Unsymmetric main-chain liquid crystal elastomers with tuneable phase behaviour: Synthesis and mesomorphism

Article abstract of DOI:10.1039/c0jm03691a

The synthesis of a new series of polysiloxane-based, main-chain liquid crystal elastomers (MC-LCEs) containing unsymmetric mesogenic units is described. The structure of the mesogenic monomers, spacers and cross-linkers has been varied systematically in order to tune the thermotropic behaviour of the networks. Three unsymmetric mesogens were studied, characterised by having two aromatic rings as the rigid core and two different alkoxy chains. Smectic A or nematic phases were observed, depending on the length of the alkoxy chains. These mesogens were combined with three different spacers, a flexible hexamethyltrisiloxane (S1) and two aromatic bis(dimethylsilyloxy)benzene with the silyl groups disposed para (S2) or meta (S3). To form the MC-LCEs, two cross-linkers were chosen, one flexible 2,4,6,8-tetramethylcyclooctasiloxane (C1) and a rigid, aromatic tris(dimethylsilyl)benzene (C2). The elastomers showed low glass transition temperatures (below 0 °C) and low clearing temperatures (between 30 and 72 °C), as such presenting liquid crystalline properties at room temperature. The transition temperatures could be tuned by a simple choice of the components; for instance, the clearing temperature decreased as aromatic spacers or cross-linkers were introduced. In contrast, the glass transition temperature did not depend on the cross-linker or mesogen used and showed only a small dependence on the structure of the spacer. The phase behaviour of the MC-LCEs was affected greatly by the spacer used and networks containing the trisiloxane spacer systematically exhibited the smectic C phase. The insertion of aromatic spacers favoured the formation of a nematic phase over a smectic phase when the flexible cross-linker, C1, was used. This behaviour was enhanced by the addition of the aromatic cross-linker C3, so that the smectic C phase was suppressed totally and only a nematic phase was observed. The synthesis of the components of MC-LCEs is presented and the structure-thermal properties relationships of the MC-LCEs are discussed.

Full text of DOI:10.1039/c0jm03691a

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PAPER
Unsymmetric main-chain liquid crystal elastomers with tuneable phase
behaviour: synthesis and mesomorphism†
a
Maria Amela-Cortes,‡ Benoıt Heinrıch, Bertrand Donnio, Kenneth E. Evans, Chris W. Smith
b
b
a
a
ꢀ
and Duncan W. Bruce
ˆ
ˆ
c
*
Received 29th October 2010, Accepted 13th January 2011
DOI: 10.1039/c0jm03691a
The synthesis of a new series of polysiloxane-based, main-chain liquid crystal elastomers (MC-LCEs)
containing unsymmetric mesogenic units is described. The structure of the mesogenic monomers,
spacers and cross-linkers has been varied systematically in order to tune the thermotropic behaviour of
the networks. Three unsymmetric mesogens were studied, characterised by having two aromatic rings
as the rigid core and two different alkoxy chains. Smectic A or nematic phases were observed,
depending on the length of the alkoxy chains. These mesogens were combined with three different
spacers, a flexible hexamethyltrisiloxane (S1) and two aromatic bis(dimethylsilyloxy)benzene with the
silyl groups disposed para (S2) or meta (S3). To form the MC-LCEs, two cross-linkers were chosen, one
flexible 2,4,6,8-tetramethylcyclooctasiloxane (C1) and a rigid, aromatic tris(dimethylsilyl)benzene (C2).
The elastomers showed low glass transition temperatures (below 0 ꢀC) and low clearing temperatures
ꢀ
(between 30 and 72 C), as such presenting liquid crystalline properties at room temperature. The
transition temperatures could be tuned by a simple choice of the components; for instance, the clearing
temperature decreased as aromatic spacers or cross-linkers were introduced. In contrast, the glass
transition temperature did not depend on the cross-linker or mesogen used and showed only a small
dependence on the structure of the spacer. The phase behaviour of the MC-LCEs was affected greatly
by the spacer used and networks containing the trisiloxane spacer systematically exhibited the smectic
C phase. The insertion of aromatic spacers favoured the formation of a nematic phase over a smectic
phase when the flexible cross-linker, C1, was used. This behaviour was enhanced by the addition of the
aromatic cross-linker C3, so that the smectic C phase was suppressed totally and only a nematic phase
was observed. The synthesis of the components of MC-LCEs is presented and the structure–thermal
properties relationships of the MC-LCEs are discussed.
temperature,11–13 light14–17 or electrical field.1,18–21 These unusual
features were predicted by de Gennes in 1975.22 These stimuli
induce a disorder in the liquid-crystalline phase that is translated
into a macroscopic deformation and, in the case of the thermally
induced systems, the change in shape is attained at the liquid
crystal-to-isotropic temperature. All these properties arise
because of the coupling of the liquid-crystalline order with the
rubber elasticity of the weakly cross-linked network,23 leading to
potential applications as sensors and actuators,16,24–28 for
example in biomedical devices such as artificial muscle.29–31
Over the years different architectures of LCEs have been
produced, for example side-chain LCEs (SC-LCEs)10,32–37 in
which the mesogenic units are attached to the polymer chain via
a flexible spacer and main-chain LCEs (MC-LCEs) where the
mesogens are part of the polymer backbone.3,13,28,38–53 Although
SC-LCEs have been studied extensively, main-chain LCEs have
recently been the subject of increasing interest since they are
expected, and have shown larger spontaneous deformations than
their side-chain analogues.6,54,55 However, they present some
challenges due to their synthesis and, in particular, because of
1. Introduction
Liquid-crystal elastomers (LCEs) have attracted much attention
due to their very special properties such as memory effects,1–5
macroscopic shape changes6–8 and shifts in phase trans-
formations.4,9 LCEs form part of the so-called ‘shape-memory
materials’3,5,10 since they have the ability to change their shape
reversibly upon application of an external stimulus, such as
aDepartment of Chemistry, University of York, Heslington, York, YO10
5DD, UK. E-mail: duncan.bruce@york.ac.uk; Tel: +44 (0)1904 324085
bCNRS-Universiteꢀ de Strasbourg (UMR 7504), Institut de Physique et
ꢀ
Chimie des Materiaux de Strasbourg, 23 rue du Loess BP 43, 67034
Strasbourg Cedex 2, France
cCollege of Engineering, Mathematics and Physical Sciences, University of
Exeter, Harrison Building, North Park Road, Exeter, EX4 4QF, UK
† Electronic supplementary information (ESI) available: DSC traces for
monomers and elastomers; X-ray diffraction patterns. See DOI:
10.1039/c0jm03691a
‡ Present address: UMR Sciences Chimiques de Rennes, UR1-CNRS
6226, Universite de Rennes 1, Campus be Beaulieu, CS74205, 35042
Rennes Cedex, France.
