clay mineral composites

MATERIALS SCIENCE & ENGINEERING Materials Science and Engineering C 6 (1998) 135-143

ELSEVIER

t2

Nematic liquid crystal/clay mineral composites Masaya Kawasumi *, Naoki Hasegawa, Arimitsu Usuki, Akane Okada Toyota Central Research and Development Laboratories, Nagakute-cho, Aichi, 480-1192, Japan

Received 14 March 1998; accepted6 August 1998

Abstract

NoveI liquid crystalline composites composed of a nematic low molar mass liquid crystal and a few percents of organized clay minerals have been prepared. The affinity of the organized clay minerals for the liquid crystal, which was evaluated by measuring their contact angles, was highly dependent on the kind of organic ammonium cations used for the organization of the clay. In the case of an organized clay having good affinity for the liquid crystal, the organized clay was dispersed homogeneously in the liquid crystal in the size of several micron meters. The composites exhibited so-called multidomain textures composed of a few micron meters domains of the liquid crystal. The composites were sandwiched between transparent conductive ITO coated glasses with 12 p~m polymer beads as a spacer and their electro-optical properties were measured. The composite cells exhibited a bistable and reversible electro-optical effect between a light scattering state and transparent state. Both the states including intermediate states between them could be selected by changing the frequency and voltage of applied electric fields. The memory effect e~bited by the composites based on the nematic liquid crystal is rather unusuaI since most of conventional nematic liquid crystals do not exhibit such a memory effect due to their low viscosity. The memory mechanism exhibited by the composites was proposed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Nematic liquid crystal; Clay mineral composites;Electric fields

1. Introduction

In recent years, organic-inorganic nanometer-composites have attracted great interest to researchers since they frequently exhibit unexpected hybrid properties derived from the two components [1-11] [12-15]. One of the promising composite systems would be hybrids based on organic polymers and inorganic clay minerals consisting of layered silicates [4-15]. In our previous works, we have synthesized nylon 6-clay hybrids (NCH) in which 10A thick silicate layers of clay minerals are dispersed homogeneously in the nylon 6 matrix [5]. The NCH exhibits various superior properties such as high strength, high modulus, high heat distortion temperature, compared to nylon 6 [6]. The most characteristic feature of the NCH is that the drastic change in these properties could be derived with few percents of the clays. The NCH was classified into biomimetic composites by Mark and Calvert [16] since the structure and process of the NCH resemble some of the biological materials such as inner surfaces of shells. It is quite interesting that there are some similarities

Corresponding author.

between the NCH and one of the ultimate structures made by long time biological evolution. The same concept as the NCH has been applied for various polymer systems such as polyimide [7], epoxy resin [8-11], polystyrene [8-13], polycaprolactone [14], acrylic polymer [8-11,15] to date. The most recent advance in this technique has made it possible to prepare hybrid materials with even nonpolar polymers such as polypropylene (PP). PP-clay hybrid was accomplished by mixing three components, i.e., PP, organized clay mineral, maleic anhydride modified PP oligomers as a compatibilizer [17]. The examples indicated above are basically classified into organic polymer-clay hybrids which are aimed at high performance structural polymer materials with high strength, high modulus, high barrier properties. This hybrid technique should be able to be applied not only for such polymer systems but Nso various types of materials including low molar mass compounds. This paper presents one of the unique attempts to obtain novel hybrid materials based on a low molar mass liquid crystal and organized clay mineral (abbreviated as LCC-X-Y; Liquid Crystal/Clay Composite; X indicates the kind of an organized clay while Y indicates the weight percent of it). First of all, we tried to select the organized clays having a good

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136

zig. Kawasumi et aI. / Materials Science and Engineering C 5 (1998) 135-143

affinity for the liquid crystals by measuring the contact angle between them and observing the stability of the LCCs. Although the clay layers in the obtained LCC based on the selected best organized clay were not perfectly exfoliated yet, the LCCs exhibited unusually stable electro-optical memory effect which is not observed in conventional nematic liquid crystal systems.

