数理解析研究所講究録 1305 巻 2003 年 139-148

139
Swelling behavior liquid crystal elastomers
in low molecular weight liquid crystals
Yusril YUSUF, Yukitada ONO Yusuke SUMISAKI and Shoichi KAI
Department Applied Physics, Faculty of Engineering
Kyushu University, Fukoka 812-8581, Japan

We experimentally investigated the swelling behavior of liquid crystal elastomers (LCEs)
and MBBA.
in two anisotropic solvents such as low molecular weight liquid crystals,$5\mathrm{C}\mathrm{B}$The length of LCEs by swelling expanded more than 1.8 times of its initial length which
depended on the director orientation. The volume change of swollen LCEs has been
investigated as function of temperature and several phase transitions were observed inboth optical and adifferential scanning calorimetry measurements. ElectrO-mechanicaleffects of swollen LCEs were also investigated in detail and drastic decrease of thecritical field for electr0-mechanical effects, 1/4000 lower than dry LCEs, was obtained.

Introduction

LCEs and gels presently attract much attention due to the volume and shapechanging properties caused by several environmental factors, such as solvent composition,, light, electric field, etc. [1]. LCE materials studied here istemperature, ionic strength,
invented and developed by Heino Finkelmann and -workers at Freiburg University,
Germany. The behavior of these materials arises ffom a coupling between the elastic
properties of the polymer chains network and the liquid-crystalline ordering of monomeric
mesogen groups as the side chains. The polymer chains network is formed by added thecross-linking agents to the system of polymer chains. There are two typical domains of LCEs
depending on the director orientation (the director is defined as the average direction of theliquid-crystalline ordering of side chains consisting of mesogenic groups). One is the
s0-called polydomain, when the mesogenic groups are macroscopically disordered in theliquid crystalline state. The other is the s0-called monodomain, when the mesogenic groups
are macroscopically ordered in the liquid crystalline state.
Most of the earlier studies are concentrated on the swelling effect of gels in isotropic
solvent, but few studies on the swelling effect of anisotropic material (i.e. LCEs) inanisotropic solvents (low molecular weight liquid crystals; LLCs). Swelling behavior ofpolymer network in liquid crystal (LC) solvents has been investigated in early study by some$\mathrm{p}\mathrm{H}$$\mathrm{c}\mathrm{o}$$\mathrm{n}$140researchers $[2, 3]$ . They investigated temperature dependence of the degree of equilibriumswelling and phase the behavior of LC in the system consisting of LC and polymer network.
In the present study, we investigated temperature dependence of volume changes of swollen
LCEs in LLCs in detail.
Additionally, spontaneous shape change of oriented side chains LCEs (monodomain)at the nematic-isotropic phase transition is firstly found by. Cladis [4]. She conclude thatthe result gave “proof of concept” to the idea of LCEs as artificial muscles when cooperativeorientation effects of the side chains (i.e. the phase transition) extended beyond atypical meshsize of the cross-linked polymer network. To check her
in this study, we deal with shapechange of swollen LCEs with LLCs under an alternating electric field as well as its
temperature dependence.
$\mathrm{P}.\mathrm{E}$$\mathrm{i}\mathrm{d}\mathrm{e}*$Experimental

Samples
LCEs, both polydomain and monodomain, were synthesized by Heino Finkelmanngroup at Freiburg University in Germany. The samples are prepared by polymer analogue
reaction of poly(methyl-hydrogen-siloxane) with an average degree of polymerization of
about 60 and the monomeric mesogen 4-buten0xy-4’-methyl0xy benzoid acid phenylester
and the cross-linking agent(H2). The cross-linker agent is the oligomeric
Poly(dimethylsiloxane) with the terminal vinyl groups. The concentration of the cross-linking
is 8% related to the reactive vinyl groups. Except the chemistry of the cross-linking agent theprocedure of the synthesis is described in [8]. The monodomain sample is mechanicallyloaded after gelation (after 3hours) to obtain the director orientation. Under these conditions
the cross-linking reaction is completed. These anisotropy was optically tested to confirm
director orientation using cross polarizers. In order to check the anisotropic swelling behavior
these samples, we prepared three types LCE samples with different geometries against bulk
director orientations . One is obtained by slicing parallel to , another by slicing
perpendicular to and the last one is apolydomain film. Hereafter we call these MONOI (i.e.
planar alignment film), MONO2 (homeotropic alignment film) and POL. All samples weresliced to thin film with the dimension of about 1.0 mm in height, 0.5 mm in width and 150
in thickness. We here define the and -directions as parallel and perpendicular to on a$(\mathrm{C}\mathrm{H}2=\mathrm{C}\mathrm{H}-\mathrm{C}\mathrm{H}2-\mathrm{C}\mathrm{H}2-\mathrm{O}-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l}-\mathrm{C}\mathrm{O}\mathrm{O}-\mathrm{p}\mathrm{h}\mathrm{e}\mathrm{n}\mathrm{y}\mathrm{l}-\mathrm{C}\mathrm{H}3)$$=\mathrm{C}\mathrm{H}-\mathrm{O}-(\mathrm{S}\mathrm{i}(\mathrm{C}\mathrm{H}3)_{2}-\mathrm{O})_{12}-\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{H}2$$\mathrm{n}$$\mathrm{n}$$\mathrm{n}$$x$$|\mathrm{m}$$y$$\mathrm{n}$plane respectively, and the -direction is perpendicular to both and the plane.
Samples are embedded in the anisotropic swelling solvents. In this study we use two
LLCs,
kind
of
4-n-phentyl-4’-cyan0biphenyland$\mathrm{z}$$\mathrm{n}$$(5\mathrm{C}\mathrm{B})$4’-meth0xy-benzilidene-4-buthyl-aniline (MBBA)

