Block Copolymer Self-Assembly in Mesostructured Silica Films

Article pubs.acs.org/Langmuir

Block Copolymer Self-Assembly in Mesostructured Silica Films Revealed by Real-Time FTIR and Solid-State NMR Héloïse De Paz-Simon,† Abraham Chemtob,*,† Céline Croutxé-Barghorn,† Séverinne Rigolet,‡ Laure Michelin,‡ Loïc Vidal,‡ and Bénédicte Lebeau‡ †

Laboratory of Macromolecular Photochemistry and Engineering and ‡Institute of Mulhouse Material Science, CNRS, LRC 7228, University of Haute-Alsace, ENSCMu, 3 bis rue Alfred Werner, 68093 Mulhouse Cedex, France S Supporting Information *

ABSTRACT: Over the past ten years, understanding the selfassembly process within mesostructured silica films has been a major concern. Our characterization approach relies on two powerful and complementary techniques: in situ time-resolved FTIR spectroscopy and ex situ solid-state NMR. As model systems, three silica/surfactant films displaying various degrees of mesostructuration were synthesized using an amphiphilic block copolymer (PEO-b-PPO-b-PEO) via a UV light induced self-assembly process. The key idea is that the hydration state of the hydrophobic PPO chain is expected to be different depending upon whether the sample is amorphous (blend) or mesostructured (segregated). With real-time FTIR experiments, we show that the methyl deformation mode can act as a signature for the PPO microenvironment so as to trace the progressive copolymer self-association throughout the irradiation time. In 1H solid-state NMR, the dependence of the 1H chemical shift on the PPO hydration state has been exploited to evidence the extent of mesostructuration.

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INTRODUCTION Silica/surfactant self-assembly is well recognized as the driving force underlying the formation of porous mesostructured silica films.1,2 Despite the wealth of structural characterization techniques,3 few are actually applicable to the investigation of the self-assembly process. Recently, spectroscopic analyses (fluorescence, 4 luminescence 5 ), X-ray scattering (2DGISAX,6−8 SAXS9,10), and reflectometry8,11 techniques allowed in situ probing of transient mesophase formation. Obviously, there is a clear need for complementary and more accessible methods to complete the description of the self-organization mechanism, in particular, for its evolution from a laboratory practice to a scalable tool applicable in a range of nanotechnological applications.3,12 The present paper highlights two powerful techniques, scarcely reported in this literature: timeresolved FTIR spectroscopy and 1H solid-state NMR. Their combination aims to provide an original picture of the selfassembly taking place during the formation of surfactant templated silica films. 1. Real-time FTIR (RT-FTIR) has been already implemented as in situ characterization technique in mesostructured silica films,8,13−17 but its potential has been limited so far by the processing conditions. In most cases, an evaporation induced self-assembly (EISA) approach is described,18,19 in which both surfactant and hydrolyzed inorganic silica precursors prepared in alcohol/water solvents are cooperatively self-assembled at the surface of the substrate through the evaporation of the solvent. The EISA’s success rests on its rapidity, the use of © 2013 American Chemical Society

conventional processing methods, and the ability to control the final mesostructure.1 Nevertheless, the real bottleneck is that self-association takes place concomitantly with evaporation (∼1 min), which makes its investigation particularly tricky. For such evaporating systems, RT-FTIR has been mainly used to monitor solvent evaporation,15 dynamic water concentration,13 and silica network condensation,8,14,16 providing limited insight into surfactant micellization and ordering. In a previous work, we introduced an alternative light induced self-assembly (LISA) route circumventing the addition of volatile compounds in the starting formulation.20 Under these conditions, the use of RTFTIR is both simplified and its interest greatly enhanced. The essential chemistry lies on the use of a photoacid generator dissolved in a mixture of alkoxy silica precursor and amphiphilic block copolymer (PEO-b-PPO-b-PEO) to form a photolatent homogeneous film. The subsequent UV irradiation liberates Brönsted superacids responsible for a photoacid-catalyzed sol− gel process, and driving the block copolymer micellization. It is well-known that the interactions between the solvent and the different blocks of the copolymer dictate the ability to form well-defined structures.21 In our case, the photoinduced formation of low-polymerized hydrophilic silicate species causes presumably a polarity increase, creating solvation conditions suitable for the block copolymer self-assembly. MesostructuraReceived: July 30, 2012 Revised: December 21, 2012 Published: January 11, 2013 1963

