九州大学大学院総合理工学府報告第 34 巻 第 2 号 1-7 頁 平成 25 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 34 , No. 2 , pp.1-7 , FEB , 2013

Observation of transient phase separation process ofliquid crystal/ polymer composites obtained byphoto-polymerization induced-phase separation and theirelectro-optical properties
Satoshi NIIYAMA*1,† and Hirotsugu KIKUCHI*2† E-mail of corresponding author: satoshi-niiyama@agc.com(Received January 15, 2013, accepted January 16, 2013)
A transient phase separated structure during the photo-polymerization induced-phaseseparation process of liquid crystal/polymer composites consisting of a continuous phase of nematic
liquid crystal and a three-dimensional polymer network was successfully observed by interrupting
UV light irradiation for photo-polymerization. Those transient composites were characterized by adifference in temperature for nematic-isotropic transition point (Tn-i) of the liquid crystal phase, acomparison of morphology for polymer phase between an initial stage and the last stage of phaseseparation, and an electro-optical performance at each period of phase separation process. It was
found that the composite even at an initial stage of phase separation showed a transparent-turbid(light scattering) operation with an electric field and the UV irradiation period required to attain the
final voltage-transmittance curve accorded well with that to give a saturation of Tn-i of the liquid
crystal phase. A schematic diagram for photo-polymerization induced-phase separation process was
proposed according to investigation into characteristics of the liquid crystal/polymer composites
having a transient phase separated structure.
Key words: Liquid crystal/polymer composite, Photo-polymerization induced-phase separation,

Transient phase structure, Electro-optical property

1.

Introduction

Anelectro-opticaleffectshowingtransparent-scattering of liquid crystal/polymer composites consisting of nematic
liquid crystal and polymer has been widelyused for smart windows for architecture
application and optical devices by theadvantage of high transparency without
polarizing plate in transparent state.
Kajiyama et al. have first proposed such acomposite, which was obtained by phaseseparationfromhomogeneous
solutioncomprising liquid crystal and dissolved
polymer in common solvent.1) Craighead et al
has reported an electro-optical effect of device
Department of Applied Science for Engineering andMaterials, Graduate student ( Present Address :Asahi Glass Co., Ltd. )*2 Department of Applied Science for Engineering andMaterials*1using the composite obtained by impregnating
liquid crystal into porous polymer film.2)
The emulsion method, in which multiple
micro-droplets comprising nematic liquid
crystal were dispersed into polymer matrix,
andthepolymerizationinduced-phaseseparation method, in which liquid crystal isphase-separated during polymerization of low
molecular weight curable compound from anisotropic homogeneous solution dissolving
liquid crystal into the compound, were
proposed to improve the contrast ratio intransparent-scattering characteristics of liquid
crystal/polymer composites by Fergason andDoane et al.3),4)
Gunjima et al. and Hirai et al. have reported
that polymer phase showing a threedimensional network having a correlation
length of several microns was formed in acontinuous liquid crystal phase in the2Transient phase structure of LC/polymer composite

composite via spinodal decomposition by highspeed photo-polymerization induced-phaseseparation
inradicalchain
photopolymerization
systemusing
acrylic
prepolymer.5),6) The bicontinuous phase typecomposite showed a higher efficiency of lightscattering because not only index-mismatching
between a liquid crystal droplet and a polymer
matrix but also that among adjacent liquid
crystal domains were available.7) Besides, it
operated with a driving voltage of 10 Vrms(Voltroot mean square) or less because smaller areaof interface between liquid crystal domain andpolymer network led to smaller fraction ofliquid crystal molecules of which alignments
were fixed at the interface with polymer phase.
Kunigita et al. have proposed a projection
display which combined projection optics with
dot-matrix display panels using the composite
driven by TFT and Niiyama et al. havereportedhysteresisonelectro-opticalcharacteristics and response time in gray scaleof device using the composite displaying amoving image.8)-10)
As to the phase-separated structure for abicontinuous phase type composite, theformation process in theoretical aspect has
been precisely investigated by Kyu et al and
Nakazawa et al. after the early study by Hirai
et al.11)-13) The effects of the polymerization
condition such as temperature, intensity oflight irradiation and so on in photopolymerizationinduced-phaseseparation
process on the phase structure have beenstudied by Li et al. and the author et al.14) On
the other hand, study in real space by anexperimental method on the formation processof the phase structure for practical liquid
crystal /polymer composites showing a low
driving voltage has been limited thoughseveral studies through analysis in Fourier
space such as diffraction and scattering
properties have been reported.15),16) Phase
separation process was closely examined inthis study through direct observation of thephasestructureandelectro-opticalcharacteristics for the composites with atransient phase-separated structure obtainedby interrupting of UV light irradiation forphoto-polymerization after a predetermined
period regarding bicontinuous phase typeliquid crystal/polymer composite obtained byphoto-polymerizationinduced-phaseseparation method showing a driving voltageof 10 Vrms or less. This is the first study on
direct observation in real space for a phasestructure change of the liquid crystal/polymer

