Fast Responding Robust Nematic Liquid Crystalline Gels Formed by a

Apr 17, 2009 - Copyright © 2009 American Chemical Society. * To whom correspondence should be addressed. E-mail: [email protected]. Cite this:J. Phys...
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J. Phys. Chem. B 2009, 113, 6647–6651

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Fast Responding Robust Nematic Liquid Crystalline Gels Formed by a Monodisperse Dipeptide: Electro-Optic and Rheological Studies Geetha G. Nair, S. Krishna Prasad,* V. Jayalakshmi, G. Shanker, and C. V. Yelamaggad Centre for Liquid Crystal Research, Jalahalli, Bangalore 560013, India ReceiVed: January 5, 2009; ReVised Manuscript ReceiVed: February 24, 2009

Realization of mechanically robust electrically fast responding liquid crystal devices with low operating voltage is one of the current research interests. Here we report a gel system comprising a commercially available nematic liquid crystal material and a new monodisperse dipeptide liquid crystalline organogelator that results in these properties. The gels exhibit nearly 2 orders of magnitude faster switching response than the pure nematic liquid crystal while having 3 orders of magnitude higher zero shear rate viscosity, and with the attractive feature that the switching threshold voltage is hardly altered. Electro-optic and rheological studies of this system are described here. Introduction Liquid crystals contained in a network are promising materials for display devices. An especially attractive material property aimed at is the faster electrical response. A popular matrix in this regard is a gel, which is made of solid network swollen by a liquid.1 Substituting the liquid with a liquid crystal yields a liquid crystal gel, which is either irreversible in nature (termed chemical gel) or reversible (physical gel). Recently there has been lot of interest in the electro-optic properties of such gels2-12 formed by physical processes such as hydrogen bonding. This is due to the fact that these gels retain the anisotropic properties of the liquid crystal, exhibit thermo-reversibility and are mechanically more rigid. A new dimension in this field is the usage of natural materials, such as polypeptides, which possess built-in mechanisms of self-assembly.12 In aqueous solutions these polypeptides form gels even at low concentrations. It has been found that the mechanical properties of these gels are quite sensitive to the molecular architecture of the peptides. However, the peptides used are polypeptides, which by nature, have polydispersity. Thus a well-determined structure-property relationship is not trivial. Here we employ a novel monodisperse homomeric dipeptide for creating the gel matrix. This dipeptide, recently synthesized and characterized by us,13 itself is liquid crystalline. The present study is on the electro-optic and rheological measurements of liquid crystal gels formed by a mixture of a commercially available nematic liquid crystal (NLC) and a very small concentration of the dipeptide. The gelation improves the dynamic characteristics of the electrooptic switching of the device employing these gels, by as much as 2 orders of magnitude. Concomitantly, a huge increase (by 3 orders of magnitude) is seen in the zero shear viscosity of the material. The combination of these properties results in a robust fast responding liquid crystal display device. Experimental Section A. Materials. The NLC used for this study is the wellknown, commercially available eutectic mixture E7 (from Merck), exhibiting the nematic phase over a wide temperature range through room temperature. The gelating material em* To whom correspondence [email protected].

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be

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Figure 1. Molecular structure of GSC98. The material exhibits an oblique columnar (Colob) mesophase upon melting.13

ployed is a monodisperse homomeric dipeptide (referred to as GSC98 here); it is an enantiomer in which the first and the second residues are derived from D-alanine.13 The molecular structure as well as the transition temperatures of this material are shown in Figure 1. Structurally, GSC98 can be regarded as an intermediate between polycatenars and taper-shaped amphiphiles as it possesses two lipophilic (half-disk shaped) segments interlinked through a peptide unit. This compound is mesogenic exhibiting a columnar phase; the mesogenicity helps in preparing mixtures with better homogeneity. Further, in the fabrication of twisted nematic devices, it is a standard practice to add a small amount of a chiral agent to the host NLC to avoid the formation of wall defects resulting from reverse twist domains. GSC98, being chiral, a property that also manifests in a macroscopic helical structure,13 obviates the need for this additional component. It is found that even at concentrations as low as 0.2% (by weight), GSC98 forms gels in E7. In the present study we have mainly used gels formed with X ) 0.2, 0.4, 0.6, 0.8, and 1, where X represents the concentration of GSC98 by wt % in E7. The procedure followed for preparing these composites is given below. B. Gel Preparation. In small quantities, GSC98 is completely solvable in E7. The required quantities of E7 and GSC98 were weighed in a glass vial. The mixture was heated to 100 °C (which is well above the isotropic temperature of E7 and the melting temperature of GSC98) and stirred constantly for homogeneous mixing. The homogeneity itself was checked by observing the sharpness of the nematic to isotropic transition under a polarizing microscope (Leica DMRXP). After mixing

