Asymmetric Organic−Inorganic Hybrid Giant Molecule: HierarchicalSmectic Phase Induced from POSS Nanoparticles by Addition ofNematic Liquid CrystalsNamil Kim,† Dae-Yoon Kim,‡ Minwook Park,‡ Yu-Jin Choi,‡ Soeun Kim,‡ Seung Hee Lee,§

and Kwang-Un Jeong*,‡

†Smart Materials R&D Center, Korea Automotive Technology Institute, Cheonan, Chungnam 330-912, South Korea‡Polymer Materials Fusion Research Center and Department of Polymer-Nano Science and Technology and §Department of BINFusion Technology, Chonbuk National University, Jeonju 561-756, South Korea

ABSTRACT: Spontaneous vertical alignment (VA) of a nematic liquid crystal medium, 4-cyano-4′-heptyloxybiphenyl (7OCB), was successfully achieved by directely adding a smallamount of cyanobiphenyl monosubstituted polyhedral oligomeric silsesquioxane (POSS-CB) giant molecules [Kim, D.-Y.; Kim, S.; Lee, S.-A; Choi, Y.-E.; Yoon, W.-J.; Kuo, S.-W.;Hsu, C.-H.; Huang, M.; Lee, S. H.; Jeong, K.-U. J. Phys. Chem. C 2014, 118, 6300−6306].The cyanobiphenyl moiety chemically attached to the pristine POSS improved the initialsolubility with the 7OCB molecules and alleviated the macroscopic aggregates. However,the phase behavior and structural evolution of the finely tuned POSS-CB giant moleculeswith 7OCB are still open questions. Based on the thermal, microscopic, and scatteringexperiments, it was realized that the POSS-based giant molecules strongly interacted withhost 7OCB at a molecular level and induced the layered structure. The POSS groups in thePOSS-CB giant molecules were laterally close-packed to create the stable two-dimensional(2D) crystalline platforms, where the tethered CB moieties provided the enough emptyspaces for 7OCB to crawl into the empty zones. Additionally, the phase behavior of the mixtures was clarified by performing thetheoretical calculation based on a combined Flory−Huggins (FH)/Maier−Saupe (MS)/phase field (PF) model.

■ INTRODUCTION

Liquid crystal display (LCD) is commonly encountered inelectronic and optical devices. Depending on the initial LCalignments, the operating LCD panel can be classified intotwisted nematic (TN), in-plane switching (IPS), and verticalalignment (VA) modes.1−8 In order to manipulate theorientation of LC molecules, the polyimide (PI) alignmentlayer is conventionally coated on the glass substrates. Thealignment layer is prepared via a series of complicatedmanufacturing steps such as coating, curing, and mechanicalrubbing processes.6−8 The dust particles and electrostaticcharges generated between the alignment layer and the rubbingpad can cause the serious problems for the applications.Therefore, the alignment-layer free LCD devices have beenhighly demanded. Among various LCD modes, the VA modehas recently received great attentions because of its excellentelectro-optical performances including high contrast, wideviewing angle, and fast response time.1−5 The LC moleculesin the VA mode align vertically on the glass substrate and shiftto the horizontal direction by applying electric voltage.Nanosized polyhedral oligomeric silsesquioxane (POSS)

particle has been proposed to generate the VA alignment ofLC molecules.9−12 Inorganic POSS nanoparticle composed ofsilicon and oxygen linkage may preferentially interact with theglass substrate, forming a homeotropic monolayer. Suppose thePOSS nanoparticles are uniformly distributed within a LC

media, the orientation of host molecules can be tuned preciselydepending on the degree of alignment of the POSSnanoparticles. However, the POSS nanoparticles are generallyvulnerable to aggregate themselves because of inherentnanoscale nature. These clusters with several micrometerdiameters can trigger light scatterings in visible wavelengthregions, resulting in reducing the contrast of LCD panel.13,14 Inour previous work, the cyanobiphenyl mesogen was chemicallylinked to the pristine POSS nanoparticle (POSS-CB) toenhance the compatibility with the 7OCB host molecules andprevent the aggregation of POSS nanoparticles (Figure 1a).15 Itwas found that the degree of alignment was mainly affected bythe POSS-CB/NLC blending ratio and processing procedures.As confirmed by conoscopic observation, the addition of POSS-CB less than 0.1 wt % readily induced the VA of 7OCB withoutcausing any light scattering. We proposed that the verticalalignment of the 7OCB molecules was driven by POSS-CB: the7OCB molecules crawled into the empty spaces among the CBtethered groups which chemically attached to the two-dimensional (2D) self-assembled POSS crystalline platformon the substrates (Figure 1b and c). However, the validity ofour hypothesis has not been sufficiently attested because of the