>,

- ---'t-,k

0.. 0

=

\


-1

RegimeI fo

-3

Table 1 Physicalpropertiesof two-frequency-addressingliquid crystallinemixture used in the experiments Name DF-05XX Nematic-smectictransitiontemperature(°C) <0 Isotropic-nematictransitiontemperature(°C) i14.6 Viscosity(20°C) (mPa.s) 41.1 Refractiveindices n~ 1.650 no 1.502 An 0.148

Regime III

\

-2

2. E x p e r i m e n t a l

The materials used for the preparation of LCCs are nematic two-frequency-addressing liquid crystal (abbreviated as TFALC, DF-05XX) purchased from Chisso and purified montmoriltonite (Kunipia-F) from Kunimine. The physical properties of TFALC are listed in Table 1. TFALC is a mixture of low molar mass liquid crystals with regimes of positive dielectric anisotropy as well as negative dielectric anisotropy (Fig. 1). Therefore, the alignment of TFALC molecule can be controlled by changing the frequency of applied electric field. Also, TFALC with a slight amount of salts exhibits a dynamic light scattering (DS) effect when subjected to an electric field whose frequency is near its crossover point (f0) or slightly higher than f0 (Regime II in Fig. 1). This behavior was used for the reversible switching of LCCs at DS mode. Montmorillonite is a layered clay mineral composed of aluminosilicates [(OH)4Sis(A13,34Mgo.66)O20-Na0.~6]. The dimensions of the unit aluminosilicate layer are only 10 in thickness, about 1000 A in width and in length. It has exchangeable cations between the layers, which are normally sodium cations. In this experiment, the montmorillonite exchanged with various organic ammonium cations including 4-cyano-(4'-biphenyloxy)undecyl anamonium cation (CBllM, Scheme 1) was used to enhance the miscibility between the montmorillonite and TFALC. Table 2 shows the clays exchanged with various types of organic ammonium cations in this experiment. The synthetic procedure of 4-(11-aminoundecyloxy)-4'-cyanobiphenyl (the organic amine used for C B l l M ) was reported previously [18]. The synthetic procedure of the other organic amines and ammoniums will be published elsewhere [19].

\ \

°

2.1. Materials

Regime II

0 .............................................

:)"OO

i

I

l

2

3

4

5

Log Frequency (Hz) Fig. 1. Dielectricanisotropyof two-frequency-addressingnematio hquid crystal (TFALC)as a functionof logarithmof frequency.

Tetra-n-butylammonium bromide (TBAB), toluene, and N,N-dimethylformamide (DMF) from Wako Pure Chemical Industries, N,N-dimethylacetamide (DMAc) from Tokyo Chemical Industry were used as received. 2.2. The preparation o f montmorillonites exchanged with organic ammoniums

All the organized clays were prepared by the same procedure. The typical procedure is as follows: sodium montmorillonite (2.00 g, cation exchange capacity: 2.38 meq.) was dispersed into 70 ml of hot water (about 50°C). 4-(11-Aminoundecyloxy)-4'-cyanobiphenyl (0.955 g, 2.62 mmol) and conc. hydrochloric acid (0.28 g) were dissolved into hot ethanol and water mixture (40 ml:10 ml). It was poured into the hot montmorillonite-water solution under

o

+NH 3

o

+ NH / 3

0

+,NH3 r/

'x

o-

04_ 0

%" kvO± 0

/, ~ I,,~I, sO

Oa_

O k

Aluminosilicate /ayer of montmoriflonite

Scheme 1. Schematic representationof the clay mineral orgardzed by 4-cyano-(4'-biphenyloxy)undecylammoniumcation.

137

M. Kawasumi et at./Materials Science and Engineering C d (1998) 135-143

Table 2 The organized clay minerals used in the experiments

Abbreviation of the organized clay mineral

Organic amine or ammonium used for the organization of the clay

Interlayer distance

The content of inorganic part (wt.%)

d(001) (A)

calculated measured value a value

8M

CH3-(CH2)7-NH2

13.7

12M

CH3-(CH2)l-NH2

16.5

18M

CH3-(CH2)17-NH2

86.3

COS ®b (TFALC)

80.6

0.930

81.4

81.7

0.839

30.4

74.6

55.9

0.602

32.8

59.7

54.2

0.607

O- (CH2)4-NH2

18.1

75.4

75.2

0.989

(CH2)I£NH2

20.4

69.1

70.5

0.989

23.5

62.4

65.7

0.993

51.5

54.5

57.8

58.8

+

CH3-(CH2)17'N(CH3)2

DSDM

2

CB4M

N

CBIIM

N C ~ O -

CB 16M

NC--@@-

C

~

O- (CH2)~6-N(CH2CH3)2

5-2(PE6)M

CH3(CH2)'C'~"COO- (CH2)CO O C O COO"(CH2)6"N(CH2CH3)2

5-4(PE6)M

CH~(CH2),O COO-(CH2,~OOCOCOC~ {CH2)~-N(CH2CH,)2 L

1

3

C

C

42.8

c

0.954

0.880

aThe calculated inorganic contents were obtained assuming that all the ions in the clay were exchanged. bOS are contact angles of TFALC on surfaces of the organized clays. CThe values could not be measured due to the shortage of the sample amounts.

