Revisiting Smectic E Structure through Swollen Smectic E Phase in

Article pubs.acs.org/JPCB

Revisiting Smectic E Structure through Swollen Smectic E Phase in Binary System of 4‑Nonyl-4′-isothiocyanatobiphenyl (9TCB) and n‑Nonane Takahito Miyazawa,† Yasuhisa Yamamura,† Mafumi Hishida,† Shigenori Nagatomo,† Maria Massalska-Arodź,‡ and Kazuya Saito*,† †

Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences, Kraków 31-342, Poland

‡

S Supporting Information *

ABSTRACT: Thermodynamic and diffraction analyses were performed to establish the phase diagram for a binary system between 4-n-nonyl-4′isothiocyanatobiphenyl (9TCB) and n-nonane. The swollen SmE structure is identified in the binary system. Upon swelling, a characteristic two-dimensional herringbone array is maintained whereas the layer spacing of SmE structure increases with the content of nnonane. Considering the difficulties in explaining the experimental findings based on the traditional model of SmE structure, a new model, lamellar with two types of sublayers consisting of aromatic core and alkyl chain moieties, is proposed.

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INTRODUCTION

Smectic E (SmE) phase is a liquid crystalline mesophase having a layer structure like smectic A (SmA) phase. The SmE phase has no fluidity and is the closest to an ordered crystal among orthogonal mesophases exhibited by calamitic mesogens.1,2 Recently, the SmE phase attracts great attention because the mesogens showing the SmE phase are favorably applied to organic semiconductors.3−6 For their development and improvement, it is important to know fundamental properties such as their structure. However, the fundamental investigation of their properties has progressed less so far, in comparison with other liquid crystalline phases, such as nematic and SmA phases. Indeed, it has just been revealed through the entropic and spectroscopic studies of two mesogenic series that alkyl chains of calamitic mesogens are fully molten even in the SmE phase.7−10 A traditional model of the SmE structure assumes that the constituent molecules are arranged in layers with orthorhombic symmetry and herringbone array.1,2 The herringbone array is characteristic of the SmE structure and has been usually drawn as the assembly of lath-like molecules as shown in Figure 1. The model seems to fit to mesogens mainly composed of aromatic core, e.g., isobutyl-4-(4′-phenylbenzylideneamino)cinnamate (IBPBAC),11 though unclear is the mechanism responsible for the interlayer correlation of molecular positions (threedimensional periodicity). For the calamitic mesogens with long alkyl chain(s), however, we need to consider packing manner of their alkyl chain(s) in some detail. Diele et al.12 systematically investigated the structure of SmE phase on a family of 4-nalkyloxy-4′-alkanoylbiphenyls with various lengths of alkyl chain. They have classified the structures into two packing © 2013 American Chemical Society

Figure 1. Traditional model of SmE phase drawn for lath-like molecules.

models from the viewpoint of the shape of mesogenic molecules. In the case of “symmetric” molecules, which have two flexible alkyl chains on both ends of the central core, the layer is divided into two sublayers consisting of the aromatic cores and the alkyl chains (category I according to Diele et al.12), as shown in Figure 2a. On the other hand, “asymmetric” molecules, which have just a flexible long alkyl chain, pack densely in an antiparallel manner within the layer (category II), as shown in Figure 2b. In both categories, molecules form layers without interdigitation, and the periodicity (layer spacing) along the stacking direction is nearly equal to the length of the molecule. 4-Alkyl-4′-isothiocyanatobiphenyl (abbreviated as nTCB, with n being the number of carbon atoms in the alkyl group) Received: June 3, 2013 Revised: June 20, 2013 Published: June 20, 2013 8293

dx.doi.org/10.1021/jp405480h | J. Phys. Chem. B 2013, 117, 8293−8299

The Journal of Physical Chemistry B

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example, a swollen SmA structure observed in a binary system between 4-(1-methylheptyloxycarbonyl)phenyl-4′-octyloxybiphenyl-4-carboxylate (MHPOBC) and n-octane.35 These suggest that the effective “repulsion” between aromatic core and alkyl chain probably works also in SmE phase of nTCB with a large n. The purpose of the present paper is to assess the traditional model12 (in more precisely, category II) of SmE phase for nTCB with a large n through making the phase diagram of a binary system between 9TCB and n-nonane. On the basis of the experimental results, we propose a new structural model of SmE phase, which naturally explains the formation of the swollen SmE phase while keeping three-dimensional “crystallinity” starting from the SmE phase in the neat system.

