Organic–Inorganic Hybrid Mesoporous Polymers Fabricated by Using

*E-mail: [email protected]. Cite this:Langmuir 28, 42, 15024- ... No additional treatment was needed to remove the templates. The reactive functional mo...
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Organic−Inorganic Hybrid Mesoporous Polymers Fabricated by Using (CTA)2S2O8 as Self-Decomposed Soft Templates Tianyou Chen, Binyang Du,* and Zhiqiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Organic−inorganic hybrid mesoporous polymers were successfully synthesized by using a templatedirected free radical polymerization technique in aqueous solution at 0−5 °C with oxidative complexes as selfdecomposed soft templates. The oxidative complexes ((CTA)2S2O8), which were formed between anionic oxidant (S2O82−) and cationic surfactant (cetyltrimethylammonium bromide, CTAB) at 0−5 °C, can be automatically decomposed due to the reduction of S2O82−. No additional treatment was needed to remove the templates. The reactive functional monomer, 3-(trimethoxysilyl)propyl methacrylate (TMSPMA), was used as main monomer. Styrene was used as the comonomer. With simultaneous free radical copolymerization of TMSPMA and styrene, condensation of methoxysilyl groups, and the self-decomposition of (CTA)2S2O8, organic−inorganic hybrid mesoporous polymers were successfully obtained. The mesoporous structures and morphologies of the resultant hybrid mesoporous polymers were found to be strongly dependent on the feed amounts of TMSPMA and styrene. In the absence of styrene, the hybrid polymer PTMSPMA exhibited mesh-like bicontinuous structures with mesopores and high surface area (335 m2/g). With the incorporation of styrene, mesoporous nanoparticles were obtained. The surface areas of the mesoporous nanoparticles decreased with the increase of styrene contents. The adsorption capabilities of such mesoporous polymers for organic dye (Congo red) and protein (bovine serum albumin) were also studied.

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(trimethylolpropane trimethacrylate).8 However, strongly corrosive chemicals such as hydrofluoric acid and sodium hydroxide are often required to etch these hard templates in order to form the mesoporous structures of the final products. These strongly corrosive chemicals might also damage the mesoporous polymeric materials. To avoid the usage of corrosive chemicals, soft templates were developed. Supramolecular aggregates of ionic surfactants or block copolymers are usually used as soft templates for the preparation of mesoporous polymers.1,4,5,9−14 Most mesoporous polymers obtained with soft templates, such as polytriallylamine,1 poly(2,4,6-triallyloxy-1,3,5-triazine),5 polyacrylonitrile,9 and poly(polyhedral vinylsilsesquioxane),10 were without wellorganized structures due to the flexibilities of the polymers and soft templates. These soft templates were usually removed by solvent extraction or controlled pyrolysis, which were timeconsuming and costly procedures. With some rigid monomers like phenol and formaldehyde, mesoporous phenolic resins with order structures could be obtained via the condensation polymerization of phenol and formaldehyde with block

INTRODUCTION Mesoporous polymers, which possess high surface areas, have wide varieties of applications including catalysis, removal of pollutant ions and volatile organic compounds, size-exclusion separation of peptides and proteins, and dielectric materials, etc.1−5 Mesoporous polymers are usually prepared by using templating techniques.6 Hard templates and soft templates have been used. For example, hard templates like MCM-48, SBA-15, and KIT-6 mesoporous silica have been employed to prepare mesoporous polydivinylbenzene, poly(ethylene glycol dimethacrylate), and poly(trimethylolpropane trimethacrylate).7,8 These hard templates with well-organized structures are suitable for the preparation of ordered mesoporous polymeric materials since the resultant mesoporous polymeric materials were expected to faithfully replicate the porous structures of the hard templates. After etching away these hard templates, order mesoporous polymeric materials could be obtained. The pore diameters of the resultant mesoporous polymeric materials could be controlled by choosing suitable templates. Kim et al. reported that ordered mesoporous polydivinylbenzene could be obtained by replicating the mesoporous MCM-48 and SBA-15 channel frameworks.7 The mesoporous silica template, KIT-6, could be used to fabricate ordered mesoporous polydivinylbenzene, poly(ethylene glycol dimethacrylate), and poly© 2012 American Chemical Society

Received: June 9, 2012 Revised: September 9, 2012 Published: October 2, 2012 15024