vigorous stirring to yield white precipitates. After 3 h, the precipitates were collected on a glass filter, washed with ethanol and two times with hot water, and freeze-dried to yield a montmorillonite exchanged with 4-(4'-cyanobiphenyl-4-oxy)undecyl anm~onium (CB 11M, Scheme 1, the content of the inorganic part, 70.5 wt.%).

2.3. The preparation and contact angle measurement of thin fihns of the organized clays The organized clay minerals were dispersed into organic solvent such as DMF or toluene to prepare 3 wt.% solution. DMF was used for 8 M, 12 M, CB4 M, CBllM, CB16M, 2(CBlt)M, 5-2(PE6)M, and 5-4(PE6)M while toluene was used for 18 M and DSDM. The solutions were

spin-coated on slide glasses by using a photoresist spinner Model K-359SD-1 manufactured by Kyowa Riken. The obtained films were dried in vacuo and used for the measurement of contact angles. The contact angles (O) of TFALC on the surfaces of the organized clays were measured by using a contact angle meter CA-A (Kyowa) at 23°C (the angles were measured 1 rain after the drop of TFALC). The contact angles were obtained by observing contact angles between TFALC droplets and thin films of organized clays.

2.4. The preparation of LCCs The LCCs based on TFALC and organized clay minerals were prepared by the same method.

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M. Kawasumi et aL/Materials Science and Engineering C 6 (1998) 135-143

The typical example is as follows: C B l l M (0.0181 g) was dispersed into DMAc and mixed with 1.0 g of TFALC and 0.200 g of TBAB/DMAc solution (TBAB/DMAC = 1.02 × 10 . 4 g/g). DMAc was evaporated in vacuo at 50°C for 10 h and the obtained composite was mechanically stirred. It was dried in vacuo at 50°C for 10 h to yield a white pasty composite (LCC-CBllM-1.25, the content of the inorganic part, 1.25 wt.%). For only C B l l M , six other LCCs with different contents of the inorganic parts (LCC-CBllM-0.20, LCCCBllM-0.50, LCC-CBllM-1.00, LCC-CBllM-1.50, LCC-CB11M-1.75. and LCC-CB11M-2.00; the contents of the inorganic parts were 0.2 wt.%, 0.5 wt.%, 1.00 wt.%, 1.50 wt.%, 1.75 wt.% and 2.00 wt.%, respectively) were prepared by the same method.

case, the microscope was used without polarizers). These were recorded on a chart recorder. The commercial power supply (100 V-60 Hz, sign wave) with voltage adjusted using a Slidac was used for applying the low frequency electric field (60 Hz) to the cell. A power amplifier Model S-4750 manufactured by the NF Electronic Instruments was used for applying the high frequency electric field (1-1.5 kHz). The light transmittance of the cell[ was calculated according to Eq. (1). LightTransmittance(%) 100× (PhotomultiplierValue of SampleCell) (1) = (PhotomultiplierValue of Blank Cell Sealedwith Water) All measurements were carried out at room temperature.

2.5. Techniques

3. Results and discussion The interlayer distances of the organized clays were measured by X-ray diffraction by using a X-ray diffractometer manufactured by Rigaku. Their inorganic contents were calculated by measuring the weights before and after burning their organic parts. An Olympus Model BHSP optical polarizing microscope (magnification, 500 × ) equipped with a Mettler FP-82HT hot stage and Mettler FP-90 central processor was used to analyze anisotropic textures. Fig. 2 shows a schematic representation of the system set up for optical measurements of the LCC cells. The substrates of the cells were 1.1 mm thick glass plates coated with a thin transparent conductive In/SnO 2 (ITO) film to allow the application of electric fields across the LCCs. The separation of the substrates was achieved by sandwiching the two plates with polymer beads spacer (diameter: 12 Ixm), thus defining the thickness of the LCCs. The electro-optical effect was measured by using the BHSP microscope equipped with a photomultiplier to monitor the optical change in the cell accompanying the application of electric fields across the samples (in this

Chart Recorder

Lens .......... ~

i i ~

Photomuttiplier LCC

. LCCCell

. . . . . . . . .