Figure 2. Previously proposed packing structures12 for SmE phase: (a) category I for symmetric mesogen; (b) category II for asymmetric mesogen.

is a calamitic mesogen consisting of a core part (a biphenyl core and isothiocyanato group as a polar one) and a flexible alkyl chain as shown in the inset of Figure 3. A family of nTCB is

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EXPERIMENTAL SECTION 9TCB was supplied by Prof. R. Dabrowski, the Military University of Technology in Warsaw, Poland, and used as obtained. Commercial n-nonane (>99%) (Tokyo Chemical Industry) was used without further purification. Compositions of binary mixture were first determined gravimetrically for differential scanning calorimetry (DSC) after confirming negligible loss of n-nonane (less than 0.1%) during sample preparation. For other experiments, the composition was deduced from the phase diagram determined by DSC because the transition temperature to the isotropic liquid (IL) was a steep continuous function of composition. The phase diagram of the 9TCB−n-nonane system was determined using a commercial DSC apparatus (TA Instruments, Q200). The samples were sealed into sealable sample pans (PerkinElmer). The samples (ranged from 1.1 to 3.5 mg) were weighed using a highly sensitive balance (Mettler-Toledo, XP2U). The mixture was kept for 5 min at 353 K to achieve homogeneous mixing prior to experiments. DSC measurements were carried out at a temperature ramp rate of 1 K min−1. The texture of the binary mixture was observed using a polarizing microscope (Olympus, BX50) under the crossed Nicole condition. The sample was sealed in quarts-glass container using silicone grease. Since the composition varied during sample preparation due to the vaporization of n-nonane, it was deduced from the transition temperature to the IL state (solution of 9TCB dissolved into liquid n-nonane for binary samples) on the basis of the phase diagram determined by DSC. Observation was performed with a temperature ramp rate of 10, 5, and 1 K min−1 using a hot stage (Linkam, LK-600PM), in which the correction was applied to temperature readings in order to achieve the consistency with those of DSC. Wide-angle X-ray diffraction (WAXD) experiments on 9TCB−n-nonane at elevated temperatures were performed using Rigaku Ultima IV powder diffractometer (Cu Kα). The temperature of the sample was controlled using a hightemperature stage equipped with a cartridge heater attached to a laboratory-made sample-plate holder, which is well isolated from the goniometer. The temperature was monitored by a thermocouple (type E), the junction of which was placed near the sample, and controlled within ±1 K. The sample was covered by Kapton film (thickness of 7.5 μm) and sealed with starch as adhesive to avoid evaporation of n-nonane. The composition was deduced based on the DSC phase diagram from the transition temperature to the IL state detected as the disappearance of the diffraction peaks. A small-angle X-ray diffractometer (SAXD), Rigaku NANOViewer, was used to determine the temperature dependences of

Figure 3. DSC results of 9TCB−n-nonane binary sample containing ca. 15 mol % n-nonane. The inset shows the molecular structure of 9TCB. The change into swollen SmE without IL is hard to be identified in cooling run, but deduced on the basis of SAXD experiments in Figure 9.

known to exhibit the SmE phase in a wide range of alkyl chain lengths (n = 2−12) as a mesophase.7,8,13−27 According to its molecular structure, nTCB is to be classified into category II because the layer spacing is nearly equal to the molecular length. We thus expect the antiparallel packing of molecules in the SmE phase of nTCB. Then, if the length of the alkyl chain becomes nearly equal to or longer than that of the core moiety, the core moiety stands nearby the fully molten alkyl chain7,8 in the antiparallel arrangement, causing the preference to SmB (HexB) structure1,2 because of a reduced orientational correlation within a layer. However, no SmB phase has been observed in any nTCB.8,13 In general, aromatic and aliphatic compounds prefer segregation in liquid mixture, i.e., macroscopic phase separation. This is also the case microscopically in thermotropic liquid crystals, quasi-binary (QB) picture of thermotropics. The present authors have demonstrated that micro segregation plays an important role in the formation of various aggregation structures in thermotropic liquid crystal such as cubic phases in thermotropics.28−33 A recent structural study on the Ia3d phase indicated that the rigid core forms a continuous frame like a jungle gym, and the molten alkyl chain fully fills the remaining space.34 A simpler structure like SmA phase is also related to the microphase segregation: SmA phase is induced in nematogenic 7CB, a member of the most famous thermotropic series nCB, by adding n-heptane.33 Besides, there is, for 8294



