Triblock Siloxane Copolymer Surfactant: Template for Spherical

Article pubs.acs.org/Langmuir

Triblock Siloxane Copolymer Surfactant: Template for Spherical Mesoporous Silica with a Hexagonal Pore Ordering M. J. Stébé,†,‡ M. Emo,†,§ A. Forny-Le Follotec,⊥ L. Metlas-Komunjer,⊥ I. Pezron,⊥ and J. L. Blin*,†,‡ †

SRSMC, UMR7565, Université de Lorraine, F-54506 Vandoeuvre-lès-Nancy cedex, France CNRS, SRSMC, UMR7565, F-54506 Vandoeuvre-lès-Nancy cedex, France § CNRS, Institut Jean Barriol, FR 2843, F-54506 Vandoeuvre-lès-Nancy cedex, France ⊥ Groupe Interface et Milieux Divisés, EA 4297 UTC/ESCOM Transformations Intégrées de la Matière Renouvelable, Université de Technologie de Compiègne, BP 20529, 60200 Compiègne Cedex, France ‡

ABSTRACT: Ordered mesoporous silica materials with a spherical morphology have been prepared for the first time through the cooperative templating mechanism (CTM) by using a silicone triblock copolymer as template. The behavior of the pure siloxane copolymer amphiphile in water was first investigated. A direct micellar phase (L1) and a hexagonal (H1) liquid crystal were found. The determination of the structural parameters by SAXS measurements leads us to conclude that in the hexagonal liquid crystal phase a part of the ethylene oxide group is not hydrated as observed for the micelles. Mesoporous materials were then synthesized from the cooperative templating mechanism. The recovered materials were characterized by SAXS measurements, nitrogen adsorption−desorption analysis, and transmission and scanning electron microscopy. The results clearly evidence that one can control the morphology and the nanostructuring of the resulting material by modifying the synthesis parameters. Actually, highly ordered mesoporous materials with a spherical morphology have been obtained with a siloxane copolymer/tetramethoxysilane molar ratio of 0.10 after hydrothermal treatment at 100 °C. Our study also supports the fact that the interactions between micelles and the hydrolyzed precursor are one of the key parameters governing the formation of ordered mesostructures through the cooperative templating mechanism. Indeed, we have demonstrated that when the interactions between micelles are important, only wormhole-like structures are recovered.

1. INTRODUCTION For the past few years, due to their properties such as the high specific surface area and pore volume, surfactants templated silica mesoporous materials have attracted much research attention, and a series of compounds have been prepared by different groups.1−13 In addition, these materials display a wide range of pore sizes and symmetries that can readily be tailored by adjusting, among other factors, the physicochemical properties of surfactant. These characteristics afford their use for several potential applications in many fields, such as adsorbents, catalysts, host matrixes for electronic and photonic devices, drug delivery, and sensors.14−19 Actually, the synthesis of these compounds combines the sol−gel chemistry and the use of assemblies of surfactant molecules as framework templates. Depending on the starting block, two mechanisms can lead to the formation of these ordered mesostructures. The first one is the cooperative templating mechanism (CTM), and in this case the building blocks are the micelles.12,20 In the initial step, the interactions between small silica oligomers and surfactants drive to the formation of hybrid organic−inorganic micelles or aggregates. Then, the condensation of the inorganic precursor at the external surface of the micelles occurs. The ordered mesophase is obtained after intermicellar condensation. Finally, the hydrothermal treatment at higher temperature © 2013 American Chemical Society

