Synthesis and adsorption properties of highly monodisperse hollow microporous polystyrene nanospheres

Xinren Kangab, Yeru Liang*b, Luyi Chenab, Weicong Maib, Zhiyong Lin*a, Ruowen Fub and Dingcai Wu*b
aCollege of Material Science and Engineering, Huaqiao University, Xiamen, 361021, P. R. China. E-mail: linzy@hqu.edu.cn; Fax: +0595-22691560; Tel: +0595-22692308
bMaterials Science Institute, PCFM Laboratory, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China. E-mail: wudc@mail.sysu.edu.cn; liangyr6@mail.sysu.edu.cn; Fax: +86-020-84112759; Tel: +86-020-84112759

Received 10th April 2014 , Accepted 28th May 2014

First published on 28th May 2014


Abstract

Hollow polymer nanospheres (HPNSs) have received an increased level of attention, not only for their fundamental scientific interest, but also for technological applications. Despite a great deal of research effort, most of the current HPNSs are suffering from a poor polydispersity as well as a particle size larger than 500 nm. Here, we report the synthesis of highly monodisperse hollow microporous polystyrene nanospheres (MHMPNSs) with diameters as low as 120 nm based on a facile hypercrosslinking strategy. We utilize the rapid formation of an almost unreactive crosslinked polystyrene outer skin during the initial hypercrosslinking process, to minimize the undesired inter-sphere crosslinking. Due to the intra-sphere hypercrosslinking, the resulting MHMPNSs possess a well-developed microporous shell structure. The MHMPNSs are able to be used as potential absorbents toward organic vapors, because of their unique hollow core and microporous shell characteristics.


Introduction

Hollow polymer nanospheres (HPNSs) have received an increased level of attention, not only for their fundamental scientific interest, but also for technological applications.1 This is motivated by their remarkable properties, including special shape, robust porosity, tunable chemical functionalities and large fraction of voids. The unique combination of these advantages of both porous materials and polymers endows HPNSs with great application potential for adsorption, separation, drug delivery, sensing and catalysis.2

Up to now, many HPNSs with designed functionality, suitable morphology and tunable porous structures have been developed.3 Despite a great deal of research effort, most of these HPNSs are suffering from a poor polydispersity and/or a particle size with larger than 500 nm. The main reason could be ascribed to the fact that during normal crosslinking treatment for introduction of pore structure into nanosphere, the undesired crosslinking between neighboring nanospheres will be inevitable when the particle size is below 500 nm and will become more and more serious with further decreasing the particle size.4 Therefore, how to avoid the inter-sphere crosslinking is vital to the synthesis of monodisperse porous polymeric nanospheres.

Most recently, a kind of monodisperse microporous polystyrene nanosphere with diameters of ca. 190 nm was developed by our group through a versatile hypercrosslinking strategy.5 In such an approach, we utilized an unreactive crosslinked polystyrene (PS) outer skin as protective layer, which formed at the very beginning of hypercrosslinking treatment, to minimize the undesired inter-sphere crosslinking. However, the as-obtained microporous polystyrene nanospheres only contained a large proportion of micropores and lacked of a hierarchically pore system, which limit their general use in many areas. Considering that a large internal volume of nanospheres could provide a storage space that can serve various functions, it is still an urgent need for preparation of highly monodisperse HPNSs with diameters below 500 nm.

Herein we report the synthesis of highly monodisperse hollow microporous PS nanospheres (MHMPNSs) with diameters as low as 120 nm based on a facile hypercrosslinking strategy. In our approach, monodisperse SiO2 nanospheres were initially prepared and functionalized as the template of hollow cavity, and then were treated with an efficient emulsion polymerization procedure to obtain the monodisperse SiO2@PS core–shell nanospheres. During the subsequent hypercrosslinking process, the undesired inter-sphere crosslinking could be minimized due to the rapid formation of almost unreactive crosslinked PS outer skin. The further intra-sphere hypercrosslinking creates a well-developed microporous network inside the PS constituents. The as-prepared MHMPNS materials with high surface area and large pore volume demonstrate good adsorption capacity toward organic vapors.