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the layer spacing (along the long molecular axis) of 9TCB−nnonane and neat 9TCB in temperature ramp conditions for comparison with DSC results. Each sample was sandwiched between Kapton films (thickness of 7.5 μm). The thickness of the sample was ca. 0.45 mm. The composition of 9TCB−nnonane was deduced from the transition temperature to the IL state. The diffraction experiments were performed using Cu Kα while scanning the temperature at a rate of 0.5 K min−1, which was achieved using the same hot stage as that used in the polarizing optical microscopy. It took 30 s for each diffraction image. Incident X-ray was normal to the Kapton films of sandwiched sample. The camera length (about 450 mm) was calibrated using the (001) diffraction of the standard silver behenate (Gem Dugout).

neat 9TCB indicates that the mesophase of the binary sample has essentially the same SmE structure as neat 9TCB. This SmE phase exhibited by binary samples is, if necessary, termed as the swollen SmE phase hereafter. In the DSC chart in heating run, the anomaly around 320 K, which is due to the phase change from the neat 9TCB crystal to the SmE phase, is divided into a long tail and a sharp peak at 320 K. The long tail reflects the dissolution of 9TCB into liquid n-nonane. The anomaly exhibiting a peak at 330 K comes from the phase change from the SmE phase to IL phase and also has a long tail on the lowtemperature side. The long tail indicates that IL phase and the SmE phase coexist in this temperature region. The so-called supercooling phenomenon is apparent in the cooling run. The DSC charts after a few cycles of measurements are depicted against xnonane in Figure 5a (heating run) and Figure

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RESULTS AND DISCUSSION Phase Diagram of 9TCB−n-Nonane Binary System. DSC measurements for 16 samples including neat 9TCB and nnonane were carried out. A typical result after a few cycles of measurements for xnonane = 15 mol % is shown in Figure 3. There are four anomalies in the heating run. Small and large anomalies around 220 K are attributed to the phase transitions in neat n-nonane. A smaller anomaly at a lower temperature is of the solid−solid phase transition36 because the possibility of eutectic melting is ruled out on the basis of appreciable supercooling observed in the cooling run. On the other hand, a larger anomaly at the higher temperature is due to the fusion (Tfus = 219.5 K37) and observed in all results of the binary samples. These facts indicate that the mixture undergoes a phase separation into the crystal of neat 9TCB and IL around 300 K on cooling. The binary sample exhibits a mesophase within the temperature range of ca. 320−330 K in heating run (ca. 330−300 K in cooling run). To characterize the mesophase, WAXD patterns of ca. 15 mol % sample were recorded as shown in Figure 4. The comparison with that of

Figure 5. DSC results of 9TCB−n-nonane binary samples for various xnonane: (a) heating and (b) cooling runs.

5b (cooling). The charts remarkably change at xnonane ≈ 30 mol % in the heating run: three anomalies are observed in xnonane < 30 mol %, whereas a single one is observed in xnonane > 30 mol %. On the other hand, the anomaly from IL phase to the SmE phase is observed up to xnonane = 80 mol % in cooling runs because of the supercooling of the swollen SmE phase coexisting with IL phase. On the basis of Figure 5b, we draw a phase diagram of 9TCB−n-nonane binary system as shown in Figure 6. Since the entrance to the swollen SmE region is hard

Figure 6. Phase diagram of 9TCB−n-nonane binary system corresponding to cooling runs. Circle is the clearing point between SmE and IL phases in neat 9TCB. Other symbols base on different experimental techniques: cross, DSC; square, SAXD; diamond, WAXD. Dotted horizontal line indicates the phase transition temperature to SmE phase from the ordered crystal in neat 9TCB.