completes the assembly of micelles and the polymerization of the silica source. The ordered mesoporous material is recovered after surfactant removal. The second approach to the preparation of ordered mesostructures uses liquid crystal phase and is labeled as the direct liquid crystal templating (LCT) pathway.3,21−23 In this case much larger surfactant concentration is necessary, and the structure of the recovered materials can be designed a priori based on the corresponding surfactant phase diagram. Except for the poly(ethylene oxide)block-poly(propylene oxide)-block-poly(ethylene oxide) (labeled as Pluronic) triblock copolymers,11−13,24−27 the use of high-molecular-weight amphiphilic polymers for the synthesis of mesoporous materials is scarce. In that case, the architecture and the composition of the block copolymers can be rationally adjusted to control the interactions between the organic and inorganic species. For instance, in a paper dealing with the synthesis of mesoporous silica using commercial poly(ethylene oxide)−polybutylene oxide−poly(ethylene oxide) (PEO− PBO−PEO) triblock copolymers, Zhao et al. have reported that amphiphiles having a critical micellar concentration (cmc) Received: October 31, 2012 Revised: January 8, 2013 Published: January 10, 2013 1618

dx.doi.org/10.1021/la304315v | Langmuir 2013, 29, 1618−1626

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and duration, as well as the quantity of silica source also play an important role in getting a well-defined architecture,50−54 we have studied their influence on the properties of the mesostructure in order to optimize the synthesis parameters.

lower than 20 mg/L template ordered mesoporous materials, while triblock copolymers with a cmc higher than 300 mg/L lead to the formation of disordered materials.24 More recently, silica mesostructures have been synthesized using diamine polypropylene amphiphiles that belong to the Jeffamine family. Indeed, Pinnavaia et al. have reported the preparation of a new group of large pore mesoporous silicas, denoted MSU-J,28,29 obtained through hydrogen-bonding pathways, from sodium silicate or tetraethyl orthosilicate as the silica source and amineterminated Jeffamine as the structure-directing agent. Depending on the synthesis conditions, the pore size and the specific surface area varied respectively from 4.9 to 14.3 nm and from 108 to 1127 m2/g. MSU-J represent the largest pore sizes observed to date for a fully three-dimensional mesoporous framework assembled from a single micellar porogen. However, no mesopore ordering is noted for MSU-J; the materials exhibit a wormhole-like framework. Mesoporous silicas with onion-like morphology were also synthesized with the same family of amphiphiles by Sayari et al.30 By grafting the myristic acid onto the polyetheramine H2N−(PO)3(EO)12.5(PO)3NH2 (Jeffamine ED900), we succeeded in preparing organized mesoporous silica.31,32 Another family of templates that has potential interests for the preparation of ordered mesoporous materials concerns the silicone surfactants, in particular the poly(dimethylsiloxane)−poly(ethylene oxide) [PDMS−PEO] or the poly(ethylene oxide)−poly(dimethylsiloxane)−poly(ethylene oxide) [PEO−PDMS−PEO] ones. These copolymers are nontoxic and environmentally compatible, and they are used in various domains as lubricants, water repellents, or antifoaming agents.33−37 They have been extensively studied, and many physicochemical data have been reported in the literature.38−43 From the mesoporous synthesis point of view the major advantage is that the dimethylsiloxane part can be polymerized or interacts with other organic polymers, offering the possibilities to design new nanomaterials. However, despite these advantages, until now they have rarely been used as template for the preparation of porous materials. For example, on the basis of TEM analysis, Xu has claimed the preparation of lamellar silica, denoted ZSU-L,44,45 using a pure silicone surfactant at neutral pH. The samples have been prepared by embedding the powder in epoxy resin and then sectioned on an ultramicrotome. Su et al. have demonstrated that the lamellar structure of ZSU-L could be an artifact.46 Moreover, no X-ray diffraction experiments were performed, so the presence of the lamellar structure was questionable. Liu et al. have reported the preparation of well-ordered polymer−silica and carbon−silica nanocomposites by using Pluronic surfactant and diblock PDMS−PEO copolymer as cotemplate and phenolic resol as a polymer precursor.47−49 Nevertheless, to the best of our knowledge, no ordered hexagonal mesoporous material has been obtained from a pure siloxane copolymer. In this paper, we have investigated the ability of the triblock copolymer OH− (OC 2 H 4 ) 12 −(CH 2 ) 3 −Si(CH 3 ) 2 −[OSi(CH 3 ) 2 ] 12 −(CH 2 ) 3 − (OC2H4)12−OH, named (EO)12−(DMS)13−(EO)12, to generate ordered mesoporous materials through the cooperative templating mechanism (CTM). When the CTM pathway is used, the characteristics of the recovered materials, such as the structure and the pore diameter, are strongly related to the phase behavior of the structure directing agent in the synthesis solvent. Therefore, in a first part of the present work, we have determined the phase diagram of the triblock copolymer (EO)12−(DMS)13−(EO)12 in water. Since the synthesis conditions, such as the hydrothermal treatment temperature

