Periodic Mesostructured Silica Films Made Simple Using UV Light

Article pubs.acs.org/JPCC

Periodic Mesostructured Silica Films Made Simple Using UV Light Héloïse De Paz-Simon,† Abraham Chemtob,*,† Céline Croutxé-Barghorn,† Séverinne Rigolet,‡ Laure Michelin,‡ Loïc Vidal,‡ and Bénédicte Lebeau‡ †

Laboratory of Macromolecular Photochemistry and Engineering, University of Haute-Alsace, ENSCMu, 3 bis rue Alfred Werner, 68093 Mulhouse, Cedex, France ‡ Institut de Science des Matériaux de Mulhouse, UMR-CNRS 7361, University of Haute-Alsace, 3 rue Alfred Werner 68093, Mulhouse, Cedex, France S Supporting Information *

ABSTRACT: Recent research has shown that silica/surfactant self-assembly combined with photoacid-catalyzed sol−gel polymerization can direct the formation of disordered mesoporous silica films. Using only a mixture of photoacid generator (PAG), amphiphilic surfactant (PEO-b-PPO-b-PEO), and methoxy oligomeric precursor, this original UVdriven method is fast and obviates the need of sol preparation and volatile compounds. Here, we report a rational basis to promote a disorder-to-order transition through the control of relative humidity (RH) and irradiance. Above a threshold RH of 35%, a longrange organization is promoted by greater interactions between the water-enriched silica phase and the PEO block. Alternatively, mesophase rearrangement and ordering are achieved by decreasing the irradiance below a limiting value (150 mW/cm2) so as to minimize the condensation rate. These two strategies show that the fundamental driving force for the creation of well-ordered hybrid mesostructures can be described both on thermodynamic and kinetic grounds. Subsequently, these optimized reaction conditions are exploited to tailor the final mesophase (hexagonal, lamellar, cubic) at different template/ inorganic ratios. In a last part, the photoinduced mesostructuration mechanism is elucidated for model lamellar films using X-ray diffraction and Fourier transform infrared spectroscopy.

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is no longer to destroy the organic template24,25 or condense preformed lyotropic silicate mesophase.26−30 Here, UV irradiation controls the entire synthesis: photoacids drive the hydrolysis and condensation of the silicate precursor, and the in-situ generation of silanol species triggers the surfactant selfassembly. Our procedure uses only nonvolatile constituents deposited in the form of a film of controlled thickness, including a photoacid generator (PAG) based on diphenyliodonium salt, a nonhydrolyzed oligomeric alkoxysilane, and an amphiphilic PEO-b-PPO-b-PEO block copolymer template. Upon UV exposure, the PAG photolysis liberates superacids, inducing both silica network formation and mesostructuration in a matter of seconds or minutes. Despite these processing advantages, disordered mesostructures, commonly referred to as wormlike, were reported in our initial investigations.21−23 As film properties are strongly related to pores geometry and accessibility,5 the precise design of pore architecture is therefore essential to many applications, especially those relying on size and shape selectivity. In this new contribution, we have established that controlling the relative humidity (RH) and the irradiance (I) provides a rational basis to promote mesophase ordering. Such approach might lay the foundation for a more

INTRODUCTION In mesoporous sol−gel materials,1−3 the control of morphology, especially film forming, is a key issue for practical applications.4−6 Interlayer dielectrics7,8 in microelectronics but also sensors,9−11 membranes,12−14 or photonic devices15,16 demand the shaping of mesoporous materials as thin films. The evaporation induced self-assembly (EISA)17 method is an effective and well-established way to afford high-surface-area and optically clear silica/surfactant films with a periodically ordered mesostructure. Films are typically synthesized from a diluted solution containing a surfactant and a silica sol prepared by hydrolysis of an ethanolic solution of alkoxysilane. During the film deposition and drying process, a rapid removal of the solvent drives the progressive growth of micellar ordered domains.18 Although this method has created considerable wealth of structures and materials, there is a continuous pressure to find simpler, more affordable, and scalable processing conditions to turn mesoporous films into real applications.19 Taking the example of organic polymer films, the transition from a thermal to a photoinitiated process20 has resulted in many benefits in regards to efficiencies (conversion rate, energy consumption, space saving) and waste minimization (solvent-free). Recently, we have demonstrated the feasibility of a photoinduced sol−gel polymerization for the synthesis of mesoporous silica films.21−23 In contrast to earlier studies, the effect of UV light © 2014 American Chemical Society

