Journal of Colloid and Interface Science 419 (2014) 107–113
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Synthesis and characterization of full interpenetrating structure mesoporous polycarbonate-silica spheres and p-phenylenediamine adsorption Lei Zu a,b, Ruirui Li b, Yue Shi b, Huiqin Lian b, Yang Liu b, Xiuguo Cui b,⇑, Zongwu Bai c a
Department of Chemistry, Yanbian University, Yanji 133002, China Beijing Key Lab of Special Elastomer Composite Materials, Department of Material Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China c University of Dayton Research Institute, University of Dayton, College Park, Dayton, USA b
a r t i c l e
i n f o
Article history: Received 15 November 2013 Accepted 25 December 2013 Available online 3 January 2014 Keywords: Mesoporous spheres Full interpenetrating structure Polycarbonate Mesoporous silica p-Phenylenediamine adsorption
a b s t r a c t As a common used and hardly emulsified amorphous thermoplastic, the bisphenol-A polycarbonates were used as the polymer candidate to form a novel monodispersed sub-micrometer mesoporous polymer–silica spheres with full interpenetrating structure. The synthesis procedure was based on a modified sol–gel approach in which the polycarbonate was plasticized in advanced by the surfactant of polymer emulsion. The mesoporous spheres possess a perfect uniform particle size and the polymer–silica spheres are held together by permanent entanglement in three dimensions. The defined crystallization of the polycarbonate was occurred when it was entrapped in the silica laminated matrix due to the plasticizing effect of the surfactant, and directly affected the thermal stability of the mesoporous spheres. The specific surface areas and pore diameters of mesoporous sphere were affected by the mass content and crystallization behavior of the polycarbonate. The p-phenylenediamine was used as adsorbate to investigate the cationic organics adsorption ability of the mesoporous spheres. The results shown that the polycarbonate-silica possess a well adsorption capacity for p-phenylenediamine by virtue of two kinds of hydrogen bond, and the maximum adsorption capacity was nearly 7.5 times larger than that of the hollow mesoporous silica. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Since the discovery of the new family of mesoporous silica M41S in 1992 [1], this kind of novel materials has received considerable attention both in academic and in industry. As a very important class of the mesoporous materials, the polymer–silica system intrigues the great interest in recent years for their tunable pore sizes, high specific surface areas, versatile possibilities of surface functionalization as well as diversity in chemical composition, monolithic morphology and inner structure [2–6]. The morphology of polymer–silica mesoporous materials has been controlled to produce composite fibers [7,8], membranes [9,10], rodlike particles [11], films [12], spheres [13–16] and hard spherical particles [17–19] to satisfy diverse practical applications. Among these various forms, spheres gained many interests for their widespread potential applications in many fields, such as catalysis [20], pollutant adsorption [21] and drug delivery [22,23]. Some researchers ⇑ Corresponding author. Fax: +86 010 81294007. E-mail addresses: [email protected] (L. Zu), [email protected] (R. Li), [email protected] (Y. Shi), [email protected] (H. Lian), [email protected] (Y. Liu), [email protected] (X. Cui), [email protected] (Z. Bai). 0021-9797/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.12.058
have synthesized polymer–silica spheres with diverse inner structures, such as the core–shell structure in which the polymer acts as the core and silica serves as the shell [24,25], and interpenetrating structure in which the polymer–silica 3D (three-dimensional) networks are held together by permanent entanglements [26]. Compared with the core–shell structure, the polymer–silica spheres with the interpenetrating structure had some excellent unique features, such as nanosized precise distribution of organic and inorganic substances thanks to the well intercalation between the polymer and silica matrix. However, although these polymer–silica spheres with various inner structures have been synthesized in recent years, but the most utilized of them are core–shell structure and the polymer is commonly used as the core template for further producing the hollow silica spheres instead of exerting their own characteristics [27–29]. There were few reports shown that the interpenetrating structure polymer–silica spheres were applied in any fields, especially the mesoporous ones which may not even have been synthesized. Therefore, the synthesis and utilization of the interpenetrating structure mesoporous polymer–silica spheres is a very creative and meaningful work.
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Commonly, the straightforward strategy to synthesize the polymer–silica mesoporous spheres is in situ polymerization or postgrafting approach [30,31]. However, although these two approaches are facial and general, but they usually could not obtain a perfect interpenetrating structure. An effective way to control the inner structure of the polymer–silica spheres is the sol–gel method in which the polymer emulsion acts as the template. David Avnir et al. have synthesized some diverse polymer–silica spheres with interpenetrating structure by this approach [32–35]. The synthesis procedure is based on hydrolysis and poly-condensation of the inorganic silica precursor in the presence of the organic polymer within droplets of an emulsion, in brief, the forming silica within the droplets entraps the dissolved polymer. The sol–gel approach for the organic polymers utilized an O/W emulsion as the template, where the oil phase was composed of the tetraethoxysilane (TEOS), precursor for the silica matrix, along with the polymer and a co-solvent, and where the water phase was an alkaline water–alcohol solution to which a proper polymer surfactant was added. By using this method, well sub-micrometer polymer–silica spheres with interpenetrating structure could be obtained. However, this approach may only be suitable for the easily dissolved and emulsified polymers but not for the other opposite ones, such as polypropylene, polylactic acid and polycarbonate. Furthermore, the synthesis procedure is under alkaline condition, so it may not be appropriate for the polymers which are easy to be hydrolyzed under alkaline condition. More importantly, the polymer–silica spheres synthesized through this approach are non-mesoporous and the specific surface areas are quite small, even less than 10 m2/g according to the results of David Avnir et al.. Accordingly, for solving these drawbacks, a new approach should be performed to synthesize the interpenetrating structure mesoporous polymer–silica spheres with a relatively large specific surface area in which the polymer is hardly dissolved and emulsified. Bisphenol-A polycarbonates, produced by the reaction of bisphenol-A and phosgene, received their name because they are polymers containing carbonate groups (AOA(C@O)AOA). As a clear and amorphous thermoplastic, owing to its chain rigidity retards the chain diffusion, the PC undergoes thermal-induced crystallization very slowly and could not be emulsified easily. Therefore, the synthesis of the PC-silica mesoporous spheres with interpenetrating structure would not be done by the David Avnir’s method. There are a great number of hydrophilic polar groups on the macromolecular chairs of PC, leading an extremely strong hydrogen bond interaction with the cationic organic compounds. As a similar interaction example, An et al. [21] synthesized PMAA-silica in which the methacrylic acid (MAA) was grafted onto the surface of silica gel to adsorb aniline. The results presented that the PMAA/silica possesses a strong adsorption ability for aniline depending on the interaction of hydrogen bond between the carboxyl groups of PMAA and aniline. According to this result, it is reasonable to presume that if composited with the silica to form mesoporous materials with large specific surface areas, the nanosized bisphenol-A polycarbonates could also be used as a potential aniline or p-phenylenediamine cationic organics adsorbent by virtue of the similar strong hydrogen bond effect. The p-phenylenediamine is one of the simplest aromatic diamine, and there are two cationic polar groups connected on the para-position of the benzene ring, so the p-phenylenediamine could form a strong hydrogen bond and/or electrostatic interaction with the other anionic organics. The p-phenylenediamine is commonly used as an intermediate in various fields [36–38], but the accumulation harmful effect has been emerged gradually, especially the p-phenylenediamine-contained wastewater. Therefore, the elimination of the p-phenylenediamine is so important to public health and environmental quality.