10.1021/jp900074e CCC: $40.75  2009 American Chemical Society Published on Web 04/17/2009

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Figure 2. Left: Pure NLC (E7). Right: X ) 0.6 gel. Note the immobility of the gel due to its high viscosity as against the freely flowing pure NLC.

at 100 °C, the sample in the vial was quenched to room temperature and kept in a refrigerator for two days to achieve gelation. C. Electro-Optic and Freedericksz Threshold Measurements. For these measurements, the sample was contained between electrically conducting indium tin oxide (ITO) coated glass plates treated with a polyimide solution and rubbed unidirectionally; the two plates had their rubbing directions orthogonal to each other to achieve the twisted nematic (TN) geometry. PET spacers of nominal thickness of 8 µm were used to define the gap between the plates. The gel formation was carried out in situ by filling the cells after homogeneous mixing of the constituents and in the isotropic state; the cells were then kept in the refrigerator for two days. All measurements reported here were performed at a temperature of 25 °C. The electro-optic experiments were carried out using the microscope lamp as the optical source. Voltage to the sample was applied using a function generator (HP34140) in conjunction with a high voltage amplifier. The transmitted light intensity through the sample (kept between crossed polarizers) detected in the forward transmission geometry using a photodiode whose output was in turn fed to an oscilloscope (Tektronix TDS420); the oscilloscope traces were employed to measure the response times. The Freedericksz threshold voltage was determined by recording the sample permittivity (ε) as a function of the applied voltage using an LCR meter (HP4284A). For these studies cells with a planar geometry instead of the TN geometry were used. The initial alignment of the molecules yields ε⊥, the permittivity perpendicular to the director (the average orientation of the nematic molecules) which changes over to ε| (permittivity parallel to the director) well above the Freedericksz threshold voltage. D. Rheological Measurements. A controlled stress rheometer (ARG2, TA Instruments) with parallel plate (8 mm diameter) geometry having an interpolate gap of 800 µm was used for all the measurements. The temperature of the sample was kept constant at 25 °C using a Peltier temperature controller. Results and Discussion A table-top rheology proof of the gel formation in a representative composite (X ) 0.6) is pictorially shown in Figure 2. While the host NLC (E7) freely flows, the composite in the gel state is highly viscous and does not flow; such an immobilization is characteristic of gelled materials. Similar results were obtained for all the composites studied here indicating that all of them form gels. A. Freedericksz Effect: Threshold Voltage. The electric field driven reorientation of a positive dielectric anisotropy nematic molecule is at the heart of the liquid crystal display devices. If the molecules are initially oriented parallel to the

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Figure 3. Threshold curves of dielectric permittivity (ε) as a function of applied AC field for E7 and for gels X ) 0.2, 0.6, and 1. For the sake of clarity, every fifth collected point is shown. Inset: Concentration dependence of Vth calculated from the threshold curves. The solid line through the data is just a guide to the eye.

glass substrates and the electric field is applied perpendicular to the substrates, the threshold voltage Vth above which the reorientation begins is given by



Vth ) π

K1 εoεa

(1)

