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Calamitic Liquid-Crystalline Elastomers Swollen inBent-Core Liquid-Crystal Solvents

By Martin Chambers,* Rafael Verduzco, James T. Gleeson, Samuel Sprunt,

and Antal Jakli

[*] Dr. M. Chambers, Prof. A. JakliLiquid Crystal Institute, Kent State UniversityKent, OH 44240 (USA)E-mail: mchambers@ijs.si

Dr. M. Chambers, Prof. J. T. Gleeson, Prof. S. SpruntPhysics Department, Kent State UniversityKent, OH 44240 (USA)

Dr. R. VerduzcoCenter for Nanophase Materials SciencesOak Ridge National LaboratoryOak Ridge, TN 37830 (USA)

DOI: 10.1002/adma.200802739

� 2009 WILEY-VCH Verlag Gmb

Liquid-crystal elastomers (LCEs) combine the properties of liquidcrystals (LCs) and rubber elasticity.[1,2] They are unique materialsin their ability to change shape spontaneously by large degrees asa consequence of their global orientational (nematic) or combinedorientational/positional (smectic) order. LCE systems are thermo-tropic, and may also be responsive to light,[3–5] magnetic fields,[6]

or electric fields.[7,8] They have possible uses as artificial musclesor sensors in addition to being intriguing materials for basicresearch.

Bent-core nematic LCs (BCNs)[9] are a relatively new class ofmesogenic materials (compared to their calamitic—or rod-shaped—cousins). They exhibit both rich phase behavior andnovel properties; they also hold the potential of fundamentallynew technological applications. In particular, some BCNs haverecently been shown to exhibit a giant flexoelectric effect[10]

(electric polarization induced by orientational distortion of theaverage direction of molecular alignment[2]). Since the reportedflexoelectric coefficients are some three orders of magnitudelarger than standard calamitics, these BCNs have excitingpotential for applications ranging from power generation tosensing. Additionally, the phase behavior of mixtures of calamitic(rod-shaped) nematics and BCNs[11] show that the range of thenematic phase can be extended to temperatures approachingambient, presumably by frustrating the formation of the crystalphase. The flexoelectric response of such mixed BCN andcalamitic LCs is still sufficiently large to be promising forapplications, even at fairly low unit concentrations of the BCN.

However, to realize practical materials that take advantage ofthis response, one needs a method of encapsulating the BCN in amechanically robust but flexible matrix that stabilizes (orruggedizes) the BCN orientational order against variations inenvironmental conditions. A well-known property of commonisotropic elastomeric systems[12,13] is their ability to swell insuitable solvents. Applied to LCEs in both isotropic[14] and

anisotropic solvents of calamitic LCs,[15] this property has provenan effective means to manipulate viscoelastic properties andphase behavior. In this study, we demonstrate, to our knowledgefor the first time, the swelling of a calamitic nematicmonodomain LCE with large amounts (greater than 30mol%)of two types of bent-core mesogens, both of which show largeflexoelectric response in their pure nematic states. LCEs swollenwith the two mesogens are additionally investigated with respectto the degree of saturation of the latter’s hydrocarbon chains. Wereport on the swelling degree (amount of bent-core materialimbibed), characteristic swelling times, the resulting phases andtransition temperatures, and the orientational order parameter ofthe swollen LCEs. This study demonstrates a potential method forcreating a robust liquid-crystalline device with enhancedflexoelectricity.

The chemical structures of the various bent-core materialsutilized in our study are presented in Figure 1a and b. Thebent-core compound 4-chloro-1,3-phenylene bis[4-(10-decenyloxy)benzoyloxy]benzoate (BCa) was synthesized and purified follow-ing a previously described procedure.[16,17] Versions withsymmetric saturated and unsaturated hydrocarbon chains(designated ‘‘uu’’ and ‘‘ss,’’ respectively) were prepared. InBCa, the arms are relatively flexible, as the outer benzene ringsare separated by ester groups. The other bent-core mesogen,4,6-dichloro-1,3-phenylene-bis 4-[{40-(9-octenyloxy)}biphenyl]carboxylate (denoted BCb), features two chlorine units in itscentral phenyl ring, and the aromatic rings of the arms aredirectly linked, making themmuchmore rigid. This material andin variants featuring unsaturated (uu) and saturated (ss) chains aswell as an asymmetric combination of the two (designated ‘‘us’’)were prepared by a synthetic route previously published.[18] Thepurity of the samples was checked by high-performance liquidchromatography using a Merck–Hitachi chromatograph equippedwith a Merck RP18 column (Cat. No. 16051).