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micrographs of the calcined films were taken with a Philips CM200 microscope working at 200 kV. Prior to observation, the powdered film was dispersed into water with ultrasounds and a few drops of the suspension were deposited at the surface of a copper observation grid. In real-time FTIR experiments, the formulations were simultaneously exposed to UV light and to an IR analytical beam. Such a technique is of interest to assess the sol−gel kinetics throughout the UV irradiation. Infrared spectra obtained in transmission were recorded with a Bruker Vertex 70 spectrophotometer equipped with a liquid-nitrogen-cooled mercury−cadmium telluride (MCT) detector. The spectra were recorded every 0.12 s during the first 18 s, then every 5 s from 18 to 600 s, using a resolution of 2 cm−1. Deconvolution of spectra in the range 1400−1360 cm−1 was realized with OPUS 6.5 software using only 100% Gaussian curves. All spectra were baseline corrected prior to integration and deconvolution. 1H (I = 1/2) MAS NMR experiments were performed on a Bruker Avance II 400 spectrometer with a double channel 2.5 mm Bruker MAS probe, a spinning frequency of 30 kHz, and a π/2 pulse duration of 3.3 μs. 1H spin− lattice relaxation times (T1) were measured with the inversion− recovery pulse sequence for each sample, and a recycling delay of 8 s ensured the quantitative determination of the proportions of the different type of protons species. Typically, 64 scans were recorded. Deconvolution of the spectrum was performed using Dmf it software.28

tion implies a nanophase segregation of the hydrophilic (PEO, silica) and hydrophobic (PPO) components. Of high interest is that FTIR can be sensitive to local polarity and hydration states,22 and their variations have been successfully used as indicators of micellization. Herein, we show the potential of rapid-scan FTIR to correlate compositional information with mesostructure formation at a high resolution time (0.12 s). 2. The second technique addressed in this feature is solidstate NMR. Several solid-state NMR experiments were conducted previously to establish the spatial separation of the two blocks in copolymer-silica self-assembled systems23,24 using proton spin diffusion,25 proton relaxation,26 and 2D 1H−13C HETCOR technique.26 Here, we have focused on the acquisition and analysis of simple 1D high-resolution 1H spectra. Through its marked sensitivity to hydration state (local environment), we show that this straightforward spin-1/2 nucleus can be used to probe the extent of nanophase segregation, and indirectly assess the level of mesostructuration.

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EXPERIMENTAL SECTION

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Film Preparation. Amphiphilic block copolymer (Pluronic P123, (PEO)20-b-(PPO)70-b-(PEO)20) was kindly donated by BASF and used as received. In a typical synthesis performed at room temperature, a variable amount of copolymer template (25, 50, and 75 wt %) was dissolved in polydimethoxysiloxane (PDMOS, ABCR) prior the addition of a PAG (Φ2I+ PF6−, 4 wt %, Sigma Aldrich). The resultant homogeneous solution was found to be stable in the absence of UV light. PDMOS is a nonhydrolyzed oligomeric silicate precursor derived from tetramethoxysilane (TMOS); further details regarding its structure and sol−gel photopolymerization have been published elsewhere.27 The initial formulation (alkoxide/PAG/copolymer) was deposited onto a glass substrate using an Elcometer 4340 automatic film applicator equipped with a wire wound bar to form a nonvolatile liquid film with an initial thickness of 10 μm. The UV insulation was carried out at room temperature under a UV conveyor using a microwavepowered mercury vapor lamp (H bulb, Fusion). The spectral output of this electrodeless microwave UV lamp is relatively similar to that of a conventional medium-pressure mercury lamp. The belt speed of the conveyor was set at 10 m/min and the lamp intensity at 100%. In these conditions, for each pass, the UV exposure time is 0.23 s and the emitted light dose is 1.46 J/cm2. (UVA [320−390 nm]: 0.45 J/cm2, UVB [280−320 nm]: 0.42 J/cm2, UVC [250−260 nm]: 0.09 J/cm2, and UVV [395−445 nm]: 0.5 J/cm2). Samples were subjected to 10 successive passes under the conveyor to yield transparent hybrid solid films. During UV irradiation, the relative humidity (RH) was carefully monitored to be in the range 27−33%. Film calcination was performed without preliminary hydrothermal treatment at 480 °C for 4 h in air. For specific time-resolved FTIR experiments, the formulations were dispensed onto a BaF2 substrate using a bar coater to produce films of similar thickness (10 μm). The in situ IR analysis was performed in transmission configuration, simultaneously with the UV irradiation triggering the polymerization process (see the Characterization section for further details on the spectrophotometric setup). In this case, the films were irradiated at a light intensity of 200 mW/cm2 by the polychromatic light of a mercury−xenon lamp (Hamamatsu, L8251, 200 W) fitted with a 365 nm reflector and coupled with a flexible lightguide. 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 window. The sample was maintained in a horizontal configuration with the same composition and experimental conditions as those photopolymerized under the UV conveyor. During this series of experiments, the RH was also carefully maintained between 27% and 33%. Characterization. XRD patterns of the 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

