Tailor-designed polyphilic promotors for stabilizing dispersions of carbon nanotubes in liquid crystals

Article abstract of DOI:10.1039/c0cc02353a

We present a potent multifunctional molecular design concept for promoting the dispersion of carbon nanotubes (CNTs) in thermotropic liquid crystals (LCs), making CNT-in-LC dispersions of unprecedented stability possible and broadening the scope of potent

Full text of DOI:10.1039/c0cc02353a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Tailor-designed polyphilic promotors for stabilizing dispersions
of carbon nanotubes in liquid crystalsw
Martin Ku
¨
hnast,ab Carsten Tschierske*a and Jan Lagerwall*b
Received 3rd July 2010, Accepted 6th August 2010
DOI: 10.1039/c0cc02353a
We present a potent multifunctional molecular design concept
for promoting the dispersion of carbon nanotubes (CNTs)
in thermotropic liquid crystals (LCs), making CNT-in-LC
dispersions of unprecedented stability possible and broadening
the scope of potential applications.
LC host, thereby functioning as a dispersion promotor. The
molecules are tailored for the purpose by following a general
design concept where a mesogenic unit and a CNT anchor
group are combined via a flexible spacer (Fig. 1). With the
right combination of these components we achieve a dramatic
stabilization of CNT suspensions in standard commercial
nematic LC mixtures, even for the case that the LC on its
own is a poor CNT solvent.
Extraordinary electrical, thermal and mechanical performance
renders carbon nanotubes (CNTs) highly attractive for a
broad range of new materials and devices.1–3 Due to their
extreme anisotropy, in shape as well as properties, the ability
to align the tubes is key to many proposed CNT applications.
This is particularly difficult—but all the more important—for
the randomly aligned and entangled CNTs resulting from
substrate-free growth, to date the only synthesis approach
that is commercially viable and allows purification and
fractionation of the nanotubes.4 Liquid crystals (LCs) represent
a unique platform for addressing this problem since their
inherent long-range order can provide macroscopic-scale
control of the orientation of nanotubes dispersed in them.5–10
Further promoted by a number of remarkable phenomena
observed in CNT-doped LCs, such as electroactive super-
elongation of CNT-clusters,11 improvement in electro-optic
response of LCs,8,12,13 CNT self-assembly into platelets,14 or
polarization of light and nanotube-induced filament formation,15
the study of LC–CNT composites has during the last few years
grown into an important and vibrant area of research. A
major obstacle limiting the speed of progress, both in terms
of research and applications, is however the lack of a reliable
and versatile method for dispersing CNTs stably at sufficient
concentration in LCs. Most liquid crystals are inadequate
hosts for CNTs16 since they have been optimized for other
purposes, e.g. to meet specific demands on optical and
elastic properties required for the proper performance of an
LC-based device. A key factor in advancing the field would
therefore be the development of a general means of stabilizing
CNT-in-LC dispersions that works with any host LC and
preferably with any CNT type.
For realizing stable suspensions of CNTs one is typically
faced with the choice between covalent functionalization of the
nanotube wall17 and the addition of a multifunctional dispersion
promotor molecule. We pursue the latter route since covalent
functionalization affects the physical properties of CNTs
negatively.18 For the case of thermotropic LC hosts this is a
much more challenging task than for the commonly encountered
case of aqueous CNT dispersion, where coating of the nano-
tubes by surfactants provides efficient stabilization. Since
CNTs as well as LCs are nonpolar, classical surfactants are
of no use, calling for a new design paradigm for molecules that
can stabilize CNT–LC suspensions.
