Micron-sized main-chain liquid crystalline elastomer actuators with ultralarge amplitude contractions

Article abstract of DOI:10.1021/ja905363f

Responsive surfaces composed of cylindrical or square micrometer-sized thermoresponsive pillars made of thiol-ene nematic main-chain liquid crystalline elastomers (LCEs) are produced by replica molding. The individual pillars behave as microactuators, sho

Full text of DOI:10.1021/ja905363f

Published on Web 09/24/2009
Micron-Sized Main-Chain Liquid Crystalline Elastomer
Actuators with Ultralarge Amplitude Contractions
Hong Yang,† Axel Buguin,† Jean-Marie Taulemesse,‡ Kosuke Kaneko,§
Ste´phane Me´ry,§ Anne Bergeret,‡ and Patrick Keller*,†
Institut Curie, Centre de Recherche, CNRS UMR 168, UniVersite´ Pierre et Marie Curie, 26 rue
d’Ulm 75248 Paris cedex 05, France, Centre des Mate´riaux de Grande Diffusion, Ecole des
Mines d’Ale`s, 6 aVenue de ClaVie`res 30319 Ale`s cedex, France, and IPCMS, UMR 7504
CNRS-Unistra, 23 rue du Loess BP 43 67034 Strasbourg cedex 02, France
Received June 30, 2009; E-mail: patrick.keller@curie.fr
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This paper contains enhanced objects available on the Internet at http://pubs.acs.org/jacs.
Abstract: Responsive surfaces composed of cylindrical or square micrometer-sized thermoresponsive pillars
made of thiol-ene nematic main-chain liquid crystalline elastomers (LCEs) are produced by replica molding.
The individual pillars behave as microactuators, showing ultralarge and reversible contractions of around
300-400% at the nematic to isotropic phase transition. The nematic main-chain LCE microactuators
described here present contractions as large as the best macroscopic systems reported in the literature.
Moreover, the contraction observed for this new system outperforms the best values already reported for
other LCE microsystems.
Introduction
Materials scientists are increasingly turning to nature as a
source of inspiration to develop new devices mimicking the
structure and function of real biological systems.1-3 Research
regularly points out that many phenomena encountered in nature,
such as the self-cleaning properties of lotus and rice leaves,4
the “sticky feet” of geckos,5 the vivid colors of the butterfly
wings,6 or the drag-reducing surface of shark skin7 to name a
few, are related to the specific micro and nanostructures of the
involved surfaces. The search for smart materials, also called
actuators or artificial muscles, that respond to external stimuli
with changes in shape or size has recently attracted considerable
attention from the material research community.8,9 Polymers
play a leading role in the domain of smart materials because
they provide several key advantages: low manufacturing cost,
high processability, flexibility, relatively low weight density,10,11
Figure 1. Responsive surface with a stimuli-driven roughness change.
etc. Combining these two domains, smart materials and solid
surfaces with special properties (low friction, specific optical
properties, etc.), could lead to the development of new
responsive surfaces with tunable micro/nanostructures, con-
trolled by external stimuli. To produce responsive surfaces such
as the one pictured in Figure 1, the surface must be covered
with micro (or nano) actuators, contracting along their long axis.
Moreover, the contraction must be large enough so that the
roughness of the surface will be substantially changed.
Nematic liquid crystalline elastomers (LCEs)12 appear as very
promising candidates to build such responsive surfaces. Nematic
LCEs are soft polymeric actuators that can contract reversibly
under the influence of various stimuli such as a change in
temperature (thermomechanical effect)12,13 or UV irradiation
(photomechanical effect).12,14 The motor for contraction is the
conformational change occurring at the level of the polymer
backbone at the nematic to isotropic phase transition: due to
the influence of the nematic order on the polymer chain, the
backbone adopts an anisotropic conformation. When the nematic
† Institut Curie.
‡ Ecole des Mines d’Ale`s.
§ IPCMS.