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copolymerization of TMSPMA with other monomers.18,19,23−27 Styrene was used as the comonomer here. With simultaneous free radical copolymerization of TMSPMA and styrene, condensation of methoxysilyl groups, and the self-decomposition of oxidative complexes ((CTA)2S2O8), organic− inorganic hybrid mesoporous polymers were successfully obtained. Note that CTAB also served as the emulsification agent for the hydrophobic vinyl monomers, i.e., TMSPMA and styrene, in aqueous solution. The mesoporous structures and morphologies of the resultant hybrid mesoporous polymers were found to be strongly dependent on the feed amounts of TMSPMA and styrene. These organic−inorganic hybrid mesoporous polymers may also have potential uses for the adsorption and separation of organic dyes or proteins.

copolymers like poly(styrene)-b-poly(4-vinylpyridine) and Pluronic F127 as soft templates.11−13 The soft templates were then removed by thermal decomposition of the template polymers, leading to the formation of mesoporous carbon structures.11−13 Hence, developing new methods for the preparation of mesoporous polymeric materials is still a challenging task. Organic−inorganic hybrid materials are new types of materials which possess and combine the properties of organic components and inorganic components at the same time, rendering them more superior microstructures and properties. Numerous efforts have been dedicated to the fabrication and characterization of organic−inorganic hybrid materials.10,15−20 In the field of mesoporous polymeric materials, various organic−inorganic hybrid mesoporous polymers have been reported in recent years. Most of them were prepared via postmodification of the mesoporous silica with functional polymers by using grafting-to or grafting-from techniques.16,17,20 For example, light-responsive copolymer, poly(Nisopropylacrylamide-co-2-nitrobenzyl acrylate) (poly(NIPAMNBAE)), was grafted onto the outer surface of mesoporous silica nanoparticles via the grafting-to technique, forming organic−inorganic hybrid nanogated ensembles, which enable the encapsulation and release of drug and biological molecules under light irradiation.16 Poly(glycidyl methacrylate) (PGMA) chains was tethered onto the surface of SBA-15 materials by using a grafting-from technique, leading to the formation of hybrid organic−inorganic PGMA−SBA-15 materials.20 The silica-based SBA-15 materials were first functionalized with aminopropyl groups and 2-bromo-2methylpropionyl bromide to form ATRP initiator species, and the tethering poly(glycidyl methacrylate) (PGMA) chains were then synthesized via the atomic transfer radical polymerization (ATRP) technique.20 Recently, Nischang et al. reported the hierarchically structured hybrid materials with a hierarchical pore space prepared via the free radical polymerization of polyhedral vinylsilsesquioxane with PEG200 as soft templates.10 The PEG200 soft templates were extracted with THF in a Soxhlet apparatus.10 Here, we report the preparation of organic−inorganic hybrid mesoporous polymers via the template-directed free radical copolymerization of vinyl monomers in aqueous solution at 0− 5 °C with oxidative complexes as self-decomposed templates. The oxidative complexes ((CTA)2S2O8) were formed between anionic oxidant (S2O82−) and cationic surfactant (cetyltrimethylammonium bromide, CTAB) at 0−5 °C. Such complexes have lamellar mesostructures and have been first applied to synthesize conducting polymers with nanostructures, like wire/ ribbon-like polypyrrole21 and nanoclip-like polyaniline, polypyrrole, and poly(3,4-ethylenedioxythiophene).22 During the chemical oxidative polymerization, the oxidative complexes were automatically decomposed due to the reduction of oxidizing anions.21,22 In the present work, the reduction of S 2 O 8 2− was accelerated by injecting an accelerant of tetramethylethylenediamine (TMEDA), which can then generate free radicals and hence initiate the free radical polymerization of vinyl monomers. The reactive functional monomer, 3-(trimethoxysilyl)propyl methacrylate (TMSPMA), was used as main monomer. The methoxysilyl groups (−Si(OCH3)3) can be hydrolyzed into the silanol (−Si(OH)3) groups, which will be subsequently condensed into the crosslinked polysilsesquioxane. TMSPMA has been widely used in the preparation of organic−inorganic hybrid materials by