In this paper, novel mesoporous PC-silica spheres with full interpenetrating structure were synthesized by a modified sol– gel approach which inspired by the achievements of David Avnir et al. The p-phenylenediamine adsorption performances were also investigated to estimate the possibility of the PC-silica served as an adsorbent for cationic organics. The full interpenetrating structure of the PC-silica mesoporous spheres was obtained by using the Pluronic P123 as the surfactant which was added into the oil phase to plasticize the PC and formed a stable polymer emulsion template. There was no stable polymer emulsion could be gained if the P123 was put into the water phase due to the plasticizing effect of the P123 was absent. Therefore, it is reasonable to infer that the polymer–silica mesoporous spheres with interpenetrating structure could be obtained via making a choice of putting the surfactant into the water or oil phase, according to the difficulty of the polymer was emulsified. For investigating the adsorption capacity of the PC-silica, the p-phenylenediamine was served as the adsorbate under the alkaline condition. 2. Experimental characterization 2.1. Materials Pluronic P123 (EO20PO70EO20; Mav = 5800), bisphenol A polycarbonate (Mw = 64 kg/mol, density = 1.20 g/cm3), tetraethyl orthosilicate (TEOS), p-phenylenediamine, cyclohexanone and hydrochloric acid (HCl) were all purchased from Aldrich Chemical Co. and used as received without any purification. 2.2. Synthesis of mesoporous PC-SiO2 In order to investigate the influence of the PC content on the structure of PC-silica, a series of samples with different PC contents was prepared by the modified sol–gel method. The samples were named S1, S2 and S3, respectively when 0.05 g, 0.1 g and 0.3 g PC were added into the synthesis system. The synthesis procedure is depicted in Fig. 1. In a typical procedure, 0.1 g PC, and 1 g P123 were placed in 5 g cyclohexanone and stirred at 60 °C for 2 h to make a transparent hydrophobic solution and then decreased the temperature to 35 °C. After 40 g ultrapure water and 4 g HCl were added at 35 °C, a stable emulsion was formed immediately, then pour 2 g TEOS into the emulsion quickly and stirred for another 24 h. After the reaction, the resulting dispersion was centrifuged at 10,000 rpm for 20 min. The precipitate was washed with ultrapure water twice and dried at 60 °C for 24 h, then washed with ethanol to remove the P123 completely by using the soxhlet extractor. The same procedure was carried out to prepare the hollow silica when the PC was absent and the precipitate was calcined at 650 °C for 3 h.
Fig. 1. Synthesis procedure schematic diagram of PC-silica.
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2.3. p-Phenylenediamine adsorption experiments The p-phenylenediamine adsorption experiments were performed by immersion method. 0.2 g PC-silica was filled in a pphenylenediamine aqueous solution with concentration of 0.5 g/L and pH of 8. The mixture was stirred at 25 °C for 6 h before the PC-silica was centrifuged out. The residual concentration of the p-phenylenediamine was determined by using a UV-spectrophotometer at a wavelength of 240 nm. 2.4. Characterization Scanning electron microscopy (SEM) observations were performed on a COXEM EM-20 microscope operating at 20 kV. Transmission electron microscopy (TEM) observations were taken on a Tecnai G2 20ST microscope operating at 200 kV. Infrared spectrometry (IR) analyses were performed on a Thermal Nicolet infrared spectrometer. Thermogravimetric analyses (TGA) were carried out using a TA instruments TGA2050 from room temperature to 800 °C, with a heating rate of 10 °C/min under nitrogen. Nitrogen adsorption–desorption isotherms were measured at 77 K using a JW-BK static physisorption analyzer after the samples were outgassed for 12 h at 100 °C. The BET surface area was calculated from the desorption branches in the relative pressure range of 0.05–0.35, and the total pore volume and average pore diameter were evaluated at a relative pressure of about 0.99. Wide angle X-ray diffraction (WAXD) patterns were obtained with a Bruker D8 diffractometer in reflection mode using Cu Ka = 0.154 nm with a voltage of 40 kV. The small-angle X-ray scattering (SAXS) measurements were taken on a Bruker Nanostar U small-angle X-ray scattering system using Cu Ka radiation (40 kV, 35 mA). The adsorption capacity of p-phenylenediamine was measured on a PGENERAL T6 UV spectrometry. 3. Results and discussion The morphology and microstructure of the samples was detected by SEM and TEM. As shown in Fig. 2, the SEM images reveal that the samples possess a similar ordered uniform spherical morphology and the average particle size is about 500 nm. Well separated spheres could be observed from both the samples S1 (Fig. 2A) and S2 (Fig. 2B), indicating a well dispersity was achieved. It is notable that some spheres were adhered together which may be due to the strong hydrogen-bond and electrostatic interaction between the drops of the emulsion. The elemental distribution of carbon and silicon in sample 3# was probed using SEM–EDX to present the distribution of PC in PC-SiO2 (supporting information). The EDX results present that the carbon is uniformly distributed among the PC-silica, indicating the distribution of PC is uniform in the PC-silica. TEM was performed to determine the structure of PC-SiO2. As shown in Fig. 3A, the adherences among the spheres could be observed clearly. A radiation pore structure is shown in Fig. 3B, directly demonstrating a kind of mesoporous structure which is ordered in the short range. Some lattice fringes are also observed as parallel lines running across the surface of the wall and wormlike channels are observable at the edges of the spheres. Moreover, it is noteworthy that there is no obvious hollow core that could be identified, suggesting the silica matrix may be very thick and the inner hollow core is relatively small. FTIR analyses were performed to investigate the chemical composition of the PC-SiO2. The interactions in a two-component system can also be studied by FTIR where the frequency shifts of the absorption bands of functional groups provide information on the nature and intensity of intermolecular interactions. As depicted
Fig. 2. SEM images of samples S1 and S2.
