Binary System Exhibiting the Nematic to Twist-Bend Nematic

May 14, 2016 - Among the salient features observed are (i) the existence of the NTB phase even when the system is loaded with a high concentration (âˆ...
0 downloads 11 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCB

Binary System Exhibiting the Nematic to Twist-Bend Nematic Transition: Behavior of Permittivity and Elastic Constants Srividhya Parthasarathi, D. S. Shankar Rao,* Nani Babu Palakurthy, C. V. Yelamaggad, and S. Krishna Prasad Centre for Nano and Soft Matter Sciences, Bengaluru 560013, India S Supporting Information *

ABSTRACT: We describe measurements of the permittivity and Frank elastic constant in the nematic phase of a binary system displaying a transition between the nematic (N) and the recently discovered twist-bend nematic (NTB) phase. Among the salient features observed are (i) the existence of the NTB phase even when the system is loaded with a high concentration (∼64 mol %) of a rodlike component; (ii) a clear signature in permittivity of the N−NTB transition; and (iii) a lower value of the bend elastic constant compared to the splay over a large phase space, with the difference between the two becoming a maximum for an intermediate mixture. These studies further support the surprising idea that the elastic features associated with bent molecules can be further augmented by suitable rodlike additives.

1. INTRODUCTION A recent prediction of a new type of nematic phase,1 labeled twist-bend nematic (N TB ), has been attracting much attention.2−5 The structure of this phase can be treated as a generalized version of the usual chiral nematic phase but with an essential difference: Whereas the axis of the helix in the latter phase is orthogonal to the director n, it makes an angle with respect to n in the NTB phase. A further point of interest is the spontaneous appearance of chirality even when the molecules are achiral. The phase thus forms a new member of liquidcrystal structures with spontaneous chirality formed by achiral molecules such as the bent-core B2 phase.6 An intriguing feature of the NTB phase is that the pitch of the helix turns out to be two to three molecules long. From a theoretical point of view, NTB is predicted to have a negative value of the Frank bend elastic constant.7 Owing to these qualities, investigations of the physical properties of the NTB phase, especially the elastic constants, have assumed importance. Although reports exist of the observation of the NTB phase in bent-core compounds,8 the corresponding phase exhibited by dimeric molecules has been well characterized.9 The structurally important aspect of these dimers is that the two monomeric arms are linked through oddparity flexible spacers, a feature that renders an average bent shape to the molecule. The importance of the bent shape due to the flexible spacer and the nature of the linker are better highlighted in symmetric dimers with cyanobiphenyl groups, wherein the compounds with methylene groups exhibit a uniaxial nematic (N) phase as well as the NTB phase but their ether-linkage counterparts exhibit only the N phase. As mentioned above, the elastic properties of materials with NTB, unusual with respect to calamitic compounds exhibiting only the N phase, makes them stand apart. For example, in the N © XXXX American Chemical Society

phase present above the NTB phase, the splay (K11) and bend (K33) elastic constants present opposite trends in their thermal dependence: Whereas K11 has the usual “increase with decreasing temperature” behavior, K33 decreases as temperature is reduced. However, there have been no clear experimental indications that K33 could be negative in the NTB phase.7 At a different level, experiments on binary mixtures composed of bent-core and rodlike molecules have shown an interesting feature, namely, K33 has a nonmonotonic thermal dependence.10−13 In a previous publication, we argued that this is due to the frustration in the packing between the two differently shaped molecules.13 Here, we demonstrate that, in a dimeric compound, exhibiting the NTB phase, the higher-temperature N phase exhibits several interesting features, some of which are similar to the N phase of rigid bent-core materials. The advantage of the dimer that emerges from these studies is that the effects of the bent shape are dependent on temperature. Our studies suggest that the difference in dimensions of the bent dimer and the rodlike component can be used to tune these behaviors.

2. EXPERIMENTAL SECTION We have investigated a binary system composed of the dimer CB7CB with a structurally similar rodlike nematogen, the wellknown n-heptyloxy cyanobiphenyl (7OCB); the structural similarity is expected to yield a more homogeneous mixing of the two materials. CB7CB was synthesized in our laboratories Received: March 24, 2016 Revised: May 13, 2016

A

DOI: 10.1021/acs.jpcb.6b03048 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

capable of applying an oscillating voltage up to 20 V. The elastic constants were extracted from capacitance−voltage profiles obtained through the latter type of experiments. The procedure, based on the Deuling analysis,13−15 was facilitated by a user-written routine scripted in MATLAB; these details were published in a previous article.16

using the procedure described in the Supporting Information. The molecular structures of these two compounds and their phase sequences are given in Figure 1. As already indicated, the