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EXPERIMENTAL SECTION

Chemicals and Materials. 3-(Trimethoxysilyl)propyl methacrylate (TMSPMA, 98%) and tetramethylethylenediamine (TMEDA, 99%) were purchased from Acros Organics and used as received. Potassium peroxydisulfate (K2S2O8), cetyltrimethylammonium bromide (CTAB), and styrene were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. Congo red (CR) was purchased from Aladdin Chemistry Co. Ltd. and used as received. Bovine serum albumin (BSA, 98%) was purchased from J&K Chemical Co. Ltd. All other reagents were of analytical grade. Synthesis of Mesoporous Polymeric Materials. Typically, 25 mg of CTAB was first dissolved in 190 mL of deionized water at room temperature in a three-neck flask under vigorous magnetic stirring. After complete dissolution of CTAB, the flask was immersed into an ice bath (0−5 °C). Oxygen was eliminated by bubbling nitrogen through the solution under vigorous stirring for 10 min. 1.50 mL of TMSPMA with various amounts of styrene were then added under vigorous magnetic stirring. The mixed solution became milky immediately due to the emulsification of the monomers by CTAB droplets. After 10 min, 10 mL of K2S2O8 aqueous solution (5 mg/mL) was added. White flocculent precipitates appeared immediately, indicating the complexation between CTAB and K2S2O8.21,22 By waiting for 10 min to allow the complete interaction of CTAB and K2S2O8, 50 μL of TMEDA was added to initiate the free radical polymerization of the monomers. The polymerization was allowed to proceed for 24 h at 0−5 °C under vigorous stirring. The final products were collected by centrifugation, washed with deionized water for three times, and then dried in vacuum at 60 °C for 24 h. Adsorption Studies for Congo Red (CR). The aqueous solution of CR with concentration of 50 mg/L was used for the adsorption studies. The adsorption behaviors of mesoporous polymers were measured according to the following procedure. Typically, 10 mg of mesoporous polymers was added into 10 mL of CR aqueous solution (50 mg/L), and the mixture was then shaken at room temperature for 24 h. The mixture solution was filtered through a syringe-driven filter with pore size of 0.45 um for the further characterizations. The residue concentration of CR in the filtered solution was measured by UV−vis spectroscopy at 495 nm. Adsorption Studies for Bovine Serum Albumin (BSA). BSA acetate buffer solutions (pH = 4.7) with the concentrations of 50, 100, 150, 200, 250, 300, 350, 400, 450, and 500 mg/L were prepared for the batch mode adsorption. The adsorption behavior of PTMSPMA was measured as following procedure. First, 10 mg samples were added into 10 mL of BSA acetate buffer solution with a predetermined concentration, and the mixture was then shaken at room temperature for 24 h. The mixture solution was separated by centrifugation at 5000 rpm for 20 min. Concentrations of BSA in solutions before and after the adsorption were determined by UV−vis spectroscopy at 277 nm. The equilibrium adsorption capacities (Qe) was determined according to the following formula: Qe = [(Ci − Ce)V]/m, wherein Ci is the initial concentration of adsorbate (BSA), Ce is the equilibrium concentration of adsorbate (BSA) in the liquid phase, V is the volume 15025

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Scheme 1. Schematic Illustration of the Preparation Process of Mesoporous PTMSPMA

ions at 0−5 °C. S2O82− ions were then added, leading to the formation of (CTA)2S2O8 complexes at 0−5 °C, which were lamellar in shape and had several micrometers in size as revealed by TEM (Figure S1a, see Supporting Information). The accelerant tetramethylethylenediamine (TMEDA) was further added into the reaction mixture in order to reduce the S2O82− ions into radicals, which could then initiate the free radical polymerization of vinyl-type monomer, i.e., TMSPMA. During the formation of radical and the proceeding of free radical polymerization, the (CTA)2S2O8 soft templates were simultaneously decomposed with the reduction of S2O82− ions, and the hydrolysis and condensation of methoxysilyl groups (−Si(OCH3)3) also led to the formation of cross-linked polysilsesquioxane. The polymerization was allowed to proceed for 24 h at 0−5 °C, and the mesoporous PTMSPMA was obtained (see below). A controlled experiment without any monomer was performed to demonstrate the decomposition of (CTA2)S2O8 templates in the presence of TMEDA. Figure S1b shows the TEM image of (CTA2)S2O8 templates after adding TMEDA for 3 h. It can be clearly seen that the (CTA2)S2O8 templates were already partially decomposed within 3 h in the presence of TMEDA. Four hybrid mesoporous polymers were then prepared with a fixed amount of TMSPMA and various amounts of styrene. The sample codes and detailed feed amounts of the monomers are listed in Table 1. Figure 1 shows the TEM images of the resultant hybrid mesoporous polymers, PTMSPMA, PTS-15, PTS-5, and PTS-2. It could be seen that PTMSPMA (Figure 1a) exhibited mesh-like structures. The mesh sizes were in the range of ca. 20−50 nm. The SEM image of PTMSPMA further indicated that the mesh-like structures were bicontinuous and three-dimensionally connected, as shown in Figure 2a.