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their high transition temperatures, which means that they are not
accessible for applications.42
hexamethyltrisiloxane, S1, 1,4-bis(dimethylsilyloxy)benzene, S2,
and 1,3-bis(dimethylsilyloxy)benzene, S3—and two silicon-
based cross-linkers—cyclic tetrameric siloxane, C1, and the
aromatic 1,3,5-tris(dimethylsilyl)benzene, C2. Compounds S2,
S3 and C2 are not available commercially and their synthesis is
described along with monomers M1 to M3. Mesomorphic
properties for M1 to M3 are reported.
Rather few investigations have been reported dealing with
structure–property relationships of these materials, but those
that have showed that small variations in the chemical compo-
sition and structure of the networks could induce significant
changes in liquid-crystalline behaviour and thermomechanical
response.39,42 Therefore, in order to tune the physical properties
of these materials, an understanding of the effect of the chemical
structure is required.
2. Experimental
2.1 Materials and apparatus
Few MC-LCEs reported in the literature show only a nematic
phase, as for example the polyether described by Bergmann
et al.,38 the photocross-linkable polyethyleneglycol system
reported by Krause et al.56 and the mixed main-chain/side-chain
of Wermter and Finkelmann.6 However, MC-LCEs derived from
siloxane spacers tend to form smectic phases, in particular the
SmC phase, as a result of sterically induced microsegregation
between the rigid mesogenic units and the flexible siloxane parts.
Indeed, SmC MC-LCEs have been reported by Sanchez-Ferrer
and Finkelmann43 and more frequently these systems show both
SmC and N phases as in the cases of Donnio et al.,41 Rousseau
et al.42 and Bispo et al.39
The solvent and reagents were obtained from commercial
suppliers and used without further purification. Anhydrous
solvents were dried by the usual procedures and used directly:
CaH2 for dichloromethane and sodium and benzophenone for
toluene and tetrahydrofuran. Preparation of all compounds was
carried out under atmosphere of nitrogen except for the synthesis
of liquid crystal polymers and elastomers. NMR spectra were
recorded on a Bruker ACF-300 spectrometer operating at
1
300 MHz for H and 75.5 MHz for 13C, and chemical shifts are
reported relative to the internal standard of the deuterated
solvent used. IR spectra were recorded on a Nicolet magna-IR
spectrophotometer 550. Elemental analyses were recorded by
a Carlo Erba EA 1110 elemental analyser.
Here are presented a variety of new MC-LCEs (E1 to E15) that
show liquid crystal properties at room temperature. These MC-
LCEs differ in the structure of the different components of the
networks, namely, the mesogenic monomer, the spacer and the
cross-linker (Fig. 1). The systematic variation of the structure of
the components forming the MC-LCE will lead to systems with
tailor-made properties for applications as soft actuators. The aim
of this work is to understand the structure–property relation-
ships of these systems. The mechanical properties of these
materials are presented in the following paper.57
Analysis by hot-stage microscopy was carried out on an
Olympus BH40 microscope equipped with a Link-Am HFS91
hot-stage, TMS92 controller and LNP2 cooling unit. DSC data
were recorded on a Perkin-Elmer DSC7 instrument using heating
and cooling rates at 10 ꢀC minꢁ1. X-Ray experiments were
carried out with a Philips PW 1730 and CuKa radiation (0.15418
nm) filtered by a graphite monochromator. The incident beam
was normal to the surface of the films.
The elastomers are prepared from three mesogenic monomers
(M1
to
M3),
three
different
spacers—1,1,3,3,5,5-
2.2 Synthesis of mesogenic monomers M1 to M3
All alkylation reactions were carried out following the Mitsu-
nobu58 reaction. A procedure for the synthesis of compound 3a is
given—other homologues were prepared similarly.
Synthesis of 4-(hex-5-enyloxy)benzoic acid, 3a. Ethyl 4-
hydroxybenzoate 2 (2.5 g, 15 mmol) and triphenylphosphine
(4.3 g, 16.5 mmol) were dissolved in THF (15 cm3) under
a nitrogen atmosphere. Then, 5-hexen-1-ol 1b was added (1.5 g,
15 mmol) to the solution and mixed with stirring at room
ꢀ
temperature. The mixture was cooled to 0 C and diisopropyla-
zodicarboxylate (3.4 g, 16.5 mmol) was added dropwise. The
reaction was stirred at room temperature and was followed by
thin-layer chromatography using ethyl acetate : hexane (2 : 1).
After 3 h, the reaction was complete and a yellow solid appeared.
The solvent was removed under vacuum and a mixture of ethyl
acetate : hexane (1 : 5, 12.5 cm3) was added, allowing the tri-
phenylphosphine oxide and the urea formed during the reaction
to precipitate. The solid formed was filtered off and the solvent
was removed in vacuo. The orange-yellow oil obtained was
purified by column chromatography with a mixture of petroleum
ether (bp 40–60 ꢀC) : acetone 95 : 5.
The ethyl ester was hydrolysed as follows. A solution of
potassium hydroxide (0.84 g, 15 mmol) in ethanol (45 cm3, 95%)
was added to a solution of ethyl 4-(hex-5-enyloxy)benzoate 3 in
Fig. 1 Chemical structure of the different components of the LCEs,
mesogenic monomers (M1 to M3), spacers (S1 to S3) and cross-linkers
(C1 and C2).
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ethanol (10 cm3) and heated under reflux for 3 h. The mixture was
then cooled to room temperature and water (15 cm3) was added.
The resulting mixture was stirred until the salt formed during the
reaction had dissolved. The aqueous solution was acidified with
hydrochloric acid (conc., 3 cm3) leading to the precipitation of
the desired product. The precipitate, which was a colourless
solid, was filtered off and further crystallised from ethanol. Yield
2.3 g (71%).
eliminated yielding an orange oil. A yellow solid was then
precipitated from ethanol (95%) at 4 ꢀC. The intermediate
benzoate ester was crystallised from ethanol and obtained in 76%
yield.
A solution of potassium hydroxide (5.1 g, 91 mmol) in ethanol
(60 cm3) was poured into a solution of 4-(but-3-enyloxy)phenyl
benzoate (8.15 g, 30 mmol) in ethanol (25 cm3, 95%). The reac-
tion was carried out under reflux for 3 h. The mixture was cooled
down and then neutralised with hydrochloric acid (10%). The
product was extracted from the aqueous solution with
dichloromethane (3 ꢂ 40 cm3). The organic phase was washed
afterwards with sodium hydrogen carbonate solution (20%, 3 ꢂ
20 cm3). The organic phase was combined and dried over
magnesium sulfate. Finally, the solvent was removed in vacuo
and a brown oil obtained. The compound was used in the next
step without further purification. Compound 6b was prepared
similarly.