composite during polymerization process by anon-destructive method.

2.
2.1

Experiment
Preparation of liquid crystal/polymer
composites

Fig.1 shows the liquid crystal material, theprepolymer and the photo-initiator used in thisstudy. The nematic liquid crystal mixture (LC)was used for the liquid crystal material andthe prepolymer (P-A) was prepared as ahomogeneous mixture in composition fraction
shown in Table 1 which comprises one
oligomer having two acryloyloxy groups andtwo monomers having one acryloyloxy group.
Urethane acrylate was used as a reactive
oligomer, which was synthesized in a presenceof a tin catalyst from 2 mol of
2-hydroxyethylacrylate and 1 mol of isocyanate
terminated urethane oligomer synthesized in apresence of the same catalyst from 2 mol of
isophorone diisocyanate and 1 mol ofpoly(propylene glycol) having number averagemolecular weight of about 1000. The monomer
A was a dehydrated compound from acrylicacid and mixed branched isomer ofmonohydroxy alcohol of which the carbon
number was 13 (Tridecanol [mixed branchedisomer], Tokyo Kasei Co., Ltd.) and
2-hydroxyethylacrylate was used for themonomer B. Uncured pre-mixture (M-A) was
prepared as a homogeneous solution in whichbenzoin isopropyl ether (BIPE) of 3 wt% to P-A
mentioned
above
was
added
as
a
photo-initiator into isotropic solution in which
P-A was completely dissolved into liquid
crystal in a predetermined amount by heating.

nematic liquid crystal material
Tn-i = 84 ℃, Δn = 0.218, Δε = 12.0, η = 25 mPa･s at 25 ℃

oligomermonomer A

monomer B

photo-Initiator

Fig.1 Physical property of liquid crystal material andchemical structures of prepolymer and photo-initiator.

平成 24 年度

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Table 1 Composition of prepolymer mixture (wt%)

Oligomer

Monomer A

Monomer B

51.2

12.2

36.6

Secondly, glass cells used for evaluation ofelectro-optical characteristics were prepared insuch a procedure hereinafter. A pair of glass
substrates on which transparent electrodes
made of ITO were formed was laminatedfacing each electrode, having distance between
those substrates of 13 μm by resin beads
spacers and being sealed by epoxy resin in
peripheral portion of the substrate. M-A
comprising 65 wt% of nematic liquid crystal
was injected into the cell after the M-A washeated to be a homogeneous isotropic solution.
P-A in the M-A was polymerized by BIPE
during of UV light irradiation for apredetermined period having intensity of 10
mW/cm2 on both sides of the cell from HgXe
lamp having main wavelength of 365 nm after
cooling the cell to be at 0.5 °C higher
temperature than that at which the liquid
crystal segregated from the isotropic solutionof the M-A during decreasing temperature inthermostatic chamber, and the liquid
crystal/polymer composite was obtained from
the M-A by photo-polymerization inducedphase separation.
2.2

2.3

Observation for phase structure

The phase structure of the liquid crystal/polymer composite in the cell was observed ina non-destructive method (Observation
method A) at 100 °C which was higher
temperature than that for the nematicisotropic transition point of the liquid crystal
material used for the composite bytransmission bright field optical microscopewith a thermal stage.