RESULTS AND DISCUSSION 1. Proof of Mesostructuration from X-ray Diffraction and TEM. As represented in Figure 1, the combined

Figure 1. XRD patterns and corresponding TEM image of calcined samples containing various P123/PDMOS weight ratios: x = 0.75 (solid line); x = 0.50 (dashed line); and x = 0.25 (dotted line).29.

characterization of the samples by XRD and TEM reveal a strong dependency between the level of mesostructuration and the concentration in surfactant. At 50 wt % P123, the XRD pattern shows clearly a broad single diffraction peak at low angle with a d-spacing of 8.2 nm. As confirmed by TEM, such a pattern is typical of a wormlike mesostructure devoid of longrange crystallographic order.30−32 Further increase in P123 content (75 wt %) produces a film with a more intense and narrow XRD peak, indicative of an increased phase segregation and degree of mesostructuration, also visible in the TEM picture. By contrast, the sample possessing 25 wt % P123 lacks any discernible mesoscopic feature, reflecting an amorphous hybrid sample. In conclusion, the mesostructuration is 1964

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evidenced above a threshold concentration of ca. 50 wt % P123 copolymer. In addition, the condensation degree of these three different films was estimated by deconvolution and integration of the Qn species from the 29Si MAS NMR spectra (not represented). Values of 76%, 81%, and 85% were obtained for sample containing, respectively, 25, 50, and 75 wt % P123, reflecting the efficiency of the photoacid-catalyzed sol−gel process. Despite the absence of long-range crystallographic order, these three systems with variable degrees of mesostructuration are of high interest as simple models to investigate the self-assembly mechanism in block copolymer/ silica hybrids using RT-FTIR and solid-state NMR. 2. Time-Resolved FTIR. Time-resolved FTIR was proved to be a suitable method to monitor in situ the compositional changes accompanying the condensation of hydrolyzed precursor species in mesostructured silica films.8,13−16 Here, this vibrational spectroscopy is used for the first time to provide insight into the self-assembly pathway itself, by exploiting its sensitivity to local environment and inter- or intramolecular interactions. The key feature is the hydration state change of the amphiphilic copolymer in the course of the structuration process. P123 has hydrophilic ethylene oxide (EO) groups, which can accommodate water molecules and silica by forming hydrogen bonds (O−H···O) and hydrophobic propylene oxide (PO) moieties. In the described procedure, P123 is initially dissolved in an unhydrolyzed methoxy oligomeric precursor (PDMOS), which represents an isotropic apolar environment for both blocks. After irradiation, two possible fates must be considered with regard to the hydration state of the PPO block: an effective self-assembly should drive a phase separation preventing their hydration process, since the hydrophobic PPO environment (micellar core) becomes isolated from the waterrich PEO/silica phase. In contrast, the formation of an amorphous silica-copolymer structure implies a molecular colocalization of the PPO block and silica phase, leading to a substantial hydration of apolar sites with the formation of Hbonds between alkyl groups and water (C−H···O−H).33 Several studies have already demonstrated that alkyl groups’ hydrophobic hydration could induce a blue-shift of the C−H stretching bands. The band shift was found to be large enough (Δυ̅ ≈ 20 cm−1) to study thermally induced phase transition of alkyl-based polymers (poly(vinyl methyl ether)22,34 and poly (N-alkyl acrylamide)35 or quantify critical micellar temperature36 and concentration37 of amphiphilic copolymers in water. These early developments suggest that specific IR bands sensitive to water−alkyl interactions may represent a powerful diagnostic marker for investigating the phase separation behavior in more complex systems such as copolymer surfactant templated silica mesophases. Instead of the C−H stretching vibrations (2800−3050 cm−1) which are the most reported in the literature, we rather focused on the CH3 symmetric deformation mode (δ(CH3) ≈ 1374 cm−1) uniquely representative of the PPO hydrophobic block (not overlapped by the IR modes of the OSi-CH3).38 FTIR spectra profiles of nonmesostructured (x = 0.25) and mesostructured (x = 0.75) films in the region 1355−1400 cm−1 at different irradiation times are reported in Figure 2. In both cases, prior to the UV irradiation the band is centered at 1374 cm−1, indicating an apolar environment39 consistent with the close contact between the PPO block and the nonreacted PDMOS precursor. Accordingly, we note an exact match in terms of band position with pure PPO homopolymer and P123 (not represented). Depending on