3. RESULTS 3.1. Thermal Variation of Permittivity. Permittivity measurements were performed at a fixed frequency of 10

Figure 1. Molecular structures and transition temperatures for the NTB material CB7CB and the rodlike nematic 7OCB.

dimer shows N and NTB phases below the isotropic (Iso) liquid, whereas 7OCB has a single mesophase, the N phase. Experiments were conducted on the pure compounds and several mixtures spanning the entire binary phase diagram. The mixtures are labeled as Xn, where n indicates the mole percentage of 7OCB in CB7CB. Samples contained in sandwich-type cells, obtained from a commercial source (AWAT, Warsaw, Poland), consisting of two indium tin oxide coated conducting glass plates separated by a gap of ∼7.5 μm, were employed. A polymer layer was deposited on the inside surfaces of the substrates to promote a unidirectional planar alignment of the liquid-crystal (LC) molecules. Measurements of the permittivity and Fréedericksz transition were taken at a constant frequency of 10 kHz with the help of a high-precision LCR meter (Agilent 4284A)

Figure 3. Thermal variation of anisotropy in permittivity εa (= ε∥ − ε⊥) with Xn for n = 0, 10, 25, 49.6, 64.1, 75.2, 89.3, and 100 for the profiles from bottom to top (Tred = TN−Iso − T, where TN−Iso is the nematic-to-isotropic transition temperature). εa increases with Xn in both the N and NTB phases. The N−NTB transition, clearly brought out in the data for Xn up to 64.1, is marked by an arrow.

Figure 2. Thermal variations of permittivity (a) parallel (ε∥) and (b) perpendicular (ε⊥) to the nematic director normalized with respect to the isotropic value (εIso) for the pure compounds 7OCB and CB7CB and several mixtures. In both panels, the profiles represent the concentration of 7OCB, Xn, varying from right to left as Xn = X0, X15.1, X25, X49.6, X64.1, and X100. For the mixture X75.2, ε⊥ data were collected down to 10 °C and depict the absence of an N−NTB transition, as discussed in the text. The ε∥ (ε⊥) value increases (decreases) with Xn. The arrows indicate the N−NTB transition as observed under the microscope, at which there is a clear change especially in ε⊥ for Xn ≤ 25, whereas the trend reverses for 49.6 ≤ Xn ≤ 64.1. B

DOI: 10.1021/acs.jpcb.6b03048 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 4. Temperature−concentration (X7OCB) phase diagram obtained using polarizing optical microscopic and dielectric studies. With increasing Xn, both the Iso−N and N−NTB transition temperatures decrease, with the decrease in the latter being more dramatic. The lines are mere guides to the eye. The NTB phase ceases to exist for Xn > 64.1%.

Figure 6. Variations in the normalized capacitance as a function of the applied voltage for X49.6 at various temperatures (T = 86.1, 85.1, 81.1, 77.1, 72.1, 69.1, and 59.1 °C for profiles from bottom to top, respectively). Inset b shows the same data on an enlarged scale in the vicinity of threshold, indicating an increase in Vth with decreasing temperature (or increasing Tred). Inset c illustrates the the large decrease in Vth with increasing Xn at a fixed Tred, depicted here for Tred = 10 °C.

Figure 5. Thermal variation of the orientational order parameter S estimated from eq 2 shown for (top) pure 7OCB and mixtures (middle) X 75.2 and (bottom) X64.1 . The inset presents the concentration dependence of S at a specific reduced temperature of Tred = 10 °C.

Figure 7. Thermal variation of the Frank splay elastic constant K11 for pure compounds and various mixtures. For all Xn, K11 shows a monotonic increase with increasing Tred (decreasing temperature); the temperature variation appears to be slightly reduced on increasing Xn. The inset shows K11 as a function of the concentration of 7OCB at a fixed Tred (= 10 °C). The behavior seems to be a quadratic reduction with Xn until 94.9%, at which point the trend reverses.

