History of the Micelles: A Key Parameter for the Formation Mechanism

Article pubs.acs.org/JPCC

History of the Micelles: A Key Parameter for the Formation Mechanism of Ordered Mesoporous Carbons via a Polymerized Mesophase Sébastien Schlienger, Claire Ducrot-Boisgontier, Luc Delmotte, Jean-Louis Guth, and Julien Parmentier* Institut de Science des Matériaux de Mulhouse UMR CNRS 7361, Université de Haute Alsace, 15 rue Jean Starcky, 68057 Mulhouse Cedex, France ABSTRACT: New insights into the formation mechanism of a polymerized mesophase (space group Im3̅m) and its derived order mesostructured carbon phase (denoted FDU-16) were evidenced by a slight modification of its synthesis conditions using proton nuclear magnetic resonance (NMR) relaxometry (T2 spin−spin relaxation time), dynamic light scattering (DLS), small-angle X-ray diffraction (SAXRD), N2 physisorption (77 K), and scanning electron microscopy (SEM). The preparation of those materials is based on the self-assembly of a prepolymer (phenol-formaldehyde resin called resol) and a pyrolysable surfactant (Pluronic F127, PEO106PPO70PEO106). The former is the carbon precursor, and the latter is the pore structuring agent and also the porogen. The resol and the F127 solutions are usually mixed at room temperature before being heated at 65 °C to promote the polymerization of the resol into a polymerized mesophase that precipates. That phase is further carbonized into a mesostructured porous carbon. By a slight change of the mixing temperature around the critical micellar temperature of the F127 amphiphilic surfactant (∼25 °C), different formation pathways were observed, visually and thanks to DLS and NMR relaxometry techniques. It is proposed that the mixing temperature influences, in the early stage of the synthesis, both the resol-bound micelles (e.g., resol localization and solubility in the micelles) and the concentration of the free resol in the solution. It leads then to different kinetics of the free resol polymerization and to different evolutions of the intermediate colloidal particle populations with precipitation times ranging from 40 to 120 h. That synthesis parameter also influences the properties of the final mesostructured carbon materials by modifying their rhombododecahedral macroscopic morphology, their body-centered cubic mesostructural arrangement, and their porosity. That study highlights the high sensitivity of surfactant templated mesostructured materials toward the mixing temperature. That key parameter is rarely investigated and deserves to be systematically mentioned in the literature to avoid unexplained problems of reproducibility.

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INTRODUCTION Mesoporous materials have great potential applications1−5 in catalysis, separation, adsorption, bioreactors, dielectrics, and sensors. Since the pioneer work of the Mobil Company on the silica materials (M41S family with MCM-41, MCM-48, and MCM-50), various pore structures have been obtained with different symmetry.6−18 The mechanism of formation is based on a self-assembly of a polymerizable inorganic precursor and a porogen agent being an amphiphilic surfactant. Those syntheses are performed by either hydrothermal or an evaporation-induced self-assembly (EISA) processes. Although different mechanisms are proposed in the literature, the generally accepted one relies on the association of micelles in solution with a silica precursor by weak attractive interactions such as hydrogen bonds, van der Waals forces, or electrostatic interactions. The micelles act as templates for the silica polycondensation. The resulting composite micelles selfassemble and yield a silica/surfactant mesostructured material. The removal of the template then leads to an ordered mesoporous silica. Those syntheses have then been adapted © 2012 American Chemical Society

to other nonsiliceous oxides such as aluminum oxide and titanium dioxide and later to other nonoxide compositions such as metal sulfides and metal phosphates1 through mainly an EISA pathway. Recently, carbon and polymeric mesoporous materials19−26 have been obtained by applying that softtemplate route with amphiphile surfactants. Those works have paved the way for the preparation of a large variety of new mesoporous polymers with interesting chemical functionalities adapted to potential applications such as captors, biocompatible micellar systems for solubilization, and drug release and biomolecules adsorption. The development of those materials requires the control of the relevant synthesis parameters to tailor the material properties to the targeted applications. This could be achieved by the understanding of their mechanisms of formation. The mechanisms of formation of siliceous mesoporous materials have been widely investigated by various Received: February 6, 2012 Revised: May 11, 2012 Published: June 1, 2012 11919