Figure 4. Wide-angle X-ray diffraction patterns recorded upon cooling 9TCB−n-nonane binary sample with xnonane ≈ 15 mol % and neat 9TCB. 8295



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to be precisely located on the basis of DSC results, the phase boundary is primarily determined while referring to the X-ray results described later. SmE Phase of 9TCB−n-Nonane Binary System. Texture of the binary sample with xnonane ≈ 18 mol % was observed in cooling run using a polarizing microscope. Figure 7a shows a

Figure 8. Temperature dependences of lattice parameters (a, b, c; open circles) of 9TCB−n-nonane binary sample with xnonane ≈ 15 mol % based on diffraction patterns in Figure 4, together with those of neat 9TCB at 328 K (closed circles). The broken line is an eye guide. The temperature dependence of layer spacing (c) of neat 9TCB obtained using SAXD (plus) is also shown.

parameter (layer spacing) seems to match that of neat 9TCB at the clearing temperature (see the broken line in Figure 8) and linearly increases with decreasing temperature. Those temperature dependences clearly show that absorption of nnonane into the SmE structure of 9TCB increases only c without an appreciable change in neither a nor b. That is, the two-dimensional herringbone array of the 9TCB within the layer is not disturbed by the absorbed n-nonane, while the absorbed n-nonane contributes to just the layer spacing. Besides, the sharpness of (111) and (201) diffractions (with mixed indices within and perpendicular to the two-dimensional layer, i.e., (|h| + |k|) × l ≠ 0) is held with the decrease in temperature. This shows that not only the intralayer periodicity but also the three-dimensional periodicity across the layers of the SmE structure is preserved upon swelling. To investigate the curious behavior in detail, the layer spacing was measured for various concentrations (ca. 4, 12, 17, and 40 mol %) using SAXD. Note that the sharpness of the primary diffraction (001) of the binary sample is comparable to that of neat 9TCB (Figure S1 in Supporting Information). The observed SmE structure in the binary system is homogeneous, accordingly. The layer spacings of the SmE structure for all sample are shown in Figure 9a,b. Experiments on the 40 mol % sample in a heating run were not performed because the sample changes directly from the crystal to IL phases. The layer spacing of the sample 4 mol % exhibits three characteristics in the heating run: (i) a rapid decrease around 319 K, (ii) a plateau up to ca. 330 K, and (iii) a linear decrease above 330 K. These behaviors can be interpreted as (i) Cr + IL to Cr + SmE, (ii) SmE phase, and (iii) SmE + IL coexistent region. Similarly, the layer spacings of 12 and 17 mol % samples observed in the SmE + IL coexistent region decrease with increasing temperature. The results on three samples indicate the same linear dependence of layer spacing against temperature in the SmE + IL coexistent region. A universal line can be drawn as an envelope in the SmE + IL coexistent region and reaches the layer spacing of neat 9TCB at the clearing temperature.

Figure 7. Textures of a 9TCB−n-nonane binary sample with xnonane ≈ 18 mol % observed in cooling by a polarizing microscopy. The texture is (a) in the SmE phase at 305 K, (b) during the crystallization while oozing n-nonane liquid (lower right in the image) at 304 K, and (c) in coexistence of neat 9TCB crystal and n-nonane liquid at 302 K.

characteristics mosaic texture of the SmE phase at 305 K. The sample underwent crystallization around 304 K on further cooling. Upon the crystallization, fluid is oozed from the texture as shown in Figure 7b. The fluid seems to be n-nonane because its fusion was observed around 220 K in the DSC experiments on heating. This phenomenon indicates that the SmE structure of 9TCB absorbs and holds the liquid n-nonane inside between ca. 305 and 340 K. WAXD patterns of the binary sample with ca. 15 mol % nnonane were recorded at nine temperatures from 328 to 312 K (Figure 4). Based on the reported indexing,13 lattice parameters (a, b, and c) of the SmE phase were determined as shown in Figure 8, in which the layer spacing (c) of neat 9TCB determined by SAXD is also shown for comparison. The a and b (intralayer periodicities) are nearly equal to those of neat 9TCB and almost independent of temperature. The c 8296