Inspired by encouraging results from studies of aqueous
nanotube dispersion using surfactants based on pyrene or
similar polycyclic aromatic moieties19–22 we have used pyrene,
anthracene and terphenyl for providing the CNT anchoring
function. These moieties are expected to have high affinity for
CNTs due to aromatic p–p stacking with the graphene
surfaces of the nanotubes. Pyrene was also used recently as
the CNT anchor group in a rare case of promotor molecules
developed for non-polar solvents23 as well as in a multi-
functional assembly promotor for CNT-based hybrid
photovoltaics.24
Since our purpose is to use standard commercial nematic
LCs as host we use a derivative of the commonly employed
cyanobiphenyl motif as our mesogenic group. As spacer group
an undecyl or a tetra(ethylene oxide) chain was used. The
resulting molecules 1a–c and 2a,b (Fig. 2) represent polyphiles
composed of three intrinsically incompatible units: the
In this Communication we present the first thrust to provide
such an instrument, by introducing a new type of polyphilic
molecule that stabilizes the interface between CNT guest and
a Martin-Luther-University Halle-Wittenberg, Institute of Chemistry –
Organic Chemistry, Kurt-Mothes Strasse 2, 06120 Halle, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de;
Fax: +49 (0)345 5527030; Tel: +49 (0)345 5525664
b Martin-Luther-University Halle-Wittenberg, Institute of Chemistry –
Physical Chemistry, von-Danckelmann-Platz 4, 06120 Halle,
Germany. E-mail: jan.lagerwall@lcsoftmatter.com;
Fax: +49 (0)345 5527400; Tel: +49 (0)345 5525836
w Electronic supplementary information (ESI) available: Synthesis
description, NMR data, phase sequences. See DOI: 10.1039/
c0cc02353a
Fig.
1 Tailor-designed polyphiles, exhibiting a CNT-anchoring
group (blue) and a mesogenic unit (red) connected via a flexible spacer
(green), promote the dispersion of CNTs in LCs.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6989–6991 6989

View Article Online
group and the linker chain. With the undecyl spacer
(compounds 2a and 2b) only marginal improvement was
observed. This was somewhat greater with the pyrene anchor
group than when terphenyl was used, as seen by the super-
natant of the pyrene sample being slightly darker than that of
the reference sample, whereas the supernatant of the sample
to which 2a had been added was nearly indistinguishable
from the reference supernatant. In contrast, promotor
molecules with tetra(ethylene oxide) spacers dramatically
improved the dispersion stability when the CNT anchor
group was either pyrene (1b) or anthracene (1c). As seen in
Fig. 3 the supernatant of the dispersions prepared with these
additives remained very dark even after four hours of
centrifugation.
Fig. 2 Structures of the new dispersion promotor molecules.
rod-like mesogenic units, which should be compatible with the
LC host but incompatible with the flexible spacers and the
anchor groups; the rigid aromatic anchoring unit which should
interact with CNTs but ideally not with the LC host; and the
flexible spacer units which should be incompatible with the
aromatic cores of the LC units as well as with the anchor
groups.
The reason for the much better performance of the additives
with tetra(ethylene oxide) spacer is most likely the stronger
incompatibility of these flexible and polar spacers with the
liquid crystal as well as with the anchor group. Almost all
previous work on CNT dispersion with the aid of pyrene-
based surfactants has been carried out with water as host
liquid, giving the hydrophobic effect an important role in
stabilizing the dispersion. This contribution disappears when
the host is a thermotropic LC because of the nonpolarity of
guest particles as well as host liquid. By incorporating a
polar and flexible oligo(ethylene oxide) spacer a nanoscale
incompatibility was introduced that separates the CNT anchor
groups from the mesogenic moieties. Spacers and mesogenic
units are thus excluded from CNTs and anchor groups,
resulting in a reinforcement of the interaction between the
latter components, similar to the action of the hydrophobic
effect in aqueous systems in promoting surfactant adsorption
onto nonpolar species. The incompatibility, which we believe
is a major factor in improving the efficiency of the CNT
dispersion promotor, is less pronounced in series 2 with alkyl
spacers, as these are non-polar and more rigid. This explains
why the series 2 molecules are much inferior promotors of
CNT solubilization in LCs.
The synthesis of the polyphiles was carried out in a modular
approach as outlined in the supporting information.