Received: October 1, 2014Revised: December 3, 2014

Article

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difficulty in distinguishing respective molecules within a thinlayer. Self-assembly of the POSS nanoparticles substituted withvarious organic materials such as block copolymers, liquidcrystals, and fullerene has been examined extensively.16−21

Phase behavior of the surfactant-modified nanoparticles isprofoundly influenced by the chemical structures and size oforganic substituents. Therefore, the understanding of the phasebehavior of amphiphilic POSS-CB giant molecules in a partiallyordered 7OCB media is immense interest. The favorableinteraction between the CB and 7OCB mesogens is expected toaffect the miscibility and structural development in a mixedstate.The main purpose of the present work is to investigate the

phase behaviors and corresponding morphological evolutionsof the POSS-CB/7OCB mixtures over the entire composition.It is informative to understand the spontaneous VA mechanismof 7OCB host molecules by the 2D self-assembled POSS-CBplatforms. First of all, the phase behavior was investigatedexperimentally using differential scanning calorimetry (DSC),polarized optical microscopy (POM), and wide-angle X-raydiffraction (WAXD) techniques. The formation of self-assembled superstructures was monitored by conductingsmall-angle X-ray scattering (SAXS) and transmission electronmicroscopy (TEM) analysis. Numerical calculation wasconducted to establish the phase diagram on the basis of acombined Flory−Huggins (FH)/Maier−Saupe (MS)/phasefield (PF) model and tested with the experimental results.

■ EXPERIMENTAL SECTIONSample Preparation and Characterization. The POSS-

CB/7OCB mixtures with different blending ratios wereprepared using a vortex mixer. The mixtures were heateduntil both constituents were melted completely and then stirredthoroughly for 30 min to ensure complete mixing. Thehomogeneous blends were gradually cooled down to ambienttemperature in order to afford the homogeneous state.The phase transition temperatures of neat POSS-CB and

7OCB and their blends were determined using differentialscanning calorimetry (DSC, PerkinElmer PYRIS Diamond)equipped with an Intracooler 2P apparatus. Temperature and

heat flow were calibrated using an indium standard and thenthe sample was scanned at the same heating and cooling rate.The morphology development at various temperatures wasmonitored using polarized optical microscopy (POM)(ECLIPSE E600POL, Nikon) coupled with a heating stage(LTS 350, Linkam). One-dimensional (1D) wide-angle X-raydiffraction (WAXD) experiment was conducted in thereflection mode using a Rigaku 12 kW rotating anode generatorcoupled with diffractometer. The X-ray beam was Cu Kα with awavelength of 0.154 nm. The peak position and width werecalibrated with silicon crystals. The samples were scanned atdifferent temperatures in the course of heating and cooling witha scan rate of 2°/min. Intensity profile of small-angle X-rayscattering (SAXS, model and manufacturer) was plotted againstq = 4π sin θ/λ. Transmission electron microscopy (TEM,JEOL-1230) with an accelerating voltage of 120 kV was utilizedto record the bright-field images of the POSS-CB/7OCBmixtures. The TEM images were taken using a digital CCDcamera.

Model Description for Numerical Calculation. Acombined Flory−Huggins ( f FH), Maier−Saupe ( fMS), andphase field ( f PF) model has been employed to describe thephase behavior of liquid−liquid demixing, nematic ordering,and crystal solidification of the POSS-CB/7OCB mixtures,respectively. The liquid−liquid demixing can be describedaccording to Flory−Huggins (FH) theory,22,23

ϕϕ

ϕϕ χ ϕ ϕ= + +f

r rln lnFH 1

11

2

22 FH 1 2

(1)

where r1 and r2 refer to the numbers of lattice sites occupied bythe respective constituents and can be related to the volumefractions through ϕ1 = n1r1/(n1r1 + n2r2) and ϕ2 = n2r2/(n1r1 +n2r2). The n1 and n2 are the numbers of constituent molecules,and n = n1r1 + n2r2. The Flory−Huggins interaction parameter,χFH, representing amorphous−amorphous interaction isdescribed by χFH = A + B/T, where A and B are constants.Maier−Saupe (MS) mean-field theory is employed to

account for nematic ordering,24−28

Figure 1. Geometric dimensions of neat POSS-CB and 7OCB molecules (a), schematic illustration of vertical alignment in mixtures of POSS-CBand 7OCB molecules (b), and orthoscopic POM image of the test cell with 0.02 wt % POSS-CB (c). The inset image of orthoscopic POM is itscorresponding conoscopic image.