Figure 2. FTIR spectra evolution for P123-templated silica film as a function of increasing irradiation time in the 1360−1400 cm−1region: (A) amorphous film (x = 0.25) and (B) mesostructured film (x = 0.75).

template concentration, we observe upon UV irradiation noticeably different band shifts. For the non-mesostructured film, a gradual broadening and shift from the initial 1374 to 1379 cm−1 is visible, while this red-shift is almost not apparent for the mesostructured sample (Δυ̅ ≈ 1 cm−1). First, these shifts can be reconciled with a medium becoming more hydrophilic, which is consistent with the replacement of methoxysilyl by silanol functions and the uptake of water into the silica network. Above all, the difference of scenario must be understood in terms of the greater hydration of the PPO segments with H-bonding interactions (C−H···O−H) in the amorphous samples. Additional evidence for the ability of the methyl deformation band to act as a signature for the PPO hydration states and indirectly to trace/monitor the copolymer self-association is provided by its deconvolution for three different weight concentrations in surfactant (x = 0.25, 0.50, and 0.75), as sketched in Figure 3. In all cases, three distinct spectral features appear at 1382, 1376, and 1371 cm−1, which can be related to distinct PPO environments. Following the assignment of Liu et al.,37 the most red-shifted band at 1382 cm−1 is ascribed to hydrated methyls (hydrophobic hydration, δ(h-CH3)), the second at 1371 cm−1 to dehydrated methyls (hydrophobic interactions, δ(dh-CH3)), and the intermediate at 1376 cm−1 to interfacial methyls in a mixed environment δ(int-CH3). The diagram in Scheme 1 depicts the local and mesoscopic structure of the copolymer−silica hybrid system with a multiplicity of PPO environments consistent with these FTIR data. Furthermore, we quantitatively analyze in Figure 3 the evolution of δ(h-CH3) and δ(dh-CH3), by plotting the variation of their integrated absorbance during the irradiation time. Of high significance is the possibility to also have in the same graphic the temporal evolution of hydrolysis. Practically, the strong peak maxima at 2840 cm−1 assigned to the −CH3 symmetric stretching mode absorption (νsym(CH3)) is ideal to 1965

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Figure 3. (Up) Evolution of the CH3 symmetric deformation mode (δ(CH3), 1360−1400 cm−1) before (dashed line) and after (solid line) UV irradiation at various weight concentrations in copolymer. (A) x = 0.25, (B) x = 0.50, and (C) x = 0.75. Deconvolution of the FTIR absorption spectrum for the as-irradiated sample is also shown. (Down) Relative evolution of the integrated absorbance of two components of the δ(CH3) band: δ(h-CH3) (blue ■, hydrated CH3, 1382 cm−1) and δ(dh-CH3) (red ▲, dehydrated methyl, 1371 cm−1) at three different weight concentrations in copolymer: (A) x = 0.25, (B) x = 0.50, and (C) x = 0.75.