Received: October 24, 2013 Revised: February 3, 2014 Published: February 7, 2014 4959

dx.doi.org/10.1021/jp410518k | J. Phys. Chem. C 2014, 118, 4959−4966

The Journal of Physical Chemistry C

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Scheme 1. Controlled-Environment Chamber Used To Afford Silica/Surfactant Mesostructured Films at Different I and RHa

a The initial liquid film composition includes PEO-b-PPO-b-PEO block copolymer (P123), PAG, and polydimethoxysiloxane (PDMOS) oligomer as silicon source.

and cyclic structures. Further details regarding its structure are available in a separate reference.21 Double face polished silicon wafers (100) from Siltronix were used as substrate. Before film deposition, the substrate was washed successively with acetone and ethanol and gently dried. Synthesis of Surfactant−Silica Mesostructured Hybrid Films. A photosensitive and homogeneous solution was prepared by mixing PAG, P123, and PDMOS in the typical weight ratio 0.02/0.5/1. Although the resultant mixture was stable over months without exposure to UV, the solution was generally deposited immediately after preparation (without aging) onto silicon wafer using a bar-coater. A nonvolatile liquid film was formed exhibiting a thickness of 2−3 μm assessed by profilometry measurements. The irradiation was performed in a controlled-environment chamber where temperature and RH were finely adjusted, as illustrated in Scheme 1. Standard experiments were carried out at 25 °C, and a saturated NaCl solution was used to produce various equilibrium RHs inside the irradiation chamber. In a typical system, the air to be humidified was provided by a diaphragm pump. The air flows through the salt solution contained in a first jar, and the humidified gas then passes through a tube into the controlled environment chamber containing the sample and finally exits through a second tube in a jar containing a hygrometer controlling the RH. Changing the gas flow rate through the chamber was found to be an easy and reliable way to control precisely the equilibrium atmosphere (20−60%). Once the desired RH condition was reached, it was maintained constant throughout the whole process. The chamber includes at the top a UV transparent BaF2 window exactly in the same axis as the film position, which enables the vertical irradiation of the sample. Illumination was performed through the focused light of a conventional medium-pressure mercury lamp coupled with a flexible quartz light guide. Films were irradiated during 600 s by the polychromatic light of a mercury−xenon lamp

rational way toward the design of experiments, in order to predict which mesostructure forms under certain conditions. In addition, the simple synthesis conditions provide a wealth of information about the kinetic and thermodynamic rules regulating the self-assembly in silica/surfactant systems.31 By determining the water content inside the film, the RH can impact the hydrophilic/hydrophobic contrast, which is one of the main thermodynamic forces driving nanophase segregation and the formation of ordered mesophase arrays. The effect of the irradiance on the condensation rates illustrates, on the other hand, that gelation kinetics can mediate the transition from a short-range order phase to organized crystalline mesophases. Using a specially developed irradiation setup (Scheme 1), photopolymerization experiments of the alkoxysilane/PAG/ template film can be conducted under controlled I (35−200 mW/cm2) and RH (20−60%). Once the conditions to achieve periodically ordered silica−surfactant mesostructures have been found, the final mesophase (hexagonal, lamellar, cubic) is tailored upon varying the template/inorganic ratio. A last part clarifies the film mesostructuring mechanism that has proven to be very different from that of the EISA method.32