1H-NMR (CDCl3): d: 1.54 (2H, m, –CH2–), 1.77 (2H, m,
–CH2–), 2.09 (2H, dt, –CH2–CH, J ¼ 14.5 Hz, 6.8 Hz), 3.95 (2H,
t, Ph–O–CH2, J ¼ 6.8 Hz), 5.06 (1H, dd, CH]CHH, Jcis ¼ 10.3
Hz, Jgem ¼ 3.2 Hz), 5.11 (1H, dd, CH]HH, Jtrans ¼ 17.1 Hz,
Jgem ¼ 3.2 Hz), 5.83 (1H, ddt, CH]CHH, Jtrans ¼ 17.1 Hz, Jcis
¼
10.3 Hz, Jgem ¼ 6.8 Hz), 6.87 (2H, AA0XX0, H4, J ¼ 7.5 Hz), 8.12
(2H, AA0XX0, H3, J ¼ 7.5 Hz).
13C-NMR (CDCl3): d: 25.6 (OCH2–CH2), 28.9 (OCH2–CH2–
CH2), 33.8 (CH2]CH–CH2), 68.44 (O–CH2), 114.6 (m-Car),
115.3 (CH2]CH), 121.8 (Cbe–COO), 132.8 (o-Car), 138.9
(CH2]CH), 164 (Cbe–O), 172.2 (COO).
Synthesis
of
4-(but-3-enyloxy)phenyl-40-(hex-5-enyloxy)-
Elemental analysis calculated for C13H16O3: C 70.9, H 7.3;
found C 70.7, H 7.4%.
benzoate, M1.
Synthesis of 4-hydroxyphenyl benzoate, 5. In a 1 dm3 three-
neck round-bottomed flask equipped with a dropping funnel,
hydroquinone (20 g, 181 mmol) was dissolved in water (520 cm3)
and to that was added potassium carbonate (37.6 g, 272 mmol).
The solution was outgassed by passing nitrogen through it for 15
min. After that, benzoyl chloride (23.2 g, 165 mmol) was added
dropwise and the mixture stirred for 20 h at room temperature.
The colourless precipitate formed during the reaction was filtered
off and dried under vacuum. The desired product, 5, was
extracted from the solid by suspending the precipitate in diethyl
ether (3 ꢂ 100 cm3) and filtering the remaining solid. The solvent
was then removed leading to a colourless solid, which was further
crystallised from toluene. Yield ¼ 21.22 g, 60%
4-(Hex-5-enyloxy)benzoic acid 4 (4.4 g, 20 mmol), 4-(but-3-
enyloxy)phenol 15a (3.3 g, 20 mmol), dicyclohexyl-carbodiimide
(4.1 g, 20 mmol) and 4-(N,N-dimethylamino)-pyridine (0.24 g,
2 mmol) were mixed in dichloromethane (100 cm3) and stirred at
room temperature for 24 h. The colourless solid that precipitated
during the reaction was filtered off and the solvent removed in
vacuo. The product was crystallised from ethanol. Yield 3.9 g
(53%).
1H-NMR (CDCl3): d: 1.52 (2H, m, H8), 1.75 (2H, m, H7), 2.06
1H-NMR (acetone-d6): d: 6.77 (2H, AA0XX0, J ¼ 6.7 Hz), 6.96
(2H, AA0XX0, J ¼ 6.7 Hz), 7.45 (2H, dt, J ¼ 7.8 Hz, J ¼ 7.5 Hz),
7.58 (1H, t, J ¼ 7.5 Hz), 8.01 (2H, d, J ¼ 7.8 Hz), 8.45 (1H, s,
–OH).
0
0
(2H, m, H9), 2.48 (2H, m, H6 ), 3.86 (4H, cs, 2H6 + 2H5 ), 4.98
0
(4H, 2Hb + 2Hc), 5.72 (2H, 2Ha), 6.75 (2H, AA0XX0, H3 , J ¼ 9
0
Hz), 6.79 (2H, AA0XX0, H4, J ¼ 9 Hz), 7.93 (2H, AA0XX0, H2 ,
J ¼ 9 Hz), 7.96 (2H, AA0XX0, H3, J ¼ 9 Hz).
13C-NMR (acetone-d6): d: 116.9 (o-COH), 123.9 (m-COH),
130.0 (COO), 131.2 (o-, m-C–COO), 143.8 (p-C–COO), 145.0 (p-
COH), 156.5 (COH), 166.3 (COO).
13C-NMR (CDCl3): d: 24.8 (C8), 28.1 (C7), 33.2, 33.0 (C9,
C60), 67.6, 67.2 (C50, C6), 113.8 (C4), 114.8, 114.6 (C80, C11),
116.7 (C30), 121.3 (C20), 122.1 (C2), 131.8 (C3), 134.0 (C70), 138.0
(C10), 144.1 (C10), 156.1 (C40), 162.1 (C5), 164.9 (C1).
CHN: calculated for C23H26O4: C 75.4, H 7.2; found: C 75.5,
H 7.2%.
CHN: calculated for C13H10O3: C 72.9, H 4.7; found: C 73.2,
H 4.5%.
Synthesis of 4-(but-3-enyloxy)phenyl benzoate, 6a.
4-(But-3-enyloxy)phenyl-40-(undec-10-enyloxy)benzoate, M2,
yield 3.7 g, 56%.
Compound 6a was synthesised following the same Mitsunobu
reaction as for compound 3a. The amounts used in this case were:
4-hydroxyphenyl benzoate (10 g, 46 mmol), triphenylphosphine
(13.2 g, 50.6 mmol), 3-buten-1-ol (3.36 g, 46 mmol), and diiso-
propylazodicarboxylate (10.2 g, 50.6 mmol) in dry THF (60 cm3).
The reaction time was 7 h at room temperature. After a mixture
of ethyl acetate : hexane (1 : 5) was added, the solid that
precipitated was removed by filtration. Finally, the solvent was
1H-NMR (CDCl3): d: 1.50 (12H, m, –CH2–), 1.85 (2H, m, H7),
0
0
2.11 (2H, m, H14), 2.62 (2H, m, H6 ), 4.09 (4H, cs, 2H6 + 2H5 ),
5.06 (4H, cs, 2Hb + 2Hc), 5.09 (2H, cs, Ha), 6.98 (2H, AA0XX0,
0
H3 , J ¼ 6.8 Hz), 7.01 (2H, AAXX0, H4, J ¼ 8.8 Hz), 7.15 (2H,
0
AA0XX0, H2 , J ¼ 6.8 Hz), 8.19 (2H, AA0XX0, H3, J ¼ 8.8 Hz).
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13C-NMR (CDCl3): d: 26.4 (C7), 29.8 (C8–C13), 34.2, 34.0
(C14, C60), 68.7, 68.0 (C6, C50), 114.7, 114.6 (C80, C16), 115.6
(C4), 117.5 (C30), 122.0 (C20), 122.9 (C3), 132.6 (C2), 134.8 (C70),
139.6 (C15), 144.9 (C10), 156.9 (C40), 163.9 (C5), 165.7 (C1).