3

Secondly, the liquid crystal was extracted
from the composite by bicyclohexyl as an
extractant via cutting plane of the glass
substrate and the composite in the cell during
immersion in it at 40 °C for 60 min. The
polymer structure was observed (Observation
method B) at room temperature by atransmission
opticalmicroscope
after
evaporation of bicyclohexyl in 266 Pa at 80 °C
for 60 min.
2.4

Electro-optical characteristics

Electro-optical characteristics of the liquid
crystal/polymer composite cell were evaluatedat 40 °C in a thermostatic chamber by
Schlieren optics having collection angle of 5
degrees shown in Fig.2. The intensity of thetransmitted light through the cell wasmeasured by a photo-diode with band-passfilter having center wavelength of 540 nm andhalf-band width of 20 nm using a halogen lamp
as an incident light source. The transmittance
was measured on applying ac voltage of 30 Hz
in rectangular waveform between thetransparent electrodes of the cell with avoltage increasing rate of 10 Vrms/min using afunctional generator (3114A, Hewlett-PackardCo.) and a high speed amplifier (4010, NF
Corp.) as a power supply for driving.

MeasuredcellLens 2

Measurement of temperature for nematic-isotropic transition point of liquid crystal
phase in transient liquid crystal/polymer
composites

The temperature for the nematic–isotropictransition point of the liquid crystal phase in
the composite cell was measured by observing
the cell sandwiched between a pair of crossed
polarizers using bright-field transmission
optical microscope (BH-2, Olympus Co., Ltd.)
while heating the cell at a heating rate of
1 °C/min by a thermal stage (FP82HT,
Mettler-Toledo Inc.).

第 34 巻 第 2 号

Lens 1

Light source(halogen lamp)

Detector
Lens 3(Photodiode)

Aperture( 5°)Band-path
f ilter

Driving circuit

Fig.2 Schematic representation of the measurement
system for electro-optical properties of liquid crystal/polymer composite cells using Schlieren optical system.

3.

Results and discussion

3.1

Phase diagram of liquid crystal andprepolymer

It is very important to evaluate a phase
diagram for a formation of phase-separation
structure because the optimum polymerization
temperature is determined based on it inpolymerization induced-phase separation.
Fig.3 shows a phase separation temperature atwhich M-A becomes turbid by segregation of

4Transient phase structure of LC/polymer composite

the liquid crystal from M-A in decreasing
temperature and a phase mixing temperature
at which the M-A becomes homogeneous state
by dissolution of the liquid crystal in M-A in
increasing temperature. The intensity of thetransmitted light from He-Ne laser as a lightsource through M-A in clear bottle wasmeasured by a photo-diode for M-A being
stirred by magnetic stirrer installed inthermobath, which was controlled intemperature increasing and decreasing rate of
1 °C/min. The segregation temperature (Ts)and the dissolution temperature (Td)correspond to temperatures at which M-Astarts to become turbid in decreasing
temperature and at which the whole of M-A
becomestransparent
inincreasing
temperature, respectively.

Temperature (℃ )

90

80
70
60

100
100
dissolution

80
80

50

segregation
40

60
60

1

2

3

5

10

20 30

50

100

40
40
Fig.4 Dependence of UV irradiation period for photopolymerization on Tn-i of LC in LC/polymer composite.
() shows Tn-i of LC just after polymerization,()
shows Tn-i of LC after 12h from polymerization.

20
20
00
0
0

20
20

40
40

60
60

80
80

100
100

LC contents (wt%)
Fig.3 Phase diagram of M-A (LC/P-A/BIPE). Solid line
shows segregation temperature on cooling and broken
line shows dissolution temperature on heating.

It was found that M-A comprising liquid
crystal of 65 wt% was positioned in the phase
diagram so as to show a segregation
phenomenon of liquid crystal from P-A since
the temperature difference between Ts and Td
was small. Besides, polymerization inducedphase separation at a temperature higher than
Ts and near Ts is required for obtaining thecomposite with uniform morphology from M-A.
3.2

liquid crystal of 65 wt% with UV lightirradiation of 10 mW/cm2 from each side of thecells at Ts+0.5 °C in decreasing temperature.
Fig.4 shows the temperatures for thenematic-isotropic transition point of the liquid
crystal phase in the composites obtained byvarious periods of UV light irradiation. The
temperature was measured just after thecompositewasobtainedbyphotopolymerization induced-phase separation andafter leaving it at room temperature for 12 h.

