Imide-siloxane block copolymer/silica hybrid membranes: preparation, characterization and gas separation properties
Introduction
An incorporation of inorganic network such as silica phase into an organic polymer matrix has been extensively studied because it displays a wide range of multifunctional properties [1], [2], [3], [4], [5]. Many new types of organic/inorganic hybrid materials have the potential to combine the desired properties of inorganic and organic systems, improving the mechanical and thermal properties of inorganic ones with the flexibility and ductility of organic polymers. These hybrid materials can be readily prepared by using sol–gel process, which offers several advantages over other techniques. The micro- and macrostructure of a hybrid composite can be controlled by the optimization of several synthetic parameters, for example, pH, concentration, water-to-alkoxide ratio, temperature, pressure, type of catalyst, and solvent at low temperature [6], [7].
Of these hybrid composites, polyimide (PI) as an organic matrix has been of special interest for its exceptional thermal and mechanical stability. It also possesses outstanding electrical properties, which has been applied to a wide range of industries such as microelectronics, aerospace, and electricity [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Particularly, in the field of membrane-based gas separation, the polyimide/silica hybrid materials have been extensively studied for applications where high chemical and thermal stability is needed [19], [20], [21], [22], [23], [24], [25]. Joly et al. [19], [23] studied the effect of silica particles on the gas transport properties of polyimide films. They made polyimide/silica films by the addition of tetramethoxysilane (TMOS) and water to poly(amic acid) solution before the thermal treatment. The PI/silica composite membrane exhibited higher gas permeabilities without a large reduction of permselectivity than those of the polyimide membrane. They explained the permeability improvement as a result of solubility enhancement in the organic matrix of the composite rather than a diffusion enhancement in the interfacial gaps between both materials (silica and polyimide). Poly(ether imide)/silica membranes were prepared by in situ growth of the inorganic network by hydrolysis and condensation of tetraethoxysilane (TEOS) and addition of amino silane [20]. An addition of 3-aminopropyl trimethoxysilane with TEOS caused the membrane morphology to become a sponge-like structure and led to a good mechanical stability of membrane under high pressure (>80 bar) when compared with pure asymmetric poly(ether imide) membrane. However, the effect of the addition of silica phase on the permselectivity was not mentioned here.
Kusakabe et al. [21] studied on the gas separation properties of the polyimide/silica membrane coated on alumina tubes. They prepared poly(amic acid) by dehydration condensation of pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) mixed with silica sol containing coupling agent at various ratios. Each hybrid sol was coated on a γ-alumina-coated tube porous tube and thermally imidized. These microcomposite membranes showed a high CO2 permeance, retaining the high permselectivity inherent to polyimide membrane at high temperature (<300 °C). Smaihi et al. [22] studied two types of poly(imide siloxane) materials prepared by condensation, imidization and sol–gel process using TMOS with two coupling agents, aminopropyltrimethoxysilane (APrTMOS) and aminopropylmethyldiethoxysilane (APrMDEOS). They showed that the gas permeation increased with the siloxane content in the polymeric matrix and higher fluxes were observed for APrMDEOS than for APrTMOS whatever the composition, due to the difference in crosslinking in the polymeric network. Thus, the hybrid composite membranes consisted of organic and inorganic components have been recently developed in order to improve the permeation characteristics, and those are regarded as presenting the advantages of each component in only one membrane.
The objective of this study is to improve the gas separation properties of polyimide/silica hybrid membrane by first introducing organosiloxane units in polyimide backbone to form poly(siloxane amic acid) (SPAA). Nanocomposite membranes have been prepared by condensation, imidization, and sol–gel reaction of SPAA containing a coupling agent in an end group with tetraethoxysiloxane (TEOS). Many types of hybrid materials were prepared and their molecular structures were characterized by Fourier transform-infrared spectrometry (FT-IR), thermogravimetric analysis (TGA), nuclear magnetic resonance ( NMR), and electron spectroscopy for chemical analysis (ESCA). The gas permeabilities and selectivities were measured at various temperatures for helium, oxygen, carbon dioxide and nitrogen. The effect of siloxane and silica content on gas permeation properties has also been investigated and discussed in this paper.
Section snippets
Materials
Pyromellitic dianhydride and 4,4′-oxydianiline were obtained from Tokyo Kasei Co. Inc. (Tokyo, Japan) and used without further purification. 3-Aminopropyl triethoxysilane (APTES) and tetraethoxysilane were obtained from Aldrich (Milwaukee, WI, USA) and used without further purification. An α,ω-aminopropyl polydimethylsiloxane (PDMS, number average molecular weight, Mn=900) was obtained from Shinetsu Chemical Co. Inc. (Tokyo, Japan) and used after vacuum distillation. Tetrahydrofuran (THF) and N
FT-IR spectra
FT-IR spectroscopy was used to study the chemical structure of the hybrid membranes containing various proportions of silica. Fig. 3 shows the FT-IR spectra of the S1PI-sil10 (with 10 wt.% SiO2), S1PI-sil30 (with 30 wt.% SiO2), and S1PI-sil50 (with SiO2 50 wt.%), respectively. All the hybrid membranes present the characteristic imide peaks at 1778 cm−1 (v(CO), in-phase, imide), 1720 cm−1 (v(CO), out-of-phase, imide), 1382 cm−1 (v(CNC), axial, imide), 1503 cm−1 (v(C6H4) or v(C6H2)), 1080 cm−1 (v
Conclusions
In this study, poly(imide siloxane)/silica hybrid membranes with covalent bonds were prepared by the sol–gel reaction. This unique feature of the present study is due to the introduction of the organosiloxane (PDMS) into a rigid polyimide backbone with coupling agent in sol–gel reaction. The addition of PDMS in polyimide improved the gas permeabilities of the hybrid membranes, indicating that the gas transport preferably occurred in the organic matrix. Meanwhile, the gas selectivities decreased
Acknowledgements
This work was supported by the Korea Institute of S & T Evaluation and Planning under a National Research Laboratory Program. J.K. Kim is grateful to the Brain Korea 21 Project for a fellowship. Authors appreciate the PDMS samples from Shinetsu Chemical Co. Inc.
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