Scheme 1. Silica-Block Copolymer Microenvironment in a Worm-Like Mesostructure Showing the Local Separation of the PEO and PPO Blocksa

a

A majority of the PPO chains are localized in a dehydrated segregated micellar core (dh-PPO), whereas a minor part of the PPO is concentrated in interfacial regions separating the silica matrix and the hydrophobic core (Int-PPO). The non-phase-separated copolymer chains in close proximity with the silica network (in minority in mesostructured films, whereas in majority in amorphous blends) are assumed to be highly hydrated (h-PPO).

with the low level of mesoscopic organization reported with this sample by the XRD and TEM data. As the sol−gel reaction occurs, the δ(dh-CH3) decreases reflecting the gradual change of environment from methoxy silicate −(SiO)x(OMe)y to hydrophilic silicate species −(SiO)x(OH)y. Due to the incapacity of P123 to segregate (self-assemble) from the silica domain, the PPO block cannot avoid hydration, as exemplified by the growth of the δ(h-CH3) mode. These data confirm the

monitor the concentration of methoxysilyl (SiO−CH3), because it is sharp, clearly resolved from the other bands, and significantly shifted compared to the analogue methyl stretching mode of the PPO at 2900 cm−1 (CH−CH3). At low P123 concentration (x = 0.25), the beginning of hydrolysis is accompanied by an intensification of the band at 1382 cm−1 (h-CH3) together with the opposite dynamic behavior for the band at 1371 cm−1 (dh-CH3). Such a scheme is fully consistent 1966

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colocalization of the two blocks with silica. They also support that the change of medium polarity for the PPO block occurs progressively to yield at the end of the UV process only a minor contribution of dehydrated methyl (1371 cm−1) representing only 10% of the band. By contrast, at higher concentrations in surfactant (x ≥ 0.50), the δ(h-CH3) band at 1382 cm−1 remains weak upon UV irradiation. Upon increasing the P123 content from 50 to 75 wt %, the δ(h-CH3) component decreases from 25% to 7%, which is in full agreement with a more resolved and sharp peak observed in XRD and suggestive of more uniform mesopores. In this second scenario, the hydrophobic interactions dominate and the PPO blocks are able to remain in an apolar environment despite the advent of the silica phase. This is consistent with the mesostructuration ability demonstrated in XRD and TEM by these copolymer-rich samples. Despite the change in medium polarity initiated by hydrolysis, the PPO blocks segregate: micelles with an isolated PPO core and a corona of hydrated PEO are formed. Although the methyl deformations hardly shift here, there is clearly a change in the environment of the PPO blocks because of the self-assembly, but they remain in a hydrophobic medium throughout the reaction. 3. Solid-State NMR. As illustrated in Scheme 1, selfassembled amphiphilic block copolymers are characterized by mesoscopic segregation in hydrophilic and hydrophobic domains, with consequences regarding the hydration state of the PPO block. In this third part, we have exploited the dependence of the isotropic chemical shift on the microenvironment to evidence the extent of mesostructuration in the three silica/copolymer films. It is well-known that the chemical shift is sensitive not only to the chemical nature of adjacent atoms, but also to physical interactions with environmental molecules such as water. For example, liquid-state 1H NMR experiments with PEO-b-PPO-b-PEO copolymer solution demonstrated that the transfer from a hydrated medium to an apolar environment during the micellization was reflected by a slight upfield shift (Δδ ≈ 0.05 ppm) of the PPO protons.41,42 The question arises whether such a dependency can be extended to solid copolymer-silica materials to distinguish different hydration states of the PPO block depending on the level of mesostructuration. The main challenge anticipated with solid-state 1H spectra are the strength of the dipolar interactions between the protons and the chemical shift anisotropy (only partially averaged by MAS),43 which cause a significant broadening of the spectra, making the detection of such a weak δ shift tricky. Figure 4 shows the 1H spectra of the three as-synthesized hybrid films recorded at fast MAS (30 kHz) to display a good resolution. The spectral feature centered at ca. 1 ppm is straightforwardly attributed to the proton resonance of the PPO methyls (full 1H MAS spectra are given in the Supporting Information, Figure S1). The proton linewidths become narrower upon increasing the template concentration, which is consistent with an increased phase separation positively affecting the chain mobility. From an amorphous blend (x = 0.25) to a mesotructured composite (x ≥ 0.50), we note the dramatic change from a very broad spectrum to a more resolved spectrum. In the amorphous sample, extended interface between the PPO chains and silica cross-linked domains contributes to enhanced dipolar interactions between protons, leading to a broad CH3 signal. In addition, we note a small upfield shift (0.02 ppm) of the PPO CH3 resonance upon increasing the surfactant weight concentration from 25 to 75 wt

