Several Types of Bilayer Smectic Liquid Crystals with Ferroelectric and

Nov 7, 2006 - The mesomorphic behavior and phase structure were examined in the mixture of two kinds of dimeric compounds, α,ω-bis(4-alkoxyanilinebe...
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J. Phys. Chem. B 2006, 110, 23911-23919

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Several Types of Bilayer Smectic Liquid Crystals with Ferroelectric and Antiferroelectric Properties in Binary Mixture of Dimeric Compounds Tatsuya Izumi, Yuu Naitou, Yoshio Shimbo, Yoichi Takanishi, Hideo Takezoe, and Junji Watanabe* Department of Organic and Polymeric Materials, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed: April 9, 2006; In Final Form: September 1, 2006

The mesomorphic behavior and phase structure were examined in the mixture of two kinds of dimeric compounds, R,ω-bis(4-alkoxyanilinebenzylidene-4′-carbonyloxy)pentane (mOAM5AMOm), by optical microscopy, X-ray diffraction, polarization switching, and second-harmonic generation measurements. One compound is 4OAM5AMO4 with a short terminal alkyl chain that forms a single-layer smectic phase (SmCAs) with a random mixing of spacer and tail groups. Another compound is 16OAM5AMO16 with a long terminal alkyl chain that forms a chiral, anticlinic, and antiferroelectric bilayer phase (SmCAb) with the bent molecules tilted to the bilayer. By mixing these two compounds, the SmCAs phase of 4OAM5AMO4 is easily destabilized, leading to the wide content region of the bilayer phases. In the bilayer regime, three other smectic phases are newly induced. Two of them are antiferroelectric and ferroelectric phases in which the molecules lie perpendicularly with respect to the layer. The other shows no polar response to an external electric field and behaves like a smectic A. The new appearance of these bilayer phases is discussed as a mixing effect of long and short tail groups.

Introduction Liquid crystals composed of bent-shaped (banana-shaped) molecules have become an important subfield in the investigation of mesogenic compounds.1,2 These materials give rise to phases that have specific peculiarities, rather different from the liquid crystals formed by conventional rod like molecules. In particular, chiral and polar phases can be achieved in achiral banana molecular systems. Seven types of banana phases have been investigated (B1-B7).3 Some of them are polar phases and great numbers of reports have been issued.4 Most are antiferroelectric in polar order.5,6 A ferroelectric phase is sometimes observed as the field-induced state,7-10 i.e., a ferro state induced by applying a high electric field to an antiferroelectric liquid crystal is maintained when the electric field is off. A few ferroelectric analogues are reported that have a fluorine substituent,11,12 chiral tails,7,13-16 an oligosiloxane unit,17,18 and other features.19,20 Most ferroelectric phases are formed by laterally substituted compounds. It is thus likely that steric hindrance may force them to form a ferroelectric structure rather than an antiferroelectric structure that can escape from spontaneous polarization, but the reason is not clear. Besides, polar smectic phases in bent shaped liquid crystals possess layer chirality by tilting of molecules to the layer. It is called the B2 phase.2 A few examples have been reported about the polar phase without the tilting of molecules.21 Twin dimers with an odd-carbon numbered alkyl spacer are another kind of bent-shaped molecule. The typical compounds are R,ω-bis(4-alkoxyanilinebenzylidene-4′-carbonyloxy)pentanes (mOAM5AMOm) with the following formula. * To whom correspondence should be addressed. Tel: 81-3-5734-2633; Fax: 81-3-5734-2888; E-mail: [email protected].