Here εo is the permittivity of free space, εa is the dielectric anisotropy, and K1 is the splay elastic constant. To determine Vth, the sample capacitance is measured as a function of the applied voltage. Figure 3 shows the dielectric permittivity as a function of applied AC field for E7 and for the gels X ) 0.2, 0.6, and 1. The threshold curve is sharp for pure E7 and the X ) 0.2 gel, but becomes broader as the gel concentration is increased. We shall return to this point later. The composition dependence of Vth calculated from such threshold curves is given in the inset of Figure 3. The most important feature is that in comparison to any previous study, the threshold voltage remains quite low for all the gels studied here. Until X ) 0.8, there is a general decrease in the Vth value as compared to that for E7, although the variation is not exactly monotonic. In fact, the work by Mizoshita et al.14 that employs non-LC gelling agents also appears to have such a feature but was not commented upon by the authors. The significant increase in Vth for higher concentrations (X > 0.8 for the present case) is again in general agreement with the literature reports,11,7 although it should be pointed out that the concentration at which such an increase is seen and the magnitude of the increase are perhaps dependent on the nature of gelating agent. For example, in the case of carboxamides,11 Vth increased by a factor of 5 even when the concentration of the gelator was only 0.2%. By comparison, the increase is less than a factor of 2 for X ) 1. We shall later see that this feature is reflected in the value of the storage modulus of the gel. In the case of polymer-stabilized NLC (PSNLC), where the NLC is confined in a network of fibrils of the polymer15 the concentration dependence of Vth was explained on the basis of a model in which the polymer fibrils constituting the network are oriented parallel to the nematic director in the field-off state and provide virtual surfaces with finite anchoring energy. (The type of fibrils formed in the PSNLC case is similar to the SEM pattern obtained for GSC98. For example, see Figure 9 of ref 13.) The mathematical expression used in this work to analyze the influence of the network on the threshold field is essentially similar to that proposed for anisotropic gels.16 The virtual surfaces counteract the electric field induced reorientation of the molecules. The effective field would then be lower than the

Robust Nematic Liquid Crystalline Gels

Figure 4. Switching times (τON and τOFF) as a function of the gel concentration, X. An AC field of 1 kHz sine, 20 Vrms was applied to samples of thickness ∼8 µm. τON is almost invariant with change of concentration whereas τOFF shows a drastic decrease with increasing X. The solid line through this set of data represents an exponential fit of τOFF to X. Inset shows electro-optic response curve obtained for the gel X ) 0.6. The maximum transmittance for X ) 0.6% gel is 75% and contrast ratio is 20.

applied one and is given by Eeff2 ) E2 - (2P/εoεa), where P describes the strength of the network contribution. Significantly, a feature that is not reported in any previous work on gel-NLC systems, but observed in the present case, is that after having a sharp increase at a certain value of X, Vth appears to reach a limiting value for higher concentrations of the gelator. Owing to this, none of the previously used theoretical models can be employed to describe the data. However, it is interesting to note that the steplike increase in Vth is seen in the concentration range that shows a double peak profile in the calorimetric measurements. Thus it is possible that the character of the gel changes for X ∼ 1 and is responsible for the sharp increase in Vth. Additionally, even for X < 1, Vth is not strictly linear with X. Whether this behavior and the fact that TNI also has a similar variation are interrelated is to be investigated. As mentioned earlier the threshold curves become broader as X increases. To a first approximation, the slope of the increase in capacitance just above Vth, is inversely proportional to the ratio of bend (K3) to splay (K1) elastic constants (neglecting the small variation in εa). The experimental data show that the slope value diminishes by a factor of more than 5 for the X ) 1 gel in comparison to that for pure E7. Since K1 (proportional to Vth) changes by less than a factor of 2 over this concentration range, we can conclude that K3 increases by a large factor (∼5) as the gel concentration is increased. It was also found that just like in the pure E7 the Freedericksz reorientation is voltageand not field-dependent for the X ) 0.6 gel material also. This observation is significant considering the fact that Chang et al.17 reported a threshold field for an elastomeric gel. Perhaps the lower concentration of the gelating material as well as the fragile nature of the network in our system is responsible for the difference in the behavior. These features require further exploration. B. Freedericksz Effect: Response Time. The switching times (τON and τOFF) were measured using the electro-optic method by applying an ac field (1 kHz sine, 20 Vrms) to the sample. A representative electro-optic response curve obtained for a gel (X ) 0.6%) is shown in the inset of Figure 4. Here τON is considered to be the time required for Itr, the transmitted light intensity, to change from 90 to 10% of the initial value upon application of the voltage, and τOFF as the time required for Itr to change from 10 to 90% after the voltage is off. The determined τON and τOFF values for all the concentrations, including pure E7, are plotted in Figure 4. While τON is essentially independent of concentration, τOFF shows a drastic decrease with increasing X. To be emphasized is the point that

