Applied Clay Science 15 Ž1999. 93–108
Liquid crystalrclay mineral composites Masaya Kawasumi ) , Naoki Hasegawa, Arimitsu Usuki, Akane Okada Toyota Central Research and DeÕelopment Laboratories, Nagakute-cho, Aichi, 480-1192, Japan Received 2 September 1998; received in revised form 16 April 1999; accepted 10 May 1999
Abstract Novel liquid crystalline composites composed of a nematic two-frequency addressing liquid crystal ŽTFALC. and a few percents of various types of organized clay minerals ŽLCC. 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. Also, the LCCs were stable enough against sedimentation or aggregation under an electric field. The LCCs were sandwiched between transparent conductive InrSnO 2 ŽITO.-coated glasses with 12 mm polymer beads as a spacer and their electro-optical properties were measured. The LCC cells exhibited a bistable and reversible electro-optical effect between a light scattering state and transparent state. The contrast between the light scattering and transparent memory states is highly dependent on the kind of the organized clays. The LCCs based on the organized clays with better affinity for the liquid crystal exhibited relatively higher contrast. The memory effect exhibited by the LCCs is rather unusual since most of conventional nematic liquid crystals do not exhibit such a memory effect due to their low viscosity. Their memory mechanism was proposed. q 1999 Elsevier Science B.V. All rights reserved. Keywords: liquid crystal; clay mineral; electro-optical effect; memory effect
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 ŽUsuki, 1989; Schmidt et al., 1990; Kojima et al., 1993; Moet and Akelah, 1993; Novak, 1993; Usuki et al., 1993; Vaia et al., )
Corresponding author. E-mail:
[email protected]
0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 2 9 - 0
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1993; Yano et al., 1993; Biasci et al., 1994; Kelly et al., 1994; Lan and Pinnavaia, 1994; Wang and Pinnavaia, 1994; Lu et al., 1995; Messersmith and Giannelis, 1995; Vaia et al., 1995.. One of the promising composite systems would be hybrids based on organic polymers and inorganic clay minerals consisting of layered silicates Ž Usuki, 1989; Kojima et al., 1993; Moet and Akelah, 1993; Usuki et al., 1993; Vaia et al., 1993; Yano et al., 1993; Biasci et al., 1994; Kelly et al., 1994; Lan and Pinnavaia, 1994; Wang and Pinnavaia, 1994; Messersmith and Giannelis, 1995; Vaia et al., 1995. . In our previous ˚ thick works, we have synthesized nylon 6–clay hybrids Ž NCH. in which 10 A silicate layers of clay minerals are dispersed homogeneously in the nylon 6 matrix ŽUsuki et al., 1993. . The NCH exhibits various superior properties such as high strength, high modulus, high heat distortion temperature, compared to nylon 6 Ž Kojima et al., 1993. . 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 same concept as the NCH has been applied for various polymer systems such as polyimide Ž Yano et al., 1993. , epoxy resin Ž Usuki, 1989; Kelly et al., 1994; Lan and Pinnavaia, 1994; Wang and Pinnavaia, 1994. , polystyrene ŽUsuki, 1989; Moet and Akelah, 1993; Vaia et al., 1993. , polycaprolactone ŽMessersmith and Giannelis, 1995. , acrylic polymer Ž Usuki, 1989; Biasci et al., 1994. to date. The most recent advance in this technique has made it possible to prepare hybrid materials with even nonpolar polymers such as polypropylene ŽPP.. PP–clay hybrid was accomplished by mixing three components, i.e., PP, organized clay mineral, maleic anhydride modified PP oligomers as a compatibilizer Ž Kawasumi et al., 1997. . 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 also various types of materials including low molar mass compounds. Recently, we prepared novel composite materials based on a low molar mass liquid crystal and organized clay mineral Žabbreviated as LCC: Liquid Crystal C lay Composite., which exhibited unusual electro-optical effect which is not observed in conventional nematic liquid crystal systems Ž Kawasumi et al., 1996a,b. . Initially, the thin cell of the LCC Ž the LCC was sandwiched between two transparent electrodes with polymer beads spacers. was untransparent due to its strong light scattering effect. When an electric field was applied to the cell, it became transparent instantly. Surprisingly, the transparent state was maintained without the electric field, thus, exhibiting memory effect. However, the transparent memory state was canceled out to the initial light scattering state only by mechanical shearing of the cell. From the practical viewpoint, reversible switching of LCCs cells by only electric fields is desirable. In this paper, in order to obtain LCCs with reversible switching capability, we prepared novel LCCs based on the two-frequency addressing liquid crystal
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Fig. 1. Dielectric anisotropy of TFALC as a function of logarithm of frequency.