141
Measurements
After sliced, LCE samples are embedded in anisotropic solvent between two.
surface glass plates. The thickness was controlled by apolymer (Mylar) spacer of 350
The swelling behavior was observed in apolarizing microscope (Nikon) equipped with the
hot stage (Mettler Teledo FP90 Central Processor) as temperature control which cansimultaneously measure its thermal property (heat capacity) by adifferential scanningby swelling is defined as the ratio of thecalorimetry (DSC). The length expansion rate
to the initial length where $i=x,y$ and .
swollen length
The electric-mechanical measurements were prepared by made an electr0-0ptical cellsurface that was treated toconsisting of two transparent ITO electrodes with very clean
and MBBA and applied an alternating electric field
be homeotropic alignment for(0,0, Ez)] at fixed temperature.
perpendicularly to the electrodes [$\mathrm{S}\mathrm{i}\mathrm{O}$$\mu \mathrm{m}$$(\alpha_{i})$$z$$l_{i0}$$l_{i}(t)$$\mathrm{S}\mathrm{i}\mathrm{O}$$5\mathrm{C}\mathrm{B}$$\mathrm{E}=$Results and discussion

SWelling effectFig. 1shows the temporal dimension changes during swelling process of three types. The time $t=0$ is atime justof slices in the low molecular weight liquid crystal. Fig. la shows variation the swelling rate of MONOI. In
embedding LCE into
$5\mathrm{C}\mathrm{B}$$x$$5\mathrm{C}\mathrm{B}$increases
direction (perpendicular to the director n) the length expansion rate %exponentially in time and saturates with the values about 1.8 after 60 minutes. Contrast to this,% is constant and no length change is observed. That is, in the direction parallel to thedirector orientation, no swelling occurs.
$(=lJl_{x\mathit{0}})$

(b)

(a)

(c)

Figure 1. Temporal length changes during swelling process of MONOl(a),the length expansion rates in x-direction
MONO2 (b) and POLY(c) in(a), (A) the length expansion rates in -direction(ay). See text for detail$5\mathrm{C}\mathrm{B}.(\bullet)$