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Figure 5. Strain amplitude (γ) dependence of the storage and loss moduli, G′ and G′′, for the X ) 0.8 gel measured with an angular frequency of 1 rad/s. The storage modulus (G′) shows a linear viscoelastic region (LVR) up to ∼2% of strain amplitude. Above a critical strain amplitude γο, G′ decreases rapidly (G′ < G′′) indicating a crossover from elastic to viscous behavior.

low concentration gelation leads to strikingly improved device characteristics. For example, X ) 0.6% shows nearly 2 orders of magnitude faster response, with hardly any change in the threshold voltage. A feature to be commented upon is that in the field on state Itr exhibits a periodic oscillation at twice the frequency (2 kHz) of the applied field (1 kHz) (see inset of Figure 4). This behavior is coupled to the director response18 with the doubling of the frequency arising due to the rms response of the nematic. In the present case, we observe this only in gel materials due to their fast field-off relaxation. In the PSNLC system,16 the concentration dependence of τOFF was described using the expression 2 τoffEth )

π2γ εaεo

(2)

Owing to the already discussed steplike variation of Vth in our data, eq 2 could not be used. Instead we find that the τOFF has an exponential dependence on the concentration of the gelator (as depicted by the solid line in Figure 4). C. Mechanical Properties. To characterize the mechanical properties of the gels, rheological measurements were performed. As an example, Figure 5 shows the strain amplitude (γ) dependence of the storage and loss moduli, G′ and G′′, for the X ) 0.8 gel. The storage modulus shows linear viscoelastic regime (LVR) up to a few percentage of the strain amplitude but decreases rapidly above a critical strain amplitude γο (18.7%), indicating an elastic behavior below (G′ > G′′) and a breakdown of the gel structure above this strain value (with G′ < G′′). More important from the device point of view is the rapid recovery of the collapsed gel seen in the transient measurements19 (Figure 6). For these studies the gel material was subjected to large oscillatory strains (γ > γο) to break the gel structure and then abruptly reducing the strain amplitude to a very small value (γ < γο, to be in the LVR region) while monitoring G′ and G′′. Upon application of a large strain, G′ decreases by 2 orders of magnitude. While G′′ also shows a decrease, its variation is much smaller which also leads to a change in the relationship from G′ > G′′ to G′ < G′′. When γ is reduced to a small value, the recovery to the elastic state (with almost the original values) takes place instantaneously, requiring less than 20 s (see inset of Figure 6), a duration that is required for our rheometer to switch between the two modes of measurement. The recovery is indeed reproducible over repeated cycles of measurement. The angular frequency dependence of the two moduli (Figure 7) obtained with small strain amplitude (within the LVR region), corroborates these features,

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Figure 6. Step strain measurements of X ) 0.8 gel showing the rapid recovery of gel structure when the gel is subjected to a large oscillatory strain of 80%. The initial low strain amplitude is 0.8% with an angular frequency of 1 rad/s. The recovery to the gel state takes place within ∼20 s (inset) and is reproducible over repeated cycles of measurement.

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Figure 8. Shear rate (γ˙ ) dependence of bulk viscosity η (open circles) for the X ) 0.8 gel. The line through this data set is a fit to Carreau model (eq 3) which describes the data well with an exponent of n ) 0.16. For comparison the angular frequency (ω) dependence of η* (open squares), determined from the oscillatory measurements is shown. The line through this data is only a guide to the eye. The feature that η* > η indicates weak gel characteristics.