Compounds 2a and 2b are liquid crystalline themselves
whereas the series with the tetra(ethylene oxide) spacer melt
directly from the crystal into an isotropic phase, cf. supporting
information. For investigation of the performance in promoting
CNT dispersion the polyphiles were first added at a concen-
tration on the order of 10 mg mLÀ1 to the target LC, here the
commercial multicomponent nematic mixture RO-TN-403/015S
(Hoffmann-Laroche), mainly composed of cyanobiphenyl,
cyanoterphenyl and phenylpyrimidine mesogens. Single-wall
CNTs (Unidym Hipco) were then added at a concentration
of 0.1 mg mLÀ1 and dispersed by means of sonication
(Dr Hielscher UIS 250V sonotrode, 250 W@24 kHz) for 20 min,
the sample kept in a water bath for cooling. To assess the
quality of the different dispersions we subjected all samples,
including a reference sample where the CNTs had been
dispersed in the pristine LC without any added promotors,
to long-term centrifugation at 12 800 g, studying the quantity
of nanotubes remaining in the supernatant as reflected by its
darkness. On its own, RO-TN-403/015S is a rather poor CNT
host, as revealed in the centrifugation test after which essentially
all nanotubes dispersed in the pure LC were in a sediment. The
supernatant had the same visual appearance as the pristine
LC, cf. Fig. 3 (sample labelled ‘ref’).
Interestingly, the tetra(ethylene oxide)-based additive with a
terphenyl anchor group (1a) did not perform well, one reason
most likely being that terphenyl is a common core in liquid
crystal molecules, i.e. it is in itself mesogenic to some extent
giving it a strong tendency to mix with the LC host. Moreover,
the non-coplanar arrangement of the three phenyl rings prevents
close contact with the CNT surface. These considerations as
well as the experimental results indicate that terphenyl is in
fact not a good CNT anchor group, despite its linearly
extended aromatic p system. More detailed characterization
of the mixtures by complementary methods is on-going and
the results will be published in a follow-up paper.
Among the samples with added dispersion promotor
the result depended strongly both on the CNT anchor
Although compounds 1b and 1c yield greatly improved
stability of the CNT-in-LC suspension, nanotube flocculation
was seen also in these suspensions after about a week, at least
with the host LC used here. Further refinements of the details
of the molecular structures of the promotor molecules are thus
called for but the present results serve as proof of the basic
concept. Depending on exactly which host LC is intended it
may be advantageous to use other mesogenic groups,
a modification which can easily be employed within our
modular approach.
Fig. 3 Dispersions of Hipco SWCNTs (0.1 mg mLÀ1) in the pristine
nematic LC (ref) and in the same LC with polyphilic additives (1a–2b)
after 20 min tip sonication followed by 4 h centrifugation.
c
6990 Chem. Commun., 2010, 46, 6989–6991
This journal is The Royal Society of Chemistry 2010

View Article Online
In summary, we have developed a new design strategy for
molecules promoting dispersion of non-functionalized carbon
nanotubes in thermotropic liquid crystals, substantially increasing
the stability of such dispersions. The generic character makes the
design concept versatile enough to be compatible with essentially
any desired combination of carbon nanotube and liquid crystal,
thereby greatly broadening the scope of potential applications of
CNT–LC composites and facilitating research on these systems.
The employment of this type of tailored polyphilic molecule may
thus open a door to removing the current main obstacle
in the exploitation of liquid crystal–nanotube suspensions, of
considerable interest for developing functional composites
enhanced through the unique properties of high concentrations
of uniformly aligned CNTs as well as for improving nematic-
based electrooptic devices.
8 M. Rahman and W. Lee, J. Phys. D: Appl. Phys., 2009, 42, 063001.
9 C. Zakri, Liq. Cryst. Today, 2007, 16, 1–11.
10 Y. Ji, Y. Huang, R. Rungsawang and E. Terentjev, Adv. Mater.,
2010, 22, 3436–3440.