Various properties of the nematic and isotropic phases of theneat BCa and BCb mesogens are described in Refs.[19,20].Unsaturation of the hydrocarbon chain generally results inlowering of the transition temperatures and increasing the flowviscosity, which is, strikingly, several orders of magnitude higherin both nematic and isotropic phases than for typical rod-shapedmesogens.[21] The transition temperatures and associatedenthalpy values of the BCa and BCb compounds are shown inTable 1. All mesogens exhibit nematic phases; BCa-ss alsoexhibits a smectic phase whose detailed in-layer structure isunknown, but is believed to be of the tilted (smectic-C) type.

A conventional poly[oxy(methylsilylene)]-based side-chainmonodomain calamitic LCE, whose components are shown in

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Figure 1. Structure of the bent-core liquid crystals, a) BCa-uu (4-chloro-1,3-phenylene bis[4-(10-decenyloxy)benzoyloxy]benzoate), b) BCb-uu (4,6-dichloro-1,3-phenylene-bis 4-[{40-(9-decenyloxy)}biphenyl] carboxylate), andalso the nematic monodomain LCE components, c) mesogen andd) crosslinker. e) Differential scanning calorimetry of the mesogen BCa-ss(solid line) and swollen LCE–BCa-ss (dashed line). f) Schematic diagram ofan LCE swollen with a bent-core LC.

able 1. Transition temperatures and associated enthalpy values for the calamitic LCE, two bent-core liquid-crystal materials, and their unsaturated andaturated variants. Values were taken from differential scanning calorimetry.

aterial TI-N (K) DHI-N (Jmol�1) TN-SmX (K) DHN-SmX (Jmol�1) TN-C (K) DHN-C (Jmol�1) TSmX-C (K) DHSmX-C (Jmol�1) TG (K)

LCE 347.0 15.0 – – – – – – 267.2

Ca-ss 362.3 15.1 335.7 130.7 – – 327.2 388.7 –

Ca-uu 345.1 8.4 – – 321.7 438.6 – – –

Cb-ss 385.7 26.1 – – 352.2 835.0 – – –

Cb-us 379.9 17.4 – – 331.0 547.7 – – –

Cb-uu 370.8 16.2 – – 321.1 1301.9 –

Figure 2. Isotropic swelling dynamics of the LCE described in the text inBCb-uu; the data points are the measured values of the swelling fractionf¼V0/V and the solid line is a fit to a double exponential decay with time.Inset: pictures of the swelling LCE at t¼ 100 and 10 000 s.

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Figure 1c and d, was prepared by the Finkelmann procedure[22]

with a crosslinking degree of 7.5%. The LCE samples wereflushed in toluene before use to remove any soluble contents. Theunswollen LCE, denoted ULCE, exhibits a nematic phasefollowed by a glassy transition (details are given in Table 1).For our swelling experiments, individual pieces of ULCEwere cutout from a single prepared strip. (Fig. 1f shows a schematicillustration of the components in the swollen nematic LCE.)

Swelling of the LCE in the bent-core solvents was carried out inthe isotropic phase of bothmaterials. The LCE is observed to swellisotropically in three dimensions. The higher the temperature inthe isotropic phase, the faster solvent diffusion becomes, as boththe solvent viscosity is lowered and the LCE network elasticity isreduced. The degree of the swelling, characterized by the ratiof¼V0/V (where V0 and V(t) are the unswollen and swollen LCEvolumes at time 0 and t, respectively), is controlled by thesolubility parameters of the LCE–bent-core mixture, while thecharacteristic swelling time is controlled by diffusion, andtherefore viscosity. The typical time variation of parameterf during swelling (shown in Fig. 2 for swelling in the BCb-uusolvent) can be fitted to a double exponential decay of the formf ¼ f1 þ f0e

�t=t0 þ f1e�t=t1 . This behavior is notably different

from that observed for swelling of LCEs in calamitic (rod-shaped)LC solvents in the isotropic phase, where a single exponentialtypically describes the time dependence of f.[23] Moreover, the

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Table 2. Change in original LCE properties when swollen in BCa (LCE–BCa) and BCb (LCE–BCb) material until saturated at T¼ 403 K. The measuredincreases in volume,DV, and weight, allow the simple calculation of the average bent-corematerial fraction in the swollen LCE. Transition temperatures andenthalpy values for all swollen LCE systems were obtained from DSC.