Figure 9. The gel concentration dependence of low shear rate (γ˙ ) 5 × 10-4 s-1) viscosity (ηo) at a frequency of 1 rad s-1 obtained from fitting the viscosity vs shear rate data in each case to Carreau model. ηo increases, for example, by nearly 3 orders of magnitude for X ) 0.8 as compared to pure E7.

Figure 7. The angular frequency dependence of the two moduli G′ and G′′ obtained with small strain amplitude (value within the LVR region). Over the frequency range studied, both the moduli have very little frequency dependence and G′ remains greater than G′′. Tan δ values lie in the range 0.13-0.3.

with the values increasing with increasing concentration of GSC98. Over the frequency range studied, both the moduli have very little frequency dependence and G′ remains greater than G′′. However, tan δ values lie in the range 0.13-0.3, and the LVR region is seen only over a small range of strain values suggesting the system to be a “weak gel”.20 All these features are qualitatively similar to those exhibited by other organogels, especially the hydrogels formed by aqueous solutions of polypeptides.9 But the highpoint of the current studies is the fact that the gelating agent, a dipeptide, is monodisperse thus providing a better control for possible fine-tuning of the gel network by molecular structural variations. Also interesting is the fact that the gel strength (defined here as the G′ value at low γ) of our system is comparable to that of polypeptide systems at similar concentration levels of the gelator. Compatible with this feature is the fact that, as SEM images show, with a protic solvent GSC98 forms a gel network with thin rodlike fibers having a diameter of couple of hundreds of nanometers.13 As pointed out earlier, in the carboxamide system studied by Muller et al.11 Vth increases by a factor of 5 even for 0.2% of the gelator concentration. This is in contrast to only a factor of 2 increase in our case, even for an order of magnitude higher concentration of the gelator. We find a similar behavior in the value of the storage modulus: while the G′ value obtained by Muller et al. 11 is ∼103 Pa, the system studied here has more than an order of magnitude smaller value. With this in mind,

we suggest that concentration dependence of the increase in Vth upon gelation is controlled by the magnitude of G′. Figure 8 shows the dependence of the bulk viscosity η on the shear rate obtained for the X ) 0.8 gel. At very large shear rates, η is independent of the shear rate (Newtonian behavior). This region is preceded by a strongly shear-rate dependent region (referred to as region II, hereafter). These two features are common for all the gels studied here. The degree of shear thinning, defined as the total decrease in η through region II, is seen to be about 3 orders of magnitude, a value comparable to that observed for polymer melts.21 In such systems, the shearrate dependence of η through all the regions can be described by the four-parameter constitutive function referred to as the Carreau model22

η(γ˙ ) ) η∞ +

η o - η∞ γ˙ 2 (1-n)⁄2 1+ γ˙ o

( ( ))

(3)

Here γ˙ o represents the shear rate value below which the viscosity approaches the low shear rate limiting value ηo and the material behaves as a Newtonian fluid. η∞ is the limiting value of the second Newtonian plateau at high shear rates, and n is the important power-law index. A standard feature of weak gel materials is that the Newtonian plateau is not observed at low shear rates. Therefore while performing the fitting we consider ηo to be the value at the lowest shear rate of 5 × 10-4 s-1 used in these experiments. The solid line through the data in Figure 8 shows that eq 3 describes the data well with an exponent of n ) 0.16. The small value of n is characteristic of strong shear thinning fluids.23 This fitting also yields the zero-rate viscosity (ηo) value for X ) 0.8 to be 4.8 × 104 Pa s. It has been

