolution. The SEM image shown in Figure 8e corresponds to the 1 h treated sample and confirms that the mesoporosity is that of a 2D hexagonal structure. This

Figure 7. (a) Adsorption/desorption isotherms and (b) corresponding pore sizes distribution obtained by EEP on S5M5-N for different periods of NH3/H2O treatment.

adsorption and desorption isotherms first reveal that the total accessible porosity increased from 10 to 33% for the treatment period ranging from 10 to 60 min. Over 1 h of treatment, the accessible porous volume of the S5M5-N film did not evolve anymore, stabilizing at 33%. Corresponding pore size distributions have been extracted from isotherms and are displayed in Figure 7b. In the first hour of the treatment, the 1827



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Figure 8. (a−d) GI-SAXS patterns of S10-N coatings depending on time of AVT treatment with corresponding structure. Red circles correspond to the ghosts caused by the evanescent waves.35 (e) SEM-FEG picture of S10-N 1 h exhibiting a 2D hexagonal structure. (f) Table of the corresponding mesostructure organizations of S10-N films as a function of the time of ammonia treatment.

AVT-induced transformation must involve a partial reorganization of both organic and inorganic phases, which will be discussed later in Section 5 of the article. One must underline here that this effect was not observed for S5M5 films. Porosity of S10-N films was assessed by EEP for different treatment times, and the results are displayed in Figure 9 and in Table 3. All isotherms displayed in Figure 9a are of type IV characteristic of a mesoporosity. The porous volume of S10-N increases progressively from 12% for 10 min to 32% for 2 h, before decreasing down to 5% for 20 h. In parallel, the corresponding pore size distributions, plotted in Figure 10, point out that a significant increase of mean pore dimension progressively took place up to 5 h of treatment. Indeed, pores enlarge from 6 to 13 nm in the first 2 h of treatment. This doubling of the pore size corresponds to the transformation from wormlike to 2Dhexagonal deduced from GI-SAXS. After that, the pore size slightly decreases down to 11.5 nm for 10 h of treatment, before it totally collapses with the loss of porosity and ordering at 20 h. As for S5M5-N, chemical analyses using FTIR were investigated to better understand the mechanism (FTIR spectra of S10-N samples are available in Supporting Information (see SI4)). They all have the characteristic bands of CH3, CH2, and CH groups and vibrations between 2850 cm−1 and 3000 cm−1 of F127 copolymer, which reveals that the organic phase could not be fully extracted whatever the treatment time, contrary to S5M5-N, which requires only 1 h of ammonia treatment to allow total removal of the copolymer. This difference can probably be attributed to the higher polarity of the pure silica matrix that allows a strong penetration of PEO chains into the inorganic walls during formation. This effect has already been reported on mesostructured F127 templated silica.40 No quantitative estimations of F127 amount remaining in the porosity can be done in this case due to the measurement in transmission where part of the infrared signal is diffused by the nonpolished face of the silicon wafer.

Figure 9. (a) Adsorption/desorption isotherms and (b) corresponding pore sizes distribution, obtained by EEP on S10-N for different periods of NH3/H2O treatment.

5. Proposition of AVT-Induced Mechanisms. An important fundamental point concerns the two distinct behaviors observed for S10 and S5M5 systems. Illustrations 1828