LCE solvent DV c (mol%) t (103 s) TI-N (K) DHI-N (Jmol�1) TN-SmX (K) DHN-SmX (Jmol) TSmX-C (K) DHSmX-C (Jmol�1) Tg (K)

BCa-ss 0.93 32 7.98 354.1 6.4 325.6 27.0 303.4 46.2

BCa-uu 0.70 39 10.00 359.0 0 338.6 10.3 302.5 3.0

BCb-ss 1.11 35 7.56 372.3 8.7 339.9 132.8 295.0 6.9

BCb-us 1.29 41 8.76 358.5 22.2 – – – – 272.0

BCb-uu 1.13 30 10.50 360.0 0.7 – – – – 269.4

Figure 3. Relative length, L/L0 (open circles), and birefringence (opensquares) versus reduced temperature T� TI-N for a) unswollen LCE (soliddiamonds curve is higher stress of 700 kPa), b) LCE–BCa-uu,c) LCE–BCa-ss, d) LCE–BCb-uu, e) LCE–BCb-us, and f) LCE–BCb-ss.

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characteristic time for the slower component of the decay in thebent-core cases (presented in Table 2) is around 8000–10 000 s,roughly 100 times longer than for swelling in calamitics (andreflecting the tremendous difference in viscosity between the twotypes of LC solvent). As expected, the lower-viscosity (saturated)bent-core materials produce a slightly quicker swelling responsein the LCE. The value of f1 also varies marginally betweenunsaturated and saturated bent-core mesogens, and between thetwo mesogen types (BCa and BCb). In particular, we findf1

BCa � 0:55 and f1BCb � 0:46. The corresponding amount of

imbibed material is 30–40mol%. This compares very favorablywith swelling in calamitics, and provides a vital first validation ofthe feasibility of producing bent-core swollen LCEs for potentialapplications.

The double-exponential character of the swelling process (Fig. 2)is an interesting result: while the overall process is very slow, thetime constant of the faster component (found in our samples tobe around 300 s) is comparable to the single characteristic timescale typically observed for swelling LCEs in ordinary calamiticsin the isotropic state. This result is consistent with prior data onBCa and BCb,[18,21] which indicate unconventional departuresfrom normal fluid LC behavior, especially in the nematic andisotropic viscosities and in flow properties, which are possibly dueto an unusual nanostructural organization of the molecules (e.g.,the presence of molecular aggregates or clusters of a nonfluidnature persisting even into the isotropic phase). One couldtherefore speculate that the dual time scales of the swellingsignify a combination of fast diffusing component of thebent-core solvent (behaving like a conventional fluid of smallmolecules) and of a substantially more viscous component(having unconventional fluid, or perhaps solid or glassy proper-ties). A more complete characterization of the short-rangestructure in the nematic and isotropic phases of bent-corecompounds are obviously necessary to confirm this speculation.

The transition temperatures and associated enthalpy values ofthe various bent-core swollen LCEs are shown in Table 2. Bothparameters weremeasured with differential scanning calorimetryand polarizing microscopy. No dual transitions are observed inthe swollen LCEs (Fig. 1e). When the unsaturated bent-corecompounds (BCa-uu and BCb-uu) are overheated for a consider-able length of time, there is a noticeable degree of polymerizationof the mesogens due to the double-bonded chains; this results insignificant increases in the isotropic–nematic (I-N) transitiontemperature in the swollen LCEs. No such effect is observed forthe saturated bent-core solvents.