The bent arrangement of the two mesogens within a molecule is strongly confined by the conformation of the spacer alkyl chain.22 Hence, the situation is similar to that of the banana molecules, but in this case, the compatibility of the tail and spacer alkyl chains has to be considered if these molecules form the smectic layer structure. As reported previously,23,24 mOAM5AMOm molecules form three types of smectic phases depending on the terminal carbon number, m, in other words, the compatibility of the tail and spacer alkyl chains. mOAM5AMOm molecules with m values of 4 and 6 form a smectic CA (SmCAs) with single-layer character where the tail and spacer alkyl groups are randomly mixed or intercalated with each other. Hence, one mesogenic layer is included in a repeating length nearly equal to half the molecular length. The mesogenic groups within a layer are tilted to the layer, but the tilt direction is opposite between the neighboring layers. The A in SmCA means anticlinic tilting of two mesogenic units to the layer. The structure of SmCAs is schematically illustrated in Figure 1a. On the other hand, mOAM5AMOm with m values of 14-18 form a SmCA phase with bilayer (or double-layer) character (so-called SmCAb). Here, the dimer molecules with bent shape themselves form each layer and then two mesogenic layers spaced by the alkyl spacer domain are included in each repeating layer (Figure 1c). The SmCAb phase is especially interesting because it is expected to be ferroelectric or antiferroelectric even in a nonchiral system.25,26 In this series of dimeric mOAM5AMOm compounds, the SmCAb phase has been found to be antiferroelectric.23,24,26,27 Further, it has been found that the moleculesare tilted to the layer normal so that each bilayer

10.1021/jp062208y CCC: $33.50 © 2006 American Chemical Society Published on Web 11/07/2006

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Figure 1. Schematic representations for molecular organization of dimeric compounds in (a) single-layer smectic CA (SmCAs) phase, (b) frustrated smectic CA (SmCAf) phase, and (c) bilayer smectic CA (SmCAb) phase with standard SmCAPA structure. (d), (e), and (f) show the schematic structures of SmAPA, SmAPF, and SmA phases with bilayer character, respectively, which were newly identified in the present mixture system.

has a chirality as well as the polarity. Thus, the situation is similar to that of banana molecules, and in its analogy, the SmCAb phase has been assigned to the homochiral SmCAPA structure.24 mOAM5AMOm molecules with intermediate m values of 8-12 show a frustrated smectic phase (SmCAf) where the bilayer is formed as in SmCAb, but an unusual density modulation takes place parallel to the layer direction to form an antidomain structure as illustrated in Figure 1b.26,28-30 In this study, we treat the mixtures of 4OAM5AMO4 forming the SmCAs phase and 16OAM5AMO16 forming the bilayer SmCAPA phase. In this mixture system, interestingly, the ferroelectric and antiferroelectric bilayer phases, which have different types of ground-state structures from that of 16OAM5AMO16, are newly formed. The detailed analyses for these new phases are performed, and their structure and properties are discussed in relation to the structural characteristics of a twin dimer molecule. Experimental Section mOAM5AMOm dimers with m values of 4 and 16 were synthesized following the methods reported in a previous paper.24 Their thermodynamic data are listed in Table 1. Binary mixtures of 4OAM5AMO4 and 16OAM5AMO16 were prepared by evaporating a chloroform solution of the two compounds at a predetermined molar ratio. The mixture content is given in the weight % fraction of 16OAM5AMO16. The calorimetric behavior was investigated with a PerkinElmer DSC-7 calorimeter at a scanning rate of 10 °C min-1. Optical microscopic observation was studied using a polarizing microscope (Olympus BX50). X-ray diffraction (XRD) photographs were taken at different temperatures using Ni-filtered CuKR radiation (Rigaku RU-200 BH). The temperature was measured and regulated within an accuracy of 1 °C using a

TABLE 1: Thermodynamic Data for Transition Temperatures and Enthalpies and Layer Spacings of 4OAM5AMO4 and 16OAM5AMO16 layer spacing/ Å

transition temperature/°C (enthalpy/kcal mol-1) n

Cr

4

•

16

•

SmCAs SmCAb

Crblue 73 (3.5) -

•

109 (17.3)

•

-

-

•

I

LC

Cr or Crblue

143 • (2.7) 121 • (4.8)