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ϕνϕ= − +⎜ ⎟

⎛⎝

⎞⎠f

rZ sln

12

MS 1

11 2

2

(2)

where s, Z, and ν are orientational order parameter, partitionfunction, and nematic interaction parameter, respectively. Thenegative term in eq 2 indicates the decrease of entropy due tolong-range orientational order of LC molecules, while thesecond term represents the enthalpic contribution due toalignment of nematic directors. Physical meaning of therespective parameters was fully demonstrated elsewhere.24

The free energy density of crystal solidification may bewritten according to the phase field (PF) model in which freeenergy of pure crystal has a Landau-type asymmetric double-well,29−32

∫ψ ψ ψ ζ ψ ζ ψ

ζ ζψ

ζ ζψ ψ

= − −

= −+

+

ψ

⎡⎣⎢

⎤⎦⎥

f W

WT T T T

( ) ( )( ) d

( ) ( )

2

( ) ( )

314

i i i i i i i i

ii i i m

ii i i m

i i

0,0

,0 , 2 ,0 , 3 4

i

(3)

where ψi represents the crystal order parameter of pureconstituents. The coefficient Wi, indicating the energy barrierfor the solidification to overcome, can be evaluated from themelting temperature (Tm) and heat of fusion (ΔHu).The contribution from coupling interaction should also be

taken into consideration.

χ ψ χ ψ ψ χ ψ α ψ ψ ϕ ϕ= − + +f s( 2 )couplingca 1

2cc 1 2 ac 2

22

21 2 1 2

(4)

The last term in eq 4 corresponding to the nematic (N)−crystal (Cr) interaction is very small relative to other terms andthus ignored in our calculation. The crystal−amorphous (χca)and amorphous−crystal (χac) interaction parameter is propor-tional to the heat of fusion of crystalline component, that is, χca∼ ΔH1

u/RT and χac ∼ ΔH2u/RT, and the crystal−crystal

interaction parameter is related to χca and χac by the geometricmean approach, that is, χcc = cw(χcaχac)

1/2, where cw signifies thedeparture from the ideality.32

By combining eqs 1−4, the total free energy of the POSS-CB/7OCB system is expressed as follows.

ϕ ψ ϕ ψ ϕ ψϕ

ϕϕ

ϕ

χ χ ψ χ ψ ψ χ ψ ϕ ϕ

ϕνϕ

= + + +

+ + − +

+ − +⎛⎝⎜⎜

⎞⎠⎟⎟

f s f fr r

rZ

s

( , , ) ( ) ( ) ln ln

( 2 )

1ln

2

1 1 2 21

11

2

22

aa cc 12

cc 1 2 ac 22

1 2

11

12

22

(5)

Order parameters (ψ1, ψ2, s2) can be determined by minimizingthe total free energy with respect to the respective orderparameters:

ϕ ψ ϕ ψψ

ϕ ψψ

∂∂

=∂

∂=

∂∂

=

f ss

f s

f s

( , , )0;

( , , )0;

( , , )0

2 1

2 (6)

The coexistent points can be determined by balancing thechemical potentials for each phase, namely, (∂f/∂ϕi)|ϕi

α = (∂f/

∂ϕi)|ϕiβ.33

■ RESULTS AND DISCUSSIONPhase Behaviors of POSS-CB and 7OCB. Figure 2

depicts the DSC thermograms of neat POSS-CB and 7OCB

obtained at a heating rate of 2.5 °C/min. Neat POSS-CBexhibits a minor endothermic peak at around 57 °C (0.3 kJ/mol) and a major endothermic peak (13.1 kJ/mol) at around160 °C. Meanwhile, neat 7OCB shows the strong and weakendothermic peaks at around 51 °C (24.2 kJ/mol) and 72 °C(1.0 kJ/mol), respectively. A minor endotherm observed near45 °C may be a consequence of the melting of unstable crystalbecause in contrast to other peaks the intensity is graduallyreduced upon increasing a heating rate. The interaction duringthe self-assembly process is mainly changed at highertemperature for POSS-CB, while the 3D long-range positionalorder is observed at lower temperature for 7OCB. Thisexplanation can be supported by investigating the morpho-logical development using optical microscopy.As shown in the inset of Figure 2a, a diamond shaped single