The relative length change (L/L0) of the monodomainunswollen nematic LCE (ULCE) shows a continuous increase

� 2009 WILEY-VCH Verlag Gmb

as temperature is decreased below the isotropic-to-nematictransition (see Fig. 3a). Here L is the length parallel to theaverage direction of alignment of the mesogenic component inthe nematic phase, and L0 is the corresponding length in theisotropic phase. Figure 3a also shows the effect on L/L0 of anexternal stress of roughly 700 kPa (applied by hanging weight).The difference in the relative length change is L/L0¼ 1.67 versusL/L0¼ 1.51 for the unstressed elastomer at ambient temperature,roughly 45 K below the I-N transition. The transition temperatureitself is marginally altered.

In the ULCE, the macroscopically observable relativechange in length is related to the global polymer backboneorder parameter QB according to the expressionL=L0 ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ 2QBð Þ= 1�QBð Þ3

p.[1] On the other hand, the optical

birefringence (or measured refractive-index anisotropy Dn) issensitive to the order parameter of the mesogenic component, Q.When extrapolated to ‘‘zero’’ temperature (i.e., crystallization orglass temperature), where we shall assume Q � 1, the orderparameter can be expressed with reasonable accuracy asQ ¼ Dn Tð Þ=Dn T ¼ 0ð Þ. For the ULCE, the linear relation

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Q ¼ aQB applies,[1] where the constant a characterizes the degree

of orientational coupling between nematically ordered mesogenand polymer backbone. Figure 3a compares results for Dn and L/L0 measured for the ULCE. The birefringence evolves in a samemanner as L/L0, and reaches a value DnULCE � 0:11, which iscomparable to previously measured values for this LCEsystem.[1,24] Use of the expressions relating Dn to Q and L/L0toQB yields a value of a ¼ 0:6 for the ULCE, which is also in goodagreement with values a� 0.7[1] reported by other investigatorson nematic LCEs.

We now turn to results for the swollen LCEs. The LCE swollenin the BCa solvents shows a decrease in the I-N transitiontemperatures from the solvent values and an increase from theLCE value, as expected (see Table 2). Both swollen LCEs alsoexhibit a smectic-like phase. This is signaled by an abruptreduction in L/L0 at lower temperatures (Fig. 3b and c), and by theappearance of a corresponding peak in differential calorimetry,which is shown in Figure 1e for LCE–BCa-ss. Crystallization ofthe LCE–BCa systems occurs near room temperature. Prelimin-ary small angle X-ray data confirm the smectic nature of theintermediate phase. As revealed in Figure 3b and c, the swollenLCE–BCa systems still retain a large shape anisotropy withL=L0 � 1:25, giving QLCE�BCa

B � 0:5QULCEB . The birefringence

shows a significant anisotropy retained after swelling, withDna�uu � 0:09 and Dna�ss � 0:07 at a temperature 25 K below theI-N transition; the temperature dependence of Dn scales with thatof the L/L0. The calculated order parameters show that theswollen LCEs have lower values of the coupling parameter a,aa�uu � 0:29 and aa�ss � 0:33, compared to the ULCE.

The LCEs swollen in the BCb solvents also show similarproperties (Table 2, Fig. 3d, e, and f). However, only the BCb-sssolvent-swollen LCE sample exhibits a smectic phase andcrystallization; all other BCb swollen LCEs exhibit a purelynematic phase and a glassy transition. The values of L/L0at a temperature 30K below the I-N transition areL=L0 � 1:28; 1:42; 1:25 for the LCE–BCb-ss, LCE–BCB-us, andLCE–BCb-uu, respectively. The optical birefringence at acomparable temperature is Dn � 0:14; 0:14; and 0:06 for theLCE–BCb-ss, LCE–BCB-us, and LCE–BCb-uu samples, respec-tively (to obtain the estimate for Dn for LCE–BCb-ss, weextrapolated from the available data, close to the transition, usingthe curve for L/L0). Again, Dn tracks the L/L0 profile withtemperature. The calculated QB and Q values show that the BCbswollen LCEs have values ab�ss � 0:48, ab�us � 0:50, andab�uu � 0:35. These values are somewhat higher than thosefound for swelling with BCa.