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the small increment (less than 0.1 nm) in the layer spacing (periodicity along the stacking direction) compared to the molecular radius of n-nonane is hard to explain. Indeed, a comparable thickness to the radius of n-alkane molecules (ca. 0.2 nm) would be necessary at least for the uniform swelling. Besides, the presence of fluid layers of n-nonane would significantly disturb the correlation along the stacking axis (three-dimensional periodicity). Namely, it is expected that diffractions such as (111) and (201) are lost. This is not the case as evident from Figure 4. Partial substitution of n-alkane molecules for nTCB would be the other possibility within the traditional model of SmE structure. Although our results indicate that the threedimensional periodicity is preserved upon swelling, it is hard to find a reason why the intralayer periodicity is scarcely affected by the partial substitution. Moreover, the substitutional inclusion of cylindrical molecules is expected to reduce the effective anisotropy of intermolecular interaction between 9TCB molecules. A transition to SmB structure is thus anticipated but not detected experimentally. The traditional model of SmE structure has some difficulties as discussed above. On the other hand, if it is abandoned, we can readily find the model that can explain all findings. Considering the molten alkyl chain in 9TCB, suppose the lamellar structure where the rigid core and alkyl chain microscopically segregate each other as in Figure 10a. One

Figure 9. Layer spacing of the SmE structure of the 9TCB−n-nonane binary system (open symbols), together with neat 9TCB (plus): (a) heating run; (b) cooling one. Data of 9TCB were taken in a cooling run.

The SmE structure was observed in a wider temperature range in cooling runs than heating runs by virtue of supercooling. The layer spacing of all samples are also on the universal line in the SmE + IL coexistent region on cooling, as shown in Figure 9b. The results on 4, 12, and 17 mol % samples indicate a deviation from the universal line at ca. 328, 318, and 314 K, respectively. These analyses lead the phase boundary between SmE and SmE + IL phases shown in Figure 6. These temperatures are the boundary temperatures between the SmE and the SmE + IL coexisting regions. After the deviation from the universal line, the layer spacing becomes almost constant against temperature in each sample. This region is the homogeneous SmE phase in which all n-nonane molecules are in the structure: the layer spacing is nearly equal to the calculated one from the volume fraction assuming noncompressibility for both of 9TCB and n-nonane. The SmE structure of 9TCB has a considerable capacity to absorb nnonane because a plateau that indicates the full consumption of n-nonane was not observed for the sample of 40 mol %. SmE Structure. The behavior of the binary system is summarized as follows: The SmE structure of 9TCB can absorb n-nonane, resulting in the formation of the swollen SmE phase. The upper limit of n-nonane content depends on temperature in the SmE + IL coexistent region. The maximum content continuously increases with decreasing temperature, leading to a continuous increase in the layer spacing. The swollen SmE structure needs to be connected continuously with that of neat 9TCB because the universal line starts from the clearing point. After fully consuming n-nonane present in the sample, the homogeneous SmE phase is formed, the layer spacing of which is almost independent of temperature. The homogeneous SmE phase oozes n-nonane upon the formation of the crystal of neat 9TCB. The intralayer periodicity of SmE phase is not affected by absorbed n-nonane while the layer spacing changes. Besides, not only the intralayer periodicity but also the threedimensional periodicity of the SmE structure is well preserved in spite of the absorption of n-nonane. What is a structure of the SmE phase to permit the whole curious phenomenon? The traditional SmE structure without interdigitation is assumed first. In this case, the molten alkyl chain of a 9TCB molecule scarcely plays a role. Putting the emphasis on the intralayer periodicity of herringbone arrangement, it is easy to imagine the absorbed alkane form a separate layer in between layers of 9TCB. This structure is very similar to Rieker’s model38 for the SmA phase containing n-alkane. Rieker assumed the structure that layers of two-dimensional fluid of mesogens and liquid alkane alternately stack. However,

Figure 10. Schematic illustrations of (a) a new model of SmE structure of asymmetric mesogen having a long alkyl chain and (b) swollen SmE structure containing n-alkane (b).

half of total number of 9TCB molecules line up randomly and the other half down also randomly. The anisotropy of interaction between the cores stabilizes the herringbone arrangement of the cores. Since the alkyl sublayers consist of alkyl chains coming from both upper and lower sublayers of cores, the structural correlation along the stacking direction is automatically guaranteed. Upon swelling, it is easy to expect that the added n-alkane favors the alkyl sublayers as shown in Figure 10b. This structure is similar to the induced SmA phase in the 7CB−n-heptane binary system33 and swollen SmA phase in a binary system between 4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate (MHPOBC) and noctane.35 Even if the amount of absorbed n-alkane becomes large, the structural correlation along the stacking direction remains through the n-alkane molecules as bridging parts. The above reasoning for three-dimensional periodicity in the SmE structure suggests that the structural correlation is not strong in SmE phase of symmetric mesogens because the 8297