in Fig. 4, the narrow absorption peak at 952 cm 1 and broad peak at 3302 cm 1 are the bending and stretching vibrating of Si-OH, respectively, indicating that the hydrolysis and condensation of the TEOS were well accomplished. The absorption peaks at 801 cm 1 and 1093 cm 1 are attributed to the asymmetric and symmetric stretching vibration of Si–O–Si, respectively, which are assigned to the siloxane bridges of the silica. The peaks at 2917 cm 1 and 2849 cm 1 are assigned to aliphatic C–H stretching, and the band at 1770 cm 1 is due to the carbonyl absorption, which confirms the existence of PC. Due to no apparently frequency shift of 1770 cm 1, it is reasonably believed that the miscibility should be mainly ascribed to weak (van der Waals) interaction between the PC and P123, which agreed with the similar results obtained by Tsuburaya and Saito [39]. The TGA results of PC-SiO2 are presented in Fig. 5, and the detailed mass loss data are listed in Table 1. It is well known that the main degradation pathways of PC can be classified into two categories: chain pyrolysis of isopropylidene bonds and hydrolysis/alcoholysis of carbonate bonds, including rearrangements of some carbonate bonds like decarboxylation or cross-linking upon heating, ultimately resulting in CO2, H2O and char [40]. As shown in Fig. 5, the mass loss of PC-silica increases in direct proportion to the content of the PC and the maximum mass loss is 65.7% (sample S3). The pure solid PC showed a typical one-step decomposition and about 20% of PC was left due to the carbonization under the nitrogen atmosphere. The degradation of sample S1 showed a
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Fig. 5. Thermal analysis of the samples.
Table 1 Specific surface area, pore diameter and pore volume data of samples. Sample SiO2 S1 S2 S3
Weight loss (%)
Surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
36.3 52.9 65.7
769.15 57.18 75.74 88.96
0.58 0.094 0.14 0.13
3.31 3.86 4.56 4.03
Fig. 3. TEM image of sample S3.
Fig. 6. WAXS patterns of samples.
Fig. 4. FTIR analysis of the samples.
continuous decomposition started at 100 °C approximately, which could be attributed to the well extension and dispersity of the PC molecular chains as indicated in WAXS measurements (Fig. 6)
and the good thermal conductance of the silica matrix. Furthermore, when the content of the PC was increased, the degradation of sample S2 followed a characteristic two-step decomposition. The low temperature decomposition process may be caused by the decomposition of the polar groups of PC, such as the carbonyl and hydroxyl, and the high temperature decomposition process could be attributed to the decomposition of the rigid aromatic carbon chains of PC. A similar thermal degradation process could also
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be observed from sample S3, but the decomposition of the polar groups was delayed greatly, which may due to the crystallization of the PC was improved (suggested in Fig. 6) and the silica matrix can act as a superior barrier to hinder the diffusion of volatile decomposition products. According to these results, it is reasonable to infer that the thermostability of PC-silica was depending on the mass content and crystallization of the PC. However, although the thermostability of PC-silica was improved along with the crystallization of the PC, but still lower than that of the semi-crystallization pure solid PC, suggesting the PC was totally entrapped in silica matrix and the silica possessed a good thermal conductance. The WAXS was performed to investigate the crystallization behavior of PC and PC-silica mesoporous spheres. The X-ray diffraction patterns are depicted in Fig. 6. There are many papers have discussed the crystallization behavior of PC with diverse polymers in recent years. For example, Tsuburaya and Saito [39] have studied the crystallization behavior of PC/PEO in detail and they found that the crystallization rate of the PC was greatly accelerated in PC/ PEO blends. The reason for this result is commonly ascribed to the increasing rate of polymeric segmental motion resulted from the plasticization effect of PEO, as proved by Gallez et al. [41]. Similarly, in our study, as shown in Fig. 6, the hollow silica possesses a broad and blunt diffraction peak at 22.4° which indicates an amorphous structure was formed. There is only a broad diffraction peak could be observed at 16.8° in solid PC, suggesting that the solid PC has a semi-crystallization structure. It can be seen clearly that the crystallization of the PC in PC-SiO2 was increased along with its mass content. When the content of the PC was 36.3% (sample S1), only a broad diffraction peak emerged clearly, indicating an
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amorphous structure of the silica matrix was formed. There is no PC crystallization diffraction peak that could also be observed owing to the well plasticization effect of P123, as the similar result reported by Li et al. [42]. Furthermore, with increasing the content of PC, the crystallization would be happened when the content of the PC was raised to 52.9% (S2), and improved significantly when it reached to 65.7% (S3). This result could also be explained by the plasticization effect of the P123 in consideration of none apparent shift of carbonyl adsorption peak in FTIR spectra. The PEO segment of P123 would accelerate the crystallization rate and increase the total crystallization of the PC. Summarized the results obtained above, it is logical to infer that the crystallization process of PC was induced by the P123 and not be suppressed within the nanosized silica pores. In the consideration of the paper which was not focused on the crystal structure of PC, the detailed investigation of the crystallization thermodynamics and kinetics is beyond the scope of this paper. Nitrogen adsorption–desorption isotherm measurements were performed to characterize the specific surface areas, pore diameters and pore volumes of the PC-SiO2 (Fig. 7). The detailed data are also listed in Table 1 for comparison. The nitrogen adsorption–desorption isotherms of the samples are of type IV, and display a distinct hysteresis loops of H2 in the range of 0.4–1.0P/P0, which indicates a mesoporous structure. There is no hysteresis loop that was observed clearly from the high pressure zone (close to 1.0P/P0), indicating the PC-silica may possess a small hollow core in the center of the silica matrix [43,44]. The pore diameter distribution curves of the samples calculated from adsorption branches show a sharp peak in the range of 3.3–3.5 nm, suggesting the mesoporous structure and narrow pore
Fig. 7. Nitrogen adsorption–desorption isotherm measurements and pore diameter distribution curves of samples.