of the BSA acetate buffer solution, and m is the mass of the PTMSPMA. Instrumental and Characterization. The morphologies of the resultant organic−inorganic hybrid mesoporous polymeric materials were obtained by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The TEM measurements were performed by using a JEOL JEM-1200 electron microscope operating at an acceleration voltage of 60 kV. The TEM samples were prepared by dip-coating with carbon-coated copper grids into the corresponding sample solutions. The solvent was gently absorbed away by a filter paper. The grids were then allowed to dry in air at room temperature before observation. SEM measurements were performed by using a Hitachi S4800 electron microscope. A droplet of the sample solution was cast onto the aluminum foil at room temperature. After about 15 min, the excess solution was gently absorbed away by a filter paper, and the aluminum foil was then allowed to dry in air at room temperature. Before SEM observation, the samples were coated with platinum vapors. Several positions of the samples were imaged, and energy dispersive X-ray analysis (EDX) was also carried out. Fourier transform infrared (FT-IR) spectra were recorded by using a Vector 22 Bruker spectrometer in the transmission mode using potassium bromide (KBr) pellets of the samples. Thermogravimetric analyses (TGA) were performed by using a SDT Q600 (TA Instruments) under the dry air at a flow rate of 100 mL/min. The temperature of the oven was first ramped to 100 °C and then held for 10 min to ensure desorption of adsorbed water, after which the temperature was further ramped to 800 °C at a rate of 10 °C/min. Powder X-ray diffraction (XRD) were performed by using a Rigaku D/ max 2550PC using Cu Kα radiation source (λ = 1.540 59 Å) with a 2θ range of 0.5°−10°. UV−vis spectra were performed on a Cary 300 instrument (Varian Australia Pty Ltd.). The concentrations of Congo red (CR) and bovine serum albumin (BSA) in the aqueous solutions were determined from the adsorption intensities at 495 and 277 nm, respectively. The nitrogen adsorption−desorption isotherms were measured by using a Micromeritics Tristar II 3020 system. The samples were pretreated at 80 °C overnight in the vacuum. The data were analyzed by the BJH (Barret−Joyner−Halenda) method with the Halsey equation for multilayer thickness. The pore size distribution was calculated from the desorption branch of the isotherm. The surface area was determined by the BET (Brunauer−Emmett−Teller) method.

Table 1. Feed Amounts of the Monomers (TMSPMA and Styrene) and the Porous Properties of the Resultant Organic−Inorganic Hybrid Mesoporous Polymers feed amounts (mL)

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RESULTS AND DISCUSSION The preparation process of the organic−inorganic hybrid mesoporous polymers by using self-decomposed soft template (CTA)2S2O8 was schematically illustrated in Scheme 1. As shown in Scheme 1, TMSPMA was first emulsified by CTA+ 15026

porous properties

sample codes

TMSPMA

styrene

BET surface area (m2/g)

PTMSPMA PTS-15 PTS-5 PTS-2

1.50 1.50 1.50 1.50

0.10 0.30 0.75

335 170 132 82

pore volume (cm3/g) 0.610 0.503 0.442 0.237

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Figure 1. TEM images of the resultant mesoporous polymers: (a) PTMSPMA, (b) PTS-15, (c) PTS-5, and (d) PTS-2.

Figure 2. SEM images of the resultant mesoporous polymers: (a) PTMSPMA, (b) PTS-15, (c) PTS-5, and (d) PTS-2.