Lens .......... ~

~

(~

"""

Glass Substrate "'..,, Coated with ITO Power Supplier Light Source

Fig. 2. Schematic representation of the system set up for the optical measurementsof LCC cells.

3.1. The organized clay minerals with various organic compounds

Table 2 presents the organized clay minerals used in this experiment, their interlayer distances, the contents of inorganic parts, and COS @ calculated from the contact angles (O) of TFALC on the surfaces of the clays. The value of COS O was used for the evaluation of the affinity of the organized clays for TFALC. If COS O is close to unity, the organized clay should have high affinity and should homogeneously disperse in TFALC. On the other hand, if COS O is low, the affinity may be too low to obtain well dispersed LCCs. The organic compounds are alkyl amines with w~rinus chain length, and alkyl amines and ammoniums with phenyl or biphenyl rings, which are similar structures to conventional liquid crystal molecules. The interlayer distances and inorganic contents of the organized clays clearly indicate almost complete intercalation of the organic amines and ammoniums into the clay. The value of COS O is dependent on the kind of the intercalated organic compounds. In the case of alkyl amines, COS O increases with decreasing their alkyl chain length. On the other hand, in the case of cyanobiphenyl atkyl amines, the length of their alkyl chains do not affect so much on COS O. The: high values of COS (9 were obtained for the clays orgaaaized with cyanobiphenyl alkyl amines. This means that these organized clays should have good affinity for TFALC and disperse in it well. 3.2. The preparation of LCCs

The LCCs were prepared by using various types of the organized clays listed in Table 2. However, the dispersibility of the clays in TFALC is highly dependent on the: kind of organic ammoniums exchanged. The composites should

I39

M. Kawasumi et al. / Materials Science and Engineering C 6 (1998) 1 3 5 - 1 4 3

Table 3 Qualitative evaluation of viscosity and stability of LCC-Xs Samples

COS O (TFALC)

Viscosity a

Stability of dispersion~

Stability of dispersion under the application of electric field (50 V-60 Hz)~

LCC-8M LCC- 12M LCC- I8M LCC-DSDM LCC-CB4M LCC-CB 11M LCC-CB 16M LCC-2(CB 11)M LCC-C5PEC6EPEC6M LCC-C5PE-3(C6EPEC6)M

0.930 0.839 0.602 0.607 0.989 0.989 0.993 0.992 0.954 0.880

high medium low medium high high high high medium medium

O × × X zx © a, zx × x

× × × × O O A O

aLow: the LCC flowed when the sample was tilted; medium: the LCCs flowed very slowly when the samples were titted, high: the LCCs did not flow. u©: no sedimentation or phase separation of the clays in the LCCs; /, : sedimentation or phase separation occurred slightly; × : sedimentation or phase separation occurred. ~O: no aggregation of the clays under 50 V-60 Hz of electric field; zx: aggregation of the clays occurred slowly; × : the clays aggregate each other very quickly.

be stable e n o u g h against sedimentation or phase separation for practical applications. The LCCs were tested qualitatively in terms of viscosity, stability without an electric field (the sample was left for a few days) and under an electric field. The results are listed in Table 3 with the value of COS O. As seen from Table 3, the LCCs based on the organized clays with relatively high COS @, m e a n i n g that the affinity of the clay surfaces for T F A L C is relatively high,

exhibited relatively high viscosity, high stability with and without the electric field. This m a y be simply due to the fact that high affinity b e t w e e n the two components stabilized the dispersion of the LCCs. However, more detailed experiments should be done to correlates the structures of exchanged cations and the stability of LCCs. The best stabilized LCCs were obtained in the case of CB 11M as an organized clay even though their dispersibility was not perfect yet as discussed in the following section. All the

20 ,urn

?-q

Fig. 3. Optical polarized micrographs of LCC-CB1 IM-1.25 and TFALC (crossed nicol, the thickness of the sample is 12 ~xm): (a) isotropic state of the LCC, (,b) nematic state of the LCC, (c) nematic state of TFALC.