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transformation and no collapsing occurred. This is due to the reduced reactivity of the S5M5 silica network toward hydrolysis/condensation/dissolution. Indeed, the inductive donor effect of the methyl group decreases the electropositive character of Si centers and thus disfavors the nucleophilic attacks.41 In addition, MTEOS allows the formation of 3/4 oxo bonds against 4/4 for TEOS which reduces the global densification. The latter chemical considerations suggest that the Ostwald ripening driven rearrangement occurs on a longer range for pure S10-N than in the case of S5M5-N. 6. Resistance to Abrasion. The ideal refractive index of 1.25 at λ = 700 nm required for AR films has been obtained after 1 h of treatment for the hybrid methylated S5M5-N film against 2 h of treatment for the pure silica S10-N sample. Because both these films are hydrophilic so far, they will be post-functionalized in a last stage to acquire the water repellence property. Before looking at the performance of these final coatings deposited on organic substrate, one must first have an idea of the mechanical properties resulting from our AVT. Indeed, mechanical properties of thin films are a key issue that must absolutely be investigated for AR applications. However, analysis of these characteristics is very challenging since, depending on the technique, different assumptions have to be made and/or different effects (e.g., substrate, interface, thickness, environment, etc.) have to be considered. In what follows, we used Si substrates and we decided to probe the intrinsic mechanical properties of the films using EEP, where the transversal deformation induced by the capillary condensation of water in the pore can be deduced from thickness measurement.30 One notices that the latter estimation does not take into account the adhesion and must be performed with hydrophilic pores, in other words, before the final hydrophobization. We had then to assume that the mechanical properties of the films would not be changed upon this final treatment. Figure 12 displays the superimposed plots of the variation of thickness upon humidity modulation for S10-N (b) and S5M5-N (a) samples after different times of AVT followed by washing with ethanol (data collected simultaneously as the adsorption/desorption isotherms in Figures 7 and 9). They all exhibited deformations induced by the capillary stresses created by adsorption and desorption of water in the porosity. Plastic and elastic deformations can be quantified by comparing the initial and final thicknesses after cycle completion. In each case, one observes that a longer AVT or a thermal treatment (S10-T) led to lower plastic deformation, confirming that higher condensation degree of the silica network leads to stiffer networks. The full reversibility of the hysteresis loops is never obtained here because the structure always undergoes a stress relaxation in the first cycle.30 After several cycles, hystereses become fully reversible and only elastic deformations, corresponding to the capillary condensation-induced thickness contraction, are measured. From the relative thickness curves the transversal Young’s moduli (E/GPa) were calculated using the method described in detail elsewhere.30 The calculated values of E are reported in Figure 13a. They fall between 1 and 7 GPa, which are an order of magnitude less than what is commonly measured on bulk silica, as a result of the porosity. On the other hand, they are close to the values corresponding to the thermally cured S10-T films that were measured at 5.9 ± 0.2 GPa. In order to complete this investigation, crockmeter tests were performed to assess film resistance over domestic abrasion as described in the Experimental Details section. Contrary to EEP measurements, films can fail by the intrinsic

Table 3. Values of Refractive Index, Thickness (Ellipsometry), Porous Volume (EEP), and Correlation Distance (GI-SAXS) Obtained for S10-N Films with Different Periods of Treatment, Followed by Ethanol Washing To Extract F127

S10-N

time of treatment

refractive index (RH = 0%, 700 nm)

porous volume (%)

d-spacing GI-SAXS (nm)

thickness (nm)

as-prepared 10 min 30 min 1h 2h 5h 10 h 20 h

1.34 1.31 1.27 1.25 1.27 1.31 1.38

12 19 29 32 30 19 5

14.0 13.0 13.5 15.2 16.5 -

160 162 168 174 185 183 171 159

Figure 10. Plots of the porosity and the pore size evolution, obtained by EEP on S10-N for different periods of NH3/H2O treatment.

of the respective mechanisms are proposed in Figure 11. Starting with the pure silica S10-N system, during AVT, NH3 and H2O vapors absorb into the films and NH4+ and OH− are generated by the dissociation equilibrium. This is accompanied by a moderate swelling of the films (see Figure 11b). In the present conditions, PEO chains of F127, which form the shell of the micelles, are intimately combined with the silica oligomers, and micelle interfaces are thus not well-defined. The presence of NH3/H2O induces a global increase of the ionic strength in both PEO and silica hygroscopic phases, globally decreasing the chemical affinity contrast. In parallel, the OH− is known to catalyze the modification of silica networks through local dissolution/condensation also addressed as Ostwald ripening. In the present case, it is evident that the latter phenomenon is responsible for the structural evolution. It is reasonable to intuit that dissolution/condensation occurs mainly at the micelle/silica interfaces. It induces the depletion between silica and PEO chains, which would result in the enlargement of the micelles and in the diminution of the curvature responsible for the transformation from spheres to cylinders. In these conditions, the transformation is also thermodynamically driven by the reduction of interfaces, since dissolution occurs preferentially at the convex parts of the interface and precipitation at the concave parts. This local process progressively restructures the porosity until the final collapsing. This later step is accompanied by the expulsion of the copolymer at the surface of the film, characterized by the apparition of an eyes-visible cloudy membrane at the film surface that disappears after washing. The same types of reactions take place in the methylated silica systems (S5M5-N) as illustrated in Figure 11, except that no mesoscopic 1829



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Figure 11. (a) Illustration of the process of the pore opening of S5M5 films during the first hour of the ammonia treatment. (b) Illustration of the ammonia action mechanism on the structure modification of S10 films.