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diameter distribution of the samples. It was shown clearly that the hysteresis loop of the hollow silica was narrower, displayed in the range of 0.4–0.6P/P0, indicating the pore size distribution of hollow silica was more regularly than the other samples, which agreed with the results of pore diameter distribution curves. As described in Table 1, the specific surface area of the mesoporous spheres was determined to be 769.15 m2 g 1 when the PC was absent, and decreased to 88.96 m2 g 1, and 57.18 m2 g 1, respectively, when the content of PC decreased from 65.7% to 36.3%. The results shown above manifest the PC was entrapped in the silica matrix to form a full interpenetrating structure, and the specific surface area of the PC-silica was impacted by the content and crystallization of PC. When the content of the PC was 36.3%, an amorphous structure was observed in sample S1, suggesting the PC molecular chain was unpacked and spread freely into the silica matrix and blocked the pores of the mesoporous spheres owing to the good plasticizing effect of P123. Furthermore, a clear crystallization process could be observed when the content of the PC was increased to 52.9%, manifesting the PC molecular chain began to stack and the rigid of the molecular chain was improved, thus the distance of the silica layers was expanded gradually. Similarly, although more PC was added into the mesoporous sphere (65.7%), the molecular chain of PC would not spread into the silica matrix freely but stack together in a very tight way, proved by the strong diffraction peak appeared in S3 (Fig. 6). The crystallization PC gave rise to occupying more space within the silica layers, so the pore volume and specific surface area of the sample S3 were all increased correspondingly. The small X-ray diffraction patterns of the PC-SiO2 samples are displayed in Fig. 8. All the samples exhibits a single reflection cor-
responding to the average pore–pore correlation distance in the small angle region, which is in consistent with the mesoporous structure suggested by the nitrogen adsorption–desorption isotherm measurements. The SAXS patterns show the reflection intensity of the hollow silica is higher than the other samples. The peak width of samples became more and more narrow and the reflection intensity was increased gradually with raising the PC content. It is notable that the reflection peak of sample S1 was very broad and the reflection intensity was also pretty low, indicating the PC-SiO2 was almost amorphous when the content of the PC was relatively low. According to these patterns in Fig. 8, it is reasonable to deduce that the crystallization of the PC had a great influence on the mesoporous structure of PC-SiO2. As can be seen in Fig. 8, a single strong reflection peak could be recognized in hollow silica, indicating a relatively regular mesoporous structure was formed when the PC was absent. This could be attributed to the self-assembly of TEOS was not affected due to the single emulsion template which formed by P123. However, when some quantity of PC was added (36.3%), the PC molecular chains were likely to intertwine with the P123 due to the plasticization effect to form an emulsion which served as the sol–gel template. In this condition, the existence of the polar groups on the PC molecular chains may change the packing parameter of P123 on the oil–water interface, and the distribution of the PC molecular chains were optional within the silica matrix, so an almost amorphous mesoporous structure was formed. Furthermore, when the content of the PC was raised gradually to 65.7%, the stack of the PC molecular chain would become very compactness due to the crystallization, so the molecular chains could not extend freely like the condition of low PC content. Although more PC was added, but the rigid PC molecular chain would not intertwine with the P123 too much due to the limit plasticization effect, thus the packing parameter of P123 on the oil–water interface was not affected greatly and the distribution of the PC became more regular correspondingly. It is notable that the diffraction angle of the PC-SiO2 was shifted to low angle, indicating the crystallization PC molecular chains enlarged the average pore–pore correlation distance of the silica matrix, so the long range order of mesoporous structure was improved. Based on these results, it is reasonable to infer that the PC molecular chains may be captivated in the laminar silica matrix, which agreed with results from the former texts. The p-phenylenediamine adsorption performances of PC-SiO2 were investigated by the UV spectrophotomrete measurements. In order to eliminate the influence of the different specific surface area, the p-phenylenediamine adsorption ability of hollow silica and PC-silica samples was assessed by converting the absolute pphenylenediamine adsorption amount into the relative adsorption
Fig. 8. SAXS patterns of samples.
Fig. 9. P-phenylenediamine adsorption performance of samples.
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amount when their specific surface area was 100 m2 g 1. As shown in Fig. 9, the hollow silica had the worst adsorption capacity, only 3.52% p-phenylenediamine could be removed when the specific surface area of the hollow silica was converted to be 100 m2 g 1. On the contrary, the PC-silica had a well p-phenylenediamine adsorption performance, which may be due to the existence of the PC. The p-phenylenediamine adsorption performance mostly relay on the hydrogen bond intensity between the PC-silica and p-phenylenediamine. According to the results of An et al. [21], it is rational to infer that there are two types of hydrogen bond forms occurring possibly between PC and p-phenylenediamine. The first one is ANH2 of the p-phenylenediamine could form a hydrogen bond (NAH hydrogen bond) with the O atom of carbonate groups in PC molecular chains. The other one is AOH of silica hydroxyl group could form a hydrogen bond (OAH N hydrogen bond) with the N atom of p-phenylenediamine. Based on this conclusion, although the hollow silica had the largest specific surface area, but the p-phenylenediamine adsorption quantity per unit area was much lower than PC-silica, according to the weak hydrogen bond formed between the silica and p-phenylenediamine. It is notable that the sample S2 had the best adsorption ability, about 7.5 times larger than that of the hollow silica. From this perspective, although the PC-silica have a relatively small specific surface area compared with the hollow silica, but the existence of the carbonate groups makes it easily form a strong hydrogen bond with the p-phenylenediamine, so the adsorption ability of the PC-silica is far stronger than that of the hollow mesoporous silica. According to this conclusion, the PC-silica has a great possibility to act as an adsorbent for cationic organics and possesses a tremendous potential application in many areas, such as medical treatment, industrial catalysis and pollutant treatment.
4. Conclusion In this paper, a novel mesoporous PC-silica spheres with full interpenetrating structure was synthesized by a modified sol–gel approach. The synthesis procedure was based on plasticizing the PC in advance by using the P123 as the plasticizer in the oil phase, and then composed with the hydrochloric acid solution to form a stable emulsion which acted as the template of the sol–gel procedure. The full interpenetrating structure was confirmed by the results of TEM, SAXS and nitrogen adsorption–desorption isotherm measurements which presented the specific surface areas of PC-silica increased along with the PC content. According to the TGA and WAXS results, the thermal stability of the PC-silica was improved due to the crystallization of the PC which was accelerated by P123. The p-phenylenediamine served as the adsorbate to assess the adsorption performance of the PC-silica, the results shown that the PC-silica had a great adsorption capacity, the maximum adsorption capacity was 7.5 times larger than that of the hollow mesoporous silica.
Acknowledgments The authors acknowledge gratefully the support of the National Natural Science Foundation of China (NSFC, Nos. 21271031, 51063009 and 51203012), and the Beijing Natural Science Foundation of China (Nos. 2092013, 2132009 and 2122015).