19.3

38.4

56.9

63.4

Mettler FP-90 hot stage. The film-to-specimen distance was determined by calibration with silicon powder. Polarization reversal current was observed using a high-speed amplifier (FLC Electronics, F20A) connected to a function generator (NF Electronic Instruments, WF1945A). The cell temperature was controlled by a temperature control unit (Mettler, FP-90). The sample was sandwiched between two glass plates with a transparent indium tin oxide (ITO) electrode. None of the polyimide layers were coated for all measurements. Optical second-harmonic generation (SHG) measurement was performed using a 10 Hz Nd:YAG laser (Continuum SureliteI) of λ ) 1064 nm as a fundamental beam. The incidence angle of the laser to the sample cell settled in a heater block was 45 degrees. The SHG signal was detected by a photomultiplier tube (Hamamatsu model-R955) and the outputs from the photomultiplier tube were accumulated by a BOXCAR integrator (Stanford Research Systems). Results and Discussion 1. Characterization of 4OAM5AMO4 and 16OAM5AMO16 Compounds. Figure 2a,b shows typical textures of the smectic phase formed by 4OAM5AMO4. On cooling from the isotropic

Bilayer Smectic Liquid Crystals

Figure 2. Optical textures (×200) observed for SmCAs of 4OAM5AMO4 and SmCAb of 16OAM5AMO16. SmCAs appears as having (a) circular focal conic textures from the isotropic phase and finally forms (b) a typical fan-shaped texture. For SmCAb, (c) helical filaments grow first from the isotropic phase and gather together to form (d) an undefined fine texture.

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Figure 4. Content dependence of layer thickness in bilayer smectic and Crblue phases of binary mixtures of 4OAM5AMO4/16OAM5AMO16. The layer thickness was determined from the spacing of first layer reflection. The molecular length of the extended chain, evaluated as a simple average of molecular lengths of two compounds, are also given by the solid line.

Figure 3. Phase behavior in binary mixtures of 4OAM5AMO4 and 16OAM5AMO16. Transition temperatures are collected using the DSC calorimeter (O), and optical microscopic observations (4).

Figure 5. Polarization reversal current responses observed for SmCAPA phase in mixture of 4OAM5AMO4/16OAM5AMO16 ) 10/90; (a) polarization reversal current obtained by applying triangle wave field and (b) POM textures at E ) on states and E ) off state. A capacitortype 2.5 µm cell with planar alignment was used under conditions of 42 Vpp and 0.5 Hz.

phase, a circular focal conic texture appears (Figure 2a) and coalesces with each other to form a fan-shaped texture (Figure 2b). The layer spacing, 19.3 Å, is equal to half the molecular length (18.5 Å), showing the single-layer type of smectic phase (SmCAs). The SmCAs phase shows no polar response to an applied electric field. It transforms to the crystalline phase on further cooling. Figure 2c,d shows textures for the smectic phase formed by 16OAM5AMO16. A helical filament texture appears from the isotropic melt like that of the B7 phase31 if the cooling rate is very low (see Figure 2c). The filaments gather together to form the undefined fine texture (Figure 2d). The layer spacing of smectic liquid crystals is 56.9 Å, which is comparable to the molecular length (64.2 Å), showing the bilayer type of smectic CA phase (SmCAb). By applying a triangular voltage to the SmCAb phase, two polarization reversal current peaks are observed, as shown later in Figure 5. The rotation in the extinction direction of the optical microscopic texture on polarization reversal switching shows the tilting of molecules to the layer. The tilt angle is around 25°, which roughly

corresponds to the angle calculated from a comparison of the molecular length and layer spacing. Thus, the SmCAb phase formed here is assigned to the homochiral SmCAPA phase with anticlinicity and antiferroelectricity. On further cooling, the bilayer SmCAPA phase transforms to the Crblue phase. This lowest-temperature solid phase shows very low birefringence, but a distinct optical rotation. These properties are also similar to those of the B4 phase in banana molecular systems and are attributed to the TGB-like helical structure.32,33 2. Phase Behavior in Mixtures of 4OAM5AMO4 and 16OAM5AMO16. Phase behavior in the mixture was examined by mixing these typical compounds of 4OAM5AMO4 and 16OAM5AMO16. The transition temperatures, associated enthalpies, and layer spacings as determined from DSC and XRD are summarized in Table 2. The transition temperatures are plotted against the mixture content of 16OAM5AMO16 in Figure 3. Phase behavior is divided into two regimes: singlelayer and bilayer regimes. The single-layer regime is very narrow; the SmCAs of 4OAM5AMO4 is destabilized by a small addition (less than 10%) of a 16OAM5AMO16 component. On