crystal at 40 °C starts to melt at 158 °C and then transforms tothe completely isotropic melt at 160 °C. No morphologicalchange associated with the mesophase transition is detectedduring the course of cooling and heating at around 57 °C. Theneat CB typically reveals the crystal−nematic and nematic−isotropic transition at around 48 and 68 °C, and therefore, allcharacteristic transitions may be destroyed when combinedwith POSS. The POSS nanoparticles strongly aggregatethemselves even though CB groups are in a molten or LCstate. Similar behaviors were reported in the POSS-based giantsurfactants.34,35 Molecular self-assembly of POSS-CB can beaccomplished at high temperature due to the stronginteractions of POSS groups. In the case of 7OCB molecules,the crystalline texture is melted to show Schlieren texture,indicative of nematic (N) phase (please see the inset of Figure2b). Upon further heating above 72 °C, the N phase transforms

Figure 2. DSC thermograms of POSS-CB (a) and 7OCB (b) obtainedat a heating rate of 2.5 °C/min. Inset images were taken during theheating process at 2.5 °C/min.

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to the isotropic melt. The DSC and POM results of neat 7OCBare almost consistent with the data in the literatures.36,37

Although the DSC and POM experiments are useful tomonitor the heat absorption/release events and morphologicalchange during the phase transitions, these techniques do notprovide direct information about the molecular structures. Thetemperature-dependent structural change of neat POSS-CB and7OCB are further investigated using 1D WAXD technique.Figures 3a shows the 1D WAXD powder patterns of neat

POSS-CB. Structures in two different length scales, that is, thenanometer scale in the low 2θ angle region between 1.5° and 9°and the sub-nanometer scale between 9° and 30°, aredistinguishable. Above 160 °C, two amorphous halos aredetected at around 8.3° (d-spacing = 1.06 nm) and 18.2° (d-spacing = 0.49 nm). The amorphous halo at 8.3° is originatedfrom the average distance among the disordered POSS groups,while the amorphous halo at 18.2° comes from the averagelateral distance between CB groups. It means that the POSSand CB groups are nanophase-separated even in the isotropicstate. Below 160 °C, these nanophase-separated domains arecrystallized, revealing the multiple Bragg reflections at 2θ = 6.2°(d-spacing = 1.42 nm), 8.3° (1.06 nm), 10.9° (0.81 nm), 12.1°(0.73 nm), 16.5° (0.54 nm), 19.1° (0.47 nm), 19.7° (0.45 nm),and 24.8° (0.36 nm). No obvious changes are detected at

around 57 °C. Therefore, self-assembly of the POSS-CB giantmolecules may be dominated by the organization of POSSgroups. The CB groups tethered at one corner of POSSnanoparticle may not affect the lateral organization of POSSand thus the melting point of POSS-CB corresponds to that ofPOSS crystals. This explanation can be further supported bythe fact that the diffraction patterns of POSS-CB below 2θ =15° are almost identical to those of other POSS-based giantsurfactants.37−42

As shown in Figure 3b, the crystalline 7OCB molecules at 30°C reveal the diffraction peak at 2θ = 3.2° (d-spacing = 2.76nm) and its corresponding higher order diffractions at 2θ = 4.3°and 6.4° due to the positional long-range order along the long-axis. On the other hand, the several diffraction peaks above 2θ= 10° are closely associated with the lateral molecularorganization. When the temperature reaches 55 °C, thecrystalline phase transforms to the nematic phase, exhibiting abroad halo at 2θ = 20.6°. Upton further heating above 70 °C,the scattering halo slightly shifts to a low angle region due toisotropization.

