Polystyrene-block-Polybutadiene-block-Polystyrene Triblock

Article pubs.acs.org/IECR

Polystyrene-block-Polybutadiene-block-Polystyrene Triblock Copolymer Meets Silica: From Modification of Copolymer to Formation of Mesoporous Silica Rentong Yu and Sixun Zheng* Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: In this work, we reported a facile preparation of mesoporous silica materials assisted by a commercial polystyreneblock-polybutadiene-block-polystyrene (PS-b-PB-b-PS) triblock copolymer. For this purpose, the PS-b-PB-b-PS triblock copolymer was first functionalized with (3-mercaptopropyl)triethoxysilane via a thiol-ene radical addition approach, and then the functionalized triblock copolymer with the midblock bearing triethoxysilane moieties was employed to perform intercomponent sol−gel reactions with tetraethoxysilane (TEOS) to obtain the organic−inorganic gels. The organic−inorganic gels with variable compositions were then used as precursors to obtain mesoporous silica via pyrolysis at elevated temperatures. The functionalized triblock copolymer was characterized by means of Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and small-angle X-ray scattering (SAXS). The results of SAXS, transmission electron microscopy, and Brunauer−Emmett−Teller measurements indicate that the mesoporous silica materials were successfully obtained and the porosity of the materials can be modulated with the mass ratios of the functionalized triblock copolymer to the precursor of silica (viz. TEOS).

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INTRODUCTION Highly porous silica materials are a class of important inorganic materials owing to their applications in the aspects of catalysis, sorption, separation, sensors, and biotechnologies.1−7 Generally, porous silica is prepared via a sol−gel process of silicon sources (e.g., tetraethoxysilane, TEOS) by the use of templates with a variety of pore-forming strategies.1−22 In the past years, considerable effort has been made to prepare micro/ mesoporous silica with larger surface areas and accessible pores. Mesoporous silica materials are generally prepared from organic−inorganic nanocomposite gels via the synergistic selfassembly between a silicon source and surfactants.23,24 The labile organic nanophases were then removed to afford the mesopores. Both ionic and nonionic surfactants can be utilized to form the self-assembled nanophases in organic−inorganic composite gels. As a class of important nonionic surfactants, amphiphilic block copolymers have attracted considerable interest in the preparation of mesoporous silica.25−40 Depending on the types of amphiphilic block copolymers, a variety of ordered or disordered nanophases can be formed in organic− inorganic nanocomposite gels via a sol−gel process. It is proposed that the amphiphilic block copolymers are selforganized into microdomains in the silicon sources (e.g., TEOS). With the occurrence of sol−gel reactions, the selfassembled microdomains would be trapped in a silica matrix. To avoid undesired macroscopic phase separation, it is required that the amphiphilic block copolymers possess both silica-philic and -phobic blocks. The affinity of copolymer blocks with silica is mainly achieved through hydrogen-bonding interactions. It is found that the silanol hydroxyl groups generated in the sol−gel process owing to the incomplete condensation of alkoxysilanes41 would preferentially form hydrogen-bonding interactions with some proton-accepting blocks of the copolymers © 2015 American Chemical Society

such as poly(ethylene oxide) (PEO). Hydrogen-bonding interactions are critical for suppression of the macroscopic phase separation of block copolymers in the sol−gel process. In fact, most of the block copolymers used here for the preparation of mesoporous silica contain PEO blocks. For instance, poly(ethylene oxide)-block-poly(propylene oxide)block-poly(ethylene oxide) (PEO-b-PPO-b-PEO) triblock copolymers are frequently used as templates to access ordered or disordered nanopores in silica.27−32 Because of lower molecular weights, nonetheless, in the utilization of commercially available PEO-b-PPO-b-PEO triblock copolymers, it is not easy to modulate the sizes of the pores in mesoporous silica materials.31,32,34,42 Recently, many investigators have explored using laboratory-made PEO-containing block copolymers to prepare mesoporous silica materials with adjustable porosity.33−40 Polystyrene-block-polybutadiene-block-polystyrene (PS-b-PBb-PS) triblock copolymers are a class of highly versatile commodity materials. PS-b-PB-b-PS triblock copolymers are generally synthesized via a sequential anionic polymerization approach. By adjustment of their compositions and molecular weights, PS-b-PB-b-PS triblock copolymers can display the properties of materials from thermoplastics to thermoplastic elastomers.43 The widespread applications of PS-b-PB-b-PS have motivated structural modification of the triblock copolymers to endow the materials with new and improved properties. For instance, PS-b-PB-b-PS triblock copolymers can be chemically modified via hydrogenation of the C−C double bonds of PB, resulting in derivative triblock copolymers with Received: December 30, 2014 Accepted: May 22, 2015 Published: June 17, 2015 6454