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Synthesis and characterization of full interpenetrating structure mesoporous polycarbonate-silica spheres and p-phenylenediamine adsorption Lei Zu a,b, Ruirui Li b, Yue Shi b, Huiqin Lian b, Yang Liu b, Xiuguo Cui b,⇑, Zongwu Bai c a
Department of Chemistry, Yanbian University, Yanji 133002, China Beijing Key Lab of Special Elastomer Composite Materials, Department of Material Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China c University of Dayton Research Institute, University of Dayton, College Park, Dayton, USA b
a r t i c l e
i n f o
Article history: Received 15 November 2013 Accepted 25 December 2013 Available online 3 January 2014 Keywords: Mesoporous spheres Full interpenetrating structure Polycarbonate Mesoporous silica p-Phenylenediamine adsorption
a b s t r a c t As a common used and hardly emulsified amorphous thermoplastic, the bisphenol-A polycarbonates were used as the polymer candidate to form a novel monodispersed sub-micrometer mesoporous polymer–silica spheres with full interpenetrating structure. The synthesis procedure was based on a modified sol–gel approach in which the polycarbonate was plasticized in advanced by the surfactant of polymer emulsion. The mesoporous spheres possess a perfect uniform particle size and the polymer–silica spheres are held together by permanent entanglement in three dimensions. The defined crystallization of the polycarbonate was occurred when it was entrapped in the silica laminated matrix due to the plasticizing effect of the surfactant, and directly affected the thermal stability of the mesoporous spheres. The specific surface areas and pore diameters of mesoporous sphere were affected by the mass content and crystallization behavior of the polycarbonate. The p-phenylenediamine was used as adsorbate to investigate the cationic organics adsorption ability of the mesoporous spheres. The results shown that the polycarbonate-silica possess a well adsorption capacity for p-phenylenediamine by virtue of two kinds of hydrogen bond, and the maximum adsorption capacity was nearly 7.5 times larger than that of the hollow mesoporous silica. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Since the discovery of the new family of mesoporous silica M41S in 1992 [1], this kind of novel materials has received considerable attention both in academic and in industry. As a very important class of the mesoporous materials, the polymer–silica system intrigues the great interest in recent years for their tunable pore sizes, high specific surface areas, versatile possibilities of surface functionalization as well as diversity in chemical composition, monolithic morphology and inner structure [2–6]. The morphology of polymer–silica mesoporous materials has been controlled to produce composite fibers [7,8], membranes [9,10], rodlike particles [11], films [12], spheres [13–16] and hard spherical particles [17–19] to satisfy diverse practical applications. Among these various forms, spheres gained many interests for their widespread potential applications in many fields, such as catalysis [20], pollutant adsorption [21] and drug delivery [22,23]. Some researchers ⇑ Corresponding author. Fax: +86 010 81294007. E-mail addresses: [email protected] (L. Zu), [email protected] (R. Li), [email protected] (Y. Shi), [email protected] (H. Lian), [email protected] (Y. Liu), [email protected] (X. Cui), [email protected] (Z. Bai). 0021-9797/$ - see front matter Ó 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2013.12.058
have synthesized polymer–silica spheres with diverse inner structures, such as the core–shell structure in which the polymer acts as the core and silica serves as the shell [24,25], and interpenetrating structure in which the polymer–silica 3D (three-dimensional) networks are held together by permanent entanglements [26]. Compared with the core–shell structure, the polymer–silica spheres with the interpenetrating structure had some excellent unique features, such as nanosized precise distribution of organic and inorganic substances thanks to the well intercalation between the polymer and silica matrix. However, although these polymer–silica spheres with various inner structures have been synthesized in recent years, but the most utilized of them are core–shell structure and the polymer is commonly used as the core template for further producing the hollow silica spheres instead of exerting their own characteristics [27–29]. There were few reports shown that the interpenetrating structure polymer–silica spheres were applied in any fields, especially the mesoporous ones which may not even have been synthesized. Therefore, the synthesis and utilization of the interpenetrating structure mesoporous polymer–silica spheres is a very creative and meaningful work.
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Commonly, the straightforward strategy to synthesize the polymer–silica mesoporous spheres is in situ polymerization or postgrafting approach [30,31]. However, although these two approaches are facial and general, but they usually could not obtain a perfect interpenetrating structure. An effective way to control the inner structure of the polymer–silica spheres is the sol–gel method in which the polymer emulsion acts as the template. David Avnir et al. have synthesized some diverse polymer–silica spheres with interpenetrating structure by this approach [32–35]. The synthesis procedure is based on hydrolysis and poly-condensation of the inorganic silica precursor in the presence of the organic polymer within droplets of an emulsion, in brief, the forming silica within the droplets entraps the dissolved polymer. The sol–gel approach for the organic polymers utilized an O/W emulsion as the template, where the oil phase was composed of the tetraethoxysilane (TEOS), precursor for the silica matrix, along with the polymer and a co-solvent, and where the water phase was an alkaline water–alcohol solution to which a proper polymer surfactant was added. By using this method, well sub-micrometer polymer–silica spheres with interpenetrating structure could be obtained. However, this approach may only be suitable for the easily dissolved and emulsified polymers but not for the other opposite ones, such as polypropylene, polylactic acid and polycarbonate. Furthermore, the synthesis procedure is under alkaline condition, so it may not be appropriate for the polymers which are easy to be hydrolyzed under alkaline condition. More importantly, the polymer–silica spheres synthesized through this approach are non-mesoporous and the specific surface areas are quite small, even less than 10 m2/g according to the results of David Avnir et al.. Accordingly, for solving these drawbacks, a new approach should be performed to synthesize the interpenetrating structure mesoporous polymer–silica spheres with a relatively large specific surface area in which the polymer is hardly dissolved and emulsified. Bisphenol-A polycarbonates, produced by the reaction of bisphenol-A and phosgene, received their name because they are polymers containing carbonate groups (AOA(C@O)AOA). As a clear and amorphous thermoplastic, owing to its chain rigidity retards the chain diffusion, the PC undergoes thermal-induced crystallization very slowly and could not be emulsified easily. Therefore, the synthesis of the PC-silica mesoporous spheres with interpenetrating structure would not be done by the David Avnir’s method. There are a great number of hydrophilic polar groups on the macromolecular chairs of PC, leading an extremely strong hydrogen bond interaction with the cationic organic compounds. As a similar interaction example, An et al. [21] synthesized PMAA-silica in which the methacrylic acid (MAA) was grafted onto the surface of silica gel to adsorb aniline. The results presented that the PMAA/silica possesses a strong adsorption ability for aniline depending on the interaction of hydrogen bond between the carboxyl groups of PMAA and aniline. According to this result, it is reasonable to presume that if composited with the silica to form mesoporous materials with large specific surface areas, the nanosized bisphenol-A polycarbonates could also be used as a potential aniline or p-phenylenediamine cationic organics adsorbent by virtue of the similar strong hydrogen bond effect. The p-phenylenediamine is one of the simplest aromatic diamine, and there are two cationic polar groups connected on the para-position of the benzene ring, so the p-phenylenediamine could form a strong hydrogen bond and/or electrostatic interaction with the other anionic organics. The p-phenylenediamine is commonly used as an intermediate in various fields [36–38], but the accumulation harmful effect has been emerged gradually, especially the p-phenylenediamine-contained wastewater. Therefore, the elimination of the p-phenylenediamine is so important to public health and environmental quality.