Experimental

Synthesis of monodisperse modified SiO2 nanospheres

SiO2 nanospheres were synthesized according to Stöber method. In a typical synthesis, two mixed solutions were prepared according to the ratio of ethanol/TEOS = 202.5 ml/22.4 ml, ethanol/NH3·H2O/water = 112.5 ml/8.4 ml/104.1 ml. The two solutions were mixed in a three-necked flask, and then stirred mechanically for 3 h at 30 °C. 2 ml of γ-methacryloxypropyl trimethoxy silane (KH570) dissolved in 100 ml of ethanol, was dropped in the three-necked flask for 8 h. Reaction was continued for 36 h. Finally, the modified SiO2 with particle size of 115 nm was obtained. The synthesis procedures of modified SiO2 nanosphere with particle size of 50 nm were exactly the same as those of the SiO2 with particle size of 115 nm, except that the ratio of ethanol/NH3·H2O was changed to be 112.5 ml/4.2 ml.

Synthesis of monodisperse SiO2@PS core–shell nanospheres with divinyl benzene precrosslinking

SiO2@PS was synthesized according to the ratio of water/styrene/divinyl benzene/sodium dodecyl benzene sulphonate (SDBS)/NaHCO3/potassium persulfate (KPS) = 100 ml/4 ml/1 ml/30 mg/240 mg/120 mg. SDBS and NaHCO3 were dissolved in deionized water in a four-necked flask at 50 °C. After mechanical stirring for 10 min, 1.2 g of modified SiO2 with ultrasonic dispersing homogenously in 10 ml of ethanol was added in the system. Further mechanical stirring for 10 min, styrene and divinyl benzene were added in the system. Temperature was increased to 72 °C after system became homogenous with stirring for 10 min, and KPS was added simultaneously. The polymerization was continued for another 12 h. All the procedure processed under the protection of N2. The product was washed in toluene via three centrifugation/redispersion cycles and then dried at 70 °C.

Synthesis of MHMPNSs

Typically, 0.3 g of SiO2@PS core–shell nanospheres were dispersed and swelled in 60 ml of CCl4 for 24 h, and then 2.5 g of AlCl3 was added to the above mixture. The reaction was carried out at 75 °C for 24 h under stirring. A mixed solution prepared according to the ratio of HCl/acetone/water = 74 ml/2020 ml/666 ml was added slowly to the resulting mixture. The product SiO2@xPS nanospheres were filtered off, washed with the mixed solution via three filtration/redispersion cycles and then dried at 70 °C. Then 0.2 g of SiO2@xPS nanospheres were added into 50 ml of HF, followed by stirring for 24 h. The final product MHMPNS was filtered off and dried at 70 °C. The synthesis procedures of MHMPNS-2 were exactly the same as those of MHMPNS, except that the template was changed to be the modified SiO2 nanosphere with particle size of 50 nm.

Characterization

Dynamic light scattering (DLS) measurements were carried out at 25 °C on a Brookheaven Zeta PALS Instrument with a 532 nm laser at a scattering angle of 90°, providing the polydispersity index (PDI) and hydrodynamic diameter (i.e., the maximum in the DLS particle size distribution curve). The concentration was 0.1 mg/1 mL ethanol for all samples. The microstructures of the samples were investigated by a Hitachi S-3400 scanning electron microscope (SEM), and FEI Tecnai G2 Spirit transmission electron microscope (TEM). N2 adsorption–desorption measurement was carried out using a Micromeritics ASAP 2020 analyzer at 77 K. The BET surface area was determined by Brunauer–Emmett–Teller (BET) theory, and the micropore surface area and external surface area were calculated by t-plot method. The total pore volume was estimated from the amount adsorbed at a relative pressure P/P0 of ca. 0.99. The pore size distribution was analyzed by original density functional theory (DFT) combined with non-negative regularization and medium smoothing. Fourier-transform infrared (FTIR) spectrum was collected at room temperature on a Bruker Equinox 55 Fourier transform infrared spectroscopy. The thermogravimetric analysis (TGA) was performed under air condition at a heating rate of 20 °C min−1. The static adsorption measurements of the samples toward organic vapors were carried out at 30 °C by a weight method. The dynamic adsorption properties of the samples was studied with an BELSORP-Max intelligent gravimetric analyzer.