ŽTFALC.. TFALC is a mixture of low molar mass liquid crystals with regimes of positive dielectric anisotropy as well as negative dielectric anisotropy ŽFig. 1.. Therefore, the alignment of TFALC molecule can be actively controlled by changing the frequency of applied electric field. Interestingly, TFALC with a slight amount of salts exhibits a dynamic light scattering Ž DS. effect when subjected to an electric field whose frequency is near its crossover point Ž f 0 . or slightly higher than f 0 ŽRegime II in Fig. 1. . This behavior was used for the reversible switching of LCCs. Also, we tried to optimize the organized clays by improving the affinity of the organized clay for TFALC. We could obtain the novel LCCs, which exhibited a bistable and reversible electro-optical effect with good contrast between light scattering and transparent memory states.
2. Experimental 2.1. Materials The materials used for the preparation of LCCs are nematic TFALC Ž DF05XX. purchased from Chisso and purified montmorillonite ŽKunipia-F. from Kunimine. The physical properties of TFALC are listed in Table 1. Montmorillonite is a layered clay mineral composed of aluminosilicates wŽOH. 4 Si 8ŽAl 3.34 Mg 0.66 .O 20 –Na 0.66 x. The dimensions of the unit aluminosilicate layer are ˚ in thickness, about 1000 A˚ in width and in length. It has exchangeonly 10 A able cations between the layers, which are normally sodium cations. In this experiment, the montmorillonite exchanged with various organic ammonium cations including 4-cyano-Ž4X-biphenyloxy. undecyl ammonium cation Ž CB11M, Scheme 1. was used to enhance the miscibility between the montmorillonite and TFALC. Table 2 shows the clays exchanged with various types of organic
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Table 1 Physical properties of TFALC mixture used in the experiments. The data of the physical properties were obtained from Chisso, Japan Name
DF-05XX
Nematic–smectic transition temperature Ž8C. Isotropic–nematic transition temperature Ž8C. Viscosity Ž208C. ŽmPa s.
-0 114.6 41.1
RefractiÕe indices ne no Dn
1.650 1.502 0.148
ammonium cations in this experiment. The synthetic procedure of 4-Ž 11aminoundecyloxy.-4X-cyanobiphenyl Žthe organic amine used for CB11M. was reported previously Ž Kawasumi et al., 1996a,b.. The synthetic procedure of the other organic amines and ammoniums was published elsewhere Ž Kawasumi et al., 1998. . Tetra-n-butylammonium bromide ŽTBAB. , toluene, and N, N-dimethylformamide ŽDMF. from Wako, N, N-dimethylacetamide ŽDMAc. from Tokyo Chemical were used as received.
Scheme 1. Schematic representation of the clay mineral organized by 4-cyano-Ž4X-biphenyloxy.undecyl ammonium cation.
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Table 2 The organized clay minerals used in the experiment
2.2. The preparation of montmorillonites exchanged with organic ammoniums All the organized clays were prepared by the same procedure. The typical procedure is as follows. Sodium montmorillonite Ž 2.00 g, cation exchange capacity: 2.38 meq. was dispersed into 70 ml of hot water Ž about 508C.. 4-Ž 11-Aminoundecyloxy.-4X-cyanobiphenyl Ž 0.955 g, 2.62 mmol. and concentrated hydrochloric acid Ž 0.28 g. were dissolved into hot ethanol and water mixture Ž40 ml:10 ml.. It was poured into the hot montmorillonite–water solution under 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-Ž 4X-cyanobiphenyl-4-oxy. undecyl ammonium Ž CB11M, Scheme 1, the content of the inorganic part, 70.5 wt.%.. 2.3. Contact angle measurements of thin films 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 8M, 12M, CB4M, CB11M, CB16M, 2ŽCB11. M, while toluene was used for 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 measurements of contact angles.