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EXPERIMENTAL PART Materials. The amphiphilic block copolymer (Pluronic P123, (PEO)20-b-(PPO)70-b-(PEO)20) was kindly provided by BASF and used as received. The PAG (Ph2I+PF6−) was purchased from Sigma-Aldrich. Polydimethoxysiloxane (PDMOS) is a nonhydrolyzed oligomer silicate precursor derived from tetramethoxysilane provided by ABCR. The 29Si liquid NMR spectrum of PDMOS exhibits four signals attributed to Q0 (Si-(OR)4, 1.8%), Q1 ((RO)3-Si-(OSi), 49.7%), Q2 ((RO)2-Si-(OSi)2, 44.4%), and Q3 ((RO)-Si(OSi)3 4.1%). However, it is difficult to provide a precise structure as PDMOS consists in a mixture of linear, branched, 4960



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(Hamamatsu, L8251, 200 W). The end of the optical guide was placed at a distance of 3 cm from the film and directed at an incident angle of 90° onto the sample. A precise adjustment of the emitted irradiance from 35 to 200 mW/cm2 was also ensured. The irradiance I, measured in mW/cm2, is defined as the total photon flux emitted in all the directions by surface area unit of the emitting source. The irradiation device was equipped with an elliptical reflector (365 nm) that efficiently reflects the UV light and lets heat rays and visible light pass through, preventing any significant increase of temperature. Using this cold reflector cutting off heat, the local temperature during the process may be increased of only 3−4 °C. The described synthesis procedure afforded chemically resistant surfactant−silica films. In contrast, fresh films arising from EISA are often made up of low-condensed molecular species, and therefore a consolidation step is often needed to increase inorganic condensation. In the dynamic study described in the Mesostructuration Mechanism section, films were characterized at different irradiation times. After the UV irradiation was stopped, the samples were left in the environmental cell for 10 min, and then X-ray diffraction (XRD) and Fourier transform infrared (FTIR) measurements were performed subsequently. For certain experiments, films were calcined at 250 °C for 4 h under air atmosphere using a heating rate of 1 °C min−1 so as to pyrolyze the surfactant. Characterization. XRD patterns of the as-prepared and calcined samples were acquired on a Philips X’pert Pro (PANalytical) diffractometer using Cu Kα radiation (λ = 0.154 18 nm; 0.5° < 2θ < 10°; 0.02°/s). TEM micrographs of the films were taken with a Philips CM200 microscope working at 200 kV. Prior to observation, the film was detached from the substrate and dispersed into water using ultrasounds and a few drops of the suspension were deposited at the surface of a copper observation grid. In FTIR spectra were acquired on irradiated films included in the environmental cell. The transmission FTIR spectra were obtained with a resolution of 4 cm−1 with a Bruker Vertex 70 spectrophotometer equipped with a liquid-nitrogen-cooled mercury−cadmium telluride detector. The hydrolysis kinetics of the methoxysilyl functions were determined using the symmetric CH3 stretching vibration which appears as an isolated and sharp band decreasing gradually in absorbance throughout UV irradiation.

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Figure 1. Effect of the RH (%) on the XRD patterns of as-irradiated films (PDMOS/P123/PAG: 1/0.5/0.02 weight ratio) under I = 200 mW/cm2 for 600 s.