Figure 4. Effect of the P123 concentration (25, 50, and 75 wt %) on the shape/chemical shift of the proton resonance of the PPO methyls (1H MAS spectrum). The spectral feature at ∼1.2 ppm can be decomposed into 3 distinct resonances at 1.32 (hydrated PPO, hCH3), 1.22 (interfacial PPO, int-CH3), and 1.16 ppm (dehydrated PPO, dh-CH3). Their respective areas are summarized in Table 1.

%. We assume that the progressive dehydration of the segregating PPO block accounts for the observed shift. A meticulous deconvolution reveals a triplet resonance, translating three different environments for the PPO methyls, in a similar fashion to what was observed in IR. The upfield and downfield spectral features at 1.16 and 1.32 ppm were assigned, respectively, to dehydrated and hydrated CH3, while the interfacial methyls contribute to an intermediate resonance at 1.22 ppm. A comparison of their relative areas at different concentrations in P123 is given in Table 1. The hydrated peak is always very broad and dominant at x = 0.25 (46%) as expected from the PPO/silica intimate contact in nonmesostructured films. However, its contribution decreases drastically as the surfactant experiences self-association at weight concentration x = 0.5 (17%), and completely vanishes at x = 0.75. In contrast, the dehydrated peak undergoes an opposite evolution, from 18% in the blend, its contribution rises to 79% in the 75 wt % P123 mesotructured sample. Figure 5 provides a comparison of the IR and NMR data. Both methods consistently reveal the coexistence of three different environments of the PPO CH3. Although the evolution of the h-, int-, and dh-CH3 components with the surfactant concentration are clearly similar with both techniques, there is no exact match due to the uncertainties arising from the deconvolution treatment. 1967

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Table 1. Data (Chemical Shift, Linewidth) Extracted from the Deconvolution of the PPO CH3 Signal in the 1H MAS Spectra Obtained for Different Copolymer/PDMOS Weight Ratios (x = 0.25, 0.50, and 0.75) x = 0.25 1

δ (ppm)

x = 0.50

H MAS line width (Hz)

relative proportion (%)

36

dh-CH3 (1.16 ppm) Int-CH3 (1.22 ppm) h-CH3 (1.32 ppm)

1

x = 0.75

H MAS line width (Hz)

relative proportion (%)

18

25

132

36

881

46

1

H MAS line width (Hz)

relative proportion (%)

59

26

79

92

24

84

21

1237

17

0

0

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ASSOCIATED CONTENT

S Supporting Information *

1

H MAS spectra. 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 5017. Notes

The authors declare no competing financial interest.

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Figure 5. Comparison between the IR and H MAS NMR results giving the proportion in hydrated (h), dehydrated (dh), and interfacial (int) PPO CH3 depending on the P123/PDMOS weight ratio: x = 0.25, 0.50, and 0.75. IR data are based on the integrated absorbance of the δ(CH3) band, while those from NMR are determined from the integrated PPO CH3 resonance at ∼1.2 ppm. Similarly, both IR band and proton signal can be decomposed into three components associated to these PPO local environments.