Results and discussion

An overall procedure for fabrication of MHMPNSs is briefly described in Fig. 1. The template of hollow cavity, i.e., SiO2 nanospheres are prepared through an improved Stöber method.6 Subsequently, the surface of the SiO2 nanospheres is functionalized by reaction with KH570 to introduce C[double bond, length as m-dash]C-containing surface emulsion polymerization initiation sites (Fig. 2A). The validity of this functionalization reaction can be obviously supported by the FTIR spectrum. According to the FTIR spectrum of Fig. S1, the modified SiO2 nanosphere shows new peaks at 1505 and 2928 cm−1, which are attributed to C[double bond, length as m-dash]C stretching vibration and –CH2 stretching vibration, respectively. These results clearly demonstrate the successful functionalization reaction on the surface of SiO2 nanospheres. The diameter of the modified SiO2 is measured to be 115 nm by means of SEM image analysis (Fig. 3A and 4A).
image file: c4ra03244f-f1.tif
Fig. 1 Illustration for preparation of monodisperse hollow microporous PS nanospheres.

image file: c4ra03244f-f2.tif
Fig. 2 TEM images of (A) modified SiO2, (B) SiO2@PS and (C) SiO2@xPS. (D) TEM image, (E) DLS curve and (F) high-resolution TEM image of MHMPNS.

image file: c4ra03244f-f3.tif
Fig. 3 SEM images of (A) modified SiO2, (B) SiO2@PS, (C) SiO2@xPS and (D) MHMPNS.

image file: c4ra03244f-f4.tif
Fig. 4 Particle size distribution curves from SEM image analysis for (A) modified SiO2, (B) SiO2@PS, (C) SiO2@xPS and (D) MHMPNS.

Subsequently, a polymeric shell is coated onto these modified SiO2 nanospheres through an emulsion copolymerization reaction of styrene and divinyl benzene. The peaks at 3025, 2923 and 1492 cm−1 corresponding to PS component are clearly shown in FTIR spectrum (Fig. S1). TEM image in Fig. 2B also confirms the successful formation of a homogeneous polymeric shell onto the SiO2 core. From SEM measurement, it could be found that the diameter of these resulting SiO2@PS core–shell nanospheres with uniform size is about 230 nm (Fig. 3B and 4B). Furthermore, DLS curve in Fig. S2A shows that the as-prepared sample demonstrates very high monodispersity characteristic with a low PDI of 0.092. These findings are in good agreement with the TEM and SEM images shown in Fig. 2B and 3B, respectively.

Afterwards, SiO2@PS is treated with –CO– crosslinking procedure based on Friedel–Crafts reaction of PS chains (Fig. S3), resulting in the core–shell structured SiO2@xPS nanospheres. Upon addition of catalyst anhydrous aluminum chloride, the colour of the reaction system at 75 °C changes quickly from milky to brick-red, and finally to black after 2 minutes. At the very beginning, the hypercrosslinking reaction occurs only on the periphery of SiO2@PS nanospheres, since the catalyst initially exists around them. This leads to the rapid formation of an almost unreactive crosslinked outer skin for each SiO2@PS core–shell nanosphere, thus minimizing the undesired crosslinking reaction between the neighboring spheres.5 After that, the catalyst gradually penetrates through the outer skin to the inside for the intra-sphere hypercrosslinking. Compared to SiO2@PS, the resulting SiO2@xPS has a weight gain of about 21%, mainly due to the introduction of carbonyl group crosslinking bridges7 (Fig. S1). DLS curve in Fig. S2B clearly shows that the SiO2@xPS demonstrates a high monodispersity characteristic with PDI of 0.005, which is well in agreement with the TEM (Fig. 2C) and SEM (Fig. 3C) observations. The diameter of SiO2@xPS is measured to be ca. 265 nm based on the SEM observation (Fig. 4C), which is a little larger than that of SiO2@PS. According to TGA analysis conducted under air, the weight percentage of SiO2 in such a composite structure is estimated to be about 28% (Fig. S4).

