Preparation of SiO2@polystyrene@polypyrrole

Journal of Colloid and Interface Science 315 (2007) 434–438 www.elsevier.com/locate/jcis

Preparation of SiO2@polystyrene@polypyrrole sandwich composites and hollow polypyrrole capsules with movable SiO2 spheres inside Tongjie Yao, Quan Lin, Kai Zhang, Dengfeng Zhao, Hui Lv, Junhu Zhang, Bai Yang ∗ Key Laboratory for Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, People’s Republic of China Received 6 February 2007; accepted 28 June 2007 Available online 13 August 2007

Abstract In this paper, we describe a flexible method for preparing conducting building blocks: SiO2 @polystyrene@polypyrrole sandwich multilayer composites and hollow polypyrrole (PPy) capsules with movable SiO2 spheres inside. First, SiO2 @polystyrene (PS) core/shell composites were synthesized, and then SiO2 @PS@PPy sandwich multilayer composites were prepared by chemical polymerization of pyrrole monomer on the surface of SiO2 @PS composites. Furthermore, hollow polypyrrole capsules with movable SiO2 spheres inside were obtained after removal of the middle PS layer. The diameter of sandwich multilayer composites could easily be controlled by adjusting the dosage of pyrrole monomer. The conductivities of composites increased with the increase of PPy content. After the insulating PS layer was selectively etched, the conductivities of hollow capsules with movable SiO2 spheres inside were much higher than those of the corresponding sandwich multilayer composites. © 2007 Elsevier Inc. All rights reserved. Keywords: Polypyrrole (PPy); Sandwich multilayer composite; SiO2 ; Polystyrene (PS); Building block

1. Introduction Self-assembled colloid crystals stand out as ideal templates for creating highly ordered three-dimensional (3D) or twodimensional (2D) structures. They have remarkable potential applications in the fields of microelectronics, chemical sensors, engineering, and biomedicine due to their special periodical dielectric structures and unique optical properties [1–6]. Various methods have been successfully established to create colloidal crystals with lattice constants of micrometers or nanometers, and many perfect structures have been fabricated [7–14]. As is well known, building blocks play an important role in self-assembly methods. 3D or 2D structures with different building blocks display different properties. For example, packed magnetic nanoparticles with regular arrays can be used in data storage [15,16]. The incorporation of metal nanoparticles into ordered structures may find applications in catalysis and photonic crystals [17–20]. Although many functional ma* Corresponding author. Fax: +86 431 85193423.

E-mail address: [email protected] (B. Yang). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.06.072

terials have been used as building blocks, little attention has been paid to conducting polymers. Once conducting polymers are used as building blocks to form ordered structures, it will enlarge their applications in electronic devices, light-emitting diodes, electrochromic devices, and transparent electrode materials [21–24]. It is well known that polypyrrole (PPy) is an outstanding conducting polymer. It can easily be synthesized by various approaches—for example, oxidative polymerization and electrochemical polymerization. In addition, PPy presents several advantages such as environmental stability, good redox properties, and the ability to give high electrical conductivities [25–28]. One of the most widely studied and applied techniques in this respect is to form core/shell composites using conducting polymers as the shells and organic or inorganic particles as the cores [29–36]. In this paper, we synthesized SiO2 @polystyrene@polypyrrole sandwich multilayer composites and hollow PPy capsules with movable SiO2 spheres inside. First, SiO2 nanoparticles grafted with 3(trimethoxysilyl)propyl methacrylate (MPS) were employed as cores in styrene emulsion polymerization to synthesize SiO2 @polystyrene (PS) core/shell composites. Then steric

T. Yao et al. / Journal of Colloid and Interface Science 315 (2007) 434–438

agent poly(N -vinylpyrrolidone) (PVP) was adsorbed onto the surface of SiO2 @PS by ultrasound as an anchor molecule and the polymerization of pyrrole monomers was performed around SiO2 @PS composites to form SiO2 @PS@PPy sandwich multilayer composites. Finally, the middle PS layer was selectively etched by tetrahydrofuran (THF) and hollow PPy capsules with movable SiO2 spheres inside were obtained.

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2.3. Preparation of hollow PPy capsules with movable SiO2 spheres inside

2. Experimental

SiO2 @PS@PPy composites were soaked in THF solution under magnetic stirring for 12 h; the PS layer between the PPy shell and the SiO2 core was selectively etched. After being washed with deionized water and ethanol several times, the black sediments were dried in vacuum overnight. Hollow PPy capsules with movable SiO2 spheres inside were obtained.