Overall, the influence of the imbibed bent-core LC reduces themesogen-backbone coupling constant; this is most likely due tothe nonchemically attached bent-core mesogens being able tofreely rotate and mediate the interaction (or packing) between thechemically attached rodlike calamitic mesogens and the polymerbackbone. However, the values of Dn recorded in the LCEsswollen with saturated BCb solvent are comparable to (or evenlarger than) the maximum Dn previously measured in the puresolvent (Dn� 0.1), as well as the value for the ULCE mentionedabove. This indicates that the mesogenic order parameter is infact enhanced in the LCEs swollen with the saturated BCb solvent.From Ref. [14a], the magnitude of f is smaller for LCE swelling inbent-core LC solvents than in calamitic LC solvents. The degree of

Adv. Mater. 2009, 21, 1622–1626 � 2009 WILEY-VCH Verlag G

spontaneous shape change with temperature is much larger forLCEs swollen with bent-cores (although still less than that for dryLCEs), as expected.

In summary, we have shown that a LCE systemmay be swollenwith substantial amounts of bent-core materials, which are ofhigh interest due to their surprisingly large flexoelectric response.In the isotropic phase of both elastomer and solvent, the LCEswells until saturation, at around f¼ 0.55 (35mol %) andf¼ 0.46 (35mol %) for the two solvents (BCa and BCb) studied.The swelling is characterized by two time scales, the shorter(�300 s) comparable to standard rodlike LC solvents, and thelonger (�8–10 000 s) perhaps reflecting the unusually highviscosity reported in the neat bent-core solvents. Differentchoices of the bent-core solvent allow some control over the phasesequence and transition temperatures of the LCEs. In particular,the BCb-ss and BCb-us swollen LCEs exhibit only nematic phases,while all other swollen LCEs studied exhibit a nema-tic–smectic-crystal phase sequence. Comparison of the globalpolymer-chain order parameter to the global mesogenic orderparameter shows that in the swollen LCEs the polymer-chainorder parameter decreases, while the overall orientational order ofthe mesogenic components is enhanced in the nematic phase.Experiments are underway to measure the flexoelectric responseof bent-core swollen LCEs.

Experimental

The LCE swelling was performed by heating the LCE and bent-coresolvents to temperatures in the isotropic phase of both materials, wherethe free energy for mixing is optimal. In particular, an LCE piece withdimensions of roughly 1� 1� 0.3mm3 was placed in the bent-core solventcontained in a custom-made hot stage and held at a fixed temperaturetypically 400 K, which is above the nematic–isotropic transition tempera-tures of the components. The increase in LCE volume with time wasfollowed using a microscope fitted with a time-delay imaging camera. Afterswelling, the surface of the swollen LCE was washed in toluene, acompatible solvent for the bent-core LCs used here, to remove any residualbent-core material on the surface. The weight of the swollen LCE wasmeasured using a microbalance, and the amount of imbibed material wascalculated.

Spontaneous change in the LCE shape due to nematic orientationalordering was observed as a function of temperature by placing the LCE filmonto a glass slide containing a thin film of silicon oil (to prevent the samplefrom sticking to the glass) in the microscope hot-stage. Digital images ofthe LCE were then taken to allow calculation of the LCE dimensions versustemperature. The optical birefringence was measured as a function oftemperature by replacing the digital camera with a spectrophotometer andmeasuring at l¼ 533 nm the depolarized transmitted light intensitythrough the LCE, as described in Ref. [24]. The refractive-index anisotropyDn was then calculated from the transmitted intensity and film thicknessversus temperature data. Enthalpy and equilibrium transition temperatureswere measured using a modulated differential scanning calorimeter (TAInstruments model TA2920 MDSC) with heat/cool rates of 10, 5, and1 K min�1.

Acknowledgements

The liquid crystal elastomer system used in this study was provided by theSlobodan Zumer group of Jozef Stefan Institute and the New Liquid CrystalMaterials Facility (http://nlcmf.lci.kent.edu) supported by the NSF (DMR0606357), the Ohio Department of Development, Kent State University,and AlphaMicron, Inc. A portion of this work was performed at Oak RidgeNational Laboratory’s Center for Nanophase Materials Sciences, which is

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sponsored by the Scientific User Facilities Division, Office of Basic EnergySciences, U.S. Department of Energy. The authors would like toacknowledge support from ONR (N00014-07-1-0440) and NSF(DMR-0606160).

Received: September 15, 2008

Revised: November 26, 2008

Published online: January 29, 2009

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