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The contact angles Ž Q . of TFALC on the surfaces of the organized clays were measured by using a contact angle meter CA-A Ž Kyowa. at 238C Žthe angles were measured 1 min. 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 the organized clay minerals indicated in Table 3 were prepared by the same method. The typical example is as follows. CB11M Ž 0.0181 g. was dispersed into DMAc and mixed with 1.0 g of TFALC and 0.200 g of TBABrDMAc solution ŽTBABrDMACs 1.02 = 10y4 grg.. DMAc was evaporated in vacuo at 508C for 10 h and the obtained composite was mechanically stirred. It was dried in vacuo at 508C for 10 h to yield a white pasty composite ŽLCC-CB11M, the content of the inorganic part, 1.25 wt.%.. The contents of the inorganic part of the other LCCs are as follows. LCC-8M: 1.18 wt.%, LCC-12M: 1.27 wt.%, LCC-DSDM: 1.56 wt.%, LCC-CB4M: 1.27 wt.%, LCC-C16M: 1.27 wt.%, LCC-2ŽCB11. M: 1.27 wt.%. 2.5. Techniques The interlayer distances of the organized clays were measured by X-ray diffraction Ž XRD. 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 Ž 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 InrSnO 2 ŽITO. 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 Ždiameter: 12 mm., 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. Ž In this case, the microscope was used without polarizers.. These were recorded on a chart recorder. The commercial power supply Ž 100 V–60 Hz, sign wave. with voltage adjusted using a Slidac was used for applying the low frequency electric field Ž 60 Hz. to the cell. A power amplifier Model S-4750 manufactured by the NF Electronic
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Fig. 2. Schematic representation of the system set up for the optical measurements of LCC cells.
Instruments was used for applying the high frequency electric field Ž 1–1.5 kHz. . The light transmittance of the cell was calculated according to Eq. Ž1. . Light Transmittance Ž % . s 100 = Ž Photomultiplier Value of Sample Cell . r Photomultiplier Value of Blank Cell Sealed with Water
Ž1.
All measurements were carried out at room temperature.
3. Results and discussion 3.1. The organized clay minerals intercalated with Õarious organic compounds Table 2 presents the organized clay minerals used in this experiment, their interlayer distances, the contents of inorganic parts, and cos Q calculated from the contact angles Ž Q . of TFALC on the surfaces of the clays. The organic compounds used for the organization of the clay are alkyl ammoniums with various chain length and alkyl ammoniums with cyanobiphenyl group. This cyanobiphenyl group is similar structure to conventional liquid crystal molecules. The value of cos Q was used for the evaluation of the affinity of the organized clays for TFALC. If cos Q is close to unity, the organized clay should have high affinity and should homogeneously disperse in TFALC. On the other hand, if cos Q is low, the affinity may be too low to obtain well dispersed LCCs. The interlayer distances and inorganic contents of the organized clays clearly indicate almost complete intercalation of the organic ammoniums into the clay. The value of cos Q is dependent on the kind of the intercalated organic compounds. In the case of alkyl ammonium, cos Q increases with decreasing their alkyl chain length. On the other hand, in the case of cyanobiphenyl alkyl
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ammonium, the length of their alkyl chains do not affect so much on cos Q . The high values of cos Q were obtained for the clays organized with cyanobiphenyl alkyl ammoniums. 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. The composites should be stable enough against sedimentation or phase separation for practical applications. The LCCs were tested qualitatively in terms of viscosity, stability without an electric field Žthe sample was left for a few days. and under an electric field. The results are listed in Table 3 with the value of cos Q . Although the organized clays were dispersed in micrometer level in all the LCCs, the stability of the dispersion is highly dependent on the kind of organic ammoniums exchanged. As seen from Table 3, the LCCs based on the organized clays with relatively high cos Q , especially, the clays intercalated with cyanobiphenyl alkyl ammoniums, meaning that the affinity of the clay surfaces for TFALC is relatively high, exhibited relatively high viscosity, high stability with and without the electric field. This may be simply due to the fact that high affinity between the two components stabilized the dispersion of the LCCs. The best stabilized LCCs were obtained in the case of CB11M as an organized clay even though their dispersibility was not perfect yet as discussed in Section 3.4. Table 3 Contact angles of the liquid crystals on the surface of the various organized clays, qualitative evaluation of viscosity and stability of LCCs Samples
cos Q ŽDF-05XX.