DOI: 10.1021/acs.iecr.5b01435 Ind. Eng. Chem. Res. 2015, 54, 6454−6466

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Industrial & Engineering Chemistry Research

polybutadiene-block-polystyrene (PS-b-PB-b-PS) triblock copolymer was kindly supplied by Baling Petrochemical Ltd. Co., China; measurement of gel permeation chromatography showed that this triblock copolymer had a molecular weight of Mn = 17800 with Mw/Mn = 1.10 and the content of PS was 43.75 wt %. According to the supplier, this PS-b-PB-b-PS sample was synthesized via an anionic polymerization approach. By adjustment of the polarity of the solvents for polymerization,58 the content of the 1,2-addition structural unit was controlled as 64.64 wt % in the PB block. The organic solvents such as 1,4-dioxane, ethanol, and tetrahydrofuran were purchased from Shanghai Reagent Co., China. Before use, tetrahydrofuran and 1,4-dioxane were refluxed over metal sodium and distilled. Modification of PS-b-PB-b-PS via Thiol-ene Radical Addition. In a flask equipped with a magnetic stirrer, PS-b-PBb-PS (1.0007 g, 10.41 mmol with respect to C−C double bonds), MPTES (24.7921 g, 103.98 mmol), AIBN (0.1706 g, 1.04 mmol), and 1,4-dioxane (48 mL) were charged with vigorous stirring until the polymer was fully dissolved in the solvent. The flask was connected onto a Schenk line to degas via three pump−thaw−freeze cycles and then immersed in a thermostated oil bath at 60 °C for 48 h. Cooled to room temperature, the reacted mixture was added dropwise into a great amount of anhydrous methanol to afford the precipitates. The precipitates were redissolved in tetrahydrofuran and reprecipitated with methanol. After being dried in vacuo at 30 °C, the product [denoted as PS-b-PBSi(OCH2CH3)3-b-PS] (3.500 g) was obtained. 1H NMR (CDCl3): δ 5.34 [2H, CH2CHCHCH2], 3.82 [6H, CH2SiOCH2CH3], 2.52 [4H, CHCH2CH2SCH2CH2CH2, 2H, CHSCH2CH2CH2], 1.68 [4H, CHCH2CH2SCH2CH2CH2, 2H, CHSCH2CH2CH2Si], 1.22 [9H, CH2SiOCH2CH3], 0.74 [2H, CH2CH2CH2SiO]. FTIR (KBr, cm−1): 1082, 1167 (Si−O−C, stretching vibration), 2970 (−CH3, stretching vibration), 1386 (−CH3, deformation vibration), 967 (−CHCH−, deformation vibration). Preparation of Mesoporous Silica. First, organic− inorganic nanocomposite gels were prepared via a sol−gel process. Typically, in a polyethylene beaker equipped with a magnetic stirrer, PS-b-PBSi(OCH2CH3)3-b-PS (0.6024 g, 5.40 mmol with respect to the ethoxyl groups), TEOS (1.4080 g, 6.76 mmol), and tetrahydrofuran (5 mL) were charged with vigorous stirring. Thereafter, deionized water (0.5807 g, 32.26 mmol) and 1.0 M HCl (0.5687 g, 15.32 mmol) were added with a syringe with continuous stirring. The flask was sealed, and the sol−gel process was carried out at room temperature for 4 weeks. After the solvent was evaporated, an organic− inorganic composite gel was obtained. Second, the above organic−inorganic composite gel was subjected to pyrolysis at elevated temperature to remove the organic components. In a furnace, the composite gel was heated from room temperature to 500 °C at the heating rate of 10 °C/ min and then heated to 550 °C at 3 °C/min. In an air atmosphere, the sample was maintained at 550 °C for 3 h to obtain the silica. Measurement and Characterization. Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR measurements were conducted on a Scientific Nicolet iS10 Fourier transform spectrometer at room temperature (25 °C). The block copolymers were first dissolved with tetrahydrofuran with a concentration of 10 wt %, and the solutions were cast onto KBr windows. The solvent was evaporated at room temperature for 2 h and then in vacuo at 30 °C for 2 h. The films of the samples

improved resistance toward thermal and oxidative degradation.44,45 To enhance the polarity of the commercial triblock copolymers, some polar groups have been introduced into the block copolymer via post-chemical-modification approaches. Of them, epoxidization of PB subchains has been demonstrated to be effective in improving the polarity of the triblock copolymers.46,47 For a similar purpose, PS-b-PB-b-PS triblock copolymers have also been modified via chlorination,48 reactive grafting with maleic anhydride49,50 or glycidyl methacrylate,51 and hydrosilylation.52 Owing to improvement of the polarity, the compatibility of PS-b-PB-b-PS triblock copolymers with polar polymers is significantly improved, and the modified triblock copolymers have been used as good compatibilizing and toughening agents for many polymer blends. It is wellknown that PS-b-PB-b-PS triblock copolymers can exhibit a series of fine microphase-separated morphologies depending on the composition.53 This microphase separation behavior allows PS-b-PB-b-PS triblock copolymers to be ideal templates for other nanostructured materials.54−56 It is expected that chemical modification of PS-b-PB-b-PS would greatly extend the utilization of PS-b-PB-b-PS triblock copolymers in this aspect. However, such an investigation remains largely unexplored. Can the PS-b-PB-b-PS triblock copolymer be used as a template to synthesize mesoporous silica with adjustable porosity? Unfortunately, the experiments showed that a simple and direct utilization would not be successful because of the lack of silica-philic blocks in this triblock copolymer. Therefore, a measurement must be taken to improve the affinity of the triblock copolymer with the precursors of silica. If successful, this approach will be promising to scale up for the industrial preparation of mesoporous silica. The purpose of this work is 2fold: (i) to modify the PS-b-PB-b-PS triblock copolymer so that commercial triblock copolymers can be readily used as a template of mesoporous silica and (ii) to report the preparation of mesoporous silica by use of the modified PS-b-PB-b-PS triblock copolymers. To this end, we explore the modification of PS-b-PB-b-PS with (3-mercaptopropyl)triethoxysilane via a thiol-ene radical addition approach to endow the midblock (viz. PB) of the triblock copolymer with reactivity with the precursors of silica (e.g., TEOS). To the best of our knowledge, there has been no previous report in this regard. It should be pointed out that Yamauchi et al.57 have recently reported the preparation of mesoporous silica materials by use of a laboratory-made poly(methylstyrene)-block-polybutadieneblock-poly(methylstyrene) (PMS-b-PB-b-PMS) triblock copolymer, an analogue of PS-b-PB-b-PS. They first hydrogenated the midblock (i.e., PB) of the triblock copolymer and then sulfonated the end blocks (viz. PS). The affinity of the modified triblock copolymer with a silica matrix was achieved via electrostatic interactions between the sulfonated end blocks and silica, which is quite different from the approach of the present work. In this work, the mesoporous silica materials would be characterized by means of small-angle X-ray scattering, transmission electron microscopy, and nitrogen sorption measurements.