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TABLE 2: Thermodynamic Data for Phase Transition Temperatures and Associated Enthalpies and Layer Spacings in Mixtures of 4OAM5AMO4 and 16OAM5AMO16 content of 16OAM5AMO16

Crblue

90

•

80

•

70

•

50

•

40

•

35

•

30

•

25

•

SmA 108 (14.8) 108 (19.3) 95 (10.0) 95 (7.4) 92 (5.2) 89 (5.1) 93 (4.3) 94 (5.1)

transition temperature/°C (enthalpy/kcal/mol) SmAPA SmAPF SmCAPA

-

-

-

•

-

•

-

•

-

•

-

•

‚ ‚

107 (0.2) 102 (0.6)

-

120 (5.4) 120 (4.2) 116 (3.5) 113 (2.3) 94 (-)

-

the other hand, the bilayer regime exists in the wide component range from 25 to 100%. This means that the bilayer structure accommodates more easily the short chain molecule. In the intermediate region from 10 to 25%, both single-layer and bilayer phases coexist. It is interesting to note that in the bilayer regime, three types of distinct smectic phases, SmAPA, SmAPF, and SmA, are newly observed in addition to the SmCAPA phase. Their detailed structures will be shown later. The standard SmCAPA structure is observed only in the narrow region of 100 to 90%. SmAPA is formed in the region of 80 to 35%, and SmAPF is formed in the region of 35 to 25%. The mixture of 35% forms SmAPA and SmAPF in the lower- and higher-temperature regions, respectively. The mixtures of 25 and 30% also show bimorphism exhibiting the SmAPF and SmA phases in order of increasing temperature. The lowest- temperature phase appearing on cooling from these bilayer smectic phases is the Crblue phase. 3. Comparison of Layer Spacings between Different Smectic Phases. At first, we refer to the X-ray data for these new phases in the bilayer regime. The layer spacings obtained from the first-order layer reflection are plotted against the mixture content in Figure 4. In the same figure, the molecular length, which was estimated as a simple average between two compounds, is also given by a solid line. It is found that the layer spacings for the phases seen in the bilayer regime are approximately equal to the molecular length. The layer spacings of the Crblue phase are almost equal to the molecular length, meaning that in this lowest-temperature phase, the molecules lie perpendicularly to the layer as in the B4 phase of banana molecules.32-34 In Figure 4, the SmCAPA phases in the content region from 100 to 90% are found to show layer spacings smaller by about 5 Å than the molecular length or the layer spacing of the Crblue phase. This indicates the tilting of molecules to the layer. The simply calculated tilt angle is around 25°, which is in an agreement with that obtained from the optoelectric switching experiment as mentioned below. Of interest is that the spacings of SmAPA and SmAPF as well as its lowesttemperature Crblue phase correspond to the molecular length. Thus, in these bilayer phases, no tilting of molecules is likely. The spacing of the SmA phase is somewhat temperaturedependent and has a value shorter than the molecular length but longer than half the molecular length. The detailed structure and properties for these new phases are described below. 4. Structural Characteristics of Bilayer Phases and their Ferroelectric and Antiferroelectric Properties. 4.1. SmCAPA Phase. The typical switching behavior of SmCAPA as observed

-

•

-

I

layer spacing/Å LC Crblue

•

55.4

60.0

-

•

57.1

57.4

-

-

•

54.9

54.8

-

-

•

50.7

50.7

-

-

•

50.1

49.4

-

•

48.4

-

•

-

•

48.4 (SmAPA) 48.4 (SmAPF) 38.0 (SmA) 46.4 (SmAPF) 38.4 (SmA) 46.6 (SmAPF)

• • •

115 (3.0) 111 (0.8) 112 (0.7)

120 (4.5)