2. MATERIALS AND METHODS The OH−(OC2H4)12−(CH2)3−Si(CH3)2−[OSi(CH3)2]12−(CH2)3− (OC2H4)12−OH, named as [(EO)12−(DMS)13−(EO)12] triblock copolymer, was provided by Momentive Performance Materials and was used as received. 2.1. Phase Diagram Determination. The phase behavior of the silicone surfactant in water has been examined by preparing samples over the whole range of surfactant/water compositions. The required amounts of the components were weighed in small glass tubes. The homogenization of the samples was achieved by mixing with a vortex stirrer, combined with heat and ultrasounds, whenever necessary. The samples were placed in a thermostatic bath for several weeks at the temperature of interest in order to reach equilibrium. Actually, it was noted by Hill that these kinds of systems can evolve with time and that a long time (more than 10 days) is needed to get the equilibrium state.38 The different phases were identified by visual inspection with a polarizing light microscope (Olympus BX 50). The boundary lines of the liquid crystal domains were evidenced by small-angle X-ray scattering (SAXS) experiments. 2.2. Mesoporous Material Preparation. Surfactant micellar solutions were prepared (from 5 to 20 wt %) at neutral pH. Tetramethoxysilane (TMOS) provided by Aldrich, used as the silica source, was added dropwise to the surfactant solution at room temperature under stirring. The surfactant/silica molar ratio (R) was varied from 0.05 to 0.20. Then, the mixture was transferred into sealed Teflon autoclaves for the hydrothermal treatment at 80 or 100 °C for 1 day. Subsequent to the hydrothermal treatment, the material was transferred into cellulose extraction cartridges and left for 48 h under Soxhlet ethanol extraction to remove the template. Finally, the material was left to air-dry. 2.3. Characterization of Liquid Crystals and Mesoporous Materials. SAXS measurements were carried out using a SAXSess mc2 (Anton Paar) apparatus. It is attached to a ID 3003 laboratory Xray generator (General Electric), equipped with a sealed X-ray tube (PANalytical, λCu(Kα) = 0.1542 nm) operating at 40 kV and 50 mA. A multilayer mirror and a block collimator provide a monochromatic primary beam. A translucent beam stop allows the measurement of an attenuated primary beam at q = 0. Mesoporous materials are introduced into a powder cell, whereas liquid crystals are put in a paste cell, before being placed inside an evacuated chamber equipped with a temperature-controlled sample holder unit. Acquisition times are typically in the range of 1−5 min for mesoporous materials and about 20 min for liquid crystals. Scattering of X-ray beam is recorded by a CCD detector (Princeton Instruments, 2084 × 2084 pixels array with 24 × 24 μm2 pixel size) placed at 309 mm from the sample holder in the q range from 0.09 to 5 nm−1. Scattering data, obtained with a slit collimation, contain instrumental smearing. Therefore, the beam profile has been determined and used for the desmearing of the scattering data. All data were corrected for the background scattering from the empty cells. Samples for transmission electron microscopy (TEM) analysis were prepared by dispersing some material in ethanol. Afterward, a drop of this dispersion was placed on a holey carboncoated copper grid. A Philips CM20 microscope, operated at an accelerating voltage of 200 kV, was used to record images. Scanning electron microscopy (SEM) was carried out with a Hitachi S-2500 at 15 keV. N2 adsorption and desorption isotherms were determined on a Micromeritics TRISTAR 3000 sorptometer at −196 °C. The pore diameter and the pore size distribution were determined by the BJH (Barret−Joyner−Halenda)55 method applied to the adsorption branch of the isotherm. 1619