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EXPERIMENTAL SECTION Materials. Tetraethoxysilane (TEOS), (3-mercaptopropyl)triethoxysilane (MPTES), hydrochloric acid (HCl), and 2,2′azobis(isobutylnitrile) (AIBN) were of reagent grade and were purchased from Shanghai Reagent Co. Before use, AIBN was recrystallized from ethanol twice. The polystyrene-block6455


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Industrial & Engineering Chemistry Research Scheme 1. Thiol-ene Radical Addition of SBS with MPTES

vacuo at 180 °C for at least 6 h before measurements. Specific surface areas were calculated using the multipoint Brunauer− Emmett−Teller (BET) method using DeltaGraph graphics software. The pore volumes and pore-size distributions were calculated with the adsorption branches of isotherms according to the method of nonlocal density functional theory (NLDFT) because the pore-volume measurements of the resulting silica materials were over the entire micro/mesoporosity range.

were sufficiently thin to be within a range where the Beer− Lambert law is obeyed. In all cases, 64 scans at a resolution of 2 cm−1 were used to record the spectra. Nuclear Magnetic Resonance (NMR) Spectroscopy. The 1 H NMR measurements were carried out on a Varian Mercury Plus 400 MHz NMR spectrometer at 25 °C. The samples were dissolved in deuterium chloroform, and the 1H NMR spectra were obtained with tetramethylsilane (TMS) as the internal reference. The solid-state 29Si NMR experiments were carried out on a Bruker Avance III 400 MHz NMR spectrometer. The high-resolution 29Si NMR spectra were obtained with crosspolarization (CP)/magic angle spinning (MAS) together with the high-power dipolar decoupling (DD) technique. The 90° pulse width of 4.0 μs was employed with free induction decay signal accumulation, and the CP Hartmann−Hahn contact time was set at 3.5 ms for all of the experiments. The rate of MAS was 4.0 kHz for measuring the spectra. The Hartmann−Hahn CP matching and DD field was 58.47 kHz. The time of the recycle delay was set as 2.0 s for the signal accumulation. The chemical shifts of all 29Si spectra were determined by taking the silicon of solid Q8M8 relative to TMS as an external reference standard. Transmission Electron Microscopy (TEM). The suspension of mesoporous silica in ethanol was dropped onto 200 mesh copper grids, and the solvent was slowly evaporated at room temperature. The specimens were then subjected to morphological observation by means of TEM analyses on a JEOL JEM2010F high-resolution transmission electron microscope at an acceleration voltage of 120 kV. Small-Angle X-ray Scattering (SAXS). The SAXS measurements were taken on a SAXS beamline of the Shanghai Synchrotron Radiation Facility, China. Two-dimensional diffraction patterns were recorded using an image-intensified CCD detector. The experiments were carried out at room temperature (25 °C) under conditions of 8 keV photon energy and 300 nm small-angle resolution. The intensity profiles were output as the plot of the scattering intensity (I) versus scattering vector, q = (4π/λ) sin(θ/2) (θ = scattering angle). Thermogravimetric Analysis (TGA). A TA thermal gravimetric analyzer (Q-5000) was used to investigate the thermal stability of organic−inorganic silica gels. The samples (about 5.0 mg) were heated in a nitrogen atmosphere from ambient temperature to 800 °C at a heating rate of 20 °C/min in all cases. Specific Surface Area Analyses. Specific surface areas and pore-size distributions were determined by nitrogen sorption at 77 K using the volumetric technique on a Micromeritics ASAP 2010 instrument (Norcross, GA). The silica samples were ground, sieved through a 200-mesh sieve, and degassed in

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RESULTS AND DISCUSSION Modification of PS-b-PB-b-PS via Thiol-ene Radical Addition. The PS-b-PB-b-S triblock copolymer was modified

Figure 1. FTIR spectra of the PS-b-PB-b-PS and PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymers.

via its thiol-ene radical addition reaction with MPTES, as depicted in Scheme 1. This reaction was carried out with AIBN as the initiator; the addition reaction occurred between C−C double bonds in the midblock (i.e., PB) of the triblock copolymer and thiol groups of MPTES, whereas the end blocks (viz. PS) of the triblock copolymer remained unchanged. Shown in Figure 1 are the FTIR spectra of PS-b-PB-b-PS and PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymers. The PB 6456


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Figure 2. 1H NMR spectra of the PS-b-PB-b-PS and PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymers.