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spherical composite micelles inside the liquid particles into a local cubic-centered arrangements (space group Im3̅m) and a macroscopic rhombododecahedral shape. This step leads to the minimization of the interfacial and the bulk free energies. (iv) The continuation of the polymerization of the polymer yields solid particles with a sustainable meso-organization and a macroscopic shape. In this complementary study, we have gone further in the investigation of the formation mechanism of the surfactanttemplated mesostructured polymer. A slight modification of one synthesis parameter has revealed different polymerizedmesophase formation pathways. Here the modified synthesis parameter is the mixing temperature between the resol and the F127 solutions. For the “classical” synthesis of the FDU-16 phase, that mixture is usually performed at room temperature (20 °C) before it is heated to the synthesis temperature (65 °C). With a mixing temperature above (40 °C) or below (5 °C) the critical micellar temperature (CMT) (∼25 °C), different evolutions of the systems were observed by in situ DLS and proton nuclear magnetic resonance (NMR) relaxometry (measurements of the spin−spin relaxation times T2). That synthesis parameter also determines the mesostructural arrangement and the morphology of the final carbon materials. A detailed formation mechanism is proposed here that fits with the different observed phenomena. This study also demonstrates the importance of a synthesis parameter too often neglected in the preparation of polymerized mesostructured materials: the mixing temperature between the amphiphile and the polymerizing species.

techniques. Different mechanisms have been proposed in the literature.1,27−31 In the case of mesostructured organic polymer materials, their mechanisms of formation have been barely studied.32−34 The system polymer/surfactant appears more complicated to self-assemble because it is constituted only of organic compounds: a polymeric precursor and a pyrolysable surfactant. Therefore, the affinity of the organic polymer for the hydrophilic corona of the micelle is not so well pronounced than for inorganic polymers such as silica. That feature was particularly evidenced during the EISA process.34 The synthesis is thus more difficult to implement. Several studies have succeeded to prepare surfactant-templated mesostructured polymers, especially from precursors based on phenolic resins 19−26 or biosourced compounds (fructose35 and tannin36) and a Pluronic surfactant ((PEO)x(PPO)y(PEO)x) using either aqueous or EISA routes. The calcination of those materials yields interesting mesoporous carbon materials. In particular, several syntheses24−26 in aqueous media, using prepolymers based on incomplete phenol-formaldehyde condensation reactions (resol), led to three mesostructured carbon materials denoted FDU-14 (space group: Ia3̅d), FDU15 (space group: P6mm), and FDU-16 (space group: Im3m ̅ ). Various parameters affect those syntheses: the resol/Pluronic ratio, the type of Pluronic, the degree of cross-linking of the resol, and the pH. The phase FDU-16 is obtained with the F127 surfactant (PEO106PPO70PEO106), the FDU-14 with the P123 surfactant (PEO20PPO70PEO20), and the FDU-15 with the P123 and a swelling agent (decane or hexadecane). Another parameter that determines the synthesis is the molar ratio phenol/sodium hydroxide. If the ratio is too high (>8.4), then the cross-linking with formaldehyde is too slow (catalysis in basic media), and no mesostructure was obtained. If the ratio is too low (50 ms), by choosing a high interpulse spacing value (2τ = 12 ms). In this way, only mobile water molecules interacting weakly (or not) with the organics are observed. Those molecules are either the free water molecules (bulk) or the (hydrodynamic) hydration water molecules that are transported with the organic aggregates (micelles, unimers, resol, ...) during their diffusion. The other water molecules, which are not detected (T2 < 50 ms), are also confined in the intra- or intermolecular spaces of the aggregates but are tightly bond with the organics. Those measurements were performed on a Bruker Minispec mq20 with a method using a Carr−Purcell− Meiboom−Gill (CPMG) sequence.41 The 1H NMR transverse relaxation time (T2) measurement were performed with an operating frequency of 20 MHz and with pulse spacing between two following 180° pulses of 12 ms. The NMR signal was measured with an average of 12 repetitions. The recycle delay of 12 s was chosen to be over 5T1. The temperature was monitored with the control unit BVT300 and checked also with the thermocouple; variations below 1° were observed. The data were treated with the software “Contin” based on the Laplace inversion,42 and different T2 populations were extracted. Their corresponding intensities (in arbitrary unit) were determined with the software “Origin” by fitting the FID with an exponential decay A(t) = ∑i Ai exp(−t/T2,i) + L0,43 where

1/T2 = 1/T2real + 1/T2

′

(1)

⎛ ⎛ Dγ 2G2τ 2 ⎞ ⎛ k τ ⎞⎞ 1 2 = ⎜1 − *tanh⎜ e ⎟⎟ke−1pb δ b2ω 2 + ⎜ ⎟ ⎝ 2 ⎠⎠ T2 ′ τke ⎝ 12 ⎠ ⎝ (2)

where τ is the time between two 180° refocusing pulses of the CPMG sequence, ke is the rate of proton exchange, pb is the fraction of exchangeable protons population, δb is the chemical shift difference in ppm between water and exchangeable protons, ω is the measurement frequency, γ is the gyromagnetic ratio, D is the self-diffusion constant, and G is the magnetic field gradient. It has been shown that this 1/T2′ component could be minored or even neglected if experiments were carried out using a low field apparatus (e.g., 20 MHz)47 and with a CPMG sequence. In our case, we are in the presence of both conditions at the same time. Moreover, because of the rather soft structure of the organic matter considered here (micelles, resol-bound micelles, free resol, etc.), the influence of the internal field gradient on the relaxation rate is believed to be negligible. That feature was checked by measuring the averaged transverse proton relaxation rates of FDU-16 solution (at different synthesis times) for different reciprocal pulse spacing 1/τ. (See figure SI1 in the supporting information of ref 39.) The negligible contribution of the 1/T2′ component was confirmed by the very slight variation of the relaxation rates 1/T2.