At a low RH value of 20%, the as-synthesized film exhibits a featureless XRD pattern consistent with an amorphous blend of copolymer chains and silica phase. Upon increasing RH to 30%, a single diffraction peak at low 2θ angle (0.75°−1.25°) appears. This latter is attributed to randomly distributed worm-like mesostructures devoid of long-range order but with a constant correlation distance.36,37 A further increase of RH has the dramatic effect of causing a “disorder-to-order” transition. Indeed, four peaks are thus observed above a threshold RH of 35%, indexed as the (100), (200), (300), and (400) reflections of a lamellar symmetry lattice. The d-spacing deduced from this pattern is 9.4 ± 0.2 nm, which is in agreement with the typical lattice parameter values encountered with P123 surfactant mesophase (dspa ≈ 10 nm).38 The samples prepared at greater RH (40% and 60%) display the same reflection peaks with only small variations in their intensities. The three TEM images displayed in Figure 2 support the conclusions deduced from the XRD data. Upon increasing RH, the progressive change from amorphous samples (20%) to wormlike mesostructures (30%) and finally well-ordered lamellar mesostructures (45%) is evident. To confirm this morphology, we mainly rely on the disappearance of the main XRD signals when these samples are calcined (see Figure S1 of the Supporting Information). Such collapse caused by the removal of surfactant provides a strong indication that the stripes are well the signature of a layered structure forming a stack of lamellae. Unlike powders, mesostructured films are known to present an anisotropic orientation induced by the substrate.39,40 Thus, 2D hexagonal structures with an orientation of the mesochannels parallel to the substrate can also give rise to similar morphology. However, this latter mesostructure is able to withstand the calcinations process.41,42 The first striking observation is the strong dependence of the mesostructuration on RH. The second major effect related to water is to drive the regular assembly of the copolymer chains in a lamellar mesophase. Under dry conditions (RH = 20%), the hydrolyzed silica phase is not sufficiently selective for the PEO blocks to induce a block segregation of the hydrophobic PPO block. Self-assembly is thus hampered by inappropriate solvation conditions in particular dehydration conditions, leading to amorphous structures. Increasing the RH up to 30% creates a water enrichment of the silica phase, which becomes more selective for the hydrophilic block since water is nonsolvent of the PPO block. The resultant phase segregation is reflected, first, by well-defined wormlike mesochannels that remain nevertheless disordered. Higher RH values lead subsequently to a disorder-to-order with the onset of ordered

RESULTS AND DISCUSSION

Synthesis of Periodically Mesostructured Silica Films Using UV Light. Effect of RH: Thermodynamic Control. It should be noticed that the initial film composition does not include water or any volatile compound, but a homogeneous mixture of silicon alkoxide precursor, nonionic surfactant, and PAG (PDMOS/P123/PAG: 1/0.5/0.02 weight ratio). Hence, the RH inside the chamber represents the only means to control water concentration within the film. A high RH implies a water-saturated film with a preferential location of water molecules in the hydrophilic parts (the PEO block and the silica network formed after UV irradiation through H-bonding) while a low RH favors water depletion. The role of water is thus threefold: to promote hydrolysis, to increase the inorganic/ surfactant volume ratio, and to enhance the mesophase curvature.33,34 As a result, RH is expected to be a critical factor for mesophase formation and ordering.35 Figure 1 shows the consequences of varying the RH from 20 to 60% on the XRD patterns of films obtained at constant I (200 mW/cm2). 4961



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Figure 2. TEM images of as-synthesized films prepared at variable RH (20%, 30%, and 45%). PDMOSP123/PAG: 1/0.5/0.02 weight ratio, t = 600 s, I = 200 mW/cm2.

In agreement with the previous experiments, a wormlike disordered mesostructures is created at a high irradiance of 200 mW/cm2, as exemplified by a small and broad peak at 2θ = 1.05°. However, clear signs showing the appearance of ordered mesophases appear upon decreasing the irradiance. At I = 150 mW/cm2, the XRD pattern exhibits already two (100) and (200) reflections indicating that a periodic lamellar mesostructure is now forming (for the mesophase confirmation, see also the TEM picture in Figure S2 of the Supporting Information). For I ≤ 50 mW/cm2, two additional (300) and (400) orders of reflection becomes visible translating a more uniform lamellar packing. In addition, the formation of a highly ordered mesostructure is accompanied by a slight decrease of the d-spacing: 8.9 ± 0.1 nm at I ≥ 150 mW/cm2 to 8.1 ± 0.2 nm at I ≤ 100 mW/cm2. The higher mobility resulting from a better organization allows the inorganic network to be more condensed, thus causing a slight contraction of the interlayer distance. The role of I on mesophase ordering can be understood on the basis of two coexisting and competitive processes: the template ordering rate (kord) and the condensation rate (kcond). A low irradiance causes a slow gelation process (kord > kcond), leaving more time for the block copolymer to self-assemble into higher organized mesophases. In contrast, a too fast condensation at high irradiance (kcond > kord) may disturb the self-assembly. The presumed fact is that if mesostructuration does not occur sufficiently early, the relative randomness and high stability of the siloxane network formed are likely to preclude any possibility for organization.46 Indeed, one of the major aspects of the EISA process, in terms of sol−gel chemistry, is to work under acid conditions where the condensation rate of the silanol species is known to be minimized.47 The goal is not to frustrate the self-assembly by gelation that would kinetically trap the system in a transient nonordered state. Also noteworthy is that the described kinetically controlled mechanism is not systematic, but essentially valid at moderate RH (30%). At a higher RH of 50% for example, the mesostructuration becomes again thermodynamically driven since a long-range organization can be achieved regardless of the value of I (Figure S3 of the Supporting Information). Revealing P123 Polymorphism. In principle, three canonical mesophases are encountered with the binary P123−silica hybrid system: lamellar, 2D hexagonal, and cubic.38 The template/silica volume ratio is known as a straightforward variable to control thermodynamically the film mesostructure. Low concentration in P123 generally yields Im3m cubic mesostructure with high curvature. By contrast, medium content in template generates preferentially 2D hexagonal (p6