Scheme 2. Preparation of Mesoporous Silica with SBS Porogen

Table 1. Characterization of the Microdomains for Composite Gels and Mesoporous Silica by Means of SAXS composite gels BCP10TEOS90 BCP20TEOS80 BCP30TEOS70 BCP40TEOS60 BCP100

fitting parameters

microdomains PS PS PS PS PS

spheres spheres spheres spheres spheres and cylinders

R R R R R

= = = = =

31.3 28.7 25.1 24.8 33.1

nm, nm, nm, nm, nm,

σ σ σ σ σ

= = = = =

mesoporous silica

0.34 0.15 0.18 0.30 0.34

MS10 MS20 MS30 MS40 MS100

fitting parameters

microdomains spherical spherical spherical spherical spherical

pores pores pores pores and cylindrical pores

R R R R R

= = = = =

34.0 32.1 33.8 30.9 34.0

nm, σ = 0.12 nm, σ = 0.33 nm; σ = 0.27 nm, σ = 0.10 nm, σ = 0.29

that at 967 cm−1 is assignable to the out-of-plane bending vibration of C−H bonds in 1,2-disubstituted alkenes (i.e., R1CHCHR2). With the occurrence of thiol-ene radical addition, notably the bands at 911 and 990 cm−1 almost

block of PS-b-PB-b-PS was characterized by the bands at 911, 990, and 967 cm−1. The bands at 911 and 990 cm−1 are attributable to the out-of-plane bending vibration of C−H bonds in monosubstituted alkenes (i.e., RCHCH2), whereas 6457


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Figure 3. 29Si CP/MAS NMR spectra of organic−inorganic gels containing 0, 20, 40, and 100 wt % PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymers.

Figure 5. SAXS profiles of the organic−inorganic gels from mixtures containing 10−100 wt % PS-b-PBSi(OCH2CH3)3-b-PS. The red lines represent the form factor scattering, which were fitted according to the Percus−Yevick closure relationship.61

Figure 4. SAXS profiles of PS-b-PB-b-PS (A); the organic−inorganic gel from plain PS-b-PBSi(OCH2CH3)3-b-PS (B); the mesoporous silica from the PS-b-PBSi(OCH2CH3)3-b-PS (C).

Figure 6. TGA curves of the organic−inorganic gels containing PS-bPBSi(OCH2CH3)3-b-PS.

disappeared, whereas that at 967 cm−1 remained less affected. This result was in good agreement with that reported by Lotti et al.59 Concurrently, there appeared several new bands at 1082, 1167, 1386, and 2970 cm−1, respectively. The bands at 1082 and 1167 cm−1 are attributable to the stretching vibration of the Si−O−C moiety, whereas those at 1386 and 2970 cm−1 are

characteristic of the plane-bending vibration of the C−H bonds in methyl groups, respectively. The results of FTIR indicate that the PS-b-PB-b-PS triblock copolymer has been successfully modified via the thiol-ene radical addition reaction and that this reaction mainly occurred in the 1,2-addition structure units of the PB block. PS-b-PB-b-PS and PS-b-PBSi(OCH2CH3)3-b-PS 6458


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Figure 8. SAXS profiles of mesoporous silica pyrolyzed from the organic−inorganic gels. The red lines represent the form factor scattering, which were fitted according to the Percus−Yevick closure relationship.61

3.82 ppm with the occurrence of the thiol-ene radical addition reaction. The signal of resonance at 0.74 ppm is assignable to the methylene groups connected to the silicon atom in 3thiopropyl groups; the peaks of resonance at 1.22 and 3.82 ppm are attributable to the methyl and methylene groups in ethoxyl groups. The peaks of resonance at 1.68 and 2.52 ppm are assignable to the protons of other methylene groups, as indicated in this figure. FTIR and 1H NMR spectroscopy indicate that PS-b-PB-b-PS has been successfully modified via the thiol-ene radical addition reaction; i.e., the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer was successfully obtained. Preparation of Mesoporous Silica. Sol−Gel Process. The above PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer was incorporated into TEOS to perform the intercomponent sol− gel reaction to obtain organic−inorganic composite silica gels; the hybrid silica gels would be used as the precursors to obtain mesoporous silica (see Scheme 2). It is expected that the midblock of the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer was capable of undergoing the mutual sol−gel reaction with TEOS, whereas PS blocks remained unaffected. All of the organic−inorganic composite gels were homogeneous and transparent, suggesting that no macroscopic phase separation occurred in the sol−gel process. The compositions of the organic−inorganic composite gels are summarized in Table 1. For the gel from PS-b-PBSi(OCH2CH3)3-b-PS, it is proposed that there could be three types of silicon nuclei, i.e., RSi(OSi)(OH)2, RSi(OSi)2(OH) and RSi(OSi)3 (denoted as T1, T2, and T3) in the product of the sol−gel process, depending on the condensation degree of the triethoxysilane moieties. For TEOS, there could be four types of resonances of silicon nuclei, i.e., Si(OSi)(OH)3, Si(OSi)2(OH)2, Si(O-