2.1. Materials

2.4. Characterization

Pyrrole and divinylbenzene (DVB) were purchased from Fluka. PVP with molecular weight of 3.6 × 105 was obtained from Sigma and used without further purification. 3-(Trimethoxysilyl)propyl methacrylate (MPS), styrene, and pyrrole were distilled under reduced pressure before being used. Tetraethyl orthosilicate (TEOS), absolute ethanol, ammonium hydroxide, sodium dodecyl benzene sulfonate (SDBS), potassium persulfate (KPS), FeCl3 ·6H2 O, and THF were analytical grade and used as received. 2.2. Preparation of SiO2 @PS@PPy sandwich multilayer composites SiO2 @PS core/shell composites with different sizes were synthesized according to the references [37,38]. PVP was dissolved in ethanol by ultrasound in the presence of SiO2 @PS composites for 1 h; then the suspension was stirred for 24 h to ensure that the surface of the composites was adequately covered by PVP. Unadsorbed PVP was removed by centrifugation. Subsequently, the composites were redispersed into deionized water, which was directly used for the pyrrolecoating step. A quantity of 1.0 g FeCl3 ·6H2 O was added into 30 ml of the above solution (containing 0.24 g SiO2 @PS composites covered by PVP) under magnetic stirring at ambient temperature. After half an hour, the pyrrole monomer was added into the system using a syringe. The suspension color turned from yellow to dark green; at last it became black. After stirring for 16 h, black SiO2 @PS@PPy sandwich composites were obtained.

A JEOL JSM-6700F scanning electron microscope (SEM) was employed to observe the surface morphologies. The mean diameter of composites was estimated by counting all particles in SEM images. The structure and shell thickness of the composites were determined by a JEOL-2010 transmission electron microscope (TEM). The FT-IR spectrum was measured at wavenumbers ranging from 500 to 4000 cm−1 using a Nicolet Avatar 360 FT-IR spectrophotometer. Thermogravimetric analysis (TGA) was conducted with a Netzschsta 449C thermogravimetric analyzer at a heating rate of 10 ◦ C per min in N2 from room temperature up to 700 ◦ C. (The temperature of SiO2 @PS composites was up to 480 ◦ C.) 3. Results and discussion 3.1. Morphology and structure The overall procedure for forming SiO2 @PS@PPy composites and hollow PPy capsules with movable SiO2 spheres inside is illustrated in Fig. 1. MPS was critical during the preparation of SiO2 @PS composites. Only pure SiO2 nanoparticles and pure PS nanoparticles were obtained in the reactor when the surface of the SiO2 spheres was not grafted with MPS. Fig. 2 shows SEM and TEM images of the SiO2 @PS composites, which display good dispersity. The mean diameters of the SiO2 cores and the composites are 80 and 230 nm, respectively. It is seen that most of the core/shell composites contain only one SiO2 core, but in

Fig. 1. The synthesis scheme for SiO2 @PS@PPy composites and hollow PPy capsules with movable SiO2 spheres inside.

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Table 1 The relationship between content of PPy and diameter of sandwich multilayer composites

Fig. 2. SEM and TEM (inset) images of SiO2 @PS composites.

Fig. 3. FT-IR spectra of (a) SiO2 , (b) PS, (c) bare SiO2 @PS composite, and (d) SiO2 @PS composite with adsorbed PVP.

several composites two SiO2 cores appear. The reason is that the SiO2 nanoparticles we used were small. Composites with more than one cores could be observed due to the aggregation of small particles when the diameter of SiO2 cores was less than 100 nm [37]. FT-IR spectra of (a) SiO2 nanoparticles, (b) PS nanoparticles, and (c) bare SiO2 @PS composites are shown in Fig. 3. In spectrum a the characteristic peak of SiO2 at 1097 cm−1 corresponds to the Si–O–Si stretching vibration. PS main peaks at 3025, 2923, 1492, 1452, and 698 cm−1 are shown in spectrum b. In spectrum c, all the PS main peaks appear and the SiO2 characteristic peak shifts from 1097 to 1106 cm−1 . SEM, TEM images and FT-IR spectra have proved that the SiO2 @PS composites were successfully prepared. To cover SiO2 @PS core/shell composites with PPy, PVP was used as a steric agent. Because of highly polar amide groups in the pyrrolidone ring and nonpolar methylene groups along its backbone, PVP adsorbed strongly onto the PS surface through hydrophobic interaction [39]. In Fig. 3, spectrum d is the FT-IR spectrum of SiO2 @PS composites adsorbed