In this paper, novel mesoporous PC-silica spheres with full interpenetrating structure were synthesized by a modified sol– gel approach which inspired by the achievements of David Avnir et al. The p-phenylenediamine adsorption performances were also investigated to estimate the possibility of the PC-silica served as an adsorbent for cationic organics. The full interpenetrating structure of the PC-silica mesoporous spheres was obtained by using the Pluronic P123 as the surfactant which was added into the oil phase to plasticize the PC and formed a stable polymer emulsion template. There was no stable polymer emulsion could be gained if the P123 was put into the water phase due to the plasticizing effect of the P123 was absent. Therefore, it is reasonable to infer that the polymer–silica mesoporous spheres with interpenetrating structure could be obtained via making a choice of putting the surfactant into the water or oil phase, according to the difficulty of the polymer was emulsified. For investigating the adsorption capacity of the PC-silica, the p-phenylenediamine was served as the adsorbate under the alkaline condition. 2. Experimental characterization 2.1. Materials Pluronic P123 (EO20PO70EO20; Mav = 5800), bisphenol A polycarbonate (Mw = 64 kg/mol, density = 1.20 g/cm3), tetraethyl orthosilicate (TEOS), p-phenylenediamine, cyclohexanone and hydrochloric acid (HCl) were all purchased from Aldrich Chemical Co. and used as received without any purification. 2.2. Synthesis of mesoporous PC-SiO2 In order to investigate the influence of the PC content on the structure of PC-silica, a series of samples with different PC contents was prepared by the modified sol–gel method. The samples were named S1, S2 and S3, respectively when 0.05 g, 0.1 g and 0.3 g PC were added into the synthesis system. The synthesis procedure is depicted in Fig. 1. In a typical procedure, 0.1 g PC, and 1 g P123 were placed in 5 g cyclohexanone and stirred at 60 °C for 2 h to make a transparent hydrophobic solution and then decreased the temperature to 35 °C. After 40 g ultrapure water and 4 g HCl were added at 35 °C, a stable emulsion was formed immediately, then pour 2 g TEOS into the emulsion quickly and stirred for another 24 h. After the reaction, the resulting dispersion was centrifuged at 10,000 rpm for 20 min. The precipitate was washed with ultrapure water twice and dried at 60 °C for 24 h, then washed with ethanol to remove the P123 completely by using the soxhlet extractor. The same procedure was carried out to prepare the hollow silica when the PC was absent and the precipitate was calcined at 650 °C for 3 h.
Fig. 1. Synthesis procedure schematic diagram of PC-silica.
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2.3. p-Phenylenediamine adsorption experiments The p-phenylenediamine adsorption experiments were performed by immersion method. 0.2 g PC-silica was filled in a pphenylenediamine aqueous solution with concentration of 0.5 g/L and pH of 8. The mixture was stirred at 25 °C for 6 h before the PC-silica was centrifuged out. The residual concentration of the p-phenylenediamine was determined by using a UV-spectrophotometer at a wavelength of 240 nm. 2.4. Characterization Scanning electron microscopy (SEM) observations were performed on a COXEM EM-20 microscope operating at 20 kV. Transmission electron microscopy (TEM) observations were taken on a Tecnai G2 20ST microscope operating at 200 kV. Infrared spectrometry (IR) analyses were performed on a Thermal Nicolet infrared spectrometer. Thermogravimetric analyses (TGA) were carried out using a TA instruments TGA2050 from room temperature to 800 °C, with a heating rate of 10 °C/min under nitrogen. Nitrogen adsorption–desorption isotherms were measured at 77 K using a JW-BK static physisorption analyzer after the samples were outgassed for 12 h at 100 °C. The BET surface area was calculated from the desorption branches in the relative pressure range of 0.05–0.35, and the total pore volume and average pore diameter were evaluated at a relative pressure of about 0.99. Wide angle X-ray diffraction (WAXD) patterns were obtained with a Bruker D8 diffractometer in reflection mode using Cu Ka = 0.154 nm with a voltage of 40 kV. The small-angle X-ray scattering (SAXS) measurements were taken on a Bruker Nanostar U small-angle X-ray scattering system using Cu Ka radiation (40 kV, 35 mA). The adsorption capacity of p-phenylenediamine was measured on a PGENERAL T6 UV spectrometry. 3. Results and discussion The morphology and microstructure of the samples was detected by SEM and TEM. As shown in Fig. 2, the SEM images reveal that the samples possess a similar ordered uniform spherical morphology and the average particle size is about 500 nm. Well separated spheres could be observed from both the samples S1 (Fig. 2A) and S2 (Fig. 2B), indicating a well dispersity was achieved. It is notable that some spheres were adhered together which may be due to the strong hydrogen-bond and electrostatic interaction between the drops of the emulsion. The elemental distribution of carbon and silicon in sample 3# was probed using SEM–EDX to present the distribution of PC in PC-SiO2 (supporting information). The EDX results present that the carbon is uniformly distributed among the PC-silica, indicating the distribution of PC is uniform in the PC-silica. TEM was performed to determine the structure of PC-SiO2. As shown in Fig. 3A, the adherences among the spheres could be observed clearly. A radiation pore structure is shown in Fig. 3B, directly demonstrating a kind of mesoporous structure which is ordered in the short range. Some lattice fringes are also observed as parallel lines running across the surface of the wall and wormlike channels are observable at the edges of the spheres. Moreover, it is noteworthy that there is no obvious hollow core that could be identified, suggesting the silica matrix may be very thick and the inner hollow core is relatively small. FTIR analyses were performed to investigate the chemical composition of the PC-SiO2. The interactions in a two-component system can also be studied by FTIR where the frequency shifts of the absorption bands of functional groups provide information on the nature and intensity of intermolecular interactions. As depicted
Fig. 2. SEM images of samples S1 and S2.