$y$

142
and
In Fig. , the length changes of MON02 by swelling are shown. Bothincrease exponentially in time and saturate with the maximum value about $1.8\pm 0.1$ . In thefilm the director of LCE is homeotropic, there is no specific direction to expand in the plane.
Thus it clearly shows that avolume expansion of monodomain LCE by swelling withindicates anisotropic property depending on directions to the director orientation in
MONO-LCE.
Similar results to MONO2 can be observed for POLY as shown in Fig. , that is,both and % increase in time and saturates with about $1.8\pm 0.1$ . In the POLY case unlikewith MON02 the dimension in direction similarly expands in time to other direction withthe rate 1.8. Thus apolydomain LCE equally expands in all directions as in isotropic gels.
.
Similar behavior was observed for swelling MBBA replacing
Fig. 2shows the volume changes ($) of monodomain and polydomain duringswelling process inand MBBA. Where is the averaged mol ffaction for all directions
as afirst step of analysis though both LCEs and LLCs are anisotropic. is the arbitrary, 29.58 $\min$relaxation time, experimentally obtained as, for monodomain: 15.57, 6.04 $\min$ (MBBA). The volume ffaction is(MBBA), and for polydomain: 8.10
) and drythat wasdefined as the ratio of the LCE volumes in the swelling process
calculated ffom the average length changes of the swollen LCE and dry LCE using a. The solid line in the figure is the result of theoretical calculation derived
relation
ffom the Flory-Rehner theory (isotropic gels) [9],$1\mathrm{b}$$\alpha_{x}$$\alpha_{y}$$5\mathrm{C}\mathrm{B}$$1\mathrm{c}$$\alpha_{x}$$l_{z}$$z$$5\mathrm{C}\mathrm{B}$$5\mathrm{C}\mathrm{B}$$\emptyset$$\tau$$\min(5\mathrm{C}\mathrm{B})$$\min(5\mathrm{C}\mathrm{B})$$\mathrm{V}\{\mathrm{f}$$V_{0}$$\phi=l_{0}^{3}/l(t)^{3}$$\mathrm{r}(\mathrm{d}\psi \mathrm{d}\mathrm{t})$(1)$\cong(1/\emptyset)[\phi+\ln(1-\phi)+\chi\beta+v\phi^{1/3}]$.
$\ovalbox{\tt\small REJECT}\wedge[searrow] v\mathrm{o}$$\ovalbox{\tt\small REJECT}\wedge[searrow]\check{\mathrm{o}\mathrm{r}\iota}$$\frac{\ovalbox{\tt\small REJECT}\triangleleft l}{>0}$$\frac{\xi 4l}{>^{\mathrm{O}}}$

(a)

(b)

Figure 2. Volume changes during swelling process of monodomain andand MBBA. The solid line indicates the theoreticalpolydomain in$5\mathrm{C}\mathrm{B}$curve (Eq. 1)

143, whereis acertain time constant, is the Flory-Huggins interaction parameter.
are number of partial chain in the LCE and volume of one lattice element
andrespectively. decreases exponentially in time in both swollen monodomain LCE (Fig. )and MBBA
and swollen polydomain LCE (Fig. ). The swelling process for both withshows similar tendency. It may indicate aform for the ffee energy between the LCEs-5CB
mixture and of the LCEs-MBBA mixture has no difference. There is some deviations of theexperimental data ffom the theoretical curve (1) (solid line in Fig.2) based on isotropic gels. It
is probably due to anisotropic properties of both LCEs and LLCs.
Temperature dependence of the dimension changes of dry and swollen monodomain
samples are shown in Fig. 3. Here, the volume change $V(D$ is normalized by the LCE
at room temperature. The typical of lengthvolumes in the equilibrium swollen statechanges in , and directions are shown in Fig. 4* and respectively. Increasing, dry monodomain gradually shrinks in x-direction
temperature, with rating of about
which is the nematic-isotropic phase transition(parallel to the director) by aroundand -directions(both aretemperature of dry LCE. In contrast, LCE stretches inperpendicular to the director) when it is heated up. Close to , drastic changes of lengths for
however in all
all direction are observed owing to phase transition. The length change at
direction disappeared for swollen LCE and no change is obtained as seen in Figs. 3* and .
, abig change for all direction is observed at nematic-isotropic
Instead, for example with
$v=n_{\mathrm{c}}\nu_{\mathrm{c}}/V_{0}$$\tau$$\chi$$v_{\mathrm{c}}$$n_{\mathrm{c}}$$2\mathrm{a}$$\emptyset$$5\mathrm{C}\mathrm{B}$$2\mathrm{b}$$V_{\mathrm{s}}$$\mathrm{b}$$x$$\mathrm{c}$

$z$

$y$

$0.7^{\mathrm{o}}\mathrm{C}/\mathrm{s}\mathrm{e}\mathrm{c}$$T_{\mathrm{c}}=84^{\mathrm{o}}\mathrm{C}$

$z$

$y$

$T_{\mathrm{c}}$$T_{\mathrm{c}}$$\mathrm{b}$$\mathrm{c}$$5\mathrm{C}\mathrm{B}$transition of
In Fig.

.