140

M. Kawasumi et at./Materials Science and Engineering C 6 (1998) 235-]43

100

kHz OFF).

matic state (b). In the isotropic state, only the partict~es of C B l l M were observed as white spots which were dispersed uniformly in the LCC. Their sizes are roughly estimated to be several micron meters. They are considered to be the aggregates of unit layers of C B l l M . Although CB 1 tM was one of the best organized clays in Table 1, the affinity of the organized clay is not high enough to achieve an exfoliated hybrid state. On the other hand, the very fine texture of the LCC was observed in the nematic state. The texture is composed of very small domains of TFALC (multi-domain structure). Since pure TFALC does not exhibit such a fine texture (Fig. 3c), C B l l M induced the multi-domain texture in the LCC, Most probably, the dispersed CB11M particles increase the defect density in TFALC since the impenetrable particle surfaces are randomly distributed throughout the bulk.

other experiments were performed with the LCCs based on CB 11M (LCC-CB 11M).

3.4. Electro-optical properties of the LCC-CBllM in DS mode

3+3. Texture of LCC-CBllM

The LCC-CB11M-1.25 cell displayed a reversible and bistable electro-optical effect based on light scattering which could be controlled by changing the frequency of an electric field. Fig. 4 shows a typical change in the light transmittance of the LCC-CB11M-1.25 cell (bold lit,e) as

r

© O r-

I I ]

Memory State I

I I I

E e-

ION/[Stat~lll

l-c-

._~

._a "~

0.)

~ ~_#> ~:Z. O ~

<>

Memory State II

1

0 100

~-

i60Hz +

0

I

5 sec

I

I .SkHz

I--[

t

Fig. 4. The changes in the transmittances of LCC-CB11M-1.25 cell and TFALC cell (broken fine) (cell gap: 12 F~m) by two frequency driving (driving sequence: 60 Hz-50 V ON, 60 Hz OFF, 1.5 kHz-100 V ON, 1.5

The obtained LCC-CBllMs were white pasty liquids. Fig. 3 shows typical polarized micrographs of the obtained LCC-CBllM-1.25 in an isotropic state (a) and in a ne-

(a) ~ m o i ~ u i a ~

l~g+uidc r , s t a l ~ t ' h e

(c)

~ ~ : ~ !n+.tl~e.~el!9~l&e~gio~ the .passes over to an isotrgpic mel~_~itae h~ ~ t u r e that scatters light is formed l l ~ ~ . A n such a way some informati]lli ~l?~tbe transparent film. This infg~a-t~ V~ct~ on a screen but may be++:-+~?pdl ~ ~ i c field or bv+a++tem~-+~ ,ill or - m o t ~ u l a r li uid cr tals ~'¢~ ' u t ~ [ + ~ ho~¢otropicatly orienled ~C rx

(d)

ions of iocal overhea~ are'c ~o[!~ser ~am, In these local regions the ~ ) s e s over to an isotropic melt and the h iion:~i ~siroyed (Figure lb). !nstea ~~dn~d~rna~n +"borneottopic t e x t ~ ~iure that scatters light is formed ! ~ ! ~ ' 9 . ! n such a way some i n f o r m a t ~ ~ & ~ transparent film. This infg/l~aa~'~! ~or~ a screen but may bei V d| Fig+ 5. Photographs of the LCC-CBI1M-1.25 cell (cell gap: 12 mm) at various states: (a) on-state I (50 V-60 Hz), (b) memory state I (60 Hz off), (c) on-state H (100 V - I . 5 kHz), (d) memory state II (1.5 kHz off).

141

M. Kawasumi e~ aI. / Materials Science and Engineering C d (2998) ]35-143

well as of a TFALC cell (broken line) when the cells were subjected to electric fields. Fig. 5 presents the photographs of the LCC-CBllM-1.25 cell at various states. When a low frequency electric field (60 H z - 5 0 V, Regime I in Fig. 1) was applied to the cell, it became transparent within 50 ms (on-state I in Figs. 4 and 5a). Even after the electric field was switched off, although small decrease in the transparency was observed, the transparent state was maintained (memory state I in Figs. 4 and 5b). When a high frequency electric field (1.5 kHz-100 V, Regime II in Fig. 1) was applied to the cell, the memory state I was cancelled out to return to the light scattering state (on-state II in Figs. 4 and 5c) within 50 ms. Again, although small increase in the transparency occurred, the light scattering state was maintained without electric field (memory state II in Figs. 4 and 5d). On the other hand, as seen from Fig. 4, TFALC cell (without the organized clay) did not exhibit such a memory effect at all. The light transmittances of the memory states I and II of LCC-CB11M-1.25 did not change after 15 h. Although the LCC is based on a nematic liquid crystal, it exhibited unusual stability of memory states. Also, after repeating the above switching sequence a hundred times, almost no changes in the transmittances of all the states from those of the initial states were observed. Fig. 6 shows the transmittance of the LCC-CB 11M-1.25 cell as a function of applied voltage. Open circles and closed circles indicate the transmittances of the cell in on-state I and memory state I, respectively (a low frequency electric field (60 Hz) was applied to the cell which had initially been in the memory state II created by applying 1.5 kHz-100 V). On the other hand, open triangles and closed triangles indicate the transmittances of the cell in on-state II and memory state II (a high frequency electric field (1.5 kHz) was applied to the cell in the memory state I created by applying 60 H z - 5 0 V). The transmittances of the cell in on-state I and the memory