Figure 13. (a) Young’s modulus values obtained by EEP and (b) crockmeter abrasion tests (higher number of cycles corresponds to higher resistance) of ammonia cured S10-N and S5M5-N films on Si wafers for various times of treatment.

cohesion (Young’s modulus) or by the adhesion with the substrate. It is thus of utmost importance to compare results from both analyses. The number of cycles from which the film started to fail is reported for various periods of treatment in Figure 13b. The Young’s modulus is related to the strain

Figure 12. Relative thickness curves of (a) S5M5-N and (b) S10-N films measured by EEP for various periods of AVT.

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After AVT, S10-N (2 h) and S5M5-N (20 h) were first treated with UV−O3 to totally remove the traces of F127 potentially remaining after the ethanol washing. They were then submitted to ClMTS vapors so as to graft trimethylsilane on the surface of the pores to ensure full water repellence. They are now denoted S10-N2H and S5M5-N20H. As expected, EEP on these samples showed no water uptake (data not shown). Antireflective properties of both S5M5-N20H and S10-N2H were assessed by comparing the reflectance in the visible range of bare PMMA substrates and coated ones (see Figure 14).

response at a stress applied for an elastic isotropic material. In our case, the strain response is a function of the energy of the chemical bonds and of the bond density in the material. For both systems, the chemical bonds at play in the matrix lattice are the same (i.e., Si−O−Si) with a Si−O energy value of 398 kJ·mol−1. Considering as insignificant the interaction of the CH3 groups of the siloxane with the oxide matrix, it is sensible to link the Young’s modulus to the condensation rate of the silica matrix. For simplicity it is also assumed, as a first approximation, that the Young’s modulus is the main contributor to the cohesive energy and that the other contributors are negligible. Young’s moduli evolution is thus dependent on the result of the OH− induced Ostwald ripening described previously. The S10-N films restructuration created by an important cleavage of Si−O−Si bond in the first 5 h may explain the dramatic decrease of Young’s modulus from 5.2 ± 0.2 GPa (1 h of AVT) to 0.7 ± 0.2 GPa (5 h of AVT). This Young’s modulus decrease is also in accordance with a mesostructure evolution from 2D hexagonal to disorder.42 After 5 h, the increase to 3.30 ± 0.2 GPa (10 h) is in accordance with the beginning of the densification of the film. The Young’s modulus of S10-N (20 h) could not be calculated due to the too low porous volume. In parallel, crockmeter tests for S10-N films in Figure 13b reveal that the best abrasion resistance is obtained for 1 and 2 h of AVT. It then decreases dramatically after 5 h, as observed by the Young’s modulus drop. This could indicate a cohesive fracture of the film, which would be corroborated by the slight increase of abrasion resistance after 10 h, itself correlated to the increase of E seen previously. A fast degradation of the thin film after 20 h of ammonia treatment could not be related to the Young’s modulus anymore but may be due to dramatic structural changes induced by an excessive reorganization at the molecular scale by Ostwald ripening. For S5M5-N films, a similar behavior is observed except that the fluctuation of E is less intense and the minimum is found at 2 h instead of 5 h. It is important to notice that, after 2 h, the stiffness is higher for S5M5-N than for S10-N. The lower variation of the modulus values confirms that Oswald ripening has a shorter-range occurrence as described previously. On the other hand, crockmeter tests exhibit a much lower abrasion resistance than S10-N. Altogether, this reveals that the hybrid film fails the abrasion probably by the adhesion but not by the cohesion. Such poor adhesion of the film is likely caused by the methyl groups that reduce the quantity of Si−O−Si bonds that can be created with the native SiO2 layer on the silicon substrate. A longer AVT would however favor the creation of these adhesion bonds, as confirmed by the progressive increase of crockmeter critical number of cycles. Crockmeter tests performed onto S10-T and S5M5-T films exhibit better resistance than ammonia treated films with numbers of cycles higher than 1000. A precise assessment of causes of failures and related mechanism would require further investigations, such as using double cantilever beam analysis and surface chemical analysis, but are out of the scope of this article. 7. Optical Properties and Durability on PMMA Substrates. According to the latter results, S10-N (2 h) and S5M5-N (20 h) layers are the most promising alternatives for abrasion resistant AR coatings on silicon wafer stabilized by AVT. Abrasion resistance, antireflectivity, and stability in aqueous conditions were conducted on PMMA coated with both types of layers.

Figure 14. Visible spectrum analysis in reflectivity at 90° incident angle of hydrophobic S5M5-N20H and S10-N2H coated on both sides of PMMA substrates.