Viscosity a
Stability of dispersion b
Stability of dispersion under the application of electric field 50 V–60 Hz c
LCC-8M LCC-12M LCC-DSDM LCC-CB4M LCC-CB11M LCC-CB16M LCC-2ŽCB11.M
0.930 0.839 0.607 0.989 0.989 0.993 0.992
high medium medium high high high high
` = = ^ ` ^ ^
= = = ^ ` ` ^
a
Low: LCC flowed when the vial with the LCC was tilted. Medium: LCC flowed very slowly when the vial with the LCC was tilted. High: LCC did not flow. b Ž`.: No sedimentation or phase separation of the clays in the liquid crystals occurred. Ž^.: Sedimentation or phase separation occurred slightly. Ž=.: Sedimentation or phase separation occurred. c Ž`.: No aggregation of the clays under electric field. Ž^.: Aggregation of the clays occurred slowly. Ž=.: The clays aggregates each other very quickly.
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3.3. Electro-optical properties of the LCCs Fig. 3 shows a typical change in the light transmittance of the LCC-CB11M cell Žbold line. as well as of a TFALC cell Žbroken line. when the cells were subjected to electric fields. Fig. 4 presents the photographs of the LCC-CB11M cell at various states. As seen from Fig. 3, TFALC cell Ž without the organized clay. did not exhibit a memory effect at all. On the other hand, in the case of the LCC-CB11M cell, when a low frequency electric field Ž 60 Hz–50 V, Regime I in Fig. 1. was applied to the cell, it became transparent within 50 ms Ž T1, on-state I in Figs. 3 and 4a. . Even after the electric field was switched off, although small decrease in the transparency was observed, the transparent state was maintained ŽT2, memory state I in Figs. 3 and 4b. . When a high frequency electric field Ž 1.5 kHz–100 V, Regime II in Fig. 1. was applied to the cell, the memory state I was canceled out to return to the light scattering state Ž T3, on-state II in Figs. 3 and 4c. within 50 ms. Again, the light scattering state was maintained without electric field Ž T4, memory state II in Figs. 3 and 4d. . Although the LCC is based on a nematic liquid crystal, it exhibited unusual stability of memory states. Table 4 summarized the light transmittances of all the states of the LCCs based on the various organized clays. The contrast indicates the difference in transmittance between memory state I and memory state II Ž T2–T4. . Fig. 5 shows the relationship between cos Q and the contrast ŽT2–T4.. Although all the LCCs exhibited similar reversible memory type switching, the contrast of switching is highly dependent on the kind of the organized clay. The LCCs based on the clay with relatively high cos Q value tend to exhibit high contrast between the two memory states. This may be due to the fact that high affinity
Fig. 3. The changes in the transmittances of LCC-CB11M cell and TFALC cell Žbroken line. Žcell gap: 12 mm. by two-frequency driving Ždriving sequence: 60 Hz–50 V on, 60 Hz off, 1.5 kHz–100 V on, 1.5 kHz off..
102 M. Kawasumi et al.r Applied Clay Science 15 (1999) 93–108 Fig. 4. Photographs of the LCC-CB11M cell Žcell gap: 12 mm. at various states: Ža. on-state I Ž50 V–60 Hz., Žb. memory state I Ž60 Hz off., Žc. on-state II Ž100 V–1.5 kHz., Žd. memory state II Ž1.5 kHz off..
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Table 4 Light transmittances and contrasts of the LCC cells at the various states LCC cell
Clay Light transmittance Ž%. Contrast Ž%. content T1 T2 T3 T4 T2–T4 Žwt.%. 60 Hz–50 V 60 Hz 1.5 kHz–100 V 1.5 kHz on off on off
LCC-C8M LCC-C12M LCC-DSDM LCC-CNBPC4M LCC-CNBPC11M LCC-CNBPC16M LCC-2ŽCNBPC11.M
1.18 1.27 1.56 1.27 1.25 1.27 1.27
43 53 50 76 85 82 68
37 51 36 65 80 75 61
7 14 19 28 11 20 12
13 31 29 19 16 30 22
24 20 7 46 64 35 39
between the two components stabilized the structures created by the application of electric fields. Fig. 6 shows the transmittance of the LCC-CB11M 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. Ž A low frequency electric field Ž60 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. ŽA high frequency electric field Ž1.5 kHz. was applied to the cell in the memory state I created by applying 60 Hz–50 V.. The transmittances of the cell in on-state I and the memory state I start to increase around 20 Vrms and saturate about 40 Vrms . On the other hand, the transmittances of the cell in
Fig. 5. Relationship between the contrast of the two memory states Žtransparent memory state I and light scattering memory II. and the cos Q values of the organized clays.