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Main-Chain Liquid Crystalline Elastomer Actuators
A R T I C L E S
Scheme 1. Synthesis of the Tetra Functional Crosslinker 2
order is destroyed, that is, at the nematic to isotropic phase
transition, the polymer backbone returns to a random coil
conformation typical of a melted polymer chain. In a mon-
odomain nematic single crystal elastomer,15 in which all the
polymer chains are, on average, parallel to each other and cross-
linked, this molecular level shape change translates to the
macroscopic sample. This coupling between nematic order and
average macromolecular shape is the strongest for main-chain
liquid crystalline polymers (LCPs) in which the mesogenic
groups are incorporated into the polymer chains.16
Several LCE-based artificial muscles have been described
over the last 10 years,12-14,17 although De Gennes introduced
the fundamental theoretical basis for LCEs as early as 1969.18
Some years ago, using a soft lithography technique called replica
molding, we succeeded in creating micrometer-sized responsive
pillars made of LCEs.19 However, due to the type of LCE used,
a side-on LCE, a relatively small contraction was observed,
around 35-40%. Clearly, to achieve the kind of surface
modification presented in Figure 1, we need to use main-chain
LCEs, for which contractions of 300-500% have been reported
in macroscopic samples.20-26 However, no micrometer or
nanosized main-chain LCE actuators have been developed, the
limitation being the synthetic scheme, which normally requires
a two-step cross-linking procedure under mechanical stress, as
originally described by Finkelmann.15 We have now succeeded
in creating, for the first time, micrometer-sized pillars made of
main-chain LCE by photopolymerization, using a thiol-ene
approach. The micrometer-sized actuators undergo an ultralarge
and reversible contraction of around 400% when heated at a
temperature close to the nematic to isotropic phase transition
of the LCE.
to develop a system that could be prepared via photopolymer-
ization. The photoinduced addition of thiols on olefins (the so-
called thiol-ene photopolymerization) is a well-known reaction
for nonmesomorphic systems, resulting in linear polymers.28
The synthesis and photopolymerization of mesomorphic com-
pounds containing vinyl and mercapto groups to give main-
chain LCP has been previously described by Lub et al.29-33
but did not receive much attention. Moreover, thiol-ene
polymers/elastomers have never been used in the context of LCE
actuators. We prepared a new nematic thiol-ene monomer 1
using the synthetic procedure of Lub et al.,33 and the new tetra
functional mesogenic cross-linker 2 by the route described in
Scheme 1.
Results and Discussion
Main-chain LCPs have been intensely studied in the past
because they present potentially interesting mechanical proper-
ties.27 However, most of these polymers were prepared by
polycondensation reactions in solution or in the melt to give
polyesters, polyamides, polyisocyanates, etc. The system we
have developed to make micrometer-sized aligned nematic LCE
actuators19 relies on the orientation, via a magnetic field, of a
nematic mixture containing a monomer, a cross-linker and a
photoinitiator, followed by a photopolymerization process to
give the aligned nematic elastomer in one step. To apply this
strategy to the preparation of main-chain LCE pillars, we needed
(15) Ku¨pfer, J.; Finkelmann, H. Makromol. Chem. Rapid Commun. 1991,
12, 717–726.
(16) Cotton, J.-P.; Hardouin, F. Prog. Polym. Sci. 1997, 22, 795–828.
(17) Xie, P.; Zhang, R. J. Mater. Chem. 2005, 15, 2529–2550, and
references therein.
(18) de Gennes, P.-G. Phys. Lett. A 1969, 28A, 725–726.
(19) Buguin, A.; Li, M.-H.; Silberzan, P.; Ladoux, B.; Keller, P. J. Am.
Chem. Soc. 2006, 128, 1088–1089.
(20) Bergmann, G. H. F.; Finkelmann, H.; Percec, V.; Zhao, M. Macromol.
Rapid Commun. 1997, 18, 353–360.
(21) Donnio, B.; Wermter, H.; Finkelmann, H. Macromolecules 2000, 33,
7724–7729.