They both successfully exhibited an almost perfect antireflective property, since a minimal reflectance of less than 1% was recorded at 575 nm (the first harmonic of the interference), which was up to 6.4% less than the bare PMMA. Crockmeter abrasion resistance of S5M5-N20H and S10-N2H films on PMMA are 30 cycles and 100 cycles, respectively. These much lower number of cycles, compared to silicon substrates (100 cycles and 1000 cycles, respectively), can be explained by the intrinsic low stiffness of PMMA leading to poor abrasion resistance, together with a weak adhesion with our AR films. It is therefore evident that the failing factor in the conception of mechanically resistant sol−gel coatings on organic substrates is the intrinsic nature of the substrate and of the adhesion strength generated at the interface. As a result, a first functionalization of PMMA surface by a coupling agent such as the 3-(trimethoxysilyl)propylmethacrylate may improve the interfacial adhesion. This is not described in the present work but will be the main focus of our future investigations. As a last investigation, S5M5-T and S10-T on Si substrates, together with S5M5-N20H and S10-N2H on PMMA and on Si substrates, were submitted to dissolution tests. Coatings were immersed into 100 mL of aqueous pH 7 phosphate buffer solutions at 70 °C for up to 48 h to simulate accelerated environmental aqueous attack. Discontinuous ellipsometry was used to measure coating thickness evolution, which is taken as being representative of the stability in water. Figure 15 gathers the observed thickness relative variations. First, the substrate nature seems not to affect the intrinsic coating stability. As expected, S5M5 coatings show globally a higher stability than S10 coatings whatever the curing processes. The postmethylation of S10 after AVT improved the resistance, but it remains much less resistant than S5M5 systems. Indeed, methylated silica networks were already reported as having good resistance toward aqueous dissolution41 due to the combination of reduced reactivity toward nucleophilic attack and hydrophobic property. S5M5-T exhibits a slightly better 1831



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resistant to chemical leaching. In terms of industrial concerns, both routes involve a certain number of steps that do not require expensive and complex facilities. It is however important to notice that the pure silica-based route required much less time of treatment than the hybrid silica-based route. Whatsoever, compromises still need to be done in the selection of the routes and conditions of treatment to better adapt the final AR coating to its functioning environment.

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ASSOCIATED CONTENT

* Supporting Information S

In situ time-resolved ellipsometric analysis during ammonia vapor treatment, GI-SAXS, and FTIR data of thin films. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(D.G.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for funding provided by CIFRE fund granted by ANRT and the AVATAR project which is supported by Polyrise SAS and DGA. M.B. et al. thank D. Montero for Semfeg microscopy conducted on a Hitachi Su-70 + Oxford XMax facilited by the IMPC (FR2482) financially supported by the C′Nano projects of the Region Ile-de-France and M. Faustini for technical help.

Figure 15. (a) Relative thickness reduction of S5M5-T and S5M5N20H on PMMA and on Si substrates after 0 h, 10 h, and 48 h of immersion in phosphate buffer solution at pH = 7. (b) Relative thickness reduction of S10-T and S10-N2H on PMMA and on Si substrates after 0 h, 1 h, and 2 h of immersion in phosphate buffer solution at pH = 7.

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REFERENCES

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stability than S5M5-N20H on the Si(100) substrate after 48 h of immersion, certainly owing to the higher matrix condensation degree. Concerning S5M5-N20H on PMMA, after 10 h of immersion, ellipsometric measurement was no more possible due to the PMMA degradation through absorption-induced swelling causing a loss of optical homogeneity. By sight, the integrity of a 48 h of immerged S5M5N20H film seems to be preserved. Here again, we emphasized the fact that the failing of such AR inorganic coatings on organic substrates often occurs by the substrate. Concerning the chemical stability properties, it is clear that hybrid S5M5 systems are the preferred ones.

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CONCLUSION Hydrophobic, antireflective films were successfully realized by a sol−gel process on thermally sensitive substrates (PMMA) thanks to an optimized room temperature ammonia vapor treatment (AVT). Two routes based on either pure or hybrid silica systems were investigated. It was shown that ammonia vapor catalysis of silica moieties requires the presence of water from humidity to locally generate OH− and to proceed to the Ostwald-ripening-type structural modification. We demonstrated that the presence of methyl groups in the silica xerogel limits the dramatic Ostwald-ripening-driven structural modification induced by the AVT. Final coatings were all water repellent and antireflective with a maximum transmittance up to 99.6% in the visible range. However, the pure silica-based route led to films with better adhesion and stiffness than the hybrid silica-based route, while the latter was much more 1832



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