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Fig. 6. Transmission properties of the LCC-CB11M cells in the on-state I Ž60 Hz on., memory state I Ž60 Hz off., on-state II Ž1.5 Hz on., memory state I Ž1.5 kHz off. as a function of applied voltage.
on-state II and the memory state II start to decrease around 20 Vrms and saturate over 60 Vrms . Any level of the transmittance in the memory states can be achieved by adjusting an applied voltage. The light transmittances of the memory states I and II of LCC-CB11M did not change after 15 h. 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. 3.4. Possible mechanism of the memory effect in DS mode Although cholesteric Ž Heilmeire and Goldmacher, 1968. and smectic ŽKajiyama et al., 1989. liquid crystalline materials have been reported to exhibit optical memory effects, most of nematic liquid crystals do not exhibit such a memory effect due to their low viscosity except few examples as follows. Polymer dispersed nematic liquid crystals with polyball type morphology have been reported to exhibit a memory effect Ž Yamaguchi and Sato, 1993. . In this case, the memory effect might be originated from the anchoring effect of the dispersed polymer. Our examples are a unique approach adding a memory effect which is different from the above examples. Let us consider the possible mechanism of the unusual electro-optical effect exhibited by the LCC. First of all, why do the LCCs exhibit such a strong light scattering? Fig. 7 shows typical polarized micrographs of the obtained LCC-CB11M in an isotropic state Ž a. and in a nematic state Ž b. . In the isotropic state, only the particles of CB11M were observed as white spots which were dispersed uniformly in the LCC. Their sizes are roughly estimated to be several micron
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Fig. 7. Optical polarized micrographs of LCC-CB11M and TFALC Žcrossed nicol, the thickness of the sample is 12 mm.: Ža. isotropic state of the LCC, Žb. nematic state of the LCC, Žc. nematic state of TFALC.
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meters. They are considered to be the aggregates of unit layers of CB11M. In the nematic state, the very fine texture of the LCC was observed. The texture is composed of very small domains of TFALC Žmultidomain structure. . Since pure TFALC does not exhibit such a fine texture Ž Fig. 7c. , CB11M induced the multidomain 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. In this 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. Secondly, why do the LCCs exhibit such a strong memory effect? Fig. 8 presents the schematic explanation of the memory mechanism. The clay particles indicate the aggregates of unit layers of an organized clay in Fig. 8. When a low frequency electric field in Regime I is applied to the cell, not only TFALC but also the clay plates align parallel to the electric field ŽFig. 8a. as demonstrated by XRD measurements previously Ž Kawasumi et al., 1996a,b. , and the cell becomes transparent. 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 Ž Fig. 8b. . 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, molecules as well as the clay plates plate occurs in the cell to exhibit a strong light scattering Ž Fig. 8c. . Turbulent motion of liquid crystals is well known as 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 DS effect. After the electric field is
Fig. 8. The schematic representations of LCC structures at various states: Ža. on-state I, Žb. memory state I, Žc. on-state II, Žd. memory state II.
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off, the alignments of the plates are randomized to produce the multidomain structure of the liquid crystal in the cell ŽFig. 8d.. Therefore, the light scattering state is maintained as memory state II. The memory states are considered to be maintained by the strong interaction between the clay plates and the liquid crystal as explained above. Therefore, it should be reasonable that the contrast between the two memory states was strongly dependent on the cos Q , which is an indication of the affinity of the liquid crystal for the clay plates.
4. Conclusions By mixing a TFALC and organophilic clay mineral, we could have created nematic LCCs 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 not require active addressing device, erasable optical storage device, and so on. The results shown in this paper would suggest the concept of polymer–clay hybrids could be extended to a wide variety of materials including liquid crystals shown in this paper.