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3. RESULTS AND DISCUSSION 3.1. The (EO)12−(DMS)13−(EO)12/Water System. The phase diagram of the (EO)12−(DMS)13−(EO)12/water system was studied between 10 and 70 °C in the whole water− surfactant composition, and it is presented in Figure 1. In the

Figure 1. Phase diagram of the (EO)12−(DMS)13−(EO)12/water system. L1: direct micellar phase; H1: hexagonal phase; and L2: reverse micellar phase.

investigated temperature range, and up to 45 wt % of surfactant, a direct micellar L1 phase is detected. Micelles formed by (EO)12−(DMS)13−(EO)12 in water have been characterized in detail by Forny-Le Follotec et al.43 The authors have shown that up to 20 wt % of polymer, at room temperature, the micelles are spherical, and their structure is described by a dense hydrophobic PDMS core, with a radius of 2 nm and a transition region from 2 to 4.5 nm, where EO and DMS segments are mixed and a shell with a thickness of 1 nm composed of the PEO chains and water. An elongated shape is observed above this concentration. In the present work, we have established the whole phase diagram and, at 20 °C, when the weight percent of (EO)12−(DMS)13−(EO)12 is increased from 62 to 80 wt %, an optically anisotropic phase is detected. The fan-shaped texture observed by optical microscopy with polarized light is characteristic of the defects of the direct hexagonal H1 phase. The hexagonal symmetry is confirmed by SAXS measurements. We have evaluated the structural parameters of H1. The hexagonal phase is composed of infinite cylinders packed in a hexagonal array. In case of direct systems, cylinders are filled by the hydrophobic chains and are covered by both headgroups and water. The hexagonal H1 phase is characterized by its typical SAXS profile with the relative peak positions, 1, √3, and 2. From geometrical considerations the distance d associated with the first peak is related to the hydrophobic radius RH by relation 1:56 VB π 3 RH 2 = VS + αVW 2d100 2

Figure 2. Hexagonal liquid crystal phase: (A) repetition distance (d, ○), cell parameter (a, ■), and hydrophobic radius (RH, ▲); (B) crosssectional area (S) as a function of α, the number of water molecules per surfactant molecule.

spacing and, thus, in the cell parameter is observed with increasing water content. This is due to the hydration of the headgroup and water could also form a film surrounding the surfactant. The hydrophobic radius is constant with α and equal to 2.8 ± 0.1 nm. The occupied surface of the surfactant molecule in the interface is found to be 1.24 ± 0.02 nm2 along the entire hexagonal phase. The hexagonal liquid crystal domain melts at about 65 °C (Figure 1). A slight increase (less than 10%) of the structural parameters of H1 is noted when the temperature is increased. Taking into account the values of the different bonds,38 we can estimate that the lengths of the extended PDMS and PEO blocks are 3.8 and 4.2 nm, respectively. The comparison of these values with the structural parameters obtained from eqs 1 and 2 can provide data about the conformation of the hydrophilic and hydrophobic chains in H1. Assuming that the PDMS chains form the inner core of the rod and the two PEO chains constitute the outer part (Figure 3A), a half molecule of surfactant should be considered to calculate the total radius of the rod. This radius is thus equal to 6.2 nm, and among them 2 nm is attributed to the PDMS part. Taking into account only a half molecule, eq 1 leads to a hydrophobic radius (RH) of 2.8 nm. Hence, the evaluated value obtained by considering an extended conformation of the hydrophobic chains (2 nm) could not be in accordance with RH determined from the X-ray experiment (eq 1). In addition, due to steric effect between the chains, the array of molecules with such a configuration to give rods packed in a hexagonal way seems to be unlikely. To give rise to the H1 phase, it is more realistic to consider that the amphiphilic molecules are selffolded, and a different hypothesis can be drawn. We can imagine that the PEO chains form a corona where all the oxyethylene units are in contact with water (Figure 3B). The radius of the cylinders is estimated to around 5.8 nm with a 1.8 nm contribution of the hydrophobic radius. Once again such a situation is not compatible with the value of RH determined from the SAXS experiments (eq 1), since RH = 2.8 ± 0.1 nm. So, this model does not correspond for the description of the