kHz for the pure compounds and several mixtures in two geometries wherein the director in the N phase was perpendicular (ε⊥) or parallel (ε∥) to the probing field; the perpendicular and parallel alignments were realized using probing fields of 0.5 and 20 V, respectively. Figure 2 shows these data for the two pure compounds and a few representative mixtures. Several interesting features can be noted: (i) Signatures of both the isotropic−N and the N−NTB (when present) transitions are clearly seen for all of the materials. The corresponding transition temperatures, TN−Iso and TN−NTB, exhibit a clear dependence on the concentration of 7OCB. Up to a loading factor of Xn = 64.1, the NTB phase

survives; the Xn = 75.2 mixture, even when supercooled to 10 °C, did not exhibit the NTB phase. (ii) The anisotropy in permittivity, εa (= ε∥ − ε⊥), is substantial in the N phase but shows an apparent reduction upon transformation to the NTB phase, as shown in Figure 3. The reduction is actually caused by the inability of the electric field (20 V) to retain the homeotropic orientation in the twist-bend phase. Attempts to obtain a surface-determined homeotropic orientation in this phase were not fruitful. (iii) The magnitude of εa increases as C

DOI: 10.1021/acs.jpcb.6b03048 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

Figure 10. Order-parameter dependence of the quantity K11/S2. The lines represent fits to eq 5, showing that the Berremann and Meiboom model describes the data very well.

Figure 8. Thermal variation of the Frank bend elastic constant for pure compounds and various mixtures Xn. The behavior can be split into two regions: A monotonic increase with Tred is observed for both 7OCB (Xn = 100) and CB7CB (Xn = 0) as well as mixtures in the concentration windows Xn< 25 and Xn > 89.3. Mixtures with intermediate concentrations present a convex-shaped anomaly with K33 initially increasing and then decreasing with temperature. The inset shows the value of K33 at Tred = 10 °C, exhibiting a nonmonotonic variation with Xn with a minimum for ∼X10.

Figure 11. Order-parameter dependence of the quantity K33/S2. The lines represent fits to eq 5, showing that the Berremann and Meiboom model describes the data very well.

thermal dependence for Xn > 25. With these data, we have constructed the temperature−concentration (T−X) phase diagram shown in Figure 4. The temperatures for both transitions obtained from the permittivity data agree with the polarizing optical microscopy (POM) observations. It can be added here that the NTB phase exhibits a striped pattern, reported by other researchers.2,4,7 Between pure CB7CB and the Xn = 50 mixture, the stripe periodicity was seen to increase from 7 to 11 μm (see Figure S7 of the Supporting Information). 3.2. Order Parameter. In the framework of the theoretical model by Maier and Meier,17 the dielectric anisotropy in the nematic phase is associated with orientational order as given by the expression

Figure 9. Concentration dependence of the elastic anistropy at three reduced temperatures. The variation is nonmonotonic, with the minimum occurring for Xn = 10. The dashed horizontal line represents the isotropy point, K33 = K11.

Xn, the concentration of 7OCB in the mixture, increases. This is to be expected, as 7OCB is more polar than CB7CB because the CN groups at the two termini of the molecule in the latter compound cancel each other’s dipolar contributions. (iv) Across the N−NTB transition, a clear change in ε∥ is seen only for certain concentrations (see Figure 2a), but a marked change in ε⊥ exists for all of the mixtures presenting the NTB phase (see Figure 2b). More importantly, for Xn < 25, ε⊥ decreases on entering the NTB phase, whereas for higher concentrations, it reverses the trend. It is interesting to note that both εa and K33 (vide infra) exhibit a nonmonotonic

⎤ ⎡ μ2 (1 − 3 cos2 β)⎥S εa = ε − ε⊥ = εo−1NhF ⎢Δα − F 2kBT ⎦ ⎣ (1) D

DOI: 10.1021/acs.jpcb.6b03048 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B Table 1. Best-Fit Parameters Obtained by Fitting S versus Kii/S2 to eq 5 n

