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Incorporation of Carbon Nanotubes into a Lyotropic Liquid Crystal by Phase Separation in the Presence of a Hydrophilic Polymer Xia Xin,*,† Hongguang Li,† Stefan A. Wieczorek,† Tomasz Szymborski,† Ewelina Kalwarczyk,† Natalia Ziebacz,† Ewa Gorecka,‡ Damian Pociecha,‡ and Robert Hozyst*,†,§ † Department III, Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland, ‡Department of Chemistry, University of Warsaw, Al. Zwirki i Wigury 101, Warsaw, Poland, and §Department of Mathematics and Natural Sciences, College of Science, Cardinal Stefan Wyszynski University, Dewajtis 5, 01-815 Warsaw, Poland
Received August 10, 2009 Single-walled carbon nanotubes (SWNTs) were incorporated into a lyotropic liquid crystal (LLC) matrix formed by n-dodecyl octaoxyethene monoether (C12E6) at room temperature through spontaneous phase separation induced by nonionic hydrophilic polymer poly(ethylene glycol) (PEG). The quality of SWNTs/LLC composite was evaluated by polarized microscopy observations and small-angle X-ray scattering (SAXS) measurements. The results obtained clearly indicated that SWNTs have been successfully incorporated into the LLC matrix up to a considerable high content without destroying the LLC matrix, although interesting changes of the LLC matrix were also induced by SWNTs incorporation. By varying the ratio of PEG to C12E6, the type of LLC matrix can be controlled from hexagonal phase to lamellar phase. Temperature was found to have a significant influence on the quality of SWNTs/LLC composite, and tube aggregation can be induced at higher temperature. When SWNTs were changed to multiwalled carbon nanotubes (MWNTs), they became difficult to be incorporated into LLC matrix because of an increase in the average tube diameter.
Introduction Because of a combination of their unusual structural, mechanical, and electronic properties, carbon nanotubes (CNTs) have shown great potential applications in physics, chemistry and life science.1-3 The freshly produced raw CNTs usually exist as ropes rather than single tubes because of the van der Waals attraction between adjacent tubes. This is especially evident for single-walled carbon nanotubes (SWNTs) with high length to width ratio. The aggregation of SWNTs is a disadvantage in their applications, because the aggregates are difficult to process into useful nanomaterial structures. In recent years, it has been shown that surfactants and amphiphilic macromolecules can prevent nanotube aggregation in aqueous solution under ultrasonication. In this way dispersions containing randomly oriented single tubes in *To whom correspondence should be addressed. E-mail:
[email protected]. edu.pl (R.H.),
[email protected] (X.X.).
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3562 DOI: 10.1021/la902960a
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Published on Web 09/03/2009
Langmuir 2010, 26(5), 3562–3568
Xin et al.
Article
Because of the high viscosity of the LLC phase, however, the incorporation of SWNTs is usually difficult, and rigorous sonication combined with a heating process was usually performed for better introduction of SWNTs. This method is inconvenient as excess heating can destroy the LLC matrix. Particularly in the case of a nonionic surfactant the LLC phase undergoes a transition to the micellar phase at high temperatures. Also the heating process causes desorption of surfactant molecules from SWNTs and subsequent tube aggregation. Therefore, approaches to create a spontaneous phase transition from the isotropic SWNTs dispersions to LLC phases are highly desired.40 In this work, we report a new method of SWNTs incorporation into LLC phase formed spontaneously by a nonionic surfactant (C12E6, n-dodecyl hexaoxyethylene glycol monoether) during phase separation in the presence of a hydrophilic polymer (PEG, poly(ethylene glycol), molecular weight: 20000). To the best of our knowledge, this is the first example of spontaneous phase separation from isotropic CNTs dispersions to well-ordered CNTs/surfactant hybrids in the presence of polymers. The type of LLC phase (hexagonal or lamellar) can be conveniently controlled by varying the ratio of PEG to C12E6. The quality of SWNTs/LLC composite as a function of SWNTs content was evaluated by polarized microscope observations and small-angle X-ray scattering (SAXS) measurements. The influences of temperature and tube diameter on the composite were also investigated. In the latter case, SWNTs were replaced by MWNTs which have a much larger tube diameter.
Experimental Section Chemicals and Materials. SWNTs, with a diameter
