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.
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.
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