Interestingly, the incorporation of styrene led to the transformation of mesh-like structure into spherical-like morphology of the mesoporous polymers. Figures 1b−d and 2b−d show the TEM and SEM images of PTS-15, PTS-5, and PTS-2, respectively. Mesoporous nanoparticles were observed; the sizes of the nanoparticles of the PTS-15, PTS-5, and PTS-2 were ca. 170 ± 30, 490 ± 73, and 630 ± 169 nm, respectively. It was understandable because styrene was a hydrophobic monomer, and the free radical copolymerization of TMSPMA

and styrene was carried out in aqueous solution. With the reduction of K2S2O8 and the initiation of free radical copolymerization of TMSPMA and styrene, the (CTA)2S2O8 complexes will be decomposed and CTAB will be released. Because of the hydrophobic nature, styrene preferred to locate inside the CTAB droplets although the concentration of CTAB (ca. 0.34 mM) used here was lower than the critical micelle concentration of CTAB (0.80 mM) in aqueous solution.28 Somehow like the emulsion polymerization, mesoporous 15027

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nanoparticles were thus formed for the copolymerization of TMSPMA and styrene in aqueous solution. The surfaces of these mesoporous nanoparticles were rough and consisted of many smaller nanoparticles. The sizes of mesoporous nanoparticles increased with increasing the amount of styrene. The exact formation mechanism of such mesoporous polymers was still unclear. It was supposed that the mesoporous structures were resulted from the synergistic effects of simultaneous free radical copolymerization of TMSPMA and styrene, condensation of methoxysilyl groups, and the self-decomposition of (CTA)2S2O8 complexes. In the control experiments, no mesoporous polymer can be obtained when only styrene, 3aminopropyltriethoxysilane, or styrene and 3-aminopropyltriethoxysilane were used. Further experiments are still needed to illustrate the formation mechanism of such hybrid mesoporous polymers. The chemical compositions, structures, and properties of such organic−inorganic hybrid mesoporous polymers were then investigated in detail. The potential applications of such mesoporous polymers in the adsorption and separation of organic dyes and proteins from the aqueous solution were also explored in the present work. Figure 3 shows the elemental composition of the hybrid mesoporous polymers, i.e., PTMSPMA, PTS-15, PTS-5, and

Table 2. EDX Results of the Resultant Hybrid Mesoporous Polymers, i.e., PTMSPMA, PTS-15, PTS-5, and PTS-2 element (atom percentage/%) sample codes

C

O

Al

Si

Pt

PTMSPMA PTS-15 PTS-5 PTS-2

61.45 65.54 68.34 75.20

19.47 19.00 10.66 8.10

10.99 7.08 16.13 13.38

7.99 8.29 4.80 3.25

0.11 0.10 0.08 0.06

transform infrared (FT-IR) spectroscopy. Figure 4 shows the FT-IR spectra of PTMSPMA, PTS-15, PTS-5, and PTS-2. For

Figure 4. FT-IR spectra of the resultant hybrid mesoporous polymers, i.e., PTMSPMA, PTS-15, PTS-5, and PTS-2.

the spectrum of the PTMSPMA, bands at 2955 and 2893 cm−1 were attributed to stretching vibrations of the −CH2− group, and the strong absorption band at 1726 cm−1 was attributed to the stretching vibration of the −CO− groups. The appearance of a weak band at 1637 cm−1, corresponding to the vibration of carbon−carbon double bonds (CC), indicated that there were still some unreacted vinyl groups in the mesoporous PTMSPMA. The absorption peaks at 818 and 1081 cm−1 have disappeared, indicating the complete decomposition of the methoxysilyl groups (Si−O−CH3). The strongest peak at 1104 cm−1 was attributed to the absorption of the Si−O−Si, which was formed by the condensation of the silanol groups. Furthermore, an intense and broad absorbance at 3200−3600 cm−1 was attributed to the O−H stretching vibration, confirming the presence of Si−OH groups. In the spectra of PTS-15, PTS-5, and PTS-2, the characteristic bands of monosubstituted benzene rings were clearly observed besides the characteristic bands of PTMSPMA. The characteristic peaks of monosubstituted benzene ring appeared at 760 and 701 cm−1, indicating the successful copolymerization of styrene and TMSPMA, leading to the formation of hybrid mesoporous copolymers. From the above analyses of FT-IR spectra, it can be concluded that (i) all methoxysilyl groups (Si−O−CH3) of TMSPMA were completely hydrolyzed into silanol groups (Si− O−H), (ii) the condensation of silanol groups was incomplete, (iii) the copolymerization of styrene and TMSPMA was successful, and (iv) the residual carbon−carbon double bonds and silanol groups were observed, which may be utilized for the further functionalization of the mesoporous polymers.10,29

Figure 3. EDX spectra of the resultant hybrid mesoporous polymers, i.e., PTMSPMA, PTS-15, PTS-5, and PTS-2.