K(2) 11

K(3) 11

49.6 64.1 75.2 89.3 94.9 100

5.5 ± 0.7 11.3 ± 1.8 10.3 ± 0.8 3.1 ± 0.4 2.8 ± 0.6 4.8 ± 0.5

7.0 ± 1.5 −5.7 ± 3.3 −2.4 ± 1.5 7.2 ± 0.8 8.9 ± 1.1 6.1 ± 0.9

K(4) 11

K(2) 33

K(3) 33

K(4) 33

± ± ± ± ± ±

5.1 ± 0.6 1.58 ± 0.9 −0.69 ± 1.4 −1.0 ± 0.3 7.3 ± 0.5 9.2 ± 1.4

8.6 ± 1.3 19.1 ± 1.8 26.9 ± 2.5 20.5 ± 0.5 8.7 ± 0.9 4.2 ± 2.6

−0.85 ± 0.04 −1.24 ± 0.05 −1.32 ± 0.06 −0.18 ± 0.01 0.17 ± 0.02 1.06 ± 0.06

0.69 1.15 0.95 0.22 0.25 0.45

0.05 0.09 0.04 0.02 0.02 0.03

respectively, to the nematic director. κ = (K3/K1) − 1; γ = (ε∥/ε⊥) − 1; and ϕ is the angle between the director and the walls of the substrate, with ϕm being the value in the midplane of the sample. Whereas the value of K11 was obtained by simply measuring Vth, extraction of K33 required a full profile fitting to eq 4 using a MATLAB script.16 Experiments were conducted in the N phases of seven mixtures apart from the two pure compounds. Profiles of the reduced capacitance [(C − C⊥)/C⊥] versus the applied voltage V at several temperatures are shown in Figure 6a for a representative material (Xn = 49.6). Similar profiles were obtained for other materials as well. In all cases, the threshold was observed to be quite sharp across the entire N range, a feature more clearly seen in the enlargement provided in Figure 6b. A common feature observed in all cases is that Vth has a monotonic variation with increasing Tred (= TN−Iso − T, where TN−Iso is the nematic−isotropic transition temperature). The temperature dependence gets weaker with increasing Xn, becoming essentially the same beyond Xn = 90. The most salient aspect of the data is that the magnitude of Vth decreases dramatically with increasing content of 7OCB, suggesting that the coupling of the director with the electric field becomes better as the dimer component is reduced, obviously owing to the increase in εa. This feature is highlighted in the inset of Figure 6c, depicting Vth as a function of Xn at fixed Tred = 10 K. The data, exhibiting a strong variation with a factor of 3 decrease between the two pure compounds, as shown in the diagram, can be described by an exponential function. Figure 7 presents the thermal dependence of the splay elastic constant for the pure compounds and several mixtures. For all of these materials, K11 exhibits a monotonic variation with temperature, a standard feature seen for both rodlike and bentcore compounds. The essentially linear thermal dependence except in the neighborhood of the transition to the isotropic phase is suggestive of the mean-field influence. Such an influence predicts the elastic constant to vary as the square of the order parameter, which, in turn, is proportional to the square root of the reduced temperature. The Xn dependence of K11 at Tred = −10 K, presented as an inset in Figure 7, exhibits a variation that is monotonically decreasing up to Xn = 89.3, beyond which there is a reversal in the trend. Another notable feature of Figure 7 is that the slope, dK11/dTred, calculated by considering data deeper in the nematic phase but away from any low-temperature transition (−40 K < Tred < −5 K), decreases as the amount of rodlike component in the mixture increases: The slope decreases by a factor of 3 between CB7CB and 7OCB. The slope for pure CB7CB is, in fact, comparable to one of the highest reported values13,19,20 for the thermal variation of K11 in a bent-core nematic phase. The bend elastic constant presents a richer thermal behavior (see Figure 8). Pure CB7CB exhibits the feature usually seen for calamitic systems and single-component bent-core nematics. Thus, it is interesting to recall that two higher homologues of

where S represents the nematic orientational order parameter; Δα is the anisotropy of the molecular polarizability; N is the number of molecules per unit volume; and h and F are the cavity field and reaction field factors, respectively. Equation 1 provides a convenient method for determining the order parameter of especially strongly polar compounds, with their large εa values and the basis that the dipolar polarizability term dominates the contribution in square brackets. This results in a simple relation between εa and S S(T ) ∝

Tεa(T ) hF 2

and, in combination with Haller’s approximation, yields γ ⎛ T ⎞ S(T ) = So⎜1 − ⎟ TNI ⎠ ⎝

(2)

Figure 5 presents the temperature dependence of S for pure 7OCB and two mixtures rich with 7OCB, namely, Xn = 75.2 and 64.1, for which Haller’s approximation18 was found to be valid; the exponent γ obtained lies between 0.12 and 0.13. The variation in the magnitude of S with concentration is seen to be quite small, unlike in the case of a binary system of rodlike and rigid bent-core molecules.13 This is to be expected because the bent nature of the odd dimer can be considered to be weaker than that of a rigid bent-core molecule. 3.3. Frank Elastic Constants. Measurements of the elastic constant were performed using the electric-field-induced Fréedericksz transformation technique, as detailed in ref 13. Essentially, the samples are contained in sandwich cells of the type mentioned earlier. In the equilibrium situation, the molecules are oriented parallel to the surface in a uniform planar configuration. Application of an electric field normal to the substrates, above a certain critical threshold voltage, Vth, induces a reorientation of the molecules, and at high enough field, the molecules are completely reoriented normal to the surface. Profiles of the probed voltage-dependent capacitance, C(V), versus the applied voltage were employed to extract Vth and the elastic constants, splay (K11) and bend (K33), by using the Deuling analysis technique. The actual expressions employed are K11 = ε0εaVth 2/π 2

