Pyrene-POSS nanohybrid as a dispersant for carbon nanotubes in solvents of various polarities: its synthesis and application in the preparation of a composite membrane
© Majeed et al.; licensee Springer. 2012
Received: 9 April 2012
Accepted: 7 June 2012
Published: 7 June 2012
In this study we report the preparation of nanohybrid dispersant molecules based on pyrene and polyhedral oligomeric silsesquioxanes for non-covalent functionalization of multi-walled carbon nanotubes (MWCNTs). The prepared dispersant improves the dispersion of MWCNTs in organic solvents with very different polarities such as tetrahydrofuran, toluene, and n-hexane. The functionalized MWCNTs were used to introduce conductivity into polydimethylsiloxane membranes which can be used for electrostatic discharge applications.
KeywordsPyrene-POSS Carbon nanotube dispersion Non-covalent functionalization Conductive membranes
The extraordinary electrical, mechanical, and thermal properties of carbon nanotubes (CNTs) make them the strongest candidates for their application in composite materials [1–3]. Good dispersion of CNTs is required for their application in many composites. However, CNTs are produced in the form of bundles where they are attracted together by van der Waals interactions. The aggregation of CNTs in the form of bundles influences the properties of the resulting composite materials e.g., ineffective stress transfer and higher percolation thresholds for electrical conductivity. Moreover, the agglomerates of carbon nanotubes may act as conventional carbon black and hence, to obtain improved material properties, the disaggregation of CNTs agglomerates is necessary [4–6].
Surface modification of CNTs is a tool to improve their dispersion in various solvents and matrices and it can be grouped into two different categories: (a) binding the functional groups on the π-conjugated skeleton of carbon nanotube via covalent bonding [7–9] and (b) physical adsorption or wrapping of a variety of functional molecules via non-covalent interactions [10–14]. Non-covalent functionalization has an advantage over covalent functionalization because no major side wall defects occur thus preserving the electronic properties of π-conjugated tubular structure of CNTs. Such functionalization involves wrapping of the CNTs’ surface by various polymers, polynuclear aromatic compounds, surfactants, or biomolecules . Ionic and biological surfactants improve the CNTs’ dispersion in aqueous solutions where CNTs are entrapped into micelles leading to a stable dispersion . Conjugated polymers interact with CNTs by π−π stacking, resulting in better dispersion of CNTs in specific organic solvents . Block copolymers, having at least one block exhibiting conjugation and other having high affinity toward solvent, lead to a better dispersion of CNTs in solvents of different polarities . Polycyclic aromatic compounds e.g., pyrene, are also well known for their π−π stacking on CNTs surface, and the attachment of pyrene to molecular species which are soluble in organic solvents or aqueous media creates the possibility to effectively disperse CNTs in these solvents .
Polyhedral oligomeric silsesquioxanes (POSS) possess an inorganic cage structure, with the possibility for a variety of functional groups of different nature to be attached to the Si8O12 core [18, 19]. The reactive functional groups on POSS have been subjected to functionalize CNTs, allowing a good dispersion of CNTs in solvents such as chloroform and tetrahydrofuran (THF) [20–22]. POSS-functionalized CNTs resulted in nanocomposites with improved CNTs dispersion and mechanical properties . In this study, 1-pyrenebutyric acid was attached covalently to POSS, and multi-walled carbon nanotubes (MWCNTs) were non-covalently modified with the resultant pyrene-POSS. The obtained hybrid material can be efficiently dispersed in various organic solvents such as n-hexane, toluene, and THF to form stable dispersions. The dispersibility of the hybrid material is provided by the presence of the POSS with aliphatic moieties having high affinity toward organic solvents.
To demonstrate the dispersing effect of pyrene-POSS also in a polymer matrix, pyrene-POSS-modified MWCNTs were used for the fabrication of conductive polydimethylsiloxane (PDMS) nanocomposite membranes. Such conductive membranes, when applied in spiral wound membrane modules with plastic housing, can provide the opportunity to neutralize electrostatic charges and hence may provide, for example, a safer separation of hydrocarbons. In order to avoid electrostatic charging, a surface resistivity between 106 Ω/□ and 109 Ω/□ or bulk electrical conductivity above 10−6 Sm−1 are required [23, 24].
It has been a challenge to uniformly disperse non-functionalized MWCNTs in a thin PDMS selective layer because of the indispersibility of unmodified MWCNTs in nonpolar solvents like n-hexane and toluene. However, pyrene-POSS provided a way to effectively disperse MWCNTs in PDMS nanocomposite membranes with the advantage of a well-preserved tubular conjugated MWCNTs structure which, in turn, conserves their electrical properties. The PDMS nanocomposite membranes were characterized by optical microscopy, scanning electron microscopy, gas permeation, and sheet resistance measurements.
1-Pyrenebutyric acid from Aldrich (Sigma-Aldrich Logistik GmbH, Schnelldorf, Germany) and aminopropylisobutyl POSS (APiB-POSS) from Hybrid Plastics Inc. (Hattiesburg, MS, USA) were purchased. The MWCNTs obtained by chemical vapor deposition with purity of >98%, surface area of 250 m2/g, tube diameter in the range of 12 to 15 nm and 8 to 12 walls were provided by FutureCarbon GmbH (Bayreuth, Germany). Dehesive 940, crosslinker V24 and catalyst OL were purchased from Wacker Silicones GmbH (WackerChemie AG, München, Germany). All the organic solvents were used as received or were additionally distilled.