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REFERENCES

(1) Grosso, D.; Cagnol, F.; Soler-Illia, G. J.; Crepaldi, E. L.; Amenitsch, H.; Brunet-Bruneau, A.; Bourgeois, A.; Sanchez, C. Fundamentals of Mesostructuring through Evaporation-Induced Self Assembly. Adv. Funct. Mater. 2004, 14, 309−322. (2) Soler-Illia, G. J. A. A.; Innocenzi, P. Mesoporous Hybrid Thin Films: The Physics and Chemistry Beneath. Chem.Eur. J. 2006, 12, 4478−4494. (3) Sanchez, C.; Boissière, C.; Grosso, D.; Laberty, C.; Nicole, L. Design, Synthesis, and Properties of Inorganic and Hybrid Thin Films Having Periodically Organized Nanoporosity. Chem. Mater. 2008, 20, 682−737. (4) Huang, M. H.; Dunn, B. S.; Soyez, H.; Zink, J. I. In Situ Probing by Fluorescence Spectroscopy of the Formation of Continuous Highly-Ordered Lamellar-Phase Mesostructured Thin Films. Langmuir 1998, 14, 7331−7333. (5) Huang, M. H.; Dunn, B. S.; Zink, J. I. In Situ Luminescence Probing of the Chemical and Structural Changes during Formation of Dip-Coated Lamellar Phase Sodium Dodecyl Sulfate Sol−Gel Thin Films. J. Am. Chem. Soc. 2000, 122, 3739−3745. (6) Doshi, D. A.; Gibaud, A.; Liu, N. G.; Sturmayr, D.; Malanoski, A. P.; Dunphy, D. R.; Chen, H. J.; Narayanan, S.; MacPhee, A.; Wang, J.; Reed, S. T.; Hurd, A. J.; van Swol, F.; Brinker, C. J. In-Situ X-ray Scattering Study of Continuous Silica−Surfactant Self-Assembly during Steady-State Dip Coating. J. Phys. Chem. B 2003, 107, 7683− 7688. (7) Dourdain, S.; Rezaire, A.; Mehdi, A.; Ocko, B. N.; Gibaud, A. Real Time GISAXS Study of Micelle Hydration in CTAB Templated Silica Thin Films. Physica B 2005, 357, 180−184. (8) Doshi, D. A.; Gibaud, A.; Goletto, V.; Lu, M.; Gerung, H.; Ocko, B.; Han, S. M.; Brinker, C. J. Peering into the Self-Assembly of Surfactant Templated Thin-Film Silica Mesophases. J. Am. Chem. Soc. 2003, 125, 11646−11655. (9) Grosso, D.; Babonneau, F.; Albouy, P.-A.; Amenitsch, H.; Balkenende, A. R.; Brunet-Bruneau, A.; Rivory, J. An in Situ Study of Mesostructured CTAB−Silica Film Formation during Dip Coating Using Time-Resolved SAXS and Interferometry Measurements. Chem. Mater. 2002, 14, 931−939. (10) Grosso, D.; Babonneau, F.; Sanchez, C.; Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Albouy, P. A.; Amenitsch, H.; Balkenende, A. R.; Brunet Bruneau, A. 26, 561., A First Insight in the Mechanisms Involved in the Self-Assembly of 2D-Hexagonal Templated SiO2 and

1

CONCLUSION

We have demonstrated the potential of using UV light (instead of solvent evaporation) as an efficient means of driving the selfassembly of alkoxide silica precursors into mesostructured films. The level of mesostructuration in the hybrid films was found to depend strongly upon the concentration in PEO-b-PPO-b-PEO triblock copolymer surfactant. Above a threshold concentration of 50 wt % in copolymer, TEM and XRD provided evidence of the transition from amorphous to worm-like mesostructured films. To structurally and compositionally investigate the selfassembly process, we used first time-resolved FTIR spectroscopy. Photoacid-catalyzed hydrolysis was kinetically followed, while micellization was studied by the gradual shift of spectral features sensitive to hydration state change in the course of the structuration process. In particular, the deformation mode of the PPO methyl provided a powerful diagnostic marker in assessing the gradual mesoscopic structuration. In a complementary approach, simple monodimensional 1H MAS spectra were implemented. The dependency of 1H chemical shift to local environment enabled the distinction of various hydration degrees of the PPO block, which could be correlated to the extent of mesostructuration. Both FTIR and solid-state NMR data were consistent with existence of three distinct PPO domains (hydrated, interfacial and dehydrated), whose relative proportions revealed the extent of self-assembly. 1968

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dx.doi.org/10.1021/la3030759 | Langmuir 2013, 29, 1963−1969