PTS-2, determined by energy dispersive X-ray analysis (EDX). Elements C, O, and Si were found, which confirmed the organic−inorganic hybrid structure of the mesoporous polymers. The element platinum (Pt) was from the coating layer, which was used to enhance the surface electronic conductivity of the samples, and the element aluminum (Al) was from the aluminum foil substrate. The percentages of each element obtained by EDX are summarized in Table 2. Note that the percentage of element calculated from EDX can only serve as semiquantitative comparison due to the large experimental uncertainty. For example, the percentages of atoms C and Si in PTMSPMA did not fit well to the formulation of TMSPMA. Nevertheless, the percentage of atom C was gradually increased with increasing the feed amount of styrene when choosing the percentage of atom Si as the reference. The chemical compositions of the organic−inorganic hybrid mesoporous polymers were further characterized by Fourier 15028

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The thermal stability of the four hybrid mesoporous polymers was investigated by thermogravimetric analyses (TGA). Figure 5 shows the TGA curves of PTMSPMA, PTS-

Figure 6. Powder X-ray diffraction (XRD) patterns of the resultant hybrid mesoporous polymers, i.e., PTMSPMA, PTS-15, PTS-5, and PTS-2. Figure 5. TGA curves of the resultant hybrid mesoporous polymers, i.e., PTMSPMA, PTS-15, PTS-5, and PTS-2.

15, PTS-5, and PTS-2. The organic component of PTMSPMA was completely decomposed at high temperature, and the residual inorganic component of PTMSPMA was ∼34 wt %. Reduced ceramic yields were observed for the PTS-15, PTS-5, and PTS-2, indicating the increase of organic content after the incorporation of styrene. The amounts of the residual inorganic component in the PTS-15, PTS-5, and PTS-2 were ca. ∼32, ∼26, and ∼22 wt %, respectively. As expected, the amount of organic component increased in the hybrid mesoporous copolymers with increasing the feed amount of styrene. The weight percentages of styrene in the PTS-15, PTS-5, and PTS-2 were then calculated to be ca. ∼2, ∼8, and ∼12 wt %, respectively. The 5 wt % decomposition temperature (Td,5 wt %) of PTMSPMA was about 255 °C. Interestingly, the copolymerization of styrene and TMSPMA led to the increase of Td,5 wt %. The Td,5 wt % values of PTS-15, PTS-5, and PTS-2 were 268, 266, and 267 °C, respectively, which were higher than that of PTMSPMA for more than 10 deg. This result indicated that the mesoporous copolymers P(TMSPMA-costyrene) had better thermal stabilities than that of mesoporous polymer PTMSPMA. The structures of the hybrid mesoporous polymers were further characterized by small-angle powder X-ray diffraction (XRD). Figure 6 shows the XRD patterns of PTMSPMA, PTS15, PTS-5, and PTS-2. For all the four samples, only broad diffraction peaks with high diffraction intensity were observed at small diffraction angle. No higher order peak was observed, indicating the weak long-range order of the hybrid mesoporous polymers. Upon increasing the styrene contents in the hybrid mesoporous copolymers, the diffraction peaks slightly shifted to larger diffraction angles and further broaden. These indicated that the incorporation of styrene decreased the long-range order of the mesoporous polymers. The long periods of PTMSPMA, PTS-15, PTS-5, and PTS-2 were then calculated from the diffraction peaks to be ca. 11.6, 10.3, 9.2, and 7.1 nm, respectively. The surface area of the hybrid mesoporous polymers was determined by nitrogen adsorption−desorption isotherms. Figure 7a shows the nitrogen adsorption−desorption isotherms

Figure 7. (a) Nitrogen adsorption−desorption isotherms of PTMSPMA, PTS-15, PTS-5, and PTS-2. (b) Barret−Joyner−Halenda (BJH) pore size distributions calculated from the corresponding desorption data in (a).

of PTMSPMA, PTS-15, PTS-5, and PTS-2. According to the IUPAC classification, PTMSPMA exhibited the form of type IV isotherms, suggesting a mesoporous structure. Desorption 15029

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Figure 8. (a) Chemical structure of Congo red (CR). (b) UV spectra of filtered mixed solutions after mixing PTS-2, PTS-5, PTS-15, and PTMSPMA, respectively, in the 50 mg/L CR aqueous solutions for 24 h.