Finally, the MHMPNS is obtained after etching off the silica core of SiO2@xPS. The peak at 1100 cm−1 representing Si–OH group disappears in FTIR spectrum (Fig. S1), indicating the core is removed completely. TEM image in Fig. 2D demonstrates that the resulting MHMPNS still retains the perfect nanospherical morphology and smooth surface, indicative of a well inheritability of the structural integrity as their parent after the removal of SiO2 template. Simultaneously, no aggregation of the spheres is observed after the removal of SiO2 templates (Fig. 3D). Therefore, the resulting MHMPNS with diameter of about 265 nm (Fig. 4D) still has good monodispersity with a very low PDI of 0.009 estimated from DLS measurement (Fig. 2E). The core size and shell thickness of MHMPNS are calculated to be 115 and 75 nm, respectively. High-resolution TEM image in Fig. 2F reveals that the shell of MHMPNS presents a unique 3D network-type microporosity, because of its hypercrosslinked structure characteristic (Fig. S5). Furthermore, the hollow structure of MHMPNSs can be easily tuned via variation of the size of SiO2 template. For example, when the size of SiO2 nanospheres are decreased to 50 nm, the resulting MHMPNS-2 sample possesses a small hollow size of ca. 50 nm (Fig. S6). The particle size for MHMPNS-2 is as low as about 120 nm based on the SEM observation, and its shell thickness and PDI are measured to be ca. 35 nm and 0.005, respectively (Fig. S6).

The porous structure of the MHMPNS is quantitatively analyzed by measurement of N2 adsorption at 77 K (see Fig. 5). An obvious adsorption uptake at low relative pressure is observed, implying a robust micropore structure. These results are in good consistence with the high-resolution TEM observation. According to the DFT pore size distribution curve in Fig. 5B, micropores of MHMPNS are mainly centered at 0.7 and 1.3 nm in diameter. The BET surface area is measured to be 335 m2 g−1, and the external surface area and micropore surface area are calculated via the t-plot method to be 183 and 152 m2 g−1, respectively. The measured total pore volume is 0.27 cm3 g−1.


image file: c4ra03244f-f5.tif
Fig. 5 (A) N2 adsorption–desorption isotherm and (B) DFT pore size distribution curve of MHMPNS.

The MHMPNS with high surface area, large pore volume and easily modifiable pore surface chemistry could show potential use in many applications. Here, the adsorption properties towards organic vapors for the MHMPNS materials are studied. Fig. 6 shows that the MHMPNS exhibits an uptake in the range of low relative pressures, further confirming that the MHMPNS has micropores. When the relative pressure increases, the adsorption capacity increases continuously, probably because of its internal core volume structure for condensation of vapors. Therefore, the MHMPNS possesses an adsorption capacity of methanol as high as 251 mg g−1, manifesting good potential application in adsorption area. The advantage of the internal core is evaluated by comparison of the adsorption performance between MHMPNS and SiO2@xPS nanospheres under the same experimental conditions. From Fig. 6 and Table S1, it can be found that MHMPNS shows much better adsorption performance towards organic vapors such as methanol, toluene and tetrahydrofuran when compared to SiO2@xPS. Even only considering the weight of polymer, the capacity of MHMPNS is still significantly larger than that of SiO2@xPS core–shell nanospheres, sufficiently demonstrating the effectiveness of the large internal volume structure in improving the adsorption properties (Fig. 6 and Table S1).


image file: c4ra03244f-f6.tif
Fig. 6 Adsorption isotherms of methanol vapor for MHMPNS and SiO2@xPS.

Conclusions

In summary, we have successfully prepared highly monodisperse hollow microporous polystyrene nanospheres with diameters as low as ca. 120 nm by using a hypercrosslinking strategy. The as-prepared MHMPNSs with a unique hollow core and microporous shell structure have high surface areas (e.g. 335 m2 g−1) and large pore volume (e.g. 0.27 cm3 g−1). It is demonstrated that these MHMPNS materials have good adsorption properties toward organic vapors. We hope that the MHMPNSs may trigger the potential use as advanced nanomaterials in many other applications including drug sustained-release, CO2 capture, gas storage and sensing of toxic molecules.

Acknowledgements

We acknowledge financial support from the project of Guangdong Natural Science Funds for Distinguished Young Scholar (S2013050014408), Program for New Century Excellent Talents in University (NCET-12-0572), Fundamental Research Funds for the Central Universities (13lgpy57), National Key Basic Research Program of China (2014CB932402), NSFC (51372280, 51173213, 51172290, 50802116, 51232005), and Program for Pearl River New Star of Science and Technology in Guangzhou (2013J2200015).

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Footnote

Electronic supplementary information (ESI) available: SEM images, DLS curves and FTIR spectrums etc. See DOI: 10.1039/c4ra03244f

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