46.4 48.0

in the mixture of 90% is shown in Figure 5a. By applying the triangular wave voltage, two switching current peaks on half a cycle are observed, indicating the antiferroelectric phase. The estimated polarization is about 600 nC cm-2. The antiferroelectric switching is also detected from the optical microscopic observation with respect to the rotation of the extinction direction (see Figure 5b). The rotation angle between two ferro states is around 45°, meaning the tilting of a molecule by 22.5° to the layer normal. This tilt angle corresponds to the calculated one (23°), obtained by a comparison of the molecular length and layer spacing. These behaviors are just the same as those observed in the SmCAb phase of 16OAM5AMO16 with the SmCAPA structure. Hence, in this SmCAPA, both the bent and tilt directions are opposite between neighboring bilayers as illustrated in Figure 1c. 4.2. SmAPA Phase. The SmAPA phase also shows the antiferroelectric switching with the polarization of about 190 nC cm-2 as presented in Figure 6a, which was observed for the

Figure 6. Polarization reversal current of SmAPA phase of 4OAM5AMO4/16OAM5AMO16 ) 50/50: (a) polarization reversal current obtained by applying triangle wave field and (b) POM textures at E ) on states and E ) off state. A capacitor-type 13 µm cell with planar alignment was used under conditions of 37 Vpp and 14 Hz.

mixture of 50%. Note that A means the perpendicular alignment of bent molecules to the bilayer and PA means the antiferroelectric polarization. Figure 6b shows microphotographs observed simultaneously on the antiferroelectric switching. It can be found that in this case the extinction direction does not rotate at all between two

Bilayer Smectic Liquid Crystals

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Figure 7. X-ray diffraction pattern observed for homeotropically aligned SmAPA phase in mixture of 4OAM5AMO4/16OAM5AMO16)50/50 with irradiation of X-ray beam parallel to smectic layer. (b) Enlarged view in a small-angle region of (a).

ferro states. Figure 7 shows the oriented X-ray pattern taken for the homeotropically aligned SmAPA phase with a beam parallel to the layer. The layer reflections with the spacing equal to the molecular length appear on the meridian, and the outer broad reflections with a spacing of 4.5 Å are split into two portions lying above and below the equator.35 This indicates that the molecules still assume the bent shape as in SmCAPA.24 According to these data, the structural model for the SmAPA phase is illustrated in Figure 1d. Here, the bent molecules lie perpendicularly to the layer with their bent direction in one bilayer opposite to that of the neighboring bilayer. On applying a positive electric field, the bent direction of molecules is directed upward (see Figure 8a), whereas on applying a negative field it is directed downward (Figure 8c). Hence, these two ferro states should be the same in texture and birefringence on viewing by microscopy, which agrees with the observations in Figure 6b. At zero field in Figure 6b, the birefringence is remarkably altered from that of ferro states. This may be caused by a tendency of the bent molecule to align its bent direction parallel to the surface on going back to the antiferro state as illustrated in Figure 8b. Thus, we conclude that the SmAPA phase is antiferroelectric, but is differentiated from the SmCAPA phase of 16OAM5AMO16 by having an orthogonal alignment of bent molecules with respect to the bilayer.36,37 The structure is schematically illustrated in Figure 1d. 4.3. SmAPF Phase. The structural characteristics of the SmAPF phase are clearly described for the mixture of 35% which shows the bimorphism forming the SmAPA and SmAPF phases. Here, PF means the ferroelectric polarization. SmAPF exists in a temperature region higher than that of SmAPA. Hence, SmAPF is a thermodynamically less-ordered phase, although a difference in the degree of structural order may be negligibly small between two phases because the transition cannot be detected by DSC thermogram. Figure 9a,b shows the polarization reversal current and textural change observed in SmAPF at 102 °C and SmAPA at 92 °C, respectively. By applying a triangular voltage wave, SmAPF shows only one current peak, whereas on cooling to SmAPA, the switching occurs with two current peaks as shown above in Figure 6. The SmAPF phase is hence ferroelectric. In fact, the optical microscopic textures clearly indicate two-state switching for the SmAPF phase and three-state switching for the SmAPA phase. The polarization is about 210 nC cm-2 in both phases. It should be noted that the texture and birefringence of SmAPF at zero field or at high field is the same as that of the ferro state of SmAPA at high field. The oriented X-ray pattern is not changed at all on the SmAPA-to-SmAPF phase transformation. The layer spacings are also the same, showing that in these phases the molecules with a bent conformation lie perpendicu-

