Preparation of silica nanospheres and porous polymer membranes with controlled morphologies via nanophase separation
© Lee et al.; licensee Springer. 2012
Received: 16 May 2012
Accepted: 29 June 2012
Published: 8 August 2012
We successfully synthesized two different structures, silica nanospheres and porous polymer membranes, via nanophase separation, based on a sol–gel process. Silica sol, which was in situ polymerized from tetraorthosilicate, was used as a precursor. Subsequently, it was mixed with a polymer that was used as a matrix component. It was observed that nanophase separation occurred after the mixing of polymer with silica sol and subsequent evaporation of solvents, resulting in organizing various structures, from random network silica structures to silica spheres. In particular, silica nanospheres were produced by manipulating the mixing ratio of polymer to silica sol. The size of silica beads was gradually changed from micro- to nanoscale, depending on the polymer content. At the same time, porous polymer membranes were generated by removing the silica component with hydrofluoric acid. Furthermore, porous carbon membranes were produced using carbon source polymer through the carbonization process.
KeywordsPhase separation Silica nanosphere Porous polymer membrane Porous carbon
Considerable efforts have been devoted to the design and fabrication of controlled organic/inorganic composites with novel properties, including optical, electrical, chemical, biological, and mechanical properties [1–4]. In these hybrid systems, phase separation occurs naturally because they are composed of two materials with totally different chemical characteristics [5–7]. When domain formation is induced by phase transition, the compatibility and interaction between organic and inorganic components are key factors to determine the uniformity and nanostructures of the final objects [8–10]. These factors contributed not only to the size of the nanostructured inorganic materials, but also to their morphologies, which can have an effect on the ultimate properties.
The composites prepared by the sol–gel-based process compared with other strategies including surface modification and development of new routes [11, 12] show the possibility of creating well-organized homogeneous inorganic structures in an organic matrix, resulting in obtaining the expected properties [13–17]. In particular, silica nanoparticles prepared by sol–gel were regarded as one of the most useful materials and were used in practical applications such as inorganic additives [18–22]. Nevertheless, the need for various sizes of silica nanoparticles with narrow size distribution has increased gradually for high technology applications.
Recently, membrane technologies have been established on a large scale, owing to the intensive results so far achieved [23–27]. A membrane refers to a separating structure serving as a selective barrier, and the unique property of membranes is to separate between two phases. For example, they separate air to remove carbon dioxide from natural gas and produce pure water from seawater via water treatment. Among the various materials (e.g., metals, ceramics, and composites) used for membranes, polymers are the most attractive materials because the permeability and selectivity of polymer can be adjustable and organized simply by solution processing [28–32]. Furthermore, Kim et al., reported the porous carbon membranes fabricated by self-assembly [33, 34].
Herein, we prepared a series of silica/polymer composites using nanophase separation based on the sol–gel process. We controlled the ratio of polymer to silica sol for fabricating silica nanospheres and porous polymer membrane simultaneously. The micro- or nanostructures of silica were tuned by controlling a mixing ratio of polymer and silica. At the same time, nanoporous polymer structures, which were reversely replicated to silica spheres, were obtained. Both silica nanospheres and/or porous polymer membranes were produced by a selective removal method, such as calcination, and a chemical etching process. In addition, porous carbon membranes were transferred from polymer sources by carbonization.
Low molecular weight poly(methyl methacrylate) (PMMA) (Mw = 75 kg/mol) and high molecular weight PMMA (Mw = 350 kg/mol) were purchased from Polymer Source Inc. (Quebec, Canada) and Sigma-Aldrich Corporation (St. Louis, MO, USA), respectively. Polyacrylonitrile (PAN) (Mw = 150 kg/mol) was supplied by Sigma-Aldrich. Analytical grade tetraorthosilicate (TEOS), hydrochloric acid (HCl), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich to synthesize silica sol. The hydrofluoric acid (HF) (J.T. Baker, Avantor Performance Materials, Center Valley, PA, USA) was diluted by deionized water before use.
Preparation of polymer/silica solution
The TEOS precursor was mixed with a diluted HCl solution in a volume ratio of 6:2.3. The diluted HCl solution was obtained by mixing 0.02 mL of a concentrated HCl with 10 mL of deionized water. THF was added to the aqueous TEOS solution in a volume ratio of 3:1 and stirred for 2 h. This solution was subsequently mixed in a volume ratio of 1:1 with a 3-wt.% polymer solution (PMMA in THF and PAN in DMF) for 2 h.