Phase Behaviors of POSS-CB/7OCB Mixtures. Figure 4shows the DSC thermograms of the POSS-CB/7OCB mixtures

with different ratios. The POSS-CB and 7OCB molecules aresimply denoted as P and N, respectively. For instance, themixture containing 10 wt % POSS-CB and 90 wt % of 7OCB issymbolized as P1N9. In mixtures, the crystal melting point(Tm) of POSS-CB and nematic−isotropic (TNI) transition of7OCB are depressed, while crystal−nematic (TCrN) transitionof 7OCB remains rather stationary. The intensity of these peaksbecomes broader and weaker by increasing counterpart. It isnoticed that a weak endothermic peak in the temperature rangeof 79−87 °C newly appears in mixtures containing 30−70 wt %of POSS-CB, as shown in the enlarged scale of Figure 4.Morphological changes of the mixtures are investigated usingoptical microscopy.POM study is undertaken on the 60/40 POSS-CB/7OCB

composition. As shown in Figure 5, upon cooling fromisotropic melt at a rate of 2.5 °C/min, the anisotropic POSS-CB crystals are suddenly evolved in the continuum of dark areaat 80 °C (Figure 5b). Similar textures are retained uponlowering temperature to 70 °C although crystal domains grow

Figure 3. Sets of 1D WAXD patterns of POSS-CB (a) and 7OCB (b)obtained at a heating rate of 2.5 °C/min.

Figure 4. DSC thermograms of POSS-CB/7OCB mixtures obtained ata heating rate of 2.5 °C/min. The inset shows a broad transition peakin the temperature range of 79−87 °C.

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in size slightly (Figure 5c). Since the anisotropic POSS-CBcrystals and isotropic 7OCB coexist, this temperature rangemay be identified as crystal + liquid (Cr1 + L2) coexistenceregion. When the temperature approaches 60 °C, the wholemicroscopic view becomes anisotropic (Figure 5d). Theenlarged view clearly shows the evolution of Schlieren textureswith line disclinations, signifying that the crystalline phase ofPOSS-CB coexists with N phase of 7OCB (Cr1 + N2). Texturalchange is no longer observable upon lowering temperaturebelow 50 °C (Figure 5e and f). The 7OCB constituent is notcrystallized during a course of cooling because of thesupercooling effect. We explore the structural evolution insub-nanometer as well as nanometer length scales using thestructure-sensitive 1D WAXD and SAXS techniques.The 1D WAXD patterns are acquired at the 60/40 POSS-

CB/7OCB composition. As shown in Figure 6, the multiplereflection peaks at 2θ = 6.1° (d-spacing of 1.447 nm), 8.2°(1.077 nm), 11.0° (0.803 nm), 12.2° (0.725 nm), 18.7° (0.474

nm), 19.2° (0.462 nm), 20.2° (0.439 nm), 22.3° (0.398 nm),and 24.0° (0.370 nm) are detected at 30 °C. The peak positionsare almost identical to those of a combined neat POSS-CB and7OCB. At 50 °C, the diffraction peaks at 2θ = 18.7°, 20.2°,22.3°, and 24.0° suddenly disappear. As manifested in Figure3b, these peaks correspond to the neat 7OCB crystals and thusthe emergence of amorphous-like broad halo above 50 °C isdue to phase transformation of the 7OCB crystals to nematicphase. It is difficult to identify the phase transition from thenematic phase to the isotropic phase because the broadscattering halo is often indistinguishable. Therefore, similardiffraction patterns are obtained in the temperature range from50 to 120 °C. Upon heating above 120 °C, the residual peaks at2θ = 8.2°, 11.0°, and 19.2° disappear completely, revealing dualamorphous halos at 2θ = 8.3° and 18.9°. The DSC and POMresults indicate that the crystalline POSS-CB molecules melt tothe isotropic phase when heated above 120 °C. However, theendothermic peak in the temperature of 79−87 °C is stillinexplicable. Various architectures such as lamellar, 2Dhexagonal, 3D cubic, and bicontinuous superstructures maybe constructed at the nanometer length scale. The formation ofself-assembled structure in mixtures of POSS-CB and 7OCBhas been exploited by monitoring the SAXS patterns at varioustemperatures.

Self-Assembly and Phase Diagram of POSS-CB/7OCBMixtures. As shown in Figure 7, no distinct scattering peak isobserved for the neat POSS-CB and 7OCB constituents in thetemperature range investigated. It indicates that the POSS-CBand 7OCB molecules themselves do not create the hierarchicalsuperstructure in several nanometer length scales. When twomolecules are mixed at the blending ratios of 60/40 and 30/70POSS-CB/7OCB, on the other hand, the scattering peak ataround q = 1.2 nm−1 is clearly discerned, which corresponds toa periodic distance of 5.23 nm (Figure 8).When the mixtures are heated above 45 °C, the scattering

peak at q = 1.3 nm−1 slightly shifts toward a smaller q value.Variation of scattering profiles is correlated to the interlayerspacing. Upon increasing temperature above the crystal−nematic transition (TCrN,2) of 7OCB, the rod-shaped molecules

Figure 5. Optical micrographs of 60/40 POSS-CB/7OCB mixture taken at 95 °C (a), 80 °C (b), 70 °C (c), 60 °C (d), 50 °C (e), and 40 °C (f) at acooling rate of 2.5 °C/min. The enlarged view in (d) clearly shows Schlieren textures with line disclinations.