(1)

where α stands for the number of water molecules per surfactant molecule and VB, Vs, and Vw respectively stand for the molar volumes of the hydrophobic part of the polymer (VB = 1062 cm3/mol), of the total part of the polymer (Vs = 2021 cm3/mol), and of water (Vw = 18 cm3/mol). The cell parameter a is given by the relation a = 2d100/√3. The cross-sectional area S can then be deduced from the relation56 S=

2VB RHNA

(2)

where NA is the Avogadro number. The obtained results at room temperature are reported in Figure 2. As shown in Figure 2A, a slight increase in the d 1620


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Figure 3. Schemes of the various potential configurations of the copolymer in the hexagonal liquid crystal phase.

studied system. To fit well with the experimental observations, it is reasonable to consider a scheme where a part of the EO group is not hydrated and two situations can be considered. First a true core−shell model (Figure 3Ca) and second a model where only one of the two PEO chains penetrates the PDMS part of the copolymer (Figure 3Cb). These representations are also supported by the results obtained by Forny-Le Follotec et al.43 for the micelle structure. 3.2. Silica Mesoporous Materials. Once the phase behavior in water of the (EO)12−(DMS)13−(EO)12 triblock copolymer has been established, the latter has been used as template to prepare mesoporous silica materials through the cooperative templating mechanism. In this case the building blocks are the micelles, so the CTM occurs at low surfactant concentrations, and the interactions between the surfactant and the inorganic precursor are at the origin of the formation of mesoporous material.20 The other parameters that should be taken into account for the design of mesoporous silica deal with the silica chemistry. In particular, the surfactant/silica molar ratio has to be optimized. To do that first, the copolymer concentration of the micellar solution has been fixed to 5 wt %. The hydrothermal treatment has been performed for 1 day at 80 or 100 °C. Concerning the silica, it has been reported that these conditions are favorable for the silica condensation and to recover ordered mesoporous materials through the CTM pathway.6,54,57 3.2.1. Structural and Morphology Characterization. Looking at the SAXS patterns of materials prepared with a (EO)12−(DMS)13−(EO)12/TMOS molar ratio of 0.1 and obtained after 1 day at 80 °C (Figure 4A) or 100 °C (Figure 4B), one can observe three reflection lines located at 7.2, 4.2, and 3.6 nm. Their relative positions are 1, √3, and 2, which can be attributed to the (100), (110), and (200) reflections of the hexagonal structure. The hexagonal arrangement of the channels is further confirmed by the TEM micrographs of the different samples (Figure 5). Indeed, either the honeycomblike arrangement (Figure 5d,g) or the hexagonal stacking of the

Figure 4. Mesoporous materials: SAXS patterns of the materials prepared from a 5 wt % (EO)12−(DMS)13−(EO)12 micellar solution with a (EO)12−(DMS)13−(EO)12/TMOS molar ratio equal to (a) 0.05, (b) 0.1, and (c) 0.20. The hydrothermal treatment is performed during 1 day at 80 °C (A) or 100 °C (B).

channels (Figure 5c,e,h) is evidenced. The 6-fold symmetry is also shown by the diffraction electron pattern (inset of Figure 5g). For the samples prepared under these conditions, an interesting feature that can be noted from the TEM and SEM images is that morphology of particles can be described by an assembly of spheres of very small diameter (

Triblock Siloxane Copolymer Surfactant: Template for Spherical

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