CHN: calculated for C28H36O4: C 77.1, H 8.3; found: C 77.0,
H 8.4%.
13C-NMR (CDCl3): d: ꢁ2.9 (Me), 136.4 (CAr–Si), 150.0 (CAr–
H).
2.3 Preparation of liquid-crystal elastomer films41
The reactions were performed in a customised centrifuge
(Hettich, Universal 32R) equipped with a custom-made stainless
steel rotor, which contained a Teflon cell with dimensions 49 mm
of diameter and 20 mm height. The inner wall of the cell was
coated with a Teflon film. The centrifuge was heated by a water-
bath circulator (Julabo, MP-5) equipped with an immersion
pump. The sample environment within the centrifuge was heated
to a temperature of 60 ꢀC, which was maintained constant.
The ratio between the vinyl groups and the hydrogen of the
silane was stoichiometric, with 10% of cross-linker used for all
the elastomers. In this way, when the functionality of the cross-
linker was four (C1), the mesogenic unit (2 mmol), cross-linker
(0.2 mmol wrt mesogenic unit) and spacer (1.6 mmol) were
solubilised in toluene (2 cm3) and for a functionality of three, the
proportions were mesogenic unit (2 mmol), cross-linker
(0.2 mmol) and spacer (1.7 mmol) in dry toluene (2 cm3).
Thus, the components were mixed in toluene and placed inside
the Teflon cell. To this, a solution of [PtCl2(COD)] in dichloro-
methane (30 ml, 1% w/w) was added before the polymerisation
started. The cell was sealed and placed inside the centrifuge. The
reaction was carried out for 2 to 6 hours at 5000 rpm. Once the
reaction was finished, the cell was removed from the centrifuge,
allowed to cool to room temperature and the swollen network
was removed. From the network film, a part was cut and
deswollen on a water surface to avoid mechanical deformation to
form polydomain, non-oriented networks. The rest of the film
was hung at one end with a clamp and the other free end of the
film was loaded in order to form aligned, monodomain networks.
Under these conditions, the solvent slowly evaporated from the
film over 24 h. The film was then placed in an oven at 30 to 50 ꢀC,
depending on the clearing temperature, and the cross-linking
reaction was allowed to run to completion over 48 h. Polydomain
networks were placed in the oven without a load, while for
monodomain films, the cross-linking reaction was completed
under load.
4-(Hex-5-enyloxy)phenyl-40-(undec-10-enyloxy)benzoate, M3,
yield 3.5 g, 51%.
1H-NMR (CDCl3): d: 1.32 (14H, m, 7 –CH2–), 1.49 (2H, m, H7),
0
0
0
1.73 (2H, m, H8 ), 1.97 (2H, H14), 2.06 (2H, H8 ), 3.96 (2H, t, H5 ,
J ¼ 6.5 Hz), 3.91 (2H, t, H6, J ¼ 6.5Hz), 4.93 (4H, 2Hb + 2Hc),
0
5.75 (2H, 2Ha), 6.85 (2H, AA0XX0, H3 , J ¼ 6.8 Hz), 6.88 (2H,
0
AA0XX0, H4, J ¼ 6.9 Hz), 7.02 (2H, AA0XX0, H2 , J ¼ 6.8 Hz),
8.04 (2H, AA0XX0, H3, J ¼ 6.9 Hz).
13C-NMR (CDCl3): d: 25.7 (C60), 26.4 (C7), 29.8 (C8–C13,
C70), 34.2, 33.9 (C14, C80), 68.7, 68.6 (C6, C50), 114.6, 114.5
(C100, C16), 115.5, 115.2 (C30, C4), 122.0 (C20), 122.9 (C2), 132.6
(C3), 139.0 (C90), 139.6 (C15), 144.8 (C10), 157.1 (C40), 163.8
(C5), 165.7 (C1).
CHN: calculated for C30H40O4: C 77.7, H 8.7; found: C 77.7,
H 8.9%.
Synthesis of 1,4-bis(dimethylsilyloxy)benzene,59,60 S2. To
a solution of hydroquinone (10 g, 91 mmol) in dry THF (15.2
cm3) was added dropwise tetramethyldisilazane (18 g, 136.5
mmol) and one drop of chlorodimethylsilane. The reaction was
carried out under reflux for 12 h, then the solvent was removed
in vacuo. The product was obtained in the form of a colourless oil
in quantitative yield.
1H-NMR (CDCl3, 300 MHz): d: 0.10 (12H, d, –CH3, J ¼ 3
Hz), 4.76 (2H, septet, Si–H, J ¼ 3Hz), 6.87 (4H, s, HAr)
IR (NaCl): ꢁn/cmꢁ1 2121 (st, nSi–H), 1256 (st, nSi–C).
1,3-Bis(dimethylsilyloxy)benzene, S3. 1H-NMR (CDCl3): d:
0.10 (12H, d, –CH3, J ¼ 3 Hz), 4.80 (2H, septet, Si–H, J ¼ 3 Hz),
6.31 (1H, s, o-C–OSi), 6.38 (2H, d, m-C–OSi, J ¼ 9 Hz), 6.95 (1H,
t, m-C–OSi, J ¼ 9 Hz).
3. Results and discussion
13C-NMR (CDCl3): d: 0 (Me), 112.2 (o-C–OSi), 114.2 (o-C–
OSi), 131.4 (p-C–OSi), 158.8 (C–OSi).
With the purpose of establishing the structure–property rela-
tionship of MC-LCEs, a matrix was constructed in which was
found the structure of each component, namely mesogenic
monomer, spacer and cross-linker, which have been changed
systematically. The elastomers were prepared in a one-pot
procedure where the mesogenic monomer, the spacer and the
cross-linker are added together and thus, polymerisation and
reticulation occur simultaneously. From the synthetic point of
view, this technique has proved to be a powerful tool for the
ready modification of the structure of the MC-LCEs. This
approach has opened the door for the synthesis of a wide variety
of MC-LCEs due to its versatility.41
IR (NaCl): ꢁn/cmꢁ1 2133 (st, nSi–H), 1257 (st, nSi–C).
Synthesis of 1,3,5-tris(dimethylsilyl)benzene,61 C2. To
a mixture of dimethylchlorosilane (4.73 g, 50 mmol) and
magnesium (1.22 g, 50 mmol) in dry THF (15 cm3) was added
a solution of 1,3,5-tribromobenzene (3.15 g, 100 mmol) in THF
(10 cm3). The rate of the addition was adjusted to maintain
a reflux. When the addition was finished the reaction was heated
under reflux for 3 h. The volatiles were removed in vacuo and the
product extracted from the solid residue of hexane (3 ꢂ 20 cm3).