Figure 6. Sets of 1D WAXD patterns of 60/40 POSS-CB/7OCBmixture obtained at a heating rate of 2.5 °C/min.

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may possess the large-amplitude of mobility to interact with CBmoieties, resulting in expansion of interlayer spacing betweenPOSS monolayers. The position and intensity of scatteringpeak almost remain unchanged up to 85 °C for 60/40 and 80°C for 30/70 POSS-CB/7OCB mixtures due to thepreservation of lamellar spacing. Upon further heating, theinduced structure disappears. The temperature-dependentSAXS results are in good agreement with the transitiontemperature measured by DSC analysis. The DSC thermo-grams revealed the weak endothermic peak at around 86 °C for60/40 and 79 °C for the 30/70 POSS-CB/7OCB composition,respectively.The TEM image of the mixture is presented in Figure 9a.

Alternative dark and gray lines are observed due to disparity inelectron density of the POSS-rich and the CB-rich phase. Thehighly ordered bilayered structure with an average spacing ofapproximately 5.2 nm is constructed, where the POSS coresmay be closely packed by the van der Waals interaction andarranged into the bilayered structure. The 7OCB moleculescrawl into the 2D bilayered POSS-CB platforms.The calculated length of neat POSS-CB and 7OCB along the

long axis is about 3 and 1.8 nm (please see Figure 1).Therefore, the layer spacing estimated from the SAXSexperiments is slightly broader than a combined length ofPOSS-CB and 7OCB. It indicates that the additional 7OCBmolecules are intercalated. Schematic molecular assemblyderived from the SAXS and TEM experiments is suggested inFigure 9b. The diameter of POSS-CB molecules occupying the

top and bottom layers is about 1 nm, and their layer spacing isabout 5.2 nm. It is reasonable to assume that the self-assembledstructure of the POSS-CB/7OCB mixtures has a cylindricalshape with unit volume of about 4.1 nm3. Ideally, about nine7OCB molecules can crawl into the 2D POSS-CB bilayeredplatforms, which corresponds to 48/52 POSS-CB/7OCB ratiowhen converted into weight fraction. In this regard, appearanceof scattering peak at the 60/40 and 30/70 POSS-CB/7OCBcomposition is understandable. Peak positions are almostidentical regardless of compositions because the number of7OCB molecules placed between POSS-CB bilayers is limited.The 7OCB molecules may be fully intercalated when the 7OCBcontent reaches 40 wt %. A sequence of second and thirdcharacteristic scattering peaks from the lamella structures isweak in the WAXD results because of quasi-long-range order ofthe layered structures. As the self-assembled structures are notinfluenced by the lateral distance of the mesogens, the newendothermic peak observed in DSC analysis is attributable tothe ordered structure formed by self-assembly of CB and 7OCBmesogens. Preferential affinity of the CB moiety to 7OCB maylead to the hierarchical smectic structure.Figure 10 shows the experimental and theoretical phase

diagram of the POSS-CB/7OCB system. Weak endothermicpeaks detected at around 57 °C from the neat POSS-CB and 45°C from the neat 7OCB are ignored in calculation because ofthe ambiguity and nonequilibrium nature. The theoreticalphase diagram is thus calculated using the following

Figure 7. Sets of SAXS patterns of POSS-CB (a) and 7OCB (b)obtained at a heating rate of 2.5 °C/min.

Figure 8. Sets of SAXS patterns of POSS-CB/7OCB mixtures in the60/40 (a) and 30/70 (b) compositions. Heating rate was maintainedat 2.5 °C/min.