References Biasci, L., Aglietto, M., Ruggeri, G., Ciardelli, F., 1994. Functionalization of montmorillonite by methyl methacrylate polymers containing side-chain ammonium cations. Polymer 35 Ž15., 3296–3304. Heilmeire, G.H., Goldmacher, J.E., 1968. Electric-field-controlled reflective optical storage effect in mixed liquid–crystal systems. Appl. Phys. Lett. 13 Ž4., 132–133. Kajiyama, T., Kikuchi, H., Miyamoto, A., Moritomi, S., Hwang, J.C., 1989. Aggregation states and bistable light switching of Žliquid crystalline polymer.rŽlow molecular weight liquid crystal. mixture systems. Chem. Lett. 5, 817–820. Kawasumi, M., Hasegawa, N., Usuki, A., Okada, A., 1996a. Novel memory effect found in nematic liquid crystalrfine particle system. Liq. Cryst. 21 Ž6., 769–776. Kawasumi, M., Usuki, A., Okada, A., Kurauchi, T., 1996b. Liquid crystalline composite based on a clay mineral. Mol. Cryst. Liq. Cryst. 281, 91–103. Kawasumi, M., Hasegawa, N., Kato, M., Usuki, A., Okada, A., 1997. Preparation and mechanical properties of polypropylene–clay hybrids. Macromolecules 30 Ž20., 6333–6338. Kawasumi, M., Hasegawa, N., Usuki, A., Okada, A., 1998. Nematic liquid crystalrclay mineral composites. Mater. Sci. Eng. C 6, 135–143. Kelly, X.P., Akelah, A., Qutubuddin, S., Moet, A., 1994. J. Mater. Sci. 29, 2274. Kojima, Y., Usuki, A., Kawasumi, M., Fukushima, Y., Okada, A., Kurauchi, T., Kamigaito, O., 1993. Mechanical properties of nylon 6–clay hybrid. J. Mater. Res. 8 Ž5., 1185–1189. Lan, T., Pinnavaia, T.J., 1994. Clay-reinforced epoxy nanocomposites. Chem. Mater. 6 Ž12., 2216–2219.
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Lu, S., Melo, M.M., Zhao, J., Pearce, E.M., Kwei, T.K., 1995. Organic–inorganic polymeric hybrids involving novel polyŽhydroxymethylsiloxane.. Macromolecules 28, 4908–4913. Messersmith, P.B., Giannelis, E.P., 1995. Synthesis and barrier properties of polyŽ e-caprolactone. layered silicate nanocomposites. J. Polym. Sci., Part A: Polym. Chem. 33, 1047–1057. Moet, A.S., Akelah, A., 1993. Polymer–clay nanocomposites: polystyrene grafted onto montmorillonite interlayers. Mater. Lett. 18 Ž1r2., 97–102. Novak, B.M., 1993. Hybrid nanocomposite materials between inorganic glasses and organic polymers. Adv. Mater. 5 Ž6., 422–433. Schmidt, H., in: Schaefer, D.W., Mark, J.E. ŽEds.., 1990. Polymer Based Molecular Composites. Materials Research Society, Pittsburgh, p. 3. Usuki, A., 1989. United States Patent 4,889,885. Usuki, A., Kojima, Y., Kawasumi, M., Okada, A., Fukushima, Y., Kurauchi, T., Kamigaito, O., 1993. Synthesis of nylon 6–clay hybrid. J. Mater. Res. 8 Ž5., 1179–1184. Vaia, R.A., Ishiim, H., Giannelis, E.P., 1993. Synthesis and properties of two-dimensional nanostructures by direct intercalation of polymer melts in layered silicates. Chem. Mater. 5 Ž12., 1694–1696. Vaia, R.A., Jandt, K.D., Kramer, E.J., Giannelis, E.P., 1995. Kinetics of polymer melt intercalation. Macromolecules 28, 8080–8085. Wang, M.S., Pinnavaia, T.J., 1994. Clay–polymer nanocomposites formed from acidic derivatives of montmorillonite and an epoxy resin. Chem. Mater. 6 Ž4., 468–474. Yamaguchi, R., Sato, S., 1993. Highly transparent memory states in polymer dispersed liquid crystal films. Liq. Cryst. 14 Ž4., 929–935. Yano, K., Usuki, A., Okada, A., Kurauchi, T., Kamigaito, O., 1993. Synthesis and properties of polyimide–clay hybrid. J. Polym. Sci., Part A: Polym. Chem. 31, 2493–2498.