PPy content (wt%)

Diameter (nm)

60 55 50 45

445 420 400 370

with PVP. Compared with spectrum (c), an additional characteristic peak appears at 1652 cm−1 corresponding to the pyrrolidone carbonyl group of the steric agent. PVP could provide active sites for pyrrole monomer loading [40,41]. Pyrrole monomers could polymerize both on the surfaces of SiO2 @PS composites and in solution. As PVP adsorbed onto the surfaces of composites, pyrrole monomers preferred to polymerize on the surface with possible formation of hydrogen bonds between the amino groups on pyrrole rings and the carbonyl groups on PVP backbones, so the process of coating PPy shells onto the composites’ surfaces was considered to be a physical course. PVP could not effectively prevent the PPy from aggregating when the hydrodynamic diameter of PVP was shorter than the thickness of the PPy shell [29]. The experimental results show when the content of PPy exceeds 60 wt% in sandwich composites, the PPy shell is too thick to form hydrogen bonds between the carbonyl groups on the PVP backbones and the amino groups on pyrrole rings, which leads to the formation of pure PPy nanoparticles originating from the polymerization of pyrrole monomers in the solution. The FT-IR spectrum of the SiO2 @PS@PPy sandwich composites (not shown in this paper) reveals that the pyrrole monomer is successfully polymerized onto the surfaces of the SiO2 @PS composites. The characteristic peaks of pyrrole ring are at 1651, 1538, and 1462 cm−1 . The peaks at 1104 and 897 cm−1 are attributed to =C–H group in-plane vibrations and out-of-plane vibrations. When pyrrole monomers are polymerized onto the surfaces of 230 nm SiO2 @PS composites, the relationship between the content of PPy and the diameter of the sandwich multilayer composites is listed in Table 1. This indicates that we can control the size of the composites ranging from 370 to 445 nm by adjusting the content of PPy. After SiO2 @PS@PPy sandwich composites were synthesized, the middle PS layer could be etched by THF and hollow PPy capsules with movable SiO2 spheres inside were successfully prepared. Fig. 4 shows SEM and TEM images of hollow PPy capsules with movable SiO2 spheres inside, and its diameter is about 370 nm. In this case, the middle PS layer almost vanishes, which also demonstrates that the PPy shell is not quite compact and is permeable to the dilute THF for the dissolution of PS. Sandwich composites of different sizes were prepared by adjusting the diameters of SiO2 nanoparticles and SiO2 @PS core/shell composites. In Fig. 5, the mean diameters of SiO2 spheres, SiO2 @PS composites, and SiO2 @PS@PPy sandwich composites are 260, 520, and 650 nm, respectively. The PPy layer is not tightly attached to the surface of the PS layer and

T. Yao et al. / Journal of Colloid and Interface Science 315 (2007) 434–438

Fig. 4. SEM and TEM (inset) images of movable SiO2 spheres in hollow PPy capsules; the arrow indicates the SiO2 sphere.

Fig. 5. SEM and TEM (inset) images of the SiO2 @PS@PPy composites with diameter 650 nm. The arrows indicate the (a) PPy layer, (b) PS layer, and (c) SiO2 sphere, respectively. We inferred that the PS and PPy layer shrank about 25 nm from the TEM image.

the interspace between them is about 25 nm, measured from the TEM image. A possible explanation of the phenomenon was that two polymer layers swelled in the solvent, and after the composites were dried in a vacuum oven to remove organic solvent and water, both the PS layer and the PPy layer shrank, which made them separate from each other. However, the separation did not appear between the PS layer and the SiO2 sphere, because the interaction between them was covalent bond, which was much stronger than the hydrogen bond between the PVP and PPy layer or the hydrophobic interaction between the PVP and PS layer. The phenomenon of the PS layer separating from the PPy layer also revealed that both the PPy shell coated on the outer of the PS layer and the PVP adsorbed on the surface of the PS layer were physical courses. 3.2. Thermogravimetric analysis The thermogravimetric analysis results are shown in Fig. 6. Curves a, b, and c correspond to the sample SiO2 @PPy com-