Temperature for nematic-isotropic transition
point of liquid crystal phase in liquid crystal/polymer composites

The liquid crystal/polymer composite cells
were prepared by photo-polymerizationinduced-phase separation of M-A comprising

It was suggested that a phase-separated
structure was formed by UV light irradiationof 1s and more since the composite cells wereobtained so as to show a turbid appearance byscattering incident light uniformly. As shownin Fig.4, Tn-i of the liquid crystal phase in thecomposite obtained by short period of UV lightirradiation was low and became higher
according to increase of the period until
irradiation period of around 30s in which Tn-i
showed a saturated temperature. It isreasonable to assume that prepolymer of P-A
remained in the liquid crystal phase led to thelowering of Tn-i in the early stage ofpolymerization induced-phase separation andthe progress of polymerization by duration of
UV light irradiation decreased the residual
prepolymer and raised Tn-i. Even saturation
temperature for Tn-i of the liquid crystal phase
was lower by about 4 °C than that of liquid
crystal material used for M-A. It wasconsidered that the lowering was probably
caused by residue of a part of P-A or平成 24 年度

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degradation fragments of a part of liquid
crystal components deteriorated by UV light in
the liquid crystal phase.
Besides, there was no change in Tn-i
between just after polymerization inducedphase separation and after leaving at room
temperature for 12h. This result clearly showsthat a transient structure of the composite
underpolymerizationinduced-phaseseparation process can be evaluated byobservation and measurement for thecomposite cell within 12h from preparation of
the cell with various periods of UV lightirradiation because the photo-polymerizationinduced-phase separation can be frozen byinterrupting UV light irradiation for photopolymerization.
3.3

Observation for morphology of liquid crystal/polymer composites

Figs.5 (a) and (b) show microscope images by“Observation method A” for the liquid crystal/polymer composites obtained by photopolymerization induced-phase separation with
UV light irradiation period of 2s and 64s,
respectively. The author et al. have discoveredthat the network structure of the polymer
phase in the composite can be easily observedby bright-field transmission optical microscopy
above a temperature in which liquid crystal
phase in the composite shows isotropic phase.
The pore sizes of the polymer networks whichwere shown as bright portions in Fig.5 (a) and(b) were found to be almost the same. Slightly
thicker network branches were observed in thecomposite with irradiation period of 64s. It was
confirmed by Fig.5 (a) and (c) that a threedimensional polymer network was formed with
UV light irradiation period of only 2s. Fig.5 (c)shows transmission optical microscope image(a)

第 34 巻 第 2 号

5by “Observation method B” for the composite
after the liquid crystal was extracted. The
images in round shape shown in nearly centerof Fig.5 (a) and (b) were resin spacers whichwere used for adjusting the distance between apair of electrodes in the cells.
3.4Electro-optical characteristics for liquid
crystal/polymer composites

Fig.6 shows optical transmittance on
increasing applied voltage for the liquid
crystal/polymer composite cells obtained byvarious UV light irradiation periods. The
composite showing transparent-turbid (lightscattering) switching by an applying andremoving voltage was obtained by UV lightirradiation period of only 1s. The thresholdvoltage at which the scattering state changed
into transparent state increased rapidly
between irradiation period of 1s and that of 4s.
The change of electro-optical characteristics
which converged after irradiation period of 30s
accorded well with the change of Tn-i shown in
Fig.4.
A change in transmittance of the composite
is caused by a change in the alignment ofliquid crystal molecules in the liquid crystal
domain held in the three-dimensional networkof polymer phase. A larger size of liquid crystal
domain is believed to lead to a lower thresholdvoltage because a larger domain has relatively
small interface between liquid crystal andpolymer. It is suggested that liquid crystal
phase comprised a part of prepolymer of P-A inthe early stage of phase separation in the
aspect of Tn-i profile. It might result in thelower threshold voltage that a substantial sizeof the liquid crystal domain in the early stage
was larger than that in the final stage of phaseseparation by prepolymer in liquid crystal

(b)

(c)

20μm
Fig.5 Optical microscope images of LC/polymer composite photo-polymerized with UV irradiation of 2s: (a) / (c), 64s: (b). Images (a)and (b) were observed at 100 ℃ ( > Tn-i of LC). Image (c) was observed after extraction of LC by bicyclohexyl.

6Transient phase structure of LC/polymer composite

Transmittance (%)

phase. It was also indicated it that thinner
network branches of polymer phase wereobserved in the composite by UV lightirradiation period of 2s in comparison with
irradiation period of 64s as shown in Fig.5.