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RESULTS AND DISCUSSION Characteristics of the As-Made and Heat-Treated Materials Prepared with Different Mixing Temperatures. First, the influence of the mixing temperature onto the mechanism of formation of the FDU-16 phase was investigated by studying the properties of the final materials. Various techniques were used such as SEM, XRD, and N2 physisorption (77 K). With the optimized synthesis of the FDU-16 polymerized mesophase (Tmixing = 20 °C), the precipitation is visually detected after 40 h of agitation at 65 °C. For Tmixing of 5 °C, precipitation is delayed with the appearance of particles only after 73 h of agitation (72 h at 65 °C and □ > △.

h instead of 20 h for Tmixing = 5, 40, and 20 °C respectively. The T2 value of the free water population is a direct measurement of the viscosity of the media and thus of the polymerization degree of the resol prepolymers present as free species (i.e., not bound with micelles) in the solution. Those delays could signify that the polymerization of the free prepolymers takes places 11924

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Figure 6. Formation mechanisms of the polymerized mesophases according to the mixing temperature (Tmixing = 5, 20, and 40 °C).

synthesis (40 h). It is worth saying that those flocs appear stables and are the only assembled mesostructures when the polymerization of the free resol in solution is delayed (Tmixing = 5 and 40 °C). At the opposite, for Tmixing = 20 °C solution, the high concentration of free resol promotes its polymerization and the formation of flocs occurs then simultaneously with the appearance of big particles. It seems that a sufficient degree of polymerization of the free resol in the solution is required to destabilize the flocs, maybe by a bridging mechanism. When the solutions have a low free resol content (Tmixing = 5 and 40 °C), longer heat treatments are then required to reach a sufficient polymerization state that causes the formation of big particles by flocs association. The delay for the floc association also leads to further cross-linking of the resol within the resol-bound micelles, with stiffening of the mesoscopic local ordering and of the shape of the flocs. At the difference of more liquid-like flocs (Tmixing = 20 °C), the subsequent association of those more rigid flocs could then prevent their arrangement into a perfect single mesocrystal. It results in mesostructured materials with shorter coherence length, as evidenced by XRD (Figure 1a,c). A model showing the influence of the mixing temperature on the formation mechanism of the self-assembly polymerized mesostructures (Im3̅m space group, FDU-16 type) is proposed in Figure 6. Micelles with a Polymerized Corona. The modification of the synthesis conditions highlights some features of the formation mechanism of polymerized mesosphase. For the synthesis of the FDU-16 phase, by varying the temperature of mixing between the F127 and the resol prepolymers solutions around the CMT, different fractions of the resol prepolymers are obtained, either as dissolved species within the micelles, as free species within the solution or as aggregates (or vesicles). It is expected that the resol solubility within the micelles decreases according to the sequence Tmixing = 5 − 20 − 40

into the micelles occurs then during their formation at 5 °C and leads to a good dissolution of the resol. The concentration of the free resol in the solution would then be lower than for Tmixing = 20 °C, where most of the hydrated micelles are formed prior to the resol addition. At the opposite, for Tmixing = 40 °C, so well above the CMT, the micelles are already present and partially dehydrated before the addition of the resol. This situation does not promote the interactions between the F127 and the resol prepolymers. Therefore, the resol is not highly dissolved within the micelles. Moreover, the free resol appears in low concentration in the solution, as previously suggested by the slow and delayed decrease in the T2 values of the free water molecules (compare Figure 5c and b). In fact, the resol is present at the beginning of the synthesis in the form of large particles (∼5 μm) observed by DLS (Figure 4c). Those large aggregates have already been detected in the pure aqueous resol solution39 or in the form of F127/resol composite vesicles.50−53 Interestingly, in our experiments, the disappearance of those aggregates (or vesicles) is concomitant with the formation of flocs having a size around 600 nm (see Figure 4c) and with the decrease in the polymerized micelles population. As previously suggested,39 the polymerized-micelle destabilization into flocs consumes the free resol polymers present in the solution. It seems here that the consumption of the free resol shifts the dissolution equilibrium of those aggregates toward their disappearance. Observation of the DLS curves (obtained in static conditions) reveals that, in all three cases, the resol-bound micelles aggregates together at the same time (∼40 h) to form flocs (Figure 4). It suggests that the degree of polymerization of the resol bound to the micelles follows the same evolution for the three solutions. The polymerized micelles have then their solubility reduced simultaneously for the three solutions. It causes a liquid−liquid phase separation into flocs for the same 11925