in Fig. 4, the narrow absorption peak at 952 cm 1 and broad peak at 3302 cm 1 are the bending and stretching vibrating of Si-OH, respectively, indicating that the hydrolysis and condensation of the TEOS were well accomplished. The absorption peaks at 801 cm 1 and 1093 cm 1 are attributed to the asymmetric and symmetric stretching vibration of Si–O–Si, respectively, which are assigned to the siloxane bridges of the silica. The peaks at 2917 cm 1 and 2849 cm 1 are assigned to aliphatic C–H stretching, and the band at 1770 cm 1 is due to the carbonyl absorption, which confirms the existence of PC. Due to no apparently frequency shift of 1770 cm 1, it is reasonably believed that the miscibility should be mainly ascribed to weak (van der Waals) interaction between the PC and P123, which agreed with the similar results obtained by Tsuburaya and Saito [39]. The TGA results of PC-SiO2 are presented in Fig. 5, and the detailed mass loss data are listed in Table 1. It is well known that the main degradation pathways of PC can be classified into two categories: chain pyrolysis of isopropylidene bonds and hydrolysis/alcoholysis of carbonate bonds, including rearrangements of some carbonate bonds like decarboxylation or cross-linking upon heating, ultimately resulting in CO2, H2O and char [40]. As shown in Fig. 5, the mass loss of PC-silica increases in direct proportion to the content of the PC and the maximum mass loss is 65.7% (sample S3). The pure solid PC showed a typical one-step decomposition and about 20% of PC was left due to the carbonization under the nitrogen atmosphere. The degradation of sample S1 showed a
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Fig. 5. Thermal analysis of the samples.
Table 1 Specific surface area, pore diameter and pore volume data of samples. Sample SiO2 S1 S2 S3
Weight loss (%)
Surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
36.3 52.9 65.7
769.15 57.18 75.74 88.96
0.58 0.094 0.14 0.13
3.31 3.86 4.56 4.03
Fig. 3. TEM image of sample S3.
Fig. 6. WAXS patterns of samples.
Fig. 4. FTIR analysis of the samples.
continuous decomposition started at 100 °C approximately, which could be attributed to the well extension and dispersity of the PC molecular chains as indicated in WAXS measurements (Fig. 6)
and the good thermal conductance of the silica matrix. Furthermore, when the content of the PC was increased, the degradation of sample S2 followed a characteristic two-step decomposition. The low temperature decomposition process may be caused by the decomposition of the polar groups of PC, such as the carbonyl and hydroxyl, and the high temperature decomposition process could be attributed to the decomposition of the rigid aromatic carbon chains of PC. A similar thermal degradation process could also
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be observed from sample S3, but the decomposition of the polar groups was delayed greatly, which may due to the crystallization of the PC was improved (suggested in Fig. 6) and the silica matrix can act as a superior barrier to hinder the diffusion of volatile decomposition products. According to these results, it is reasonable to infer that the thermostability of PC-silica was depending on the mass content and crystallization of the PC. However, although the thermostability of PC-silica was improved along with the crystallization of the PC, but still lower than that of the semi-crystallization pure solid PC, suggesting the PC was totally entrapped in silica matrix and the silica possessed a good thermal conductance. The WAXS was performed to investigate the crystallization behavior of PC and PC-silica mesoporous spheres. The X-ray diffraction patterns are depicted in Fig. 6. There are many papers have discussed the crystallization behavior of PC with diverse polymers in recent years. For example, Tsuburaya and Saito [39] have studied the crystallization behavior of PC/PEO in detail and they found that the crystallization rate of the PC was greatly accelerated in PC/ PEO blends. The reason for this result is commonly ascribed to the increasing rate of polymeric segmental motion resulted from the plasticization effect of PEO, as proved by Gallez et al. [41]. Similarly, in our study, as shown in Fig. 6, the hollow silica possesses a broad and blunt diffraction peak at 22.4° which indicates an amorphous structure was formed. There is only a broad diffraction peak could be observed at 16.8° in solid PC, suggesting that the solid PC has a semi-crystallization structure. It can be seen clearly that the crystallization of the PC in PC-SiO2 was increased along with its mass content. When the content of the PC was 36.3% (sample S1), only a broad diffraction peak emerged clearly, indicating an
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amorphous structure of the silica matrix was formed. There is no PC crystallization diffraction peak that could also be observed owing to the well plasticization effect of P123, as the similar result reported by Li et al. [42]. Furthermore, with increasing the content of PC, the crystallization would be happened when the content of the PC was raised to 52.9% (S2), and improved significantly when it reached to 65.7% (S3). This result could also be explained by the plasticization effect of the P123 in consideration of none apparent shift of carbonyl adsorption peak in FTIR spectra. The PEO segment of P123 would accelerate the crystallization rate and increase the total crystallization of the PC. Summarized the results obtained above, it is logical to infer that the crystallization process of PC was induced by the P123 and not be suppressed within the nanosized silica pores. In the consideration of the paper which was not focused on the crystal structure of PC, the detailed investigation of the crystallization thermodynamics and kinetics is beyond the scope of this paper. Nitrogen adsorption–desorption isotherm measurements were performed to characterize the specific surface areas, pore diameters and pore volumes of the PC-SiO2 (Fig. 7). The detailed data are also listed in Table 1 for comparison. The nitrogen adsorption–desorption isotherms of the samples are of type IV, and display a distinct hysteresis loops of H2 in the range of 0.4–1.0P/P0, which indicates a mesoporous structure. There is no hysteresis loop that was observed clearly from the high pressure zone (close to 1.0P/P0), indicating the PC-silica may possess a small hollow core in the center of the silica matrix [43,44]. The pore diameter distribution curves of the samples calculated from adsorption branches show a sharp peak in the range of 3.3–3.5 nm, suggesting the mesoporous structure and narrow pore
Fig. 7. Nitrogen adsorption–desorption isotherm measurements and pore diameter distribution curves of samples.