100
80
32s/64s

4s

60
8s

40

1s

the polymer rich phase comprising liquid
crystal just after phase separation. It isconsidered that the composite just after phaseseparation has a phase structure in which a
continuous liquid crystal phase comprising
prepolymer, which shows a low Tn-i, is held in
a continuous polymer network having slightly
thin polymer branches, and the liquid crystal
phase is divided into multiple liquid crystal
domains having a rather large diameter incomparison with that in the final stage ofphase separation.

16s
2s

20
0
0

2

4

6

8

10

Applied voltage (Vrms)
Fig.6
Electro-optical properties of LC/polymer
composites obtained by photo-polymerization inducedphase separation with various periods of UV irradiation(2s/4s/8s/16s/32s/64s), which are indicated by arrows in
the figure.

On the other hand, the slope of the V-T curve,that is, the change in the transmittance versus
applied voltage (T/Vrms) became larger
between UV light irradiation periods of 4s andthat of 30s with almost no change of thethreshold voltage. It might lead to an increaseof T/Vrms that dielectric anisotropy of theliquid crystal domain was restored to theoriginal value of the liquid crystal material by
purification of liquid crystal phase with
progress of polymerization induced-phaseseparation.
3.5

phase Aphase BLC comprising prepolymer

Polymer swelling LC

Fig.7
Schematic diagram of proposed model for
structure of LC/polymer composite obtained by photopolymerization induced-phase separation in the earlystage of phase separation.

Schematic diagram for phase separation ofliquid crystal/polymer composites

Fig.7 shows a schematic diagram ofestimated composite structure in the earlystage of photo-polymerization induced-phaseseparation. This model is based on theexperimental examinations of the change for
Tn-i of liquid crystal phase in the composite
during phase separation process, theobservation of the polymer phase and thechange for the electro-optical characteristics.
Basic structure of a three-dimensional network
and its correlation length in the polymer phase
is assumed to be determined by a freezing ofconcentration fluctuation between the liquid
crystal rich phase comprising prepolymer andAlmost pure liquid crystal
Fig.8
Schematic diagram of proposed model for
structure of LC/polymer composite obtained by photopolymerization induced-phase separation in the laststage of phase separation.

With the progress of polymerization inducedphase separation, the liquid crystal phase is平成 24 年度

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purified by decrease of prepolymer remained in
the liquid crystal phase according to theprogress of polymerization reaction. The
purification of liquid crystal phase leads to anincrease in Tn-i and thicker branches in thepolymer phase. Fig.8 shows a schematic
diagram of estimated composite structure in
the last stage of photo-polymerization inducedphase separation. The liquid crystal phasebecomes almost pure liquid crystal andpolymer network with thick branches isformed as shown in this model.
The change of electro-optical characteristics
during phase separation process is wellunderstood by this proposed model regarding
formation process of the phase structure. It isconsidered that change of those electro-opticalcharacteristics is comparatively small because
the pore sizes of polymer network have been
already
determined
just
after
phase
separation.

4.

Summary and Conclusion

The liquid crystal/polymer composites with atransient phase-separated structure wereobtained by interrupting UV light irradiationforphoto-polymerizationafterapredetermined period during the photopolymerizationinduced-phaseseparation
process in this study for formation by photopolymerizationinduced-phaseseparation
regarding the composites consisting of abicontinuous phase of nematic liquid crystal
and a three-dimensional polymer network.
The phase-separated structure showing
transparent and scattering by an applied
voltage was obtained by UV light irradiation
period of only 1s. Tn-i of liquid crystal phase of
the composite became higher with the progressof polymerization induced-phase separation
and reached into a steady temperature after
UV light irradiation period of around 30s. Low
threshold voltage was observed in the earlystage of phase separation and the thresholdvoltage also became higher according topolymerization. UV light irradiation period in
which
the
changeofelectro-opticalcharacteristics converged almost accorded withthe change of Tn-i. Almost the same basic
structure of polymer network in the polymer
phase was observed both in the early stage andthe final stage of polymerization inducedphase separation by a transmission optical
microscopy with non-destructive method at atemperature in which the liquid crystal phaseshowed isotropic phase.

第 34 巻 第 2 号

7The
transient
structureanditscharacteristics in the liquid crystal/polymer
composite are well understood by the proposedmodel regarding formation of phase structure,in which basic structure of polymer network in
the polymer phase is determined by a freezingof concentration fluctuation between the liquid
crystal phase comprising prepolymer and thepolymer phase comprising liquid crystal justafter phase separation, and the liquid crystal
phase is purified with the progress ofpolymerization induced-phase separation.
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