The solvent was removed in vacuo. The product was purified by
distillation under reduced pressure. Yield ¼ 75%.
In this work each monomer has been combined with each
spacer and cross-linker and the influence of their chemical
structure on the thermal behaviour is discussed.
1H-NMR (CDCl3): d: 0.21 (18H, d, –CH3, J ¼ 3 Hz), 4.29 (3H,
septet, Si–H, J ¼ 3 Hz), 7.58 (3H, s, HAr).
8430 | J. Mater. Chem., 2011, 21, 8427–8435
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3.1 Synthesis of unsymmetric mesogenic monomers M1 to M3,
spacers S2 and S3 and cross-linker C2 (Schemes 1 and 2)
The synthesis of the unsymmetric monomers followed a conver-
gent strategy. Thus, a carboxylic ester was alkylated under
Mitsunobu conditions and then deprotected to give a carboxylic
acid (Scheme 1), while the phenol derivative was prepared
starting from the benzyl-ester-protected hydroquinone, again
using Mitsunobu conditions for O-alkylation. After depro-
tection, three different monomers M1, M2 and M3 were
obtained by esterification.
Scheme 2 Reactions for the synthesis of the siloxane spacers S2 and S3
and cross-linker C2.
Spacers S2 and S3 were synthesised in quantitative yield by the
chlorodimethylsilane-catalysed reaction between 1,1,3,3-tetra-
methyldisilazane and either hydroquinone (for S2) or resorcinol
(for S3) in THF under reflux (Scheme 2). These compounds were
obtained as colourless oils without further purification. Cross-
linker C2 was synthesised by reaction of 1,3,5-tribromobenzene
Table 1 Transition temperatures of the mesogenic monomers
Monomers
Thermal behavioura
with
a Grignard reagent formed in situ from chloro-
M1
M2
M3
Cr 43.5 (60.1) N 53.9 (3.9) Iso
Cr 57.9 (63.4) SmA 66.2 (4.1) Iso
Cr 54.2 (72.3) SmA 75.6 (13.6) Iso
dimethylsilane (Scheme 2); C2 was obtained as a colourless liquid
in 60% yield.
a
Transition temperatures (ꢀC) and enthalpies (DH in
J ) in
gꢁ1
parentheses. Data correspond to the second heating: Cr, crystal phase,
SmA, smectic A phase, N, nematic phase, and Iso, isotropic liquid.
3.2 Liquid crystal properties of the unsymmetric monomers,
M1, M2 and M3
The mesomorphic behaviour of the new, unsymmetric monomers
was determined by polarised optical microscopy and DSC. The
phase behaviour and the thermal stability depended on the length
of the alkyl chain. Monomer M1 showed the typical texture of
the nematic phase by microscopy consistent with the short alkyl
chain lengths. As the length of the alkyl chain increased, the
formation of smectic phases was favoured and, indeed, mono-
mers M2 and M3 showed focal conical textures characteristic of
a smectic A phase. The thermal parameters of the three mono-
mers were evaluated by DSC and the values for the second
heating are shown in Table 1. As expected, the mesogenic
monomers showed low transition temperatures and, as the length
of the alkyl chain increased, the liquid crystal-to-isotropic tran-
sition temperature increased from 53.9 ꢀC, for M1, to 75.6 ꢀC, for
M3. Regarding the thermal stability, monomers M1 and M2
showed similar liquid-crystalline range (10 ꢀC) whereas for
monomer M3 the stability improved two-fold.
bonds of the mesogenic monomers and the silane group of the
spacers and cross-linkers. All elastomers were prepared with 10%
cross-linker C1 or C2 having four or three linking sites, respec-
tively.
The elastomers prepared were analysed by the estimation of
the sol content and the degree of swelling. The sol content esti-
mation gives an idea of the effectiveness of the cross-linking
reaction since the chains that do not form part of the network are
soluble in certain solvents and can be extracted from the swollen
network. As such, the higher the sol content, the lower the
reticulation. In these networks, the sol content varied between 5
and 25 wt% (Table 2); previous work has shown that the meso-
morphic properties of MC-LCEs are not affected by sol
content,39 and this is the case here too. Elastomers E1 and E2
showed the lowest sol contents of 6 and 5 wt%, respectively,
although overall the synthesised elastomers showed a sol content
in the range described in the literature for these systems.39,41 This
high soluble content can be due to the purity of the precursor
olefins and side reactions, as explained by Sanchez-Ferrer and
Finkelmann,43 who avoided this problem by using a high-purity
terminal vinyl ether. Regarding the swelling ratio (q), two
3.3 Preparation and characterisation of MC-LCE
MC-LCEs were prepared following a one-pot hydrosilylation
reaction catalysed by [PtCl2(COD)] and involving the double
Scheme 1 Synthetic route for the synthesis of the mesogenic monomer.
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Table 2 Transition temperatures of the elastomers prepared with different monomers, spacers and cross-linkers
Elastomer
Componentsa
Swelling ratiob/q
Sol content/wt%
Thermal behaviourc
E1
E2
E3
E4
E5
E6
E7
E8
M1$S1$C1
M2$S1$C1
M3$S1$C1
M1$S2$C1
M2$S2$C1
M3$S2$C1
M1$S3$C1
M2$S3$C1
M3$S3$C1
M2$S1$C2
M3$S1$C2
M2$S2$C2
M2$S3$C2
M3$S2$C2
M3$S3$C2
4
3
4
—
4
4
—
4
—
7
—
6
3
5
g ꢁ26 SmC 30 (4) Iso
g ꢁ24 SmC 72 (8) Iso
g ꢁ27 SmC 71 (15) Iso
g ꢁ4.8 Iso
10
—
10
24
—
25
—
20
—
18
25
22
—
g ꢁ6 SmC 44 N 53 (8) Iso
g ꢁ9 SmC 60 (12) Iso
g ꢁ22 SmC 43.6 (3) Iso
g ꢁ14 SmC 37 N 50 (7) Iso
(g ꢁ9.2) Cr 83 Isod
E9
E10
E11
E12
E13
E14
E15
g ꢁ22 SmC 63 (11) Iso
(g ꢁ11) Cr 82 Isod
g ꢁ7.2 N 31.6 (8) Iso
g ꢁ16 N 30.6 (7) Iso
g ꢁ13 N 47.5 (10) Iso
(g ꢁ3)Cr 79.6 Isod
9
6
—
a
b
Relative composition as set out in the text. Swelling ratio is defined as q ¼ a2 ꢂ ak, where ak and at are the stretching factors parallel and
t
c
perpendicular to the stretching direction. Transition temperatures (ꢀC) and enthalpies (DH in J gꢁ1) in parentheses. Data correspond to the second
heating: g, glassy phase. Note that for elastomers E9, E11 and E14, a melting point is measured on first heating, after which cooling leads to the
formation of a glassy phase. The crystalline state is not recovered in subsequent thermal cycles. Elastomers E4, E7 and E9 were not obtained in film form.
d
different groups of networks can be distinguished depending on
the cross-linker used. Elastomers cross-linked with C1 (func-
tionality of four, E1 to E9) showed a swelling ratio of four, while
with C2 (functionality of three, E10 to E15), the swelling ratio
was higher and varied between 6 and 9. These results are in
agreement with those found in the literature for MC-LCEs6,39,41
and suggest an anisotropic swelling behaviour for monodomain
networks. Thus, for a monodomain, the network swells more in
the direction perpendicular to the stretching direction than in the
parallel.