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diameter distribution of the samples. It was shown clearly that the hysteresis loop of the hollow silica was narrower, displayed in the range of 0.4–0.6P/P0, indicating the pore size distribution of hollow silica was more regularly than the other samples, which agreed with the results of pore diameter distribution curves. As described in Table 1, the specific surface area of the mesoporous spheres was determined to be 769.15 m2 g 1 when the PC was absent, and decreased to 88.96 m2 g 1, and 57.18 m2 g 1, respectively, when the content of PC decreased from 65.7% to 36.3%. The results shown above manifest the PC was entrapped in the silica matrix to form a full interpenetrating structure, and the specific surface area of the PC-silica was impacted by the content and crystallization of PC. When the content of the PC was 36.3%, an amorphous structure was observed in sample S1, suggesting the PC molecular chain was unpacked and spread freely into the silica matrix and blocked the pores of the mesoporous spheres owing to the good plasticizing effect of P123. Furthermore, a clear crystallization process could be observed when the content of the PC was increased to 52.9%, manifesting the PC molecular chain began to stack and the rigid of the molecular chain was improved, thus the distance of the silica layers was expanded gradually. Similarly, although more PC was added into the mesoporous sphere (65.7%), the molecular chain of PC would not spread into the silica matrix freely but stack together in a very tight way, proved by the strong diffraction peak appeared in S3 (Fig. 6). The crystallization PC gave rise to occupying more space within the silica layers, so the pore volume and specific surface area of the sample S3 were all increased correspondingly. The small X-ray diffraction patterns of the PC-SiO2 samples are displayed in Fig. 8. All the samples exhibits a single reflection cor-
responding to the average pore–pore correlation distance in the small angle region, which is in consistent with the mesoporous structure suggested by the nitrogen adsorption–desorption isotherm measurements. The SAXS patterns show the reflection intensity of the hollow silica is higher than the other samples. The peak width of samples became more and more narrow and the reflection intensity was increased gradually with raising the PC content. It is notable that the reflection peak of sample S1 was very broad and the reflection intensity was also pretty low, indicating the PC-SiO2 was almost amorphous when the content of the PC was relatively low. According to these patterns in Fig. 8, it is reasonable to deduce that the crystallization of the PC had a great influence on the mesoporous structure of PC-SiO2. As can be seen in Fig. 8, a single strong reflection peak could be recognized in hollow silica, indicating a relatively regular mesoporous structure was formed when the PC was absent. This could be attributed to the self-assembly of TEOS was not affected due to the single emulsion template which formed by P123. However, when some quantity of PC was added (36.3%), the PC molecular chains were likely to intertwine with the P123 due to the plasticization effect to form an emulsion which served as the sol–gel template. In this condition, the existence of the polar groups on the PC molecular chains may change the packing parameter of P123 on the oil–water interface, and the distribution of the PC molecular chains were optional within the silica matrix, so an almost amorphous mesoporous structure was formed. Furthermore, when the content of the PC was raised gradually to 65.7%, the stack of the PC molecular chain would become very compactness due to the crystallization, so the molecular chains could not extend freely like the condition of low PC content. Although more PC was added, but the rigid PC molecular chain would not intertwine with the P123 too much due to the limit plasticization effect, thus the packing parameter of P123 on the oil–water interface was not affected greatly and the distribution of the PC became more regular correspondingly. It is notable that the diffraction angle of the PC-SiO2 was shifted to low angle, indicating the crystallization PC molecular chains enlarged the average pore–pore correlation distance of the silica matrix, so the long range order of mesoporous structure was improved. Based on these results, it is reasonable to infer that the PC molecular chains may be captivated in the laminar silica matrix, which agreed with results from the former texts. The p-phenylenediamine adsorption performances of PC-SiO2 were investigated by the UV spectrophotomrete measurements. In order to eliminate the influence of the different specific surface area, the p-phenylenediamine adsorption ability of hollow silica and PC-silica samples was assessed by converting the absolute pphenylenediamine adsorption amount into the relative adsorption
Fig. 8. SAXS patterns of samples.
Fig. 9. P-phenylenediamine adsorption performance of samples.
L. Zu et al. / Journal of Colloid and Interface Science 419 (2014) 107–113
amount when their specific surface area was 100 m2 g 1. As shown in Fig. 9, the hollow silica had the worst adsorption capacity, only 3.52% p-phenylenediamine could be removed when the specific surface area of the hollow silica was converted to be 100 m2 g 1. On the contrary, the PC-silica had a well p-phenylenediamine adsorption performance, which may be due to the existence of the PC. The p-phenylenediamine adsorption performance mostly relay on the hydrogen bond intensity between the PC-silica and p-phenylenediamine. According to the results of An et al. [21], it is rational to infer that there are two types of hydrogen bond forms occurring possibly between PC and p-phenylenediamine. The first one is ANH2 of the p-phenylenediamine could form a hydrogen bond (NAH hydrogen bond) with the O atom of carbonate groups in PC molecular chains. The other one is AOH of silica hydroxyl group could form a hydrogen bond (OAH N hydrogen bond) with the N atom of p-phenylenediamine. Based on this conclusion, although the hollow silica had the largest specific surface area, but the p-phenylenediamine adsorption quantity per unit area was much lower than PC-silica, according to the weak hydrogen bond formed between the silica and p-phenylenediamine. It is notable that the sample S2 had the best adsorption ability, about 7.5 times larger than that of the hollow silica. From this perspective, although the PC-silica have a relatively small specific surface area compared with the hollow silica, but the existence of the carbonate groups makes it easily form a strong hydrogen bond with the p-phenylenediamine, so the adsorption ability of the PC-silica is far stronger than that of the hollow mesoporous silica. According to this conclusion, the PC-silica has a great possibility to act as an adsorbent for cationic organics and possesses a tremendous potential application in many areas, such as medical treatment, industrial catalysis and pollutant treatment.
4. Conclusion In this paper, a novel mesoporous PC-silica spheres with full interpenetrating structure was synthesized by a modified sol–gel approach. The synthesis procedure was based on plasticizing the PC in advance by using the P123 as the plasticizer in the oil phase, and then composed with the hydrochloric acid solution to form a stable emulsion which acted as the template of the sol–gel procedure. The full interpenetrating structure was confirmed by the results of TEM, SAXS and nitrogen adsorption–desorption isotherm measurements which presented the specific surface areas of PC-silica increased along with the PC content. According to the TGA and WAXS results, the thermal stability of the PC-silica was improved due to the crystallization of the PC which was accelerated by P123. The p-phenylenediamine served as the adsorbate to assess the adsorption performance of the PC-silica, the results shown that the PC-silica had a great adsorption capacity, the maximum adsorption capacity was 7.5 times larger than that of the hollow mesoporous silica.
Acknowledgments The authors acknowledge gratefully the support of the National Natural Science Foundation of China (NSFC, Nos. 21271031, 51063009 and 51203012), and the Beijing Natural Science Foundation of China (Nos. 2092013, 2132009 and 2122015).
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