isotherm of PTMSPMA exhibited a sharp increase at 0.45 < P/ P0 < 0.88 with a hysteresis loop, which could be regarded as type H2. The corresponding pore size distribution, which was estimated from the desorption branch of the isotherms, showed two peaks with the first peak centered at 4 nm and the second sharp peak at 10 nm (Figure 7b). It was worthy to note that the pore sizes estimated from the desorption branch of the isotherms were smaller than the mesh sizes observed by SEM and TEM (i.e., 20−50 nm) because the nitrogen adsorption− desorption isotherms were much sensitive to the small mesopores. When styrene was copolymerized with TMSPMA, the N2-sorption isotherms of PTS-15 could be assigned to type IV isotherms with H3 type hysteresis loops. This implied that the aggregates of the particles or adsorbents contained slitshaped pores. The corresponding pore size distribution of PTS15 also showed two peaks with the first peak centered at 4 nm and the second broad peak at 13 nm. With increasing the amount of styrene in the mesoporous polymers, the H3 type hysteresis loop became unclear for PTS-5, indicating a smaller population of mesoporous structures, which was confirmed by the pore size distributions (Figure 7b). However, the desorption branch of PTS-5 clearly exhibited a sharp increase at relative pressures P/P0 of 0.92−0.99, suggesting the existence of macroporous structures. Besides the first peak at 4 nm, the pore size distribution of PTS-5 also exhibited a weak second peak centered at 65 nm, which supported the existence of macroporous structures in PTS-5. For PTS-2, only the peak at 4 nm was observed. From Figure 7b, it can be seen that the second peak shifted to larger value of pore sizes from 10 nm for PTMSPMA to 65 nm for PTS-5 and broaden with increasing the amount of styrene. For PTS-2 with the largest amount of styrene, the second peak was even disappeared. Interestingly, the first peak at 4 nm was always observed for PTMSPMA, PTS-15, PTS-5, and PTS-2, regardless of the amount of styrene in the mesoporous polymers, which could be attributed to the silsesquioxane frameworks formed by the hydrolysis and condensation of methoxysilyl groups during the free radical polymerization. The BET surface areas and pore volumes calculated by the BJH method of the four hybrid mesoporous polymers are summarized in Table 1. The BET surface area and pore volume of PTMSPMA reached 335 m2/g and 0.610 cm3/ g, respectively. With increasing the styrene contents in the mesoporous polymers, the BET surface area and pore volume strongly decreased. The above results indicated that the structures and properties of the organic−inorganic hybrid mesoporous polymers

obtained here were strongly dependent on the chemical compositions of the mesoporous polymers, namely, the styrene contents in the mesoporous polymers. In the absence of styrene, the resultant hybrid polymer of TMSPMA (PTMSPMA) exhibited mesh-like bicontinuous structures with mesopores and high surface area (335 m2/g). With the incorporation of styrene, mesoporous nanoparticles were obtained. The surface areas of the mesoporous nanoparticles decreased with the increase of styrene contents. In order to explore the potential applications of such mesoporous polymers in the separation of organic dyes and proteins from the aqueous solution, the adsorption behaviors of the mesoporous polymers were studied for the adsorption of Congo red (CR) and bovine serum albumin (BSA). Figure 8a shows the chemical structure of Congo red (CR). CR has a molecular size of 2.65 × 1.10 × 0.25 nm, and the surface area of single Congo red molecule (SC) was ca. 2.92 nm2. Figure 8b shows the UV spectra of the filtered mixed solutions after mixing PTS-2, PTS-5, PTS-15, and PTMSPMA in the 50 mg/L CR aqueous solutions for 24 h. Clearly, the residual concentrations of Congo red after the adsorption were much lower than the original Congo red solution (50 mg/L CR), indicating that Congo red molecules were indeed adsorbed by the mesoporous polymers. The adsorption capabilities were in the order of PTMSPMA > PTS-15 > PTS-5 > PTS-2. This result implied that the mesoporous polymers with larger surface area could adsorb more Congo red molecules. The adsorption capacities for Congo red with concentration of 50 mg/L and contacting times of 24 h were ca. 32.5, 27.7, 24.9, and 22.8 mg/g for PTMSPMA, PTS-15, PTS5, and PTS-2, respectively. The percentage of adsorbed Congo red was 65%, 55%, 50%, and 46% for PTMSPMA, PTS-15, PTS-5, and PTS-2, respectively. The PTMSPMA with largest surface area (335 m2/g) and large pore size (10 nm) was thus chosen to test its adsorption capability of bovine serum albumin (BSA, size of 4.0 × 4.0 × 14.0 nm).30,31 Figure 9 shows the adsorption isotherm of BSA on PTMSPMA for the adsorption measurements of mesoporous PTMSPMA in BSA acetate buffer solutions at room temperature. BSA was indeed adsorbed by the PTMSPMA. The adsorption capacity was ca. 39.7 ± 1.0 mg/g for 500 mg/L BSA acetate buffer solution, which was much higher than that of the porous silica monolith (∼3.4 mg/ g).30 Four different models, namely Langmuir, Freundlich, Redlich−Peterson, and Tempkin models, were used to fit the adsorption isotherm of BSA onto the mesoporous PTMSPMA (see Supporting Information). The Langmuir model could be 15030