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(5) Funahashi, M.; Hanna, J. Mol. Cryst. Liq. Cryst. 2001, 368, 303− 310. (6) Kurotaki, K.; Haruyma, H.; Takayashiki, Y.; Hanna, J. Chem. Lett. 2006, 35, 1194−1195. (7) Horiuchi, K.; Yamamura, Y.; Sumita, M.; Yasuzuka, S.; MassalskaArodz, M.; Saito, K. J. Phys. Chem. B 2010, 114, 4870−4875. (8) Yamamura, Y.; Adachi, T.; Miyazawa, T.; Horiuchi, K.; Sumita, M.; Massalska-Arodź, M.; Urban, S.; Saito, K. J. Phys. Chem. B 2012, 116, 9255−9260. (9) Adachi, T.; Yamamura, Y.; Hishida, M.; Ueda, M.; Ito, S.; Saito, K. Liq. Cryst. 2012, 39, 1340−1344. (10) Adachi, T.; Saitoh, H.; Yamamura, Y.; Hishida, M.; Ueda, M.; Ito, S.; Saito, K. Bull. Chem. Soc. Jpn. 2013, DOI: 10.1246/ bcsj.20130122. (11) Leadbetter, A. J.; Mazid, M. A. Mol. Cryst. Liq. Cryst. 1980, 61, 39−60. (12) Diele, S.; Tosch, S.; Mahnke, S.; Demus, D. Cryst. Res. Technol. 1991, 26, 809−817. (13) Jasiurkowska, M.; Budziak, A.; Czub, J.; Massalska-Arodź, M.; Urban, S. Liq. Cryst. 2008, 35, 513−518. (14) Massalska-Arodź, M.; Wüflinger, A.; Büsing, D. Z. Naturforsch. 1999, 54a, 675−678. (15) Urban, S.; Czuprynski, K.; Dabrowski, R.; Gestblom, B.; Janik, J.; Kresse, H.; Schmalfuss, H. Liq. Cryst. 2001, 28, 691−696. (16) Massalska-Arodź, M.; Schmalfuss, H.; Witko, W.; Kresse, H.; Würflinger, A. Mol. Cryst. Liq. Cryst. 2001, 366, 221−227. (17) Würflinger, A.; Urban, S. Liq. Cryst. 2002, 29, 799−804. (18) Urban, S.; Czub, J.; Dąbrowski, R.; Kresse, H. Liq. Cryst. 2005, 32, 119−124. (19) Ishimaru, S.; Saito, K.; Ikeuchi, S.; Massalska-Arodź, M.; Witko, W. J. Phys. Chem. B 2005, 109, 10020−10024. (20) Urban, S.; Czub, J.; Dąbrowski, R.; Wüflinger, A. Phase Transitions 2006, 79, 331−342. (21) Jasiurkowska, M.; Budziak, A.; Czub, J.; Urban, S. Acta Phys. Pol., A 2006, 110, 795−805. (22) Pełka, R.; Yamamura, Y.; Jasiurkowska, M.; Massalska-Arodź, M.; Saito, K. Liq. Cryst. 2008, 35, 179−186. (23) Roland, C. M.; Bogoslovov, R. B.; Casalini, R.; Ellis, A. R.; Bair, S.; Rzoska, S. J.; Czuprynski, K.; Urban, S. J. Chem. Phys. 2008, 128, 224506. (24) Jasiurkowska, M.; Sciesinski, J.; Czub, J.; Massalska-Arodź, M.; Pełka, R.; Juszynska, E.; Yamamura, Y.; Saito, K. J. Phys. Chem. B 2009, 113, 7435−7442. (25) Maeda, Y.; Urban, S. Phase Transitions 2010, 83, 467−481. (26) Jasiurkowska, M.; Zieliński, P.; Massalska-Arodz, M.; Yamamura, Y.; Saito, K. J. Phys. Chem. B 2011, 115, 12971−12977. (27) Urban, S.; Roland, C. M. J. Non-Cryst. Solids 2011, 357, 740− 745. (28) Saito, K.; Sorai, M. Chem. Phys. Lett. 2002, 366, 56−61. (29) Sorai, M.; Saito, K. Chem. Rec. 2003, 3, 29−39. (30) Saito, K. Pure Appl. Chem. 2009, 81, 1783−1798. (31) Saito, K.; Sato, A.; Sorai, M. Liq. Cryst. 1998, 25, 525−530. (32) Kutsumizu, S.; Morita, K.; Yano, S.; Nojima, S. Liq. Cryst. 2002, 29, 1459−1468. (33) Yamaoka, Y.; Taniguchi, Y.; Yasuzuka, S.; Yamamura, Y.; Saito, K. J. Chem. Phys. 2011, 135, 044705. (34) Nakazawa, Y.; Yamamura, Y.; Kutsumizu, S.; Saito, K. J. Phys. Soc. Jpn. 2012, 81, 094601. (35) Hiraoka, K.; Kato, A.; Hattori, H.; Sasaki, Y.; Omata, Y.; Koguma, K.; Ban, N.; Oshima, M. Mol. Cryst. Liq. Cryst. 2009, 509, 743−750. (36) Finke, H. L.; Gross, M. E.; Waddington, G.; Huffman, H. M. J. Am. Chem. Soc. 1954, 76, 333−341. (37) NIST Chemistry WebBook, http://webbook.nist.gov/ chemistry/. (38) Rieker, T. P. Liq. Cryst. 1995, 19, 497−500. (39) Pensec, S.; Tournilhac, F.-G.; Bassoul, P. J. Phys. II 1996, 6, 1597−1605.