(a)

100 On State I 8O

60 E r-. ,< 1-

2O 0

l-'-

l

1.0

60

~

.

¢

40

= =~

30

.

.

.

.

.

On State II

20

"zX~'~'....

lo T

.0

T

0.5

t

1.0

/

Fig. 7. (a) Saturated transmittance of the LCC-CB11M cells in the on-state I (60 Hz ON) and memory state I (60 Hz OFF) as a function of the C B l l M content, (b) saturated transmittance of the LCC cells in the on-state II (1 kHz ON), memory state I (I kHz OFF) as a function of the CBI 1M content (based on an inorganic part).

state I start to increase around 20 Vrms and saturate about 40 Vrms (the equilibrium values of the transmittance of on-state I and memory state I are 87% and 83%, respec-

70

1.5kHz-OFF

Z0

,.

_

2o: - o - o o

~"

50

>

40

O

o/

I

I

I

40

60

80

V90 / / O / 0

30 20

o/

10

@____----0

00'.0

20

I

2.0

Clay Content (%)

> X~

a

1.5

80

IX', /

4O

2.0

Memory State II

50 O

.

60

60

I

1.5

Clay Content (wt.%)

sd

r-

i

0.5

(b)

,.j_, 4.,.0K3_0.0...~..~60Hz-ON

.=_ E 03

I

0.0

lOO

(D 0

Memory State

40

.e

o/

I

* i" / V10 ,

0.5

o/

/ I

1.0

,

I

1.5

r

1

2.0

l

100

Voltage (Vrms) Fig. 6. Transmission properties of the LCC-CB11M-I.25 cells in the on-state I (60 Hz ON), memory state I (60 Hz OFF), on-state 1t (1.5 Hz ON), memory state I (!..5 kHz OFF) as a function of applied voltage.

Clay Content (%) Fig. 8. Plots of Vi0 (the voltage at which the light transmittance becomes the i0% of the maximum value) and Vgo (the voltage at which the light transmittance becomes the 90% of the maximum value) of the LCCCBI IM cells in on-state I (60 Hz) as a function of the CB11M content (based on an inorganic part).

142

M. Kawasumiet al. / Materials Science and Engineering C 5 (J998) 135-143

100

V90

80 >

>

60

/

40

z~/

20

•

O{

,0

•

• ~"i'/ i

i

0.5

1.0

increases abruptly from around 1.0 wt.%, thus decreasing the memory effect. These results indicate the most effective clay content to obtain the highest contrast between memory state I and memory state II is around 1.25 wt.%. Fig. 8 show the plots of Vt0 (the voltage at which the light transmittance becomes the 10% of the maximum value) and I190 (the voltage at which the light transmittance becomes the 90% of the maximum value) of the LCC cells in on-state I against the clay content. Both V]0 and V90 increase with increasing the clay content. The same trend was observed for the plots of V]0 and Vg0 of the LCC cells in on-state II as indicated in Fig. 9.

S

Vlo

~

i

i

r

1.5

2.0

Clay Content (%) Fig. 9. Plots of Vm and Vg0 of the LCC-CB11M cells in on-state II (1 kHz) as a function of the CBllM content (based on an inorganic part).