Synthesis of nanostructured silica and polymer membranes
The resulting homogeneous solution was cast into a Teflon container and dried in a vacuum oven at 60°C for 6 h. The solid samples were produced after the evaporation of all solvents. As-synthesized polymer/silica composites were treated in two different ways to selectively remove one of the components. Calcination proceeded at 500°C for 3 h in air condition to obtain pure silica particles. On the other hand, polymer membranes were prepared by immersing the samples in a diluted 5 wt.% HF solution and subsequently rinsed several times with deionized water. Porous carbon membranes were prepared by a carbonization process (850°C for 3 h in an argon environment) of PAN/silica composites. A scanning electron microscope (SEM) (NanoSEM 230, FEI Company, Hillsboro, OR, USA) operating at 10 kV was used to characterize the surface morphologies of as-prepared silica/polymer composites, nanostructured silica, and polymer membranes. Raman spectrum was recorded on a JASCO spectrometer (NRS 3000; JASCO Inc., Easton, MD, USA) to investigate the characteristics of carbon materials. An He-Ne laser was operated at λ = 632.8 nm.
Results and discussion
When the sample seen in Figure 2b that has increasing polymer content was employed, PMMA membranes with smaller pore size were fabricated. In a similar manner, PMMA membranes of Figure 6c with uniform pores can be prepared from the samples seen in Figure 2c. Morphologies of polymer membranes seen in Figure 6 are the same as the replicated silica structures seen in Figure 2. Depending on the applications, nanostructured silica and/or polymer membranes can be selectively left over or removed.
We have successfully synthesized uniform-sized silica spheres and porous polymer membranes using a concept of nanophase separation. Incompatibility between polymer and silica sol induced the nanophase separation, resulting in the formation of polymer/silica composites. In this manner, the size of silica spheres could be tuned in the range of 1.6 μm to 80 nm by controlling the mixing ratio of polymer to silica sol after calcination process. Concurrently, a selective chemical etching of the same polymer/silica composites led to the formation of porous polymer membranes. Moreover, when polymer that can be used as a carbon source was used to make polymer/silica composites, followed by a chemical etching in HF solution, macroporous carbon membranes were successfully fabricated. This simple but straightforward process can be used in other applications, such as photonic bandgap, antireflection coating, lithium-ion batteries, and so on.
This work was supported by the WCU (R31-2008-000-20012-0) programs.
- Imazato S: Antibacterial properties of resin composites and dentin bonding systems. Dent Mater 2003, 19: 449. 10.1016/S0109-5641(02)00102-1View Article
- Bledzki AK, Gassan J: Composites reinforced with cellulose based fibres. Prog Polym Sci 1999, 24: 221. 10.1016/S0079-6700(98)00018-5View Article
- Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, Stach EA, Piner RD, Nguyen ST, Ruoff RS: Graphene-based composite materials. Nature 2006, 442: 282. 10.1038/nature04969View Article
- Qiao Y, Bao S-J, Li CM, Cui X-Q, Lu Z-S, Guo J: Nanostructured polyaniline/titanium dioxide composite anode for microbial fuel cells. ACS Nano 2008, 2: 113. 10.1021/nn700102sView Article
- Zhang Q, Archer LA: Poly(ethylene oxide)/silica nanocomposites structure and rheology. Langmuir 2002, 18: 10435. 10.1021/la026338jView Article
- Nakanishi K: Pore structure control of silica gels based on phase separation. J Porous Mater 1997, 4: 67. 10.1023/A:1009627216939View Article
- Lipatov YS, Nesterov AE, Ignatova TD, Nesterov DA: Effect of polymer–filler surface interactions on the phase separation in polymer blends. Polymer 2002, 43: 875. 10.1016/S0032-3861(01)00632-2View Article
- Fu S-Y, Feng X-Q, Lauke B, Mai Y-W: Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of composites. Composites: Part B 2008, 39: 933. 10.1016/j.compositesb.2008.01.002View Article
- Goltner-Spickermann C: Non-ionic templating of silica: formation mechanism and structure. Curr Opin Colloid & Interface Sci 2002, 7: 173. 10.1016/S1359-0294(02)00046-8View Article
- Park C, Hyun DC, Lim MC, Kim SJ, Kim YR, Paik HJ, Jeong U: Continuous production of functionalized polymer particles employing the phase separation in polymer blend films. Macromol Rapid Commun 2011, 32: 1247. 10.1002/marc.201100199View Article