lamellar mesodomains. In agreement with this progressive structuration and ordering sequence, several studies in dilute solutions have demonstrated that block copolymer micellization is strongly dependent on the ability for the solvent to swell selectively certain blocks and segregate others.43 In the case of Pluronics, the selectivity relies on the strength of interaction (hydrogen bonding) of the silica phase and adsorbed water molecules with PEO chain segments. In other words, as the RH increases, the number of hydrogen bonding moieties increases because of a higher concentration in water, and the efficacy in promoting segregation/ordering of copolymer is thus enhanced. An analogy of the role of water can be made with that of small molecule additives bearing hydrogen-bonding sites for PEO that can induce an disorder−order transition.44,45 On the other hand, other authors have interpreted the effect of a higher RH (higher water concentration) on nonthermodynamic grounds.18 They postulated that a greater dilution of the silica and surfactant species might result in a more fluid environment suitable to mesophase rearrangement and the subsequent formation of well-ordered mesostructures. Effect of the Irradiance: Kinetic Control. Although RH can affect hydrolysis and condensation rates, its impact on mesophase ordering can be reconciled here with a thermodynamically driven self-assembly. To gain a complete picture of this photoinduced system, we have investigated whether a kinetic parameter such as I can induce, in a similar way, an ordering effect. One advantage of our process is that sol−gel kinetics are photochemically controlled. Thus, any change of irradiance may slow down or accelerate the PAG photolysis rate with a direct effect on catalyst concentration and reaction rates. Figure 3 shows the XRD patterns of as-irradiated films obtained at various I values ranging from 35 to 200 mW/cm2, while maintaining the RH value constant at 30%.

Figure 3. XRD patterns of as-irradiated P123/silica films prepared under different irradiances. PDMOS/P123/PAG: 1/0.5/0.02 weight ratio, t = 600 s, RH = 30%. 4962



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mm) structure while lamellae are formed at higher concentration. To foster this polymorphism, three silica/surfactant hybrid films including various copolymer/silica weight ratios (x = 0.25, 0.35, and 0.50) have been prepared and analyzed by XRD (Figure 4). In order to promote self-assembly and mesophase ordering, these experiments were performed under optimized conditions as regards to I (35 mW/cm2) and RH (50%).

nonlamellar structure is formed. At a lower P123 concentration (x = 0.25), it seems that a cubic mesostructural ordering takes place. Our interpretation is based on TEM observation and the XRD pattern of the as-irradiated films showing only a single intense peak in the 2θ range of 1°−1.2° indexed as the (200) reflections of the Im3m cubic space group. Interestingly, we find with this photoinduced sol−gel process the typical phase sequence lamellar → 2D hexagonal → cubic, which is fully consistent with the thermodynamics of amphiphilic aggregation of P123 in solution.48 Insight into the Mesostructuration Mechanism. Like the EISA method, our photoinduced process gives a prominent role to template self-organization to segregate the silica species into ordered hydrophilic mesodomains. However, the absence of silica sol, water, and solvent in the initial formulation suggests a substantially different mechanism. In a view to shed light on the mesostructuring process, we have investigated the temporal evolution of the XRD patterns throughout the irradiation, as shown in Figure 6. For clarity reasons, experimental conditions (x = 0.50, RH = 50%, and I = 35 mW/cm2) have been chosen to promote the formation of model lamellar mesostructures.