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resultant hybrid polymer of TMSPMA (PTMSPMA) exhibited mesh-like bicontinuous structures with mesopores and high surface area (335 m2/g). With the incorporation of styrene, mesoporous nanoparticles were obtained. The surface areas of the mesoporous nanoparticles decreased with the increase of styrene contents. Such organic−inorganic hybrid mesoporous polymers may be used in the adsorption or separation of Congo red and BSA. Other functional monomers can be also introduced for tailoring the functionalities and mesoporous structures of the resultant hybrid mesoporous polymers. Currently, further investigations with other comonomers are ongoing.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of (CTA) 2 S 2 O 8 complexes and partially decomposed (CTA)2S2O8 complexs, the standard curve for UV adsorption of BSA, adsorption isotherm models, and corresponding fitting parameters for the adsorption of bovine serum albumin. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Adsorption isotherm fitted with Langmuir (black line), Freundlich (red line), Redlich−Peterson (green line), and Tempkin (blue line) models for the adsorption of mesoporous PTMSPMA in BSA acetate buffer solutions at room temperature. Qe is the solid-phase adsorbate concentration at equilibrium, and Ce is the adsorbate concentration in the aqueous phase at equilibrium.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

applied to monolayer adsorption of substrate over a homogeneous surface.32 Freundlich model is used to express multilayer adsorption.33 The Redlich−Peterson model combines the feature of both the Langmuir and Freundlich models, and the mechanism of adsorption is hybrid.34 The Tempkin model takes into account the effects of some indirect adsorbate/adsorbate interactions on adsorption isotherms.35 The correlation coefficients (R2 value) were 0.89, 0.85, 0.88, and 0.90 for Langmuir, Freundlich, Redlich−Peterson, and Tempkin models, respectively. The sum of the squares of the errors (SSE, see eq 5 in Supporting Information) has been widely used to determine and evaluate the fit of the isotherm equation to the experimental data.36−38 Here, the SSEs were ca. 82.7, 115.7, 79.6, and 77.4 for Langmuir, Freundlich, Redlich− Peterson, and Tempkin models, respectively. General speaking, the model was thought to be “good” for describing the experimental data if a high value of R2 and low value of SSE are obtained by fitting the model to the experimental data. Based on such criteria, Figure 9 suggested that the adsorption behavior of BSA onto PTMSPMA was more close to the prediction of the Tempkin isotherm. It was understandable since there were some BSA/BSA interactions during the adsorption of BSA onto the mesoporous PTMSPMA. Furthermore, the Freundlich model fitted the experimental data in the worst way, suggesting that the adsorption of BSA was not a multilayer adsorption. BSA is a charge protein so that multilayer adsorption of BSA was not expected.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 20874087 and 21074114) for financial support. We thank Prof. Xurong Xu in the Department of Chemistry, Zhejiang University, for the use of the SDT Q600 Instrument.

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REFERENCES

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CONCLUSIONS Organic−inorganic hybrid mesoporous polymers were successfully synthesized by using a template-directed free radical polymerization technique with oxidative complexes (CTA)2S2O8 as self-decomposed soft templates in aqueous solution at 0−5 °C. The (CTA)2S2O8 complexes were automatically decomposed due to the reduction of S2O82−, which generated free radicals and hence initiated the polymerization of the monomers. No additional treatment was needed to remove the templates. TMSPMA and styrene were chosen as main and comonomer. In the absence of comonomer, the 15031

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