Mesomorphic behaviour: effect of spacer structure. Compared
to the mesomorphic behaviour of the liquid crystal monomers
shown in Table 1, reticulation favours the formation of SmC
phase. The mesomorphic behaviour of the LC networks depen-
ded greatly on the spacer used. Networks containing trisiloxane
spacer S1 (E1 to E3) formed a SmC phase due to the segregation
between rigid cores of the mesogenic molecules and the flexible
spacer. Elastomers containing mesogenic monomers M1 (E1, E4
and E7) and M3 (E3, E6 and E9) formed smectic C phases and,
depending on the aromatic spacer used (S2 for M1 and S3 for
M3), the formation of liquid crystal phases was suppressed.
The family of elastomers that contained M2 as the mesogenic
monomer was shown to be more variable in the phase behaviour
and the phase formed depends on the spacer. The elastomers that
contained S2 were smectic C, but interestingly as aromatic
molecules were added as spacers (S2 and S3, E5 and E8), the
networks tended to form a nematic phase above the smectic C.
Concerning the clearing temperature and the phase stability,
different trends were observed since both depended on the
mesogenic monomer and the spacer used. The addition of trisi-
loxane spacer S1 stabilised the liquid-crystalline phase in the case
of monomer M2 (from 66.2 ꢀC for monomer to 72 ꢀC) and had
little effect on the elastomer containing M3 (entries 2 and 3). In the
case of monomer M1, the introduction of spacer S1 (elastomer
E1) lowered the clearing temperature from 53.9 ꢀC for the
monomer to 30 ꢀC for the elastomer. Thus, the introduction of an
odd parity alkoxy chain may have affected the stabilisation of the
mesophase. Interestingly, when the more rigid aromatic spacers
S2 and S3 were used, the clearing temperature fell. Moreover,
elastomers E6 and E8 showed the same mesomorphism.
3.4 Thermal properties of MC-LCEs
As described in the literature, MC-LCEs show systematically
both SmC and N phases.38–43 That the SmA phase of the
monomer is replaced by a SmC phase in the polymer is accounted
for by the tilt enforced to compensate for the much greater cross-
sectional area of the siloxane chain compared with an alkyl
chain. The liquid-crystalline properties of the new MC-LCEs
showed not only that mesogenic units dictate their behaviour but
also that the spacer and the cross-linker used had a major effect
on the stability and identity of the mesophases. The liquid crystal
properties of the networks prepared are summarised in Table 2.
The remaining elastomers exhibited a second-order transition,
ꢀ
between ꢁ27 and ꢁ6 C, corresponding to the glass transition
temperature (Tg) and were liquid-crystalline at room tempera-
ture. The results show that Tg depended strongly on the flexibility
of the spacer, but much less so on the mesogenic monomer used.
As expected, for the most flexible spacer, S1, the networks
showed the lowest Tg (ꢁ27 to ꢁ24 ꢀC; E1 to E3). The value of Tg
increased as the more rigid, aromatic spacers S2 and S3 were
used. Networks containing spacer S2 (E4 to E6) showed the
highest Tg, which was attributed both to the rigid nature of the
spacer and its overall linear shape (1,4-disubstitution), allowing
good organisation of the polymer chains. Tg was much lower for
elastomers E7 to E9 that use the non-linear spacer S3, as the 1,3-
disubstituted component led to less well organised packing. On
the other hand, Tg was not affected by the nature or connectivity
of the cross-linker.
Mesomorphic behaviour: effect of the cross-linker structure. In
order to compare and verify the trends observed in the previous
section, the same study was carried out for elastomers cross-
linked with the more rigid aromatic cross-linker, C2 (which
possesses a functionality of three), and mesogenic monomers M2
and M3 (Table 2, E10 to E15).
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Fig. 2 X-Ray diffraction patterns for elastomer E5: (a) SmC phase at room temperature, (b) nematic phase at 50 ꢀC, and (c) isotropic phase at 59 ꢀC.
Radial variation, 4, of the intensity of the small angle (dotted line) and wide angle (solid line) reflections corresponding to the X-ray patterns.
The most noticeable result was that cross-linker C2 favoured
the formation of the nematic phase, the exception being E10
(which used flexible spacer, S1), where a SmC phase was found.
Regarding the clearing temperature and phase stability, aromatic
cross-linker C2 had little effect on ꢀthe clearing temperature
compared to the monomer M2 (66.2 C). Also, it was observed
that the addition of aromatic spacers S2 and S3 (for elastomers
E12, E13andE14) destabilisedthe mesomorphicphase, compared
to that of the monomer M2 or M3 (66.2 ꢀC and 75.6 ꢀC, respec-
tively) and clearing temperatures as low as 30 ꢀC were found.
Elastomers E11 and E12 exhibited the same mesomorphism.
For the systems described in the literature and for the
networks described here, the formation of SmC or both SmC and
N phases was independent of whether the mesogenic monomer
had two or three aromatic rings, the length of the alkyl chain and/
or the degree of substitution in the ring. Comparing these
systems with those containing mesogens with two aromatic
rings,41,43 in all cases it was found that clearing temperatures were
between 50 and 70 ꢀC. In these networks, this temperature can be
tuned by modification of the spacer and/or the cross-linker, and
temperatures as low as 30 ꢀC have been found. As reported in the
literature,39 the use of rigid cross-linkers destabilises the meso-
morphic phase in these systems also.
the aromatic spacer, a nematic phase was observed above the
SmC phase. Here, the presence of a nematic phase was promoted
by changing the cross-linker from a flexible siloxane to a more
rigid aromatic unit.
3.5 XRD measurements
The mesophases of the elastomers were characterised by small-
angle X-ray diffraction (XRD) for aligned samples, with the
incident beam being normal to the surface of the films and the
stretching direction. XRD patterns at different temperatures for
E5 are shown in Fig. 2; other data are found in the ESI†.