11 S. Jeong, K. Park, S. Jeong, H. Jeong, K. An, C. Nah, D. Pribat,
S. Lee and Y. Lee, Nano Lett., 2007, 7, 2178–2182.
12 H. Y. Chen, W. Lee and N. A. Clark, Appl. Phys. Lett., 2007, 90,
033510.
13 I. S. Baik, S. Y. Jeon, S. H. Lee, K. A. Park, S. H. Jeong, K. H. An
and Y. H. Lee, Appl. Phys. Lett., 2005, 87, 263110.
14 H. S. Jeong, Y. K. Ko, Y. H. Kim, D. K. Yoon and H.-T. Jung,
Carbon, 2010, 48, 774–780.
15 G. Scalia, C. von Buhler, C. Hagele, S. Roth, F. Giesselmann and
¨
¨
J. P. F. Lagerwall, Soft Matter, 2008, 4, 570–576.
16 S. Schymura, M. Kuhnast, V. Lutz, S. Jagiella, U. Dettlaff-
¨
Weglikowska, S. Roth, F. Giesselmann, C. Tschierske, G. Scalia
and J. Lagerwall, Adv. Funct. Mater., 2010, DOI: 10.1002/
adfm.201000539.
17 O. Trushkevych, N. Collings, T. Hasan, V. Scardaci, A. Ferrari,
T. Wilkinson, W. Crossland, W. Milne, J. Geng, B. Johnson and
S. Macaulay, J. Phys. D: Appl. Phys., 2008, 41, 125106.
18 D. Tasis, N. Tagmatarchis, V. Georgakilas and M. Prato,
Chem.–Eur. J., 2003, 9, 4000–4008.
Financial support from the ‘‘Excellenzcluster nanostrukturierte
Materialien’’ and the ‘‘Graduiertenforderungsprogramm’’ of
¨
the Land Sachsen-Anhalt is gratefully acknowledged.
19 J. Liu, O. Bibari, P. Mailley, J. Dijon, E. Rouviere,
F. Sauter-Starace, P. Caillat, F. Vinet and G. Marchand, New J.
Chem., 2009, 33, 1017–1024.
Notes and references
1 R. Hu, B. Cola, N. Haram, J. Barisci, S. Lee, S. Stoughton,
G. Wallace, C. Too, M. Thomas, A. Gestos, M. Cruz, J. Ferraris,
A. Zakhidov and R. Baughman, Nano Lett., 2010, 10, 838–846.
2 W. Yang, K. Ratinac, S. Ringer, P. Thordarson, J. Gooding and
F. Braet, Angew. Chem., Int. Ed., 2010, 49, 2114–2138.
3 P. Diao and Z. Liu, Adv. Mater., 2010, 22, 1430–1449.
4 C. Backes, F. Hauke, C. Schmidt and A. Hirsch, Chem. Commun.,
2009, 2643–2645.
5 I. Dierking, G. Scalia, P. Morales and D. Leclere, Adv. Mater.,
2004, 16, 865–869.
6 G. Scalia, ChemPhysChem, 2010, 11, 333–340.
7 J. P. F. Lagerwall and G. Scalia, J. Mater. Chem., 2008, 18,
2890–2898.
20 S. Meuer, L. Braun and R. Zentel, Chem. Commun., 2008,
3166–3168.
21 C. Ehli, G. Rahman, N. Jux, D. Balbinot, D. Guldi, F. Paolucci,
M. Marcaccio, D. Paolucci, M. Melle-Franco, F. Zerbetto,
S. Campidelli and M. Prato, J. Am. Chem. Soc., 2006, 128,
11222–11231.
22 Y. Tomonari, H. Murakami and N. Nakashima, Chem.–Eur. J.,
2006, 12, 4027–4034.
23 Y. Ji, Y. Huang, A. Tajbakhsh and E. Terentjev, Langmuir, 2009,
25, 12325–12331.
24 J. Klare, I. Murray, J. Goldberger and S. Stupp, Chem. Commun.,
2009, 3705–3707.
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6989–6991 6991