Nuclear magnetic resonance (NMR) spectra of the compounds were recorded on Bruker AV-300 (Bruker Biospin GmbH, Karlsruhe, Germany) at 300 MHz using CDCl3. Fourier transform infrared spectroscopy (FTIR) was conducted using a Bruker Equinox 55 (Bruker Optics, Bremen, Germany). Thermogravimetric analysis (TGA) measurements were done using Netzsch TG209 F1 Iris (NETZSCH-Gerätebau GmbH,Selb, Germany), under constant argon flow of 20 mL/min at a constant heating rate of 20 °C/min. The transmission electron microscopy (TEM) characterization of MWCNTs was carried out in bright-field and energy filtered modesusing an FEI Tecnai G2 F20 (FEI Company, Eindhoven, The Netherlands) operated at 200 kV. Optical microscopy images were taken on Leica DMLM (Leica Microsystems GmbH, Wetzlar, Germany),and samples were analyzed under reflection mode. Scanning electron microscopy studies on the membranes were carried out using LEO 1550 VP from Zeiss (Carl Zeiss Inc., Oberkochen, Germany). The sample preparation for cross-section analysis was done under cryogenic conditions. The surface conductivity was analyzed with a four-point measurement equipment from Jandel, Linslade, UK. Gas permeances of CH4, N2, O2, CO2, and C2H6 were characterized at 23 °C using a constant volume variable pressure method facility.
Synthesis of pyrene-POSS nanohybrid
Functionalization of MWCNTs
Pyrene-POSS (300 mg) was dissolved in 50 mL THF, and dry MWCNTs were added to reach 1:1 weight ratio. The resultant mixture was sonicated for 5 h using Sonorex Super RK 255 H ultrasonic bath from Bandelin (Bandelin Electronic GmbH & Co. KG, Berlin, Germany), operated at a frequency of 35 KHz and an effective power of 160 W. The temperature was maintained at 20–25 °C during sonication. After sonication, MWCNTs were washed at least five times with THF to remove non-adsorbed pyrene-POSS followed by vacuum drying at room temperature for 24 h.
The functionalization of MWCNTs was also carried out at 1:0.2, 1:0.6, 1:2, and 1:3 weight ratios between MWCNTs and pyrene-POSS, respectively, to investigate the effect of the increased quantity of pyrene-POSS on its degree of adsorption on MWCNTs.
Fabrication of nanocomposite membranes
Appropriate amounts of pyrene-POSS-modified MWCNTs (PP-MWCNTs) were dispersed in 25 g toluene by sonication. Dehesive 940 was added to the PP-MWCNTs’ dispersion to maintain a 4 wt.% concentration of PDMS/PP-MWCNTs and 1, 2, 3, 4, and 5 wt.% loading of the effective MWCNTs with respect to PDMS. The mixture was stirred for 3 h to obtain a homogeneous dispersion, and crosslinker and catalyst were added to the solution followed by brief vigorous stirring to homogenize the system. The PDMS nanocomposite solution was dip-coated on top of a polyacrylonitrile (PAN) microporous membrane (average pore size 20 nm and 15% surface porosity); the solvent was evaporated at room temperature, and PDMS was cross-linked in an oven at 70 °C for 1 h.
Results and discussion
Covalent functionalization of carbon nanotubes with APiB-POSS has been reported in literature for the case of MWCNTs dispersion in THF . In the present study, we report the preparation of nanohybrid dispersant molecules based on APiB-POSS and pyrene for the physical surface modification of MWCNTs. Besides conserving the intrinsic properties of the MWCNTs, we aim to improve their dispersibility not only in a polar solvent like THF, but also in non-polar solvents like toluene and n-hexane. The improved dispersion of MWCNTs in non-polar solvents provides a way to fabricate PDMS nanocomposite films by solvent evaporation from a mixed solution of polymer and nanofiller.
Characterization of pyrene-POSS
Dispersion of MWCNTs in solvents of different polarities
TGA data for different ratios of MWCNTs and pyrene-POSS
MWCNTs/pyrene- POSS ratioa
MWCNTs surface area ratiob
Mass loss (percent)c
Pyrene-POSS content (mmol g−1)d
Pyrene coverage (percent)e
Flat Pyrene-POSS coverage (percent)e
L-shaped pyrene-POSS coverage (percent)e
Characterization of PDMS nanocomposite membranes
Pyrene-POSS was synthesized via amidation reaction between 1-pyrenebutyric acid and aminopropyli sobutyl POSS. The successful synthesis of pyrene-POSS hybrid is evident from NMR, FTIR, and MWCNTs dispersion analyses. TGA and TEM analyses proved the adsorption of pyrene-POSS nanohybrids on MWCNTs. Digital photographs and optical micrographs of PDMS nanocomposite membranes containing pyrene-POSS functionalized MWCNTs show homogeneously coated samples compared to samples containing purified MWCNTs. Defect-free conductive PDMS nanocomposite membranes were prepared which can reduce the probability of electrostatic discharges in gas separation applications. Moreover, the ability of pyrene-POSS to disperse MWCNTs in different solvents opens the way of dispersibility improvement of other carbon-based materials and provides the opportunity to fabricate polymer nanocomposites by solvent evaporation.
The authors would like to thank Sabrina Bolmer for SEM analysis and Silivio Neumann, Bahadir Gacal, Gunter Lührs, Ahnaf Usman Zillohu, and Muntazim Khan for technical support. This work was financially supported by the project “High aspect ratio carbon-based nanocomposites” (HARCANA) within the European Community’s 7th Framework Programme for Research and Technological Development under the Grant Agreement number NMP3-LA-2008-213277.
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