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Fig. 6. TGA curves of (a) SiO2 @PPy composites, (b) SiO2 @PS composites, and (c) SiO2 @PS@PPy sandwich particles.

posites (they are not hollow PPy capsules with movable SiO2 spheres inside—we coated PPy onto the surface of SiO2 directly, according to Ref. [40]), SiO2 @PS composites, and SiO2 @PS@PPy sandwich composites. The weight loss below 200 ◦ C is attributed to the evaporation of water and residual organic solvent. In curve a, composites begin decomposing at 300 ◦ C, which corresponds to the decomposed temperature of PPy, and its content is about 45 wt%. Similarly, from curve b, PS begins decomposing at 330 ◦ C. When the temperature reaches about 480 ◦ C, the PS shell completely vanishes, and the residuals are SiO2 nanoparticles, the content of which is about 2.4 wt%. Comparing curve c with curves a and b, the SiO2 @PS@PPy composites begin decomposing at 316 ◦ C; when the temperature is up to 340 ◦ C, the decomposition rate increases, which is caused by degradation of the PS layer. At last, 1.3 wt% SiO2 residues are left behind. From curve c, it can be confirmed that there are three different substances in composites, and this is also evidence that the sandwich composites were prepared. From the thermogravimetric analysis, we obtained thermodynamical information on the composites and the content of each layer. The PPy content in composites was used in conductivity analysis. 3.3. Conductivity As a conducting polymer, the conductivity of PPy composites is an important issue. The samples were dried in a vacuum oven overnight and then pressed into disks at room temperature. The conductivity of SiO2 @PS@PPy sandwich composites was determined using standard four-point probe techniques. The research on conductivity of composites with low PPy content had been done by other researchers. For example, Armes’ group loaded 1 wt% PPy onto the PS surface [29]. The conductivity of the composite was less than 10−6 S/cm, since PS particles were not completely covered by PPy shells and were exposed to the air. Interparticle charge carrier transport occurred via the surface of the composites, and insulating PS

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Table 2 The conductivities of composites with different content of PPy before and after the middle PS layer is etched PPy content in sandwich composites (wt%)

Conductivity of sandwich composites (S/cm)

Conductivity after PS layer etched (S/cm)

37.5% 44.2% 50.6%

1.29 3.28 4.90

18.09 24.07 27.41

hampered the electron transport, which caused a reduction of conductivity. We investigated the conductivity of composites with high PPy content (obtained from TGA). Various amounts of PPy were loaded onto the surfaces of 230-nm SiO2 @PS composites. Conductivities of sandwich multilayer composites before or after the middle PS layers were etched are listed in Table 2. When PPy content increased from 35.7 to 50.6 wt%, the conductivities of the sandwich multilayer composites improved from 1.20 to 4.90 S/cm. After the PS layer was etched, the conductivities of the corresponding composites increase from 18.09 to 27.41 S/cm. Because the content of insulating materials is less than before, the conductivity of hollow PPy capsules with movable SiO2 spheres inside is efficiently improved compared with that of the corresponding sandwich multilayer composites. It can be inferred the conductivities of the SiO2 @PS@PPy composites and hollow PPy capsules with movable SiO2 spheres inside increase with increased PPy content. So the content of PPy in composites is a main factor affecting the conductivity of the composites. 4. Conclusions We successfully synthesized SiO2 @PS@PPy sandwich composites and hollow PPy capsules with movable SiO2 spheres inside. The diameters of the sandwich multilayer composites ranged from 370 to 445 nm by adjustment of the content of PPy coated onto 230-nm SiO2 @PS core/shell composites. The conductivity increased obviously from 1.20 to 4.90 S/cm when the PPy content in composites increased from 35.7 to 50.6 wt%. After the PS middle layer was selectively etched, the conductivity of corresponding hollow capsules with movable SiO2 spheres inside ranged from 18.09 to 27.41 S/cm. It is expected that SiO2 @PS@PPy sandwich composites and hollow PPy capsules with movable SiO2 spheres inside would have potential applications as conducting building blocks in selfassembly. Acknowledgments This work was supported by the National Nature Science Foundation of China (Grants 20534040, 20674026, and 90401021) and the program for Changjiang Scholars and Innovative Research Team in University (No. IRT0422).

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