For all elastomers that showed a SmC phase, a pattern char-
acteristic of a layered structure was observed as indicated by the
presence of the inner reflection at low angles corresponding to the
layer spacing (ESI†); the outer reflection corresponded to
the side-to-side distance between mesogens. As the film was
stretched uniaxially to form a monodomain, the mesogens
become oriented in the direction parallel to the applied external
stress, observed by the presence of two diffuse crescents in the
˚
wide-angle range (d-spacing ¼ 4.5 A) (Fig. 2a). At small angles,
four, sharp reflections typical of a smectic C phase were
observed, in which the layer normal was tilted at an angle with
respect to the mesogens, suggesting a chevron-like structurex
(Fig. 3). The sharp reflections were indicative of a very high
Concerning the siloxane spacer, the present materials showed
the same behaviour as the systems described by Rousseau et al.,42
where trisiloxane spacer S1 and an aromatic 1,4-substituted
siloxane spacer, similar to S2, were used. They observed that the
use of the trisiloxane spacer formed a SmC phase, whereas using
x This has also been modelled as a conical arrangement in ref. 44. The
two are rather similar.
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behaviour of the networks. Unsymmetric monomers did not
favour the formation of the nematic phase as might have been
expected, rather a SmA phase was formed when a long alkyl
chain was used.
The MC-LCEs were liquid-crystalline over a wide temperature
range, showing low Tg values and low clearing temperatures (up
to 72 ꢀC). The elastomers that contained the flexible spacer (S2)
systematically presented the smectic C phase. The introduction
of an aromatic ring into the spacers (S4 or S5) promoted the
formation of the nematic phase. That an aromatic group should
have such an effect was reinforced by the observation that the use
of an aromatic cross-linker (C2) also gave rise to elastomers that
showed only a nematic phase. On the other hand the presence of
a more rigid spacer and/or cross-linker destabilised the meso-
Fig. 3 Aligned ‘chevron-like’ structure of SmC phase.
Table 3 X-Ray data for elastomers containing cross-linker C1a
ꢀ
ꢀ
/
q/ꢀ
morphic phase and thus clearing temperatures as low as 30 C
˚
Elastomer Phase(s) dm/A d001/A d002/A lcal/A qcalc
˚
˚
˚
were found.
E5
E6
E2
E3
E8
SmC, N 4.5
30.4
33.9
30.1
31.6
31.5
15.0
16.3
—
15.5
15.0
38.2
40.5
34.9
35.6
38.2
37
33
30
27
35
33
37
30
26
37
In this report, it is shown that the mesomorphic behaviour of
the MC-LCEs can be tuned by the chemical composition of the
components, namely the mesogenic monomer, the spacer and the
cross-linker and the accessible working temperatures found make
them good candidates as sensors and actuators. The effect of the
structure and thus the mesomorphic behaviour on the mechan-
ical and thermomechanical properties of the new unsymmetrical
MC-LCEs will be discussed in the following paper.
SmC
SmC
SmC
4.5
4.3
4.3
SmC, N 4.4
a
dm: d-spacing between mesogens; d001 and d002: interlayer distances
measured from X-ray data; lcal: interlayer distances from geometry
optimisation; qcalc: calculated, and q: measured tilted angle from X-ray
experiments.
Acknowledgements
degree of alignment, while the fact that there are four of them
points to the chevron structure rathꢀer than a true monodomain.
When the sample was heated to 50 C in the nematic phase, the
sharp inner reflections give way to diffuse lines, corresponding to
the presence of smectic fluctuations within the nematic phase,
their domain size decreasing on heating (cybotactic groups,
Fig. 2b). The mesogens orientation along the stretching direction
is maintained in the mesophase as evidenced by the two large
diffuse half-crescents confined in the equatorial plan (i.e. the
nematic director is aligned perpendicular to the X-ray beam and
parallel to the surface of the film, Fig. 2b).
The authors thank the European Union for support and grant
funding through the RTN project ‘Functional liquid crystal
elastomers’ (FULCE-HPRN-CT-2002-00169) and Johnson
Matthey for generous loans of K2[PtCl4].
References
1 A. Lendlein and S. Kelch, Angew. Chem., Int. Ed., 2002, 41, 2034.
2 E. R. Zubarev, S. A. Kuptsov, T. I. Yuranova, R. V. Talzore and
H. Finkelmann, Liq. Cryst., 1999, 26, 1531.
The interlayer distance obtained from the low-angle distribu-
tion for the elastomers cross-linked with C1 varied between 30.1
3 I. A. Rousseau and P. T. Mather, J. Am. Chem. Soc., 2003, 125,
15300.
4 F. J. Davis and G. R. Mitchell, Polymer, 1996, 37, 1345.
5 C. H. Legge, F. J. Davis and G. R. Mitchell, J. Phys. II, 1991, 1, 1253.
6 H. Wermter and H. Finkelmann, e-Polymers, 2001, 013, 1.
7 S. M. Clarke, A. Hotta, A. R. Tajbkhsh and E. V. Terentjev, Phys.
Rev. E: Stat., Nonlinear, Soft Matter Phys., 2002, 65, 021804.
8 S. M. Clarke, E. V. Terentjev, I. Kundler and H. Finkelmann,
Macromolecules, 1998, 31, 4862.
9 M. Warner and E. V. Terentjev, Liquid Crystal Elastomers, Clarendon
Press, Oxford, 2003.
10 P. Xie and R. Zhang, J. Mater. Chem., 2005, 15, 2529.
˚
˚
and 33.9 A for d001 and between 15 and 16.3 A for d002, in
agreement with those distances reported in the literature.42 The
tilt angles, q, calculated from the azimuthal intensity distribution
at low angle, were between 26 and 37ꢀ.
These tilt angles are in the same range to those reported in the
literature39,41–43 (q ¼ 30–45ꢀ). The measured tilt angles q are in
good agreement with the theoretical values of qcalc, calculated
from the fully extended chain conformation tilted at the angle q
(Fig. 2, Table 3).
€
11 J. Kupfer and H. Finkelmann, Macromol. Chem. Phys., 1994, 195,
1353.
12 D. L. Thomsen III, P. Keller, J. Naciri, R. Pink, H. Jeon,
D. K. Shenoy and B. R. Ratna, Macromolecules, 2001, 34, 5868.
13 P. Beyer, E. V. Terentjev and R. Zentel, Macromol. Rapid Commun.,
2007, 28, 1485.
4. Conclusions
14 T. Ikeda and O. Tsutsumi, Science, 1995, 268, 1873.
15 T. Oge and R. Zentel, Macromol. Chem. Phys., 1996, 197, 1805.
16 M. Camacho-Lopez, H. Finkelmann, P. Palffy-Muhoray and
M. Shelley, Nat. Mater., 2004, 3, 307.
17 R. Y. Yin, W. X. Xu, M. Kondo, C. C. Yen, J. Mamiya and T. Ikeda,
J. Mater. Chem., 2009, 19, 3141.
A series of new unsymmetric MC-LCEs having different chem-
ical composition have been synthesised. The mesogenic mono-
mer, the spacer and the cross-linker have been varied
systematically with the purpose of determining the structure–
properties relationships of these systems.