Figure 7. TEM images of mesoporous silica: (A) MS10; (B) MS20; (C) MS30; (D) MS40; (E) MS100.

were subjected to 1H NMR spectroscopy, and the 1H NMR spectra are shown in Figure 2. For PS-b-PB-b-PS, the signals of resonance in the range of 0.8−2.5 ppm are assignable to the protons of methylene and methine groups in the main chains of PS and PB blocks as indicated; the peaks in the range of 6.2− 7.5 ppm are attributable to the protons of aromatic rings of PS blocks. In addition, the signals of resonance in the range of 4.6−5.8 ppm are assignable to the protons of methylene and methine groups of C−C double bonds in the PB block. Of them, the signals of resonance at 4.94 and 5.56 ppm are assignable to the protons of methylene and methine groups of vinyl groups in 1,2-addition structural units, whereas the resonance at 5.34 ppm is assignable to the protons of the methine in the double bonds of 1,4-addition structural units. With the thiol-ene radical addition reaction, notably the signals of resonance at 4.94 and 5.56 ppm completely disappeared, whereas that at 5.34 ppm was still discernible. This observation suggests that all of the vinyl groups in 1,2-addition structural units have fully undergone the thiol-ene radical addition reaction. In contrast, the peak of resonance at 5.34 ppm was still discernible, implying that the addition reaction of the double bonds in the 1,4-addition structural units did not go to completion under the present conditions (see the inset in Figure 2). This result was in good agreement with that of FTIR spectroscopy (see Figure 1). In the meantime, there appeared several new signals of resonance at 0.74, 1.22, 1.69, 2.52, and 6459


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Figure 9. Nitrogen sorption isotherms of the mesoporous silica materials.

Si)3(OH), and Si(OSi)4 (denoted as Q1, Q2, Q3, and Q4) in the product of the sol−gel process depending on the condensation degree. In order to investigate the intercomponent reaction, organic−inorganic composite gels were subjected to solid-state 29Si NMR spectroscopy. Shown in Figure 3 are the 29Si CP/MAS NMR spectra of organic−inorganic composite gels from plain PS-b-PBSi(OCH2CH3)3-b-PS, TEOS, and their mixtures. For the gel from the PS-bPBSi(OCH2CH3)3-b-PS triblock copolymer, two signals of silicon resonance at −61.5 and −53.3 ppm were detected. The former is assignable to the silicon nucleus of the completely condensed triethoxysilane (viz. T3), whereas that at low field (viz. −55.6 ppm) is assignable to that of the incompletely condensed triethoxysilane (viz. T2).60 For the gel from TEOS,

Table 2. Results of BET Measurements for Mesoporous Silica mesoporous silica

composite gel

[BCP]a: [TEOS] (wt)

Vpore (cm3/g)

SBET (m2STP/g)

silica MS10 MS20 MS30 MS40 MS100

BCP0TEOS100 BCP10TEOS90 BCP20TEOS80 BCP30TEOS70 BCP40TEOS60 BCP100

0:100 10:90 20:80 30:70 40:60 100:0

0.24 0.29 0.30 0.29 0.12

418 450 480 425 160

a BCP represents the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer.

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Figure 10. Incremental pore-volume distribution data obtained by fitting the DFT model to sorption isotherms.

three signals of silicon resonance were detected at −111.2, −101.4, and −91.6 ppm, respectively. These signals of resonance are assignable to the silicon nucleus of the Q4, Q3, and Q2 structures, respectively. For the gels from the mixture of PS-b-PBSi(OCH2CH3)3-b-PS with TEOS, two groups of silicon resonance signals were displayed in the ranges of −40 to −80 and −90 to −120 ppm, respectively. The former resulted from PS-b-PBSi(OCH2CH3)3-b-PS, whereas the latter resulted from TEOS. Nonetheless, the 29Si CP/MAS NMR spectra of the composite silica gels from mixtures of PS-

b-PBSi(OCH2CH3)3-b-PS with TEOS were not the simple superposition of the spectra from PS-b-PBSi(OCH2CH3)3-b-PS and TEOS gels. It is noted that all of the T peaks shifted to high field, whereas the Q peaks remained less affected. This observation indicates that there were intercomponent sol−gel reactions between PS-b-PBSi(OCH2CH3)3-b-PS and TEOS and the covalent bonds were indeed formed between polysilsesquioxane from PS-b-PBSi(OCH2CH3)3-b-PS and silica from TEOS. 6461