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°C. Those prepolymers bound to the micelles polymerize roughly at the same speed, for the three mixing temperatures, because the destabilization of these colloidal solutions occurs in all cases around 40 h with the formation of flocs. It is believed that the driving force of that phenomenon is related to the higher hydrophobic character of the polymerized micelles compared with pure micelles.39 Free Resol Polymerization. As previously mentioned, the portion of the free resol present in the solution is influenced by the mixing temperature Tmixing. For the optimized synthesis (Tmixing = 20 °C), its concentration is relatively high and promotes its fast polymerization in solution. After 40 h of synthesis, the conditions are set simultaneously for flocs formation (due to the micelles instability) and for flocs association into big particles (related to the sufficient crosslinking of the free resol). For Tmixing = 5 and 40 °C, the free resol concentration is expected to be lower than for Tmixing = 20 °C solution for the reasons previously mentioned. The polymerization in solution is then delayed compared with the one of the optimized process, and it does not reach a sufficient level of cross-linking to destabilize the flocs into big particles (by a bridging mechanism for instance). Further time is required to complete the free resol polymerization and to promote flocs association into big particles (>70 h of synthesis). This mechanism is supported by the experimental conditions used by Fang et al. to prepare mesostructured polymeric nanospheres.54 The authors used the same conditions as those for the FDU-16 phase for the first 18 h; polymerized micelles and also partially cross-linked free resol are expected. After this step, the solution was then diluted by a factor 11 and heated to 130 °C for 24 h. Here the high temperature could promote the polymerization of the resolbound micelles and the destabilization of the micellar solution into flocs (here the mesostructured nanospheres), whereas the dilution could delay the free resol polymerization and the subsequent floc association into big particles Mesophase Arrangements. The various degrees of complexation of the resol within the micelle determine the properties of the polymer-bound corona and the mutual interactions between micelles. The high sensitivity of the F127 system to “impurities” leads then to different mesophase arrangements with ordered (bcc) or disordered mesostructures. In our previous study on FDU-16 mesophase,39 we have shown that the just formed big particles are in a liquid-like state that turns progressively into a solid state after ∼20 h at 65 °C. A similar feature occurs here. For Tmixing = 20 °C, the formation of flocs and their association into big particles occur simultaneously. The liquid-like state of the big particles allows a facile reorganization of the ordered mesostructure (bcc) and of the macroscopic rhombododecaedral shape. For the two other mixing temperatures, flocs association occurs only at least 24 h later than their formation, giving them enough time to stiffen. The assembly of more rigid flocs into a macroscopic mesocrystal is then more difficult to achieve, yielding mesostructures with limited long-range ordering and more irregular macroscopic crystalline shapes, as observed by XRD and SEM, respectively.

at which the amphiphile and the polymerizable species (resol) are mixed together (Tmixing = 5, 20, and 40 °C), keeping constant the main synthesis temperature (65 °C) and the other synthesis conditions. Whatever the mixing temperature, the mechanism of formation occurs via polymer-bound micelles formed in the early stage of the process. The mixing temperature influences the amount of resol prepolymers dissolved into those micelles and also its concentration as a free species in the solution. The kinetics of polymerization of the free resol then depends on the mixing temperature. The progressive polymerization of the resol bound to the micelles leads to a more hydrophobic corona that causes a phase separation into flocs (∼500 nm). When the degree of polymerization of the free resol species is sufficient, these flocs associate together into bigger particles (∼1 μm) that precipitate. Although these three syntheses lead to a more-orless well-defined rhombododecaedral shape, the mesostructures display an ordered arrangement (bcc) with variable long-range orderings depending on the amounts of the resol either dissolved in the micelles or adsorbed on the surfaces of the micelles and of the flocs. The mixing temperature parameter is usually not considered to be a key parameter in the process of mesostructured polymerized materials by the self-assembly of amphiphiles in an aqueous media. It is also rarely mentioned in the experimental part of publications. In fact, that study highlights that this parameter deeply determines the mechanism of formation and the characteristics of the final materials (ordering of the mesostructure, macroscopic shape, particle size). It could explain the problem of reproducibility sometimes encountered in the synthesis of such materials.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +33 3 89 60 87 02. Fax: +33 3 89 60 87 99. E-mail: julien. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Agence Nationale de la Recherche (ANR) for supporting this research program through the ANR08-BLAN-0189-018 contract.

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CONCLUSIONS Mesoporous carbon nanoparticles and polymerized mesophases are materials with numerous potential applications. We have investigated their mechanisms of formation in aqueous media by modifying slightly one synthesis parameter: the temperature 11926

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