Figure 4. Influence of the P123/PDMOS weight ratio (x) on the XRD patterns of the as-synthesized silica/surfactant hybrid films. PDMOS/ P123/PAG: 1/x/0.02 weight ratio, t = 600 s, RH = 50%, I = 35 mW/ cm2.

The XRD pattern of the x = 0.50 sample exhibits a sequence of very sharp low-angle diffraction peaks suggestive of a lamellar mesostructure. The first intense peak (100) at 2θ = 1.06° corresponds to the interlayer distance, which was evaluated to be 8.3 nm. At higher angles, the other peaks are attributed to second (200), third (300), and fourth (400) order diffractions. Our attribution was confirmed by TEM (Figure 5) showing stacks of sheets well indicative of a lamellar phase disappearing after the surfactant removal. For the x = 0.35 sample, five peaks are observed which could be indexed as the (100), (110), (200), (210), and (300) reflections of a 2D hexagonal structure. In contrast to the (110) peak, the (100), (200), and (300) diffractions are highly intense, suggesting a preferential orientation of the channels parallel to the substrate. Accordingly, the TEM image of this film shows a mixed arrangement of hexagonally packed mesostructures and a high density of stripes suggesting the alignment of mesochannels parallel to the substrate. To support this interpretation, the influence of calcination on the film structure was examined. Accordingly, XRD analysis (Figure S4 of the Supporting Information) demonstrated the persistence of the mesostructure after calcination, providing the direct evidence that a

Figure 6. XRD patterns of as-synthesized silica films exposed to various irradiation times ranging from 0 to 600 s. PDMOS/P123/ PAG: 1/0.5/0.02 weight ratio, RH = 50%, I = 35 mW/cm2. The subscript “a” refers to the proto-lamellar phase arising from the selfassembly of hydrolyzed species, while the subscript “b” corresponds to the same lamellar phase which has been subjected to condensation reactions.

Prior to irradiation (t = 0 s), the precursor film does not exhibit any diffraction peak, which indicates that the PDMOS/ P123/PAG mixture is isotropic and does not contain any preformed surfactant mesophase. Thus, nonhydrolyzed

Figure 5. TEM images of the “as-prepared” silica films containing various P123/PDMOS weight ratios (x = 0.25, 0.35, and 0.50). PDMOS/P123/ PAG: 1/x/0.02 weight ratio, t = 600 s, RH = 50%, I = 35 mW/cm2. 4963