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Industrial & Engineering Chemistry Research Microphase Separation in Organic−Inorganic Composite Gels. All of the organic−inorganic composite gels from PS-bPBSi(OCH2CH3)3-b-PS and its mixtures with TEOS were subjected to SAXS. Shown in Figure 4 are the SAXS profiles of PS-b-PB-b-PS, the gel from the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer and the mesoporous silica from PS-bPBSi(OCH2 CH 3) 3-b-PS. For PS-b-PB-b-PS, the intense primary scattering peak was detected at qm = 0.14 nm−1 and other scattering peaks were discernible at q = 0.22 and 0.42 nm−1. The SAXS results indicate that PS-b-PB-b-PS was microphase-separated and the PS (or PB) microdomains were arranged in a lamellar lattice. According to the position of the primary scattering peak, Bragg’s spacing dm can be estimated to be L = 44.9 nm. Compared to PS-b-PB-b-PS, a single scattering peak at q = 0.22 nm−1 was exhibited in the SAXS profile of the gel from PS-b-PBSi(OCH2CH3)3-b-PS. Notably, the primary scattering peak shifted to the position with a higher qm value; i.e., the average distance between adjacent PS microdomains was shortened to L = 28.5 nm. The decreased d spacing suggests that the lamellar microdomains in PS-b-PB-b-PS could be transformed into spherical (or cylindrical) microdomains owing to an increase in the volume fraction of the midblock after the PB block was bonded with (3thiopropyl)triethoxysilane moieties via thiol-ene radical addition. It is proposed that in the composite gel the spherical (or cylindrical) PS microdomains could be dispersed into a continuous PB−polysilsesquioxane matrix. The results of SAXS indicate that the gel from PS-b-PBSi(OCH2CH3)3-b-PS was still microphase-separated. The fact that no multiple scattering peaks were detected suggests that a disordered nanostructure was formed in the organic−inorganic composite gel. The SAXS profiles of organic−inorganic composite gels from mixtures of PS-b-PBSi(OCH2CH3)3-b-PS with TEOS are shown in Figure 5. Notably, the scattering phenomena were displayed in all cases. The SAXS results indicate that all of these organic−inorganic composite gels were microphase-separated; i.e., the PS microdomains could be dispersed in the continuous matrixes composed of silica and polysilsesquioxane networks. The scattering peaks shifted to the positions with the lower q values with decreasing content of the PS-b-PBSi(OCH2CH3)3b-PS triblock copolymer, indicating that the average distance between the adjacent PS microdomains increased with increasing content of TEOS. According to the Bragg equation, the long periods in the silica materials from the gels containing 100, 40, 30, 20, and 10 wt % PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymers were calculated to be L = 28.5, 14.3, 15.3, 25.1, and 27.3 nm−1, respectively. In all cases, the broad and flat scattering peaks were exhibited, which resulted from the form factor scattering of the PS microdomains dispersed in a silica matrix. Notably, the values of the scattering vector were significantly higher than that of the gel from PS-b-PBSi(OCH2CH3)3-b-PS (viz. q = 0.22 nm−1); i.e., the average distances between the adjacent PS microdomains in organic− inorganic composite gels were much lower than that in the gel from the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer. The nanostructures of organic−inorganic composite gels from mixtures of PS-b-PBSi(OCH2CH3)3-b-PS and TEOS could be affected by the following two factors: (i) the content of the silica generated from TEOS and (ii) the sizes of the PS microdomains in situ formed in the sol−gel reactions. In the composite gels from the mixture of PS-b-PBSi(OCH2CH3)3-bPS with TEOS, the average distance between the adjacent PS microdomains could be increased with the content of the silica

formed via the sol−gel process; i.e., the q values decreased with increasing content of silica if the sizes of the PS microdomains remained unchanged. Nonetheless, the sizes of the PS microdomains in the composite gels could be changed with the occurrence of the intercomponent sol−gel reaction. It is plausible to propose that the PS microdomains could be formed with sizes smaller than that in the gel only from the PS-bPBSi(OCH2CH3)3-b-PS triblock copolymer. The decreased sizes of the PS microdomains are responsible for an increase in the volume fraction of the silica-philic blocks in the form of PB−polysilsesquioxane, which was produced via hydrolysis and the polycondensation reaction of PBSi(OCH2CH3)3. In this case, the measured average distance between adjacent PS microdomains would be increased because of the formation of PS microdomains with smaller sizes; i.e., the q values of the composite gels would be higher than that of the gel only from the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer. In the present case, the observation that the q values of the resulting composite gels were significantly higher than that of the gel from the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer suggests that the latter factor dominated the formation of PS microdomains. In fact, this inference was indeed confirmed with the curve-fitting results of the SAXS profiles, as summarized in Table 1 (we will be back to this issue latter on). Pyrolysis of Organic−Inorganic Nanocomposite Gels. The above organic−inorganic nanocomposite gels were used as precursors to obtain mesoporous silica via pyrolysis at elevated temperatures. The purpose of pyrolysis is to remove the organic portions of the organic−inorganic nanocomposite gels, whereas the silica backbone remained invariant. The organic portions in the composite gels included the PS microdomains, the main chains of PB blocks, and 3-thiopropyl groups bonded onto the PB block. The matrixes of the organic−inorganic gels were composed of polysilsesquioxane and silica. To determine the temperatures degrading all of these organic portions, the organic−inorganic nanocomposite gels were subjected to TGA, and the TGA profiles are shown in Figure 6. All of these gels displayed the initial degradation at ca. 180 °C and the second degradation at ca. 300 °C. The initial degradation steps could be associated with the subsequent condensation of silanol hydroxyls in the matrixes of the organic−inorganic gels due to incomplete condensation under the conditions of the present sol−gel process. The second mass loss steps resulted from thermal degradation of the organic portions in the gels. Notably, thermal decomposition occurred until the samples were heated to about 550 °C, at which the plateaus of degradation appeared. The appearance of the degradation plateaus indicates that the organic portions in these gels were decomposed to completion; i.e., all of the organic−inorganic gels were converted into inorganic silica, which was evidenced by the observation that all of the silica materials obtained were purely of white color. The yields of silica increased with increasing percentage of TEOS, and they were 73.0, 62.5, 56.9, 55.2, and 36.6% for BCP10TEOS90, BCP20TEOS80, BCP30TEOS70, BCP40TEOS60, and BCP100 gels, respectively. With the results of TGA, the conditions of pyrolysis were thus determined. All of the organic−inorganic nanocomposite gels were subjected to pyrolysis at elevated temperature in a furnace, and the conditions of pyrolysis were used as described in the Experimental Section. It is expected that the removal of the PS microdomains contributed to the formation of mesopores (or nanopores) with sizes of 2−50 nm diameter, 6462