(28) Hoyle, C. E.; Lee, T. Y.; Roper, T. J. Polym. Sci., Part A: Polym.
Chem. 2004, 42, 5301–5338.
(22) Wermter, H.; Finkelmann, H. e-Polym. 2001, 013.
(23) Tajbakhsh, A. R.; Terentjev, E. M. Eur. Phys. J. E. 2001, 6, 181–
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(29) Lub, J.; Broer, D. J.; Van Den Broek, N. Liebigs Ann. 1997, 2281–
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(30) Lub, J.; Broer, D. J.; Martinez Antonio, M. E.; Mol, G. N. Liq. Cryst.
1998, 24, 375–379.
(24) Ahir, S. V.; Tajbakhsh, A. R.; Terentjev, E. M. AdV. Funct. Mater.
2006, 16, 556–560.
(31) Lub, J.; Broer, D. J.; Allan, J. F. Mol. Cryst. Liq. Cryst. 1999, 332,
259–266.
(25) Bispo, M.; Guillon, D.; Donnio, B.; Finkelmann, H. Macromolecules
2008, 41, 3098–3108.
(32) Wilderbeek, H. T. A.; Goossens, J. G. P.; Bastiaansen, C. W. M.;
Broer, D. J. Macromolecules 2002, 35, 8962–8968.
(33) Wilderbeek, H. T. A.; Van Der Meer, M. G. M.; Jansen, M. A. G.;
Nelissen, L.; Fischer, H. R.; Van Es, J. J. G. S.; Bastiaansen, C. W. M.;
Lub, J.; Broer, D. J. Liq. Cryst. 2003, 30, 93–108.
(26) Krause, S.; Zander, F.; Bergmann, G.; Brandt, H.; Wertmer, H.;
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and references therein.
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A R T I C L E S
Yang et al.
Figure 2. Experimental setup used to prepare the responsive pillars.
Both compounds presented a nematic mesophase. In particu-
lar, monomer 1 had a broad nematic mesophase (from 59 to 80
°C), with an additional monotropic smectic A mesophase.
We first checked that the linear main-chain thiol-ene polymer
obtained by polymerizing monomer 1 possessed the required
nematic mesophase. Indeed, solution polymerization of mono-
mer 1 in presence of a thermal radical initiator gave a nematic
polymer with a nematic to isotropic phase transition around 170
°C. (See Supporting Information for the detailed synthesis and
characterization of compounds 1 and 2 as well as for the
thiol-ene main-chain polymer.)
Then, using the setup described in Figure 2, the structured
surfaces were prepared. A small amount of a mixture composed
of the nematic monomer 1 (94.5 mol %), the cross-linker 2 (5
Figure 3. (a) and (b) SEM images shown at two magnifications of a surface covered with large cylindrical pillars; (c) SEM image of a surface covered with
small cylindrical pillars; (d) SEM image of a surface covered with square pillars.
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Main-Chain Liquid Crystalline Elastomer Actuators
A R T I C L E S
mol %) and the photoinitiator 2-benzyl-2-(dimethylamino)-4′-
morpholinobutyrophenone (0.5 mol %) was heated to its
isotropic phase (85 °C) on a clean microscope coverslip
positioned atop a rare earth permanent magnet (∼1 T NdFeB
alloy (Aldrich)). A PDMS soft mold, prepared by replica
molding, was then pressed down on the melted sample, which
filled the inner structure of the mold. Four different molds were
used, with various hole shapes and sizes.
The temperature was slowly decreased (- 0.5 °C/min) down
to 70 °C to allow the formation of the nematic mesophase.
During the slow cooling process, the applied magnetic field
ensured the alignment of the nematic director parallel to the
long axis of the pillars. The sample was then irradiated through
the mold using a UV lamp (30 mW cm-2, λ ) 365 nm; ELC-
4001 light curing unit; Electro-Lite corporation) for 30 min to
promote the photopolymerization of the monomer mixture. After
cooling to room temperature, the PDMS mold was peeled off,
leaving a thin glassy polymer film covered by a regular array
of pillars, as seen by scanning electron microscopy (SEM)
(Figure 3 and Figure SI-5 in Supporting Information).