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PDMOS must be regarded as a good solvent of both PEO and PPO blocks inhibiting the block copolymer self-assembly. This result clearly evidence that UV light is a unique and effective trigger to mesostructuration and polymerization. After only 15 s of irradiation, the appearance of three (100), (200), and (300) reflection peaks marks the amorphous-to-order transition and the onset of a periodic P123/silica lamellar mesophase displaying a d-spacing of 9.2 nm. The intense and narrow (100) peak indicates that highly ordered mesostructures have been already generated at this early stage. A hydrolysis degree reaching 70% is found by FTIR analysis (see Characterization section for details), illustrating that the emergence of silanol species participate in the self-assembly process. A complete hydrolysis of the methoxysilyl functions is achieved by continuing the irradiation until 30 s, and the corresponding XRD pattern shows in this case a two-maximum peak in the 1°−1.5° 2θ range. The deconvolution of this broad signal (not represented) makes clearer the formation of a second lamellar mesophase with a smaller interlayer of 8.2 nm compared to the first lamellar mesophase (d = 9.2 nm). At t = 45 s, the progressive disappearance of the low angle peak (2θ = 0.9°) and the concomitant intensification of the second peak at larger angle (2θ = 1.1°) signifies the transformation (contraction) of the incipient lamellar structure. At t = 60 s, the proto-lamellar phase has disappeared, and the XRD pattern shows a single lamellar phase with a stable interlayer distance of 8.2 nm. From these XRD data, it is apparent that the onset of mesostructuration must lag behind the photogeneration of hydrolyzed species. Exposition to UV light liberates Brönsted superacids catalyzing hydrolysis reactions. The polarity increase resulting from the conversion of hydrophobic alkoxy precursors to hydrophilic silanol species drives the surfactant self-assembly. It seems that the reaction proceeds via a rapid preliminary hydrolysis of PDMOS to form poorly condensed transient silanol species. As shown in the XRD pattern obtained at t = 15 s, this creates suitable solvation conditions for the formation of a proto-lamellar structure. Additionally, the emergence of two distinct lamellar phases at t = 30 and 45 s reveals significant insight into the self-assembly. Their evolution is consistent with a two-step mechanistic sequence decoupling hydrolysis and condensation reactions. First, transient mesostructures surrounded by poorly condensed hydrophilic silica species are generated, which can subsequently undergo Si−O−Si condensation in a manner which is compliant with a preorganized template. Such mechanism is apparent from the disappearance of a proto-lamellar mesophase (dspa ≈ 9.2 nm) and the formation of a more cross-linked and contracted mesostructures (dspa ≈ 8.2 nm). The estimated Δdspa of ca. 1 nm between the two mesophases is totally in agreement with a shrinkage of the silica network due to more extended condensation reactions.49 Furthermore, the condensation of the silica network can be evidenced from the bands in the 1000−1250 cm−1 IR region mainly assigned to Si−O−Si antisymmetric stretching modes (Figure 7). Their gradual intensification with irradiation time is the sign that condensation occurs between Si−OH groups. In contrast to hydrolysis completed in just 30 s, condensation reactions proceed more slowly and progressively throughout the irradiation (600 s). As a result, these FTIR data tend to confirm a two-step sequence separating hydrolysis and condensation.

Figure 7. Temporal evolution of the IR spectra in the range 1000− 1300 cm−1. PDMOS/P123/PAG: 1/0.5/0.02 weight ratio after various irradiation time: () 30, (- - -) 45, (···) 60, and (- · -) 600 s, RH = 50%, and I = 35 mW/cm2.

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CONCLUSION We have developed and optimized a UV-mediated pathway to surfactant templated periodic mesostructured films. The processing conditions are critical to successfully self-assemble the amphiphilic block copolymer and order the resulting mesophases over large surface area. The local irradiation of films in an environmental chamber enabled to evidence the structuring effect of two key parameters: relative humidity and irradiance. It is believed that higher humidity conditions allow the two blocks of the copolymer to self-assemble in separate and ordered domains since water is a nonsolvent for PPO but a good solvent for PEO. The disorder-to-order phase transformation is also sensitive to condensation kinetics. Decreasing the irradiance slow downs the photoacid formation rate in a way to permit the reorganization of the organic surfactant species into highly ordered mesophases. XRD combined with FTIR was employed to study the light-induced self-assembly mechanism. Noteworthy is the role of the in-situ photogenerated silanol species in the generation of mesophases. In absence of solvent, these species apparently serve as nonvolatile hydrophilic fluids that replace water to induce the mesostructuration. By studying model lamellar silica/surfactant films, a twostep mechanistic sequence was evidenced: transient mesostructures arising from hydrolyzed species are first created and fixed subsequently by condensation reactions.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns of as-irradiated (dotted line) and calcined (solid line) P123/silica films (Figure S1); TEM images of as prepared surfactant/silica films under two different irradiances: 200 mW/ cm2 (A) and 150 mW/cm2 (B) (Figure S2); XRD patterns of as-prepared silica films prepared at various irradiances (35−200 mW/cm2 under RH = 50%. (Figure S3); XRD patterns of asirradiated and calcined P123/silica films. PDMOS/PAG/ P123:1/0.02/0.35 weight ratio, RH = 50%, I = 35 mW/cm2 (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +33 3 8933 5030; Fax +33 3 8933 5034 (A.C.). Notes

The authors declare no competing financial interest. 4964



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