Figure 8. Illustration of SmAPA phase showing antiferroelectric response. At E ) off state, the molecules lie with their bent direction parallel to the surface as in (b). On applying a positive electric field (E+ ) on state), the bent direction of molecules is directed upward as in (a), whereas on application of a negative field (E- ) on state) it is directed downward as in (c). Because the molecular axis is perpendicular to the layer, the birefringence and texture of (a) and (c) are the same as those observed in Figure 6(b).

larly to the layer. Thus, it can be concluded that each bilayer of the SmAPF phase possesses the same character as that of the SmAPA phase, but the bilayers are stacked with the same bent (polar) directionality as illustrated in Figure 1e. In other words, the SmAPA-to-SmAPF transition is regarded as an antiferroelectric-to-ferroelectric phase transition. The decisive evidence for the transformation of antiferroelecric SmAPA to ferroelectric SmAPF is given from SHG responses. Figure 10 shows the temperature dependence of the SHG signal observed without an external electric field. Note that previous field alignment such as rubbing or electric treatment was not applied on preparing the samples. The hightemperature SmAPF phase shows a clear SHG, while no SHG is observed for the low-temperature SmAPA phase although SHG can be detected for its ferro states achieved at the high electric field. SHG activity for SmAPF and inactivity for SmAPA were also confirmed in other mixtures with different contents. 4.4. SmA Phase. We finally refer to the SmA phase, which is observed at higher temperatures than the SmAPF phase in the mixtures of 25-30%. Figure 11 shows the texture of SmA appearing from the isotropic melt as observed in the mixture of 25%. A fine fanlike texture was observed in the homogeneous region (Figure 11a). Of interest is that no birefringence is

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Figure 9. Polarization reversal current responses of (a) SmAPF at 102 °C and (b) SmAPA phase at 92 °C in mixture of 4OAM5AMO4/ 16OAM5AMO16 ) 65/35 obtained by applying triangle wave field. Textures at E+, E- ) on state and E ) off state are also given. A capacitortype 9 µm cell with planar alignment was used under conditions of 18 Vpp and 30 Hz.

Figure 10. Temperature dependence of SHG intensity observed in mixture of 4OAM5AMO4/16OAM5AMO16 ) 65/35.

observed in the homeotropic alignment (Figure 11b), whereas on cooling to the SmAPF phase, a clear birefringence can be

seen as in all other bilayer phases (Figure 11d). This means that the SmA phase possesses an uniaxial structure.

Bilayer Smectic Liquid Crystals

J. Phys. Chem. B, Vol. 110, No. 47, 2006 23917 The observed layer spacing is temperature-dependent; it is changed from 38 to 40 Å on decreasing the temperature from 110 to 104 °C. These values are fairly larger than half the molecular length but smaller than the 47 Å of SmAPF. It is thus considered that the bilayer phase is still formed here. The smaller spacing may be due to the significant intercalation of tail groups. No polar response to the external electric field also differentiates the SmA phase from the three other bilayer phases. The details of the SmA structure are not yet clear, but this type of bilayer phase has been often observed in other binary mixture systems of odd-numbered twin dimers where the difference in terminal chain length is relatively large.38 Conclusion

Figure 11. Optical micrographs (×200) of (a) homogeneously and (b) homeotropically aligned SmA (at 110 °C) in mixture of 4OAMO5AMO4/16OAM5AMO16 ) 75/25. (c) and (d) Textures of the lowertemperature SmAPF (at 100 °C) observed at the same positions of (a) and (b), respectively.

Figure 12 shows oriented X-ray patterns of SmA and SmAPF phases in the same mixture with a content of 25%. The wideangle broad reflection also undergoes a significant change on the transformation; the reflection is observed near the equatorial line in SmA whereas it is split into two portions lying above and below the equator in the lower-temperature SmAPF phase. This confirms the uniaxial orientation of mesogens in SmA, which results from the alteration of the bent conformation to the more extended conformation with two mesogenic groups within a molecule parallel to each other.