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[I(q)] depends on the square of the contrast difference (Δρ2), the number of scattering particles (N), the shape and size of the scattering particle described by the form factor [P(q)], and interdomain correlations accounted for by a structure factor [S(q)]:

whereas decomposition of other organic portions resulted in microporosities with sizes lower than 2 nm. Morphologies and Surface Area Analysis of Mesoporous Silica. All of the silica materials were subjected to morphological observation by means of TEM, and the TEM images are shown in Figure 7. It is seen that all of the silica materials displayed mesoporous structures. For silica from the gel of the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer, the spherical (and/or cylindrical) pores were dispersed in continuous silica matrixes with sizes of 20−30 nm diameter (Figure 7E). In all cases, for the silica materials from the composite gels of PS-b-PBSi(OCH2CH3)3-b-PS with TEOS, the spherical pores were formed with diameters of 10−20 nm. The quantity of pores increased with increasing content of PSb-PBSi(OCH2CH3)3-b-PS (Figure 7A−D). It is noted that the diameters of the spherical nanopores were significantly smaller than those of the mesopores in the silica from the gel of PS-bPBSi(OCH2CH3)3-b-PS. The above mesoporous silica materials were subjected to SAXS, and the SAXS profiles are shown in Figure 8. In all cases, the scattering phenomena were displayed, indicating that the silica materials indeed possessed nanoporous (or mesoporous) structures. Notably, the SAXS profiles of the silica materials were quite similar to those of the corresponding organic− inorganic gels, suggesting that under the present conditions of pyrolysis the backbone of the silica remained less affected after the organic portions in the organic−inorganic gels. For mesoporous silica from the gel of the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer, the primary scattering peak appeared at q = 0.41 nm−1 (see curve C in Figure 4). Notably, the q value was much higher than that of the PS-b-PBb-PS triblock copolymer (viz. q = 0.22 nm−1), suggesting that the average distance between adjacent mesopores in the silica was significantly smaller than that between adjacent PS microdomains in the organic−inorganic gel. This observation was also found for the other silica materials and the corresponding composite gels containing 40, 30, 20, and 10 wt % PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymers (see Figures 5 and 8). According to the position of the primary scattering peak, Bragg’s spacing dm values were calculated as L = 15.3, 13.1, 14.3, 21.7, and 22.4 nm−1 for the silica materials from the gels containing 100, 40, 30, 20, and 10 wt % PS-bPBSi(OCH2CH3)3-b-PS, respectively. The fact that these values of dm spacing were quite smaller than those of the corresponding gels indicates that the average distances between adjacent mesopores in the silica materials were significantly lower than those between adjacent PS microdomains in the corresponding hydrogels. The decrease in the interdomain distances is responsible for shrinkage of the silica networks compared to the silica matrix in the gels after removal of the organic portions. The SAXS results indicate that mesoporous silica materials were successfully obtained. In view of the TEM results, we fitted the measured SAXS data with a model of hard spheres with polydispersity to obtain information about the sizes of the mesopores (and/or the PS microdomains) in the silica (and/or the gels). In this curve fitting, the structure factor for hard spheres was fitted according to the Percus−Yevick closure relationship.61 The effect of interaction considered in this model is only the excluded volume present in a dispersion of hard spheres; the polydispersity is taken into account by simply averaging the partial structure factors of the single components.62 In this model, the intensity of scattering at a given scattering vector

I(q) = Δρ2 NP(q) S(q)

(1)