Robust Nematic Liquid Crystalline Gels demonstrated that in weak gels the Cox-Merz superposition is not applicable.19 This is indeed true for the present system which exhibits the viscosity (η*) obtained from oscillatory measurements to be greater than that (η) from steady state shear measurements (see Figure 8). The concentration dependence of ηo obtained from a similar fitting is given in Figure 9. ηo increases by nearly 3 orders of magnitude for X ) 0.8 as compared to pure E7, establishing that these gels are mechanically quite robust. Conclusions We have reported here a unique gel system comprising a nematic liquid crystal and a new liquid crystalline organogelator that is a monodisperse dipeptide. The combination of nearly 2 orders of magnitude faster electrical switching, low Freedericksz threshold, mechanical robustness as quantified by 3 orders of magnitude higher shear viscosity makes these gels attractive from the viewpoint of device applications. References and Notes (1) See, for example: Low Molecular Mass Gelators; Fages, F., Ed.; Springer: Berlin, 2005. (2) For a review, see: Kato, T.; Hirai, Y.; Nakaso, S.; Moriyama, M. Chem. Soc. ReV. 2007, 36, 1857–1867. (3) Zhao, Y.; Guan, L. Liq. Cryst. 2003, 30, 81–86. (4) He, J.; Yan, B.; Yu, B.; Bao, R.; Wang, X.; Wag, Y. J. Colloid Interface Sci. 2007, 316, 825–830. (5) Kato, T. Science 2002, 295, 2414–2418.

J. Phys. Chem. B, Vol. 113, No. 19, 2009 6651 (6) Deindorfer, P.; Eremin, A.; Stannarius, R.; Davis, R.; Zentel, R. Soft Matter 2006, 2, 693–698. (7) Fan, Y.; Ren, H.; Lang, X.; Lin, Y.; Wu, S. T. Appl. Phys. Lett. 2004, 85, 2451–2453. (8) Inn, Y. W.; Denn, M. M. J. Rheol. 2005, 49, 887–895. (9) Breedveld, V.; Nowak, A. P.; Sato, J.; Deming, T. J.; Pine, D. J. Macromolecules 2004, 37, 3943–3953. (10) Li, J.; Stannarius, R.; Tolksdorf, C.; Zentel, R. Phys. Chem. Chem. Phys. 2003, 5, 916–923. (11) Muller, M.; Schopf, W.; Rehberg, I.; Timme, A.; Lattermann, G. Phys. ReV. E. 2007, 76, 061701. (12) Deming, T. J. Soft Matter 2005, 1, 28–35. (13) Yelamaggad, C. V.; Shanker, G.; Ramana Rao, R. V.; Shankar Rao, D. S.; Krishna Prasad, S.; Suresh Babu, V. V. Chem.sEur. J. 2008, 14, 10462–10471. The dipeptide used was thermally and hydrolytically stable. (14) Mizoshita, N.; Hanabusa, K.; Kato, T. Displays 2001, 22, 33–37. (15) Kossyrev, P. A.; Qi, J.; Priezjev, N. V.; Pelcovits, R. A.; Crawford, G. P. Appl. Phys. Lett. 2002, 81, 2986–2988. (16) Hikmet, R. A. M.; Boots, H. M. J. Phys. ReV. E. 1995, 51, 5824– 5831. (17) Chang, C.-C.; Chien, L.-C.; Meyer, R. B. Phys. ReV. E. 1997, 56, 595–599. (18) Xia, Y.; Verduzco, R.; Grubbs, R. H.; Kornfield, J. A. J. Am. Chem. Soc. 2008, 130, 1735–1740. (19) Yoshida, M.; Koumura, N.; Misawa, Y.; Tamaoki, N.; Matsumoto, H.; Kawanami, H.; Kazaoui, S.; Minami, N. J. Am. Chem. Soc. 2007, 129, 11039–11041. (20) Ross-Murphy, S. B. J. Rheol. 1995, 39, 1451–1463. (21) See, for example: Larson, R. G. The Structure and Rheology of Complex Fluids; Oxford University Press: Oxford 1999. (22) Carreau, P. J. Trans. Soc. Rheol. 1972, 16, 99–127. (23) (a) Yasuda, K.; Armstrong, R. C.; Cohen, R. E. Rheol. Acta 1981, 20, 163–178. (b) Kojic, N.; Bico, J.; Clasen, C.; McKinley, G. H. J. Exp. Biol. 2006, 209, 4355–4362.

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