The results clearly described the characteristics of phase behavior in the mixture system. The single-layer SmCAs phase in 4OAM5AMO4 with a short tail is easily destabilized by an addition of less than 10% of 16OAM5AMO16 with a long tail. This is reasonable because the short spacer zones with the carbon number n ) 5 hardly accommodate the long tail group with the carbon number m ) 16. Thus, the mixtures of two compounds with a big difference in tail length prefer to form the bilayer smectic phases with the microsegregation of spacer and tail groups. This is really the observed trend in the present study where the bilayer phases appear in the wide content region of 25-100%. The bilayer phase composed of the bent twin dimers possesses the polar property because the bent molecules are usually packed with the same bent directionality. Furthermore, as a general trend, the molecules are tilted to the bilayer and its tilt direction, as well as the bent direction, is opposite between neighboring bilayers. Thus, the bilayer phase has been assigned to the homochiral, anticlinic, and antiferroelectric SmCAPA phase. Of interest is that the mixtures form three other bilayer structures.

Figure 12. X-ray diffraction patterns for homeotropically aligned (a),(b) SmA (at 110 °C) and (c),(d) SmAPF (at 100 °C) phases formed from mixture of 4OAM5AMO4/16OAM5AMO16 ) 75/25. The X-ray beam is irradiated parallel to the layer in a homeotropically aligned cell. (b) Enlarged view in a small-angle region of (a), and (d) enlarged view in a small-angle region of (c).

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Izumi et al. phase exists in a temperature region higher than that of the antiferroelectric SmAPA phase, suggesting that the delicate difference in the interaction causes the ferro-antiferroelectric phase transformation, although no changes are seen at SmAPA - SmAPF transition even in DSC. However, the result of polarization reversal measurement clearly shows this transition by textural change and change of switching current. In the mixture of 25-30%, the SmAPF transforms to the simple SmA bilayer phase with increasing temperature. In the SmA phase, the uniaxial alignment of mesogenic groups may overcome the conformational constraint that has forced the twin molecule to assume the bent-shaped conformation. The layer thickness is fairly smaller than the extended molecular length, so that a significant overlapping of alkyl tails may take place in this phase. The SmA-SmAPF transition is clearly observed by POM, XRD, polarization reversal current, and SHG. The result indicates that these phases are thermodynamically distinguishable from each other. Finally, we note that mixing of two compounds in a twin dimer system will be a convenient method of preparing new attractive types of polar phases. Although the SmAPA phase has been reported, this mixing system is quite an easy way to form SmAPA. Furthermore, the SmAPF and SmA phases have never been reported as far as we know. References and Notes

Figure 13. Schematic illustration showing interaction between tail groups protruding from neighboring bilayers in (a) single-component system and (b) binary-mixture system.

In two of them, the bilayer consists of molecules with bent conformation as in the SmCAPA phase, but the molecules lie perpendicularly to the bilayer. One is an antiferroelectric SmAPA phase appearing in the wide content region of 16OAM5AMO16 from 35 to 80%, and another is a ferroelectric SmAPF phase appearing in the content region of 25-35%. Thus, we know the distinct mixing effect that mixing of the two different tails in the tail zone forces the orthogonal packing of bent molecules into a bilayer. Considering that the molecular length is nearly equal to the calculated length based on the trans conformation of alkyl groups, we speculate that the molecules between neighboring bilayers are interacting only at the ends of tail groups. In a single-component system, the ends of tails are concentrated at the limited space between bilayers so that no significant orientation correlation may be required between the tail groups sticking out of the neighboring layers (see Figure 13a). The delicate interaction at the ends of tails makes the molecules tilt to the bilayer and the polar interaction forces the antiferroelectric alignment, resulting in the standard SmCAPA structure. In the mixture, on the other hand, the long tail groups from neighboring layers have to intercalate mutually to effectively fill the vacant space produced by the short tail groups (Figure 13b). Such an intercalation, however, would be energetically unfavorable in the SmCAPA structure because the alternate tilting of molecules would significantly interrupt the tail groups from aligning uniaxially in the intercalating zone. We thus speculate that the SmAPA structure may be formed in the mixtures to attain the uniaxial alignment of mutually intercalating tails. The mixture of 35% shows the bimorphism that the ferroelectric SmAPF

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