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experimental parameters: r1 = 2, r2 = 1, A = −1.0, χca = 0.45 at150 °C, ΔH1

u = 13.1 kJ/mol at Tm,1 = 160 °C, ΔH2u = 24.2 kJ/

mol at TCrN,2 = 51 °C, TNI,2 = 72 °C. The calculated solid lines(liquidus and solidus) describe well the experimental results(denoted by various symbols). It shows the eutectic phasebehavior, consisting of single phase crystal (Cr1) and nematic(N2), and crystal + liquid (Cr1 + L2) and liquid + nematic (L1 +N2) coexistence regions. The broad coexistence regions indicatethat the POSS-CB and 7OCB molecules are partially miscible.By the aid of numerical calculation, the phase boundaries of thesingle and coexistence regions are clearly discerned.From the DSC, SAXS, and TEM experiments, the formation

of the layered structures is clearly observed by interactionbetween CB groups and 7OCB molecules and thus thesubmerged coexistence regions involving induced smectic phase(iSm) are also added. Induced smectic phase occurs when theinteraction between two different mesogens is strong enough.Phase behavior of binary mesogenic mixtures involving inducedsmectic phase is well reported in the literature.43,44 Guided bythe references, the dual UCST envelopes develop in the

intermediate concentrations, where the narrow iSm phase isencompassed by the Cr1 + iSm and Cr1 + Cr2 + iSmcoexistence regions. In general, the Cr2 + iSm coexistenceregion appears instead of the Cr1 + Cr2 + iSm coexistenceregion, but the POSS-CB crystal (Cr1) is identified over thebroad composition range from the DSC, POM, and WAXDresults in our system. Therefore, the mixtures reveal Cr1 + iSm,iSm, Cr1 + Cr2 + iSm along with narrow single crystal (Cr1,Cr2), nematic (N2), and iSm at room temperature. Uponincreasing temperature, the 7OCB crystal transforms to thenematic and liquid phase, but the POSS-CB crystal remainsunchanged, leading to the Cr1 + N2 + iSm and Cr1 + L2 + iSmcoexistence regions.

■ CONCLUSIONS

The phase behavior and corresponding structural developmentof the POSS-CB/7OCB mixtures were investigated over thewhole composition range. The CB substituent generated theempty space by alleviating aggregation of the POSS cores aswell as improved the compatibility with 7OCB mesogens. Withaddition of the 7OCB molecules, the CB group in POSS-CBand 7OCB tend to form a locally ordered phase, as witnessedby the DSC and SAXS experiments. The TEM image clearlyshowed the construction of the layered structures with theperiodic distance of 5.2 nm, where the 7OCB moleculescrawled into the space between the 2D self-assembled POSSplatforms. On the basis of experimental results and numericalcalculations, a eutectic phase diagram was established with theformation of induced smectic phase. The constructed phasediagram revealed the various coexistence regions such as crystal+ liquid (Cr1 + L2), crystal + induced smectic phase (Cr1 +iSm), crystal + liquid + induced smectic (Cr1 + L2 + iSm),crystal + nematic + induced smectic (Cr1 + N2 + iSm), crystal +crystal + induced smectic (Cr1 + Cr2 + iSm), along with narrowsingle crystal (Cr1, Cr2), nematic (N2), and induced smectic(iSm) regions. Optical properties of the POSS-CB/7OCBmixtures and their self-assembly performances may beinformative for the potential applications in LC displayrequiring low temperature process, high contrast, and fastresponse time.

Figure 9. Bright-field cross-sectional TEM image of 2D bilayered structure (a) and schematic illustration of the self-assembled layered structures (b)of POSS-CB/7OCB mixtures.

Figure 10. Experimental (denoted by ○, ◇, △, □) and theoretical(solid line) phase diagram of POSS-CB/7OCB mixtures, exhibitingcrystal + liquid (Cr1 + L2), liquid + nematic (L1 + N2), crystal + liquid+ induced smectic (Cr1 + L2 + iSm), crystal + nematic + inducedsmectic (Cr1 + N2 + iSm), crystal + crystal + induced smectic (Cr1 +Cr2 + iSm), crystal + induced smectic (Cr1 + iSm), and narrow singlecrystal (Cr1, Cr2), nematic (N2), induced smectic (iSm) regions.

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■ AUTHOR INFORMATIONCorresponding Author*E-mail: kujeong@jbnu.ac.kr.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was mainly supported by Basic Science Research(2013R1A1A2007238), KIST Institutional program(2Z04320), and BK21 Plus program, Korea. D.-Y.K. appreciatesthe support from Global Ph.D. Fellowship Program.

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