tively). On the other hand, the transmittances of the cell in on-state II and the memory state II start to decrease around 20 Vrms and saturate over 60 Vrms. (The equilibrium values of the transmittance of on-state II and memory state II are 10% and 17%, respectively.) Any level of the transmittance in the memory states can be achieved by adjusting an applied voltage. 3.5. Dependence of electro-optical properties of the LCC on the clay content In Fig. 7a and b, the transmittances of the LCC cells were plotted as a function of the clay content. Fig. 7a indicates the saturated maximum transmittances of the cells in transparent on-state I (white circle) and memory state I (black circle), while Fig. 7b indicates the saturated minimum transmittances of the cell in opaque on-state II and memory state II. As seen from Fig. 7a, the maximum transmittances of the LCC cells in both the transparent states tend to increase with decreasing the clay content. On the other hand, with decreasing the clay content, the minimum transmittance of the LCC cells in the on-state II increases gradually, while that in the memory state II

3.5. Possible mechanism of the memoo, effect in DS mode Although cholesteric [20] and smectic [21] liquid crystalline materials have been reported to exhibit eptical memory effects, most of nematic liquid crystals do not exhibit such a memory effect due to their low viscosity except for few examples as follows. Polymer dispersed nematic liquid crystals with polyball type morphology have been reported to exhibit a memory effect [22]. In this case, the memory effect might be originated from the anchoring effect of the dispersed polymer. The other examples [23,24] use surface effects to produce memory effect. Our examples is a unique approach adding a memory effect which is different from the above examples. Let us consider the possible mechanism of the memory effect in the LCC. Fig. 10 presents the schematic explanation of the memory mechanism. The clay particles indicate the aggregates of C B l l M in Fig. 10. When a love frequency electric field in Regime I is applied to the cell, not only TFALC but also C B l l M plates align parallel to the electric field (Fig. 10a) as demonstrated by XRD measurements previously [25], and the cell becomes transparent. The mechanism for the CB11M to couple to the applied field has not been clarified yet. However, the shape of the clays should play an important roll to align parallel to the electric field.

I, ,,',,,i~',,I/, ;,,,!,, ,', Electric Field

11 i'll[,,

II,

,11"

,',L;".,'I',;, ~1,

~I~.~

wr

'

I'1

'" l,!, '!,

I~l ,,I ~ ,l'~ll,

'

,.,~I I, ~

~,.'S~..,2~<,,

'l~l~,ll'l'~ I i~ ,1[[ "1~ i !~1 ,,,

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M. Kawasumi et al./Materials Science and Engineering C 6 (1998) 135-143

Even after the electric field is switched off, the oriented plates maintain their orientation due to their bulkiness. As a result, TFALC molecules maintain their homeotropic alignment to produce the memory state I (Fig. 10b). On the other hand, when a high frequency electric field is applied, a turbulent motion of TFALC, which was confirmed by the in-situ observation of the cell by the optical microscope, as well as C B l l M plate occurs in the cell to exhibit a strong light scattering (Fig. 10c). Turbulent motion of liquid crystals is well-known as dynamic scattering (DS) which is enhanced by the presence of a slight amount of salt in the system. This is due to the fact that DS effect arises from not only a dielectric effect but also a current effect of ions in liquid crystals. In this case, TBAB was added to enhance the dynamic scattering effect. After the electric field is off, the alignments of the plates are randomized to produce the multidomain structure of the liquid crystal in the cell (Fig. 10d). In the multidomain, the nematic director of each domain is considered to orient randomly in space. The fluctuation of refractive indexes arising from the random nematic directors becomes the cause of light scattering. Therefore, the light scattering state is maintained as memory state II. However, if there is no enough amount of the clay to maintain the multidomain structure, the memory effect becomes weak as discussed above. The increase in driving voltage with increasing clays content should be attributed to the fact that the change of alignment of clay particles as well as TFALC molecules should take place to change the state as discussed above. Therefore, LCC requires stronger electric field than TFALC alone does. Also the interaction between TFALC and clay particles might increase the threshold voltage of TFALC in the LCC.

4. Conclusions By mixing a two-frequency-addressing liquid crystal and organophilic clay mineral, we could have created nematic liquid crystalline composites (LCC) in which the plates of the clay mineral were homogeneously dispersed in micron meter level. The LCC cells exhibited a bistable and reversible light scattering effect which could be controlled by changing the frequency and voltage of an applied electric field. This new material would be a potential candidate for advanced applications such as a light controlling glass, a high information display device which does

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not require active addressing device, erasable opticat storage device, and so on. The results shown in this paper would suggest the concept of polymer-clay hybrids, which is considered one of the biomimetic materials, can be extended to a wide variety of materials including liquid crystals shown in this paper. This fact would mean that all concepts in biomimetic materials and systems should be extended to next fruitful stages and would provide us a new chance of creating wide variety of functional and high performance materials.

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