To characterize their contraction, pillars were cut off the
surface with a razor blade. When heated above the nematic to
isotropic transition temperature (around 170 °C for the main-
chain LCE), the pillars underwent an ultralarge contraction
(Figure 4).
In the first stages of the contraction, the pillar kept its
cylindrical shape (Figure 4a, b, and c), but at the end of the
process, it reached the shape of “a coin” (Figure 4d). Further-
more, in some cases, this coin, being not able to stand on its
“edge”, flipped down on its “face” (Figure 4d, pillar on the
right). On cooling down, the pillar returned to its original shape
and size (Figure 4e).
Keeping in mind that the total volume of the elastomeric pillar
(Poisson ratio ≈ 0.5) remains nearly constant during the nematic
to isotropic transition, one can then measure the thickness of
the “edge” of the coin at full contraction and, as a consequence,
calculate the contraction which could be estimated to reach
300-400%.
The nematic main-chain LCE microactuators described here
present contractions as large as the best macroscopic systems
reported in the literature for mechanically oriented samples.20-26
Moreover, the observed contractions (up to 400%) outperform
the best values already reported for other LCE microsystems
(ca. 40%).19,34-36
Conclusion
With these novel nematic main-chain LCE microactuators,
we are one step closer to the responsive surfaces like the one
drawn in Figure 1. One possible drawback for the system
described here is its high nematic to isotropic transition and
glass transition temperatures. However, high working temper-
atures could be considered an asset for development of a
responsive polymer surface that could also be capable of shape
memory.37 Using the considerable knowledge accumulated over
the years in the liquid crystal field, the next step will be to design
Figure 4. Contraction of isolated nematic main-chain LCE pillars heated
up to the isotropic (a-d), then cooled back to room temperature (e). Scale
bare is 100 µm. A movie in AVI format shows this in motion.
new thiol-ene monomers and polymers with lower transition
temperatures.
(34) Elias, A. L.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J.; Brett,
M. J. J. Mater. Chem. 2006, 16, 2903–2912.
The straightforward method described here to prepare LCE
microactuators with ultralarge contractions paves the way for
the development of LCE-based responsive surfaces mimicking
natural surfaces with specific properties. In addition, since the
micrometer-sized actuators can be manipulated individually, they
(35) Van Oosten, C. L.; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J.
Eur. Phys. J. E 2007, 23, 329–336.
(36) Elias, A. L.; Brett, M. J.; Harris, K. D.; Bastiaansen, C. W. M.; Broer,
D. J. Mol. Cryst. Liq. Cryst. 2007, 477, 137–151.
(37) Liu, C.; Qin, H.; Mather, P. T. J. Mater. Chem. 2007, 17, 1543–1558.
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A R T I C L E S
Yang et al.
Institute) for the gift of the master used in the preparation of the
small cylindrical pillars mold and Dr. J. Plastino (Curie Institute)
and Dr. E. Tsai (University of Colorado at Boulder) for their critical
reading of the manuscript.
could be used in microfluidics as active elements to design
micropumps or microvalves.
Our procedure could be easily extended to the preparation
of azo main-chain LCE microactuators with photomechanical
properties, using the photochemical approach we previously
developed to prepare azo nematic side-on LCE films.38 Future
work will address these goals.
Supporting Information Available: Experimental part S2,
which presents the synthesis of monomer 1, cross-linker 2, and
thiol-ene polymer, informations on the setup used to prepare
the films and additional SEM pictures of the surfaces.
This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. This research was supported in part by the
ANR (ANR-07-MAPR-0020)). We thank Dr. P. Silberzan (Curie
(38) Li, M.-H.; Keller, P.; Li, B.; Wang, X.; Brunet, M. AdV. Mater. 2003,
15, 569–572.
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