$5\mathrm{C}\mathrm{B}(\mathrm{r}\mathrm{N}\mathrm{I}\cong 34.5^{\mathrm{o}}\mathrm{C})$$3\mathrm{d}$, temperature dependence of the volume changes of monodomain LCE isshown. Increasing temperature, the volume changes in dry sample slightly increase almost. Close to Tc, abig expansion of the volume occurs owing to its
linearly till
nematic-isotropic transition and then saturates wit the value about 1.08. It is known that theisotropic gels show no volume changes with increasing temperature. This result clearly
indicates that the phase transition of LCE plays an important role in the temperaturedependence ofthe volume changes in the monodomain sample.
On the other hand, drastic difference for swollen LCEs ffom dry samples can beobserved. There exists almost no change of the length in -direction in swollen LCE except a. In y-direction,
and avery small dip atofjump indicating its shrink at
however, the length change is also small but different ffom in -direction. There is no change
at all but small jump indicating elongation at .
at
However, the length change of -direction has avariety. There is abig jump in theconsiderable decrease, andandchange indicating shrink at , in temperature between
around again jump indicating elongation. We do not yet understand these mechanisms forshrinking and elongating. Due to these anomalous changes in $x,y$ and -directions, the volume. According to the DSC
change shows complicate temperature dependence as sown in$80^{\mathrm{o}}\mathrm{C}$$x$$T_{\mathrm{N}1}$$T_{\mathrm{A}}\cong 45^{\mathrm{o}}\mathrm{C}$$5\mathrm{C}\mathrm{B}$$x$$T_{\mathrm{A}}$$T_{\mathrm{N}1}$$z$$T_{\mathrm{N}1}$$T_{\mathrm{N}1}$$T_{\mathrm{A}}$$T_{\mathrm{B}}$$z$$\mathrm{F}\mathrm{i}\mathrm{g}.3\mathrm{d}$144Tmperatlre$[^{\mathrm{o}}\mathrm{C}]$$\mathrm{T}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\iota \mathrm{r}\mathrm{e}$

(b)

(a)

Temperature$[^{\mathrm{o}}\mathrm{C}]$

Tempelatlm$[^{\mathrm{o}}\mathrm{C}]$$[^{\mathrm{o}}\mathrm{C}]$

(d)

(c)

Figure 3. Dependence of volume changes of dry and swollen monodomain
LCEs on temperature (d). (a) typical length change in -direction, (b) in-direction, (c) in -direction. See text for detail.
$x$

$y$

$z$

and two broad and small bumps are obtained atand. It therefore suggests that some phase transition may occur. At this moment
however, we do not determine what those phases are. To determine these unknown states

measurement, asharp peak is observed at$T_{\mathrm{A}}$$T_{\mathrm{N}1}$$T_{\mathrm{B}}$more investigation is necessary.
Temperature dependence of the volume changes in polydomain LCE is shown in
Fig.5. Unlike the case of dry monodomain sample, the volume change of adry sample does
not show any big change at and the volume is constant over whole temperature range. It issimilar to isotropic gels and therefore polydomain LCE could be an isotropic elastomer. Abig
volume change occurs however for swollen polydomain. Two big shrinks are observed at, heating further the volume increases by nearand
$T_{\mathrm{c}}$$T_{\mathrm{N}1}$$(5\mathrm{C}\mathrm{B})(\cong 34.5^{\mathrm{o}}\mathrm{C})$$T_{\mathrm{A}}\cong 39.0^{\mathrm{o}}\mathrm{C}$$T_{\mathrm{c}}$145

Temperature$[^{\mathrm{o}}\mathrm{C}]$

Figure 4. Dependence of volume changes of dry
and swollen polydomain LCEs on temperature.
(nematic-isotropic transition temperature of LCE). According to DSC measurements, similar

peak and bump are observed but more investigation will be required.

ElectrO-mechanical effects
Applying an electric field to the swollen LCEs with LLCs, LLC molecules will easily
rearrange their orientation parallel to the fields. This electr0-mechanical effect is mediated bymobile LLCs ffee to move in and out of the LCEs, the LC side chains could communicate
with each other on length scales extending beyond atypical mesh size of the cross-linked
polymer network, then aweak electric field, rather than temperature changes, could also
trigger aspontaneous shape change. The investigations in shape changes of liquid-crystalline
polymers by electric fields were reported by some researchers. In 1986, Zentel [6] reported
his observation on shape variation of cross-linked liquid-crystalline polymers, which areswollen with nematic LLCs, by electric fields. Subsequently, Barnes et al. [6] reported theirlargest shape change of about 20% contraction of an elastomer swollen in
were isotropic. Later, in 1994 Kishi et(cyanohexyl-biphenyl) when both elastomer andal. [7] reported the quantitative results of shape changes of swollen polydomain LCEs under. Below we will described our resent results on thisacting a dc electric field,$6\mathrm{C}\mathrm{B}$$6\mathrm{C}\mathrm{B}$$\mathrm{E}=0.3\mathrm{M}\mathrm{V}/\mathrm{m}$subject.
(after) was embedded in
Asliced polydomain film (the thickness is 20) to observe the electr0-mechanical effects.
swelling sample expands to about
Spontaneous shape changes were observed when an alternating electric field applied
perpendicularly to the electrodes. In Fig. 5* the variation of shape changes (determined by$5\mathrm{C}\mathrm{B}$$\mu \mathrm{m}$$\sim 40\mu \mathrm{m}$

146
20

.
.