In the curve-fitting of the SAXS profiles for the organic− inorganic gels, we assumed that the PB blocks around the PS microdomains have essentially the same scattering density as the silica matrix (i.e., they are contrast-matched) and that the scattering results almost from the contrast between the PS microdomains and the silica matrix. The scattering of the microdomains (viz. PS microdomains and/or mesopores) is represented by the spherical form factor convoluted with a Gaussian distribution with a standard deviation (σ) to account for the polydispersity in the radii of the spherical mesopores. The interpore interaction was modeled as hard-sphere potentials between the mesopores with disordered packing in the silica matrix. Also shown in Figures 5 and 8 are the fitted SAXS curves (the red bold lines) by combining both the form factor [P(q)] and structural factor scattering [S(q)]. Notably, the model can fit the measured data quite well, yielding the fitting parameters summarized in Table 1. The fitting results showed that the radii of the mesopores were in the range of 25−34 nm with standard errors of 0.10−0.34. It is seen that the size of the PS microdomains (and/or mesopores) in the gel of the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer (or in its silica) was significantly higher than those of the PS microdomains in the composite gels (or the corresponding silica). In addition, the radii of the mesopores in the silica materials were slightly larger than those of the PS microdomains in the corresponding organic−inorganic gels. The observation could be interpreted in terms of the formation of a number of micropores around the PS microdomains in the silica matrix after the organic portion was decomposed. It should be pointed out that the sizes of the microdomains for the gel and mesoporous silica from the PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymer were subject to some underestimation because there were some cylindrical microdomains besides the spherical mesopores. The specific surface areas of mesoporous silica were measured with nitrogen sorption experiments. Shown in Figure 9 are the nitrogen sorption isotherms of the nanoporous silica materials, and the structural information derived from the nitrogen adsorption data is summarized in Table 2. Apart from MS100, these silica materials exhibited typical type IV sorption isotherms with a H2-type hysteresis loop, suggesting that the silica materials possessed some cagelike pores with small windows. The MS100 sample displayed adsorption behavior different from that of the other samples. For MS100, the adsorption first occurred at low relative pressure (P/P0 < 0.1) and finally at higher relative pressure (P/P0 > 0.9); the closure between the adsorption and desorption isotherms was not achieved, which was characteristic of type I adsorption.63−65 This kind of isotherm is generally observed for the adsorption of gases on porous solids containing micropores, the sizes of which do not exceed the molecular diameter of the adsorbate. Complete filling of these micropores results in the formation of a molecular monolayer. The MS100 sample exhibited a major increase in the adsorption volume at pressures close to the saturation vapor pressure. This behavior is responsible for the capillary condensation of nitrogen in secondary pores (i.e., 6463



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ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of China (Grants 51133003 and 21274091) is gratefully acknowledged. The authors thank the Shanghai Synchrotron Radiation Facility for support under Projects 10sr0260 and 10sr0126.

mesopores with radii of 20−50 nm) with some contribution of multilayer adsorption at the surface of these pores. In the silica resulting from the organic−inorganic gel of PS-b-PBSi(OCH2CH3)3-b-PS, the micropores were created with the removal of PS microdomains. In addition, there were some micropores with very small sizes. The extremely low size could result from removal of the organic portion (viz. PB backbone) and oxidation of polysilsesquioxane. From the adsorption branches of the nitrogen sorption isotherms, the BET surface areas are calculated to be 418, 450, 480, 425, and 160 m2/g for the mesoporous silica materials from nanocomposite gels containing 10, 20, 30, 40, and 100 wt % PS-b-PBSi(OCH2CH3)3-b-PS triblock copolymers, respectively. The values of the porous volume (V) of mesoporous silica materials from the gels of PS-b-PBSi(OCH2CH3)3-b-PS and its mixtures were measured in the range of 0.12−0.24 cm3/g (see Table 2). In this work, the pore volumes and pore-size distributions were calculated with the adsorption branches of isotherms according to the method of NLDFT66−68 because the pore volumes of the resulting silica materials were over the entire micro/ mesoporosity range. Shown in Figure 10 are plots of the incremental pore volume as a function of the pore width. Notably, the broad distribution of the pore volume was found for all received silica materials. In these samples, some pores had widths of 20−40 nm, whereas other pores had widths of 4−7 nm. It is proposed that the pores with large width were formed with removal of the PS microdomains and can be well characterized by means of SAXS and TEM. The micropores with extremely small size could mainly result from the decomposition of other organic portions of the block copolymer. The results of the BET measurements indicate that the materials of mesoporous silica were successfully obtained.

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REFERENCES

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CONCLUDING REMARK A commercially available PS-b-PB-b-PS triblock copolymer was successfully modified with MPTES via a thiol-ene radical addition reaction. With this reaction, the midblock (viz. PB) of the triblock copolymer was transformed to a new subchain bearing (3-thiopropyl)triethoxysilane moieties, whereas the end block (i.e., PS) remained invariant. The derivative triblock copolymer was then employed to perform intercomponent sol−gel reactions with TEOS to obtain organic−inorganic nanocomposite gels with various compositions. The organic− inorganic nanocomposite gels were successfully used as precursors to obtain the mesoporous silica materials via pyrolysis at elevated temperatures. The results of SAXS, TEM, and BET measurements indicate that the mesoporous silica materials were successfully obtained and the porosity of the silica materials can be modulated with the compositions of the organic−inorganic nanocomposite gels. The approach of functionalization for a commercial PS-b-PB-b-PS triblock copolymer is facile and effective, which could be suitable for the scalable preparation of mesoporous silica materials with controllable porosity.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-21-54743278. Fax: +8621-54741297. Notes

The authors declare no competing financial interest. 6464


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Polystyrene-block-Polybutadiene-block-Polystyrene Triblock

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