18 M. Chambers, H. Finkelmann, M. Remskar, A. Sanchez-Ferrer,
B. Zalar and S. Zumer, J. Mater. Chem., 2009, 19, 1524.
19 H. Koerner, G. Price, N. A. Pearce, M. Alexander and R. A. Vaia,
Nat. Mater., 2004, 3, 115.
The MC-LCEs studied showed that small changes in the
chemical structure either of the mesogenic monomer, the spacer
or the cross-linker had an important effect on the thermal
8434 | J. Mater. Chem., 2011, 21, 8427–8435
This journal is ª The Royal Society of Chemistry 2011

View Article Online
20 A. Kaiser, M. Winkler, S. Krause, H. Finkelmann and
A. M. Schmidt, J. Mater. Chem., 2009, 19, 538.
41 B. Donnio, H. Wermter and H. Finlemmann, Macromolecules, 2000,
33, 7724.
21 S. Courty, J. Mine, A. R. Tajbkhsh and E. V. Terentjev, Europhys.
Lett., 2003, 64, 654.
42 I. A. Rousseau, H. Qin and P. T. Mather, Macromolecules, 2005, 38,
4103.
22 P. G. de Gennes, C. R. Seances Acad. Sci., Ser. B, 1975, 281, 101.
23 J. E. Mark, Prog. Polym. Sci., 2003, 28, 1205.
24 A. Sanchez-Ferrer, T. Fischk, M. Stubenrauch, H. Wurmus,
M. Hoffmann and H. Finkelmann, Macromol. Chem. Phys., 2009,
210, 1671.
25 M. Yamada, M. Kondo, J. I. Mamiya, Y. L. Yu, M. Kinoshita,
C. J. Barret and T. Ikeda, Angew. Chem., Int. Ed., 2008, 47, 4986.
26 M. Yamada, M. Kondo, R. Miyasato, Y. Naka, J. I. Mamiya,
M. Kinoshita, A. Shishido, Y. L. Yu, C. J. Barret and T. Ikeda, J.
Mater. Chem., 2009, 19, 60.
27 C. Ohm, M. Brehmer and R. Zentel, Adv. Mater., 2010, 22, 3366.
28 K. A. Burke and P. T. Mather, J. Mater. Chem., 2010, 20, 3449.
29 J. Naciri, A. Shirivasan, H. Jeon, P. Nikolov, P. Keller and
B. R. Ratna, Macromolecules, 2003, 36, 8499.
30 D. K. Shenoy, D. L. Thomsen III, A. Shirivasan, P. Keller and
B. R. Ratna, Sens. Actuators, A, 2002, 96, 184.
43 A. Sanchez-Ferrer and H. Finkelmann, Macromolecules, 2008, 41,
970.
44 P. Beyer, L. Braun and R. Zentel, Macromol. Chem. Phys., 2007, 208,
2439.
45 W. H. de Jeu, E. P. Obraztsov, B. I. Ostrovskii, W. Ren,
P. J. MacMullan, A. C. Griffin, A. Sanchez-Ferrer and
H. Finkelmann, Eur. Phys. J. E, 2007, 24, 399.
ꢀ
46 H. Yang, A. Buguin, J.-M. Taulemesse, K. Kaneko, S. Mery,
A. Bergeret and P. Keller, J. Am. Chem. Soc., 2009, 131, 15000.
47 R. Ishige, K. Osada, H. Tagawa, H. Niwano, M. Tokita and
J. Watanabe, Macromolecules, 2008, 41, 7566.
48 H. P. Patil, J. Liao and R. C. Hedden, Macromolecules, 2007, 40, 6206.
49 G. Cordoyiannis, A. Sanchez-Ferrer, H. Finkelmann, B. Rozic,
S. Zumer and Z. Kutnjak, Liq. Cryst., 2010, 37, 349.
50 F. Zander and H. Finkelmann, Macromol. Chem. Phys., 2010, 211,
1167.
31 D. K. Shenoy, D. L. Thomsen III, P. Keller and B. R. Ratna, J. Phys.
Chem. B, 2003, 1, 107.
51 D. Rogez, H. Brandt, H. Finkelmann and P. Martinoty, Macromol.
Chem. Phys., 2006, 207, 735.
32 R. Zentel, Angew. Chem., Int. Ed. Engl., 1989, 28, 1407.
33 K. Urayama, Macromolecules, 2007, 40, 2277.
34 A. R. Tajbkhsh and E. M. Terentjev, Phys. Rev. E: Stat., Nonlinear,
Soft Matter Phys., 2002, 65, 041720.
52 P. Beyer, E. M. Terentjev and R. Zentel, Macromol. Rapid Commun.,
2007, 28, 1485.
53 D. Ribera, M. Giamberini, A. Serra and A. Mantecon, J. Polym. Sci.,
Part A: Polym. Chem., 2006, 44, 6270.
35 H. Finkelmann, E. Nishikawa, G. G. Pereira and M. Warner, Phys.
Rev. Lett., 2001, 87, 015501.
54 S. V. Ahir, A. R. Tajbakhsh and E. M. Terentjev, Adv. Funct. Mater.,
2006, 16, 556.
36 J. Cviklinski, A. R. Tajbakhsh and E. M. Terentjev, Eur. Phys. J. E,
2002, 9, 427.
37 M. H. Li, P. Keller, B. Li, X. Wang and M. Brunet, Adv. Mater., 2003,
15, 569.
38 G. H. F. Bergmann, H. Finkelmann, V. Percec and M. Zhao,
Macromol. Rapid Commun., 1997, 18, 353.
39 M. Bispo, D. Guillon, B. Donnio and H. Finkelmann,
Macromolecules, 2008, 41, 3098.
55 A. R. Tajbakhsh and E. V. Terentjev, Eur. Phys. J. E, 2001, 6, 291.
56 S. Krause, R. Dersch, J. H. Wendorff and H. Finkelmann, Macromol.
Rapid Commun., 2007, 28, 2062.
57 M. Amela-Cortes, C. W. Smith, D. W. Bruce and K. E. Evans, J.
Mater. Chem., 2011, DOI: 10.1039/c0jm03692g, following paper.
58 O. Mitsunobu and Y. Yamada, Bull. Chem. Soc. Jpn., 1967, 40, 2380.
59 P. F. Hudrlik and D. K. Minus, J. Organomet. Chem., 1996, 521, 157.
60 P. D. Murphy, T. Taki, T. Sogane, R. Metzler, T. G. Squires and
B. C. Gerstein, J. Am. Chem. Soc., 1979, 101, 4055.
61 H. Pleino, J. Organomet. Chem., 1992, 435, 21.
40 C. Ortiz, M. Wagner, N. Bhagarva, C. K. Ober and E. J. Kramer,
Macromolecules, 1998, 31, 8531.
This journal is ª The Royal Society of Chemistry 2011
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