segregation is realized without interdigitation in the packing of category I. This issue is however left for future works. In fact, serious attempts to reveal SmE structures using diffraction techniques were reported only for an interdigitated SmE structure having a longer layer spacing than the molecular length for asymmetric mesogens with perfluoroalkyl chain.39,40 Finally, the cause of the observed swelling is commented on. As described above, the saturated amount of the absorbed nnonane decreases upon heating. A similar dependence was reported for swelling of SmA phase.35 The behavior is in contrast to that of lyotropic liquid crystals formed in neutrally charged lipid−water systems, where the repulsion between lipid layers is enhanced by thermal agitation.42−44 Namely, QB mixtures in thermotropic systems are not the same completely as lyotropic lipid−water systems. It is also noted that the swelling is driven by not the dissolution (mixing) of n-alkane into the molten alkyl chain of 9TCB but the enthalpic gain like physical adsorption (physisorption).41 In this respect, it is interesting to note that the smectic structure resembles to layered materials such as graphite and some clays.

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CONCLUSION Thermodynamic and diffraction analyses and optical observations were performed to establish the phase diagram of a binary system between 9TCB and n-nonane. The binary phase diagram between 9TCB and n-nonane shows that the SmE structure absorbs n-alkane while preserving the characteristic layered structure with herringbone array and three-dimensional periodicity. Considering the difficulties in explaining the experimental findings based on the traditional model of SmE structure, a new model, lamellar with two sublayers consisting of aromatic core and alkyl chain moieties, is proposed. The proposed model largely differs from the traditional model of the SmE phase, which resembles a crystal consisting of lath-like molecules. In this respect, the question of whether the proposed model is of reality is crucially important for confirming the “liquid crystallinity” of the SmE phase.

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ASSOCIATED CONTENT

S Supporting Information *

SAXD patterns of SmE phase in neat 9TCB and in a binary system between 9TCB and n-nonane. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by Grant-in-Aid for Scientific Research (B) 22350056 from Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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(40) Pensec, S.; Tournilhac, F.-G.; Bassoul, P.; Durliat, C. J. Phys. Chem. B 1998, 102, 52−60. (41) Morrison, R. The Chemical Physics of Surfaces, 2nd ed.; Plenum Press: New York, 1990. (42) Helfrich, W. Z. Naturforsch. 1978, 33a, 305−315. (43) Petrache, H. I.; Gouliaev, N.; Tristram-Nagle, S.; Zhang, R.; Suter, R. M.; Nagle, J. F. Phys. Rev. E 1998, 57, 7014−7024. (44) Hishida, M.; Seto, H.; Yamada, N. L.; Yoshikawa, K. Chem. Phys. Lett. 2008, 455, 297−302.

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Revisiting Smectic E Structure through Swollen Smectic E Phase in

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