$\overline{\underline{\Xi \mathrm{a}}}15$$\cdots$...

.

$\nwarrow\wedge$

10$\dot{5}$$.A^{\mathrm{o}}\ovalbox{\tt\small REJECT}\ovalbox{\tt\small REJECT} 5$$T_{\mathrm{N}\mathrm{I}\backslash _{0}}$

0

-2

-8
$- 64T-T_{\mathrm{N}1}(^{\mathrm{o}}\mathrm{K})$$\nabla[\mathrm{V}\mathrm{o}1\mathrm{t}]$

(b)

(a)

Figure 5. ElectrO-mechanical effect of swollen polydomain LCE.
is expressed in (a),Displacement versus applied voltage (V) at. See text(b) temperature dependence of in field$T=26^{\mathrm{o}}\mathrm{C}$$\delta$$([=50\mathrm{H}\mathrm{z}, V=50\mathrm{V})$for detail.
displacement of sample shape ffom its equilibrium swollen state) ffom zero volts position is. The solid line is afit to:
shown. Temperature is controlled at$\delta$$26^{\mathrm{o}}\mathrm{C}$$X0$$=12\mathrm{J}8$(2)$[1-\exp(V/7.97)]$, the shape change increases and saturates
Increasing voltage, with fixed ffequency (. According to the fitting, saturation is about. The
with the maximum value about 13
threshold voltage is about 1.OV, 1/4000 times smaller than that of dry LCE.
and $V=$Fig.
shows temperature dependence of shape change in field (. As temperature is increased, shape change increases slightly. Maximum contraction isachieved at temperature just before the nematic-isotropic phase transition. In the isotropic
phase, no displacements were observed.
The spontaneous shape changes of swollen LCEs are induced by the reorientation ofanisotropic solvent molecules inside the LCE. This reorientation influences many mesogenicside chains cooperatively which in turn reorient to change the network shape making it thicker
along and thinner perpendicular to E. Since in the isotropic phase the reorientation ofissolvent molecules could not occurs, it does not driving the shape change. Thoughthat it is still
isotropic, the mesogenic side chains are still in nematic phase until about.
possible to contraction but should be applied in high voltage$f=50\mathrm{H}\mathrm{z}$$20\mathrm{V}$$\mu \mathrm{m}$$f=50\mathrm{H}\mathrm{z}$$5\mathrm{b}$$50\mathrm{V})$$\mathrm{E}$$5\mathrm{C}\mathrm{B}$$84^{\mathrm{o}}\mathrm{C}$$(\sim 4000\mathrm{V})$147$-_{1}-.\underline{-n_{8}\Phi 0}\mathrm{b}$Figure 6. Voltage dependence of inverse
relaxation time of swollen polydomain
LCE when the field is switched “on”.

Fig. 6shows applied voltage dependence of the relaxation times when the field is
switched “on”. Increasing the voltage, the inverse of relaxation time increase monotonically(linear or probably square). Above 10 , the image analysis could not be done, because of thelimitation of our image software (NIH-Image). The response of spontaneous shape changes$\mathrm{V}$speed less than asecond when the field is switched off.
Summary and Conclusions

We have presented in this study the swelling behavior of LCEs with LLCs and found
the following facts.
(1) The swelling process may be described by the Flory-Rehner theory.
(2) The complex volume changes of swollen LCEs are observed, which may indicate avariety

of different phase transitions. However to determine them more detailed studies are necessary.
(3) The threshold field for electr0-mechanical effects has been lowered by swollen LCEs ofwhich value becomes 1/4000 times smaller than that of dry LCEs.
Finally we would like to mention about the mechanism of the volume changes bytemperature. Due to rubber elasticity of LCEs, elevating temperature dry LCEs may shrinkwith strong elasticity. Therefore if there are no other effects like phase transitions of solventsand LCEs, the volume of dry LCEs is basically either constant or monotonically shrinking.
The experimental facts such as jumps of the volume change are due to the interaction changes
between LCE networks and solvents LLCs by phase transitions. Also increasing elasti

148
constants, LCEs exclude absorbing LLCs. The detailed mechanisms are left in future works.

We would like to thank Profs. P. Cladis, H. Finkelmann and H. R. Brand for supplying LCEs,
many valuable suggestions and discussions.
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