- Caruso F: Nanoengineering of particle surfaces. Adv Mater 2001, 13: 11. 10.1002/1521-4095(200101)13:1<11::AID-ADMA11>3.0.CO;2-NView Article
- Hu J, Chen M, Wu L: Organic–inorganic nanocomposites synthesized via miniemulsion polymerization. Polym Chem 2011, 2: 760. 10.1039/c0py00284dView Article
- Sun J, Akdogan EK, Klein LC, Safari A: Characterization and optical properties of sol–gel processed PMMA-SiO2 hybrid monoliths. J Non-Cryst Solids 2007, 353: 2807. 10.1016/j.jnoncrysol.2007.05.158View Article
- Yeh J-M, Hsieh C-F, Yeh C-W, Wu M-J, Yang H-C: Organic base-catalyzed sol–gel route to prepare PMMA-silica hybrid materials. Polym Int 2007, 56: 343. 10.1002/pi.2143View Article
- Zou H, Wu S, Shen J: Polymer/silica nanocomposites: preparation, characterization, properties, and applications. Chem Rev 2008, 108: 3893. 10.1021/cr068035qView Article
- Morikawa A, Lyoku Y, Kakimoto M, Mai Y: Preparation of new polyimide-silica hybrid materials via the sol–gel process. J Mater Chem 1992, 2: 679. 10.1039/jm9920200679View Article
- Caruso RA, Antonietti M: Sol–gel nanocoating: an approach to the preparation of structured materials. Chem Mater 2001, 13: 3272. 10.1021/cm001257zView Article
- Lu Y, Yin Y, Mayers BT, Xia Y: Modifying the surface properties of superparamagnetic iron oxide nanoparticles through a sol–gel approach. Nano Lett 2002, 2: 183. 10.1021/nl015681qView Article
- Bogush GH, Tracy MA, Zukoski CF: Preparation of monodisperse silica particles: control of size and mass fraction. J Non-Cryst Solids 1988, 104: 95. 10.1016/0022-3093(88)90187-1View Article
- Barbe C, Bartlett J, Kong L, Finnie K, Lin HQ, Larkin M, Calleja S, Bush A, Calleja G: Silica particles: a novel drug-delivery system. Adv Mater 2004, 16: 1959. 10.1002/adma.200400771View Article
- Rosenholm JM, Sahlgren C, Linden M: Towards multifunctional, targeted drug delivery systems using mesoporous silica nanoparticles – opportunities & challenges. Nanoscale 1870, 2010: 2.
- Stöber W, Fink A: Controlled growth of monodisperse silica spheres in the micron size range. J Colloid Interface Sci 1968, 26: 62. 10.1016/0021-9797(68)90272-5View Article
- Ulbricht M: Advanced functional polymer membranes. Polymer 2006, 47: 2217. 10.1016/j.polymer.2006.01.084View Article
- Freeman BD: Basis of permeability-selectivity tradeoff relations in polymeric gas separation membranes. Macromolecules 1999, 32: 375. 10.1021/ma9814548View Article
- Sokalski T, Ceresa A, Zwickl T, Pretsch E: Large improvement of the lower detection limit of ion-selective polymer membrane electrodes. J Am Chem Soc 1997, 119: 11347. 10.1021/ja972932hView Article
- Tanaka M, Sackmann E: Polymer-supported membranes as models of the cell surface. Nature 2004, 437: 656.View Article
- Lee K-S, Jeong M-H, Lee J-P, Kim Y-J, Lee J-S: Synthesis and characterization of highly fluorinated cross-linked aromatic polyethers for polymer electrolytes. Chem Mater 2010, 22: 5500. 10.1021/cm101405hView Article
- Li Q, Jensen JO, Savinell RF, Bjerrum NJ: High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Prog Polym Sci 2009, 34: 449. 10.1016/j.progpolymsci.2008.12.003View Article
- Ham H, Chung I, Choi Y, Lee S, Kim S: Macroporous polymer thin film prepared from temporarily stabilized water-in-oil emulsion. J Phys Chem B 2006, 110: 13959. 10.1021/jp0616361View Article
- Bakker E, Buhlmann P, Pretsch E: Polymer membrane ion-selective electrodes-What are the limits? Electroanalysis 1999, 11: 915. 10.1002/(SICI)1521-4109(199909)11:13<915::AID-ELAN915>3.0.CO;2-JView Article
- Park J, Lee S, Han T, Kim S: Hierarchically ordered polymer films by templated organization of aqueous droplets. Adv Funct Mater 2007, 17: 2315. 10.1002/adfm.200601141View Article
- Widawski G, Rawiso M, Francois B: Self-organized honeycomb morphology of star-polymer polystyrene films. Nature 1994, 369: 387. 10.1038/369387a0View Article
- Lee S, Park J, Lim B, Mo C, Lee W, Lee J, Hong S, Kim S: Highly entangled carbon nanotube scaffolds by self-organized aqueous droplets. Soft Matter 2009, 5: 2343. 10.1039/b817477fView Article
- Lee S, Kim H, Hwang J, Lee W, Kwon J, Bielawski C, Ruoff R, Kim S: Three-dimensional self-assembly of graphene oxide platelets into mechanically flexible macroporous carbon films. Angew Chem Int Ed 2010, 49: 10084. 10.1002/anie.201006240View Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.