(3)

C(V ) − C⊥ 2γ Vth 1 + γ sin 2 ϕm × =γ− C⊥ π V

∫0

ϕm

⎡ (1 + κ sin 2 ϕ)(1 − sin 2 ϕ) ⎤1/2 ⎢ ⎥ cos ϕ dϕ ⎢⎣ (1 + γ sin 2 ϕ)(sin 2 ϕm − sin 2 ϕ) ⎥⎦

(4)

where εo is the permittivity of free space and εa = ε∥ − ε⊥ is the dielectric anisotropy of the sample, with ε ∥ and ε ⊥ corresponding to the dielectric constant measured in terms of the capacitances parallel (C∥) and perpendicular (C⊥), E

DOI: 10.1021/acs.jpcb.6b03048 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

anisotropy, taken as K33 − K11 at different Tred values, decreases with concentration, becoming most negative for Xn = 10 and then monotonically increasing, depicting the dominance of the bend elastic constant over the splay elastic constant (see Figure 9). In the vicinity of Xn = 70, the two elastic constants become comparable, reducing the anisotropy to zero. 3.4. Influence of the Order Parameter. Considering the inadequacy of the simple mean-field argument of Kii (i = 1, 3) being simply proportional to square of the order parameter S, Berreman and Meiboom23 introduced a higher-order dependence on the order parameter and provided the following expression

the CBnCB dimer series, namely, CB9CB21 and CB11CB,22 were reported to exhibit the opposite trend, namely, K33 decreases as the temperature is lowered in the nematic phase. However, the slope of this variation appears to be quite different for the three CBnCB compounds, decreasing from −0.4 pN/K for CB11CB to −0.15 pN/K for CB9CB, and extrapolating the trend to the presently studied CB7CB yields a value of +0.16 pN/K, which is comparable to the observed value of +0.1 pN/K in the present experiments. It can be noted here that, in the representation used, a positive value for the slope indicates that K33 increases as the temperature is lowered (or Tred is increased). Recall that the positive slope for K33 (value decreasing as the temperature is lowered) in bent-core nematics is argued10 to be due to the better compatibility of the bent-core molecules to the bend deformation. If the same argument were to be employed here owing to the reason that the odd parity of the ether linkage between two parts of the dimer would lead to an overall bent shape of the molecule, the effect should be stronger for shorter linkers than longer ones. This lends credence to our argument, described in an earlier article,13 that, although the absolute values are lower than for calamitics and the validity of the relation K33 < K11 could be due to molecular compatibility to bend deformation, the slope of the temperature dependence is not entirely dependent on such compatibility alone. With hardly an exception, singlecomponent rigid bent-core nematics exhibit a positive value of dK33/dTred [Tred = (TIso − T), where TIso is the isotropic− bent-core-nematic transition temperature], and the negative slope often arising below a trend reversal point (resulting from a convex-shaped K33 vs Tred profile) is almost always seen in binary mixtures of calamitic and bent-core nematic materials. We have argued13 that the convex-shaped profile in bent-core− calamitic systems arises from the frustration in the packing of the two differently shaped molecules. Indeed, as we do see such convex-shaped profiles in the present study when 7OCB is present (see Figure 8), the feature seems to more general, appearing whether the bent-shaped molecule is a rigid bentcore molecule or a conformation-caused soft bent-core molecule such as CB7CB. Generalizing our previous argument, we propose that the linkers in dimers play the role of the calamitic molecules. Varying the linker length is thus similar to changing the concentration of the calamitic component in a binary system and can thus change the sign of the dK33/dTred term. When the concentration of 7OCB is increased in the binary mixture, several interesting features appear (see Figure 8) in the thermal behavior of K33. Whereas mixtures with an excess of one component, namely, Xn = 10, 25, 89.3, and 94.9 (as well as pure 7OCB), exhibit a monotonic temperature dependence, the intermediate-concentration mixtures (Xn = 49.6, 64.1 and 75.2) display the convex-shaped profile mentioned above. The negative slope achieved after attaining the maximum value of K33 increases as the concentration of the calamitic increases, with the slope equaling −0.05, −0.14, and −0.16 pN/K for Xn = 49.6, 64.1, and 75.2, respectively. This suggests that the frustration in the molecular packing caused by the shape incompatibility of the two components is a nonmonotonic function of the calamitic concentration. More interesting is the nonmonotonic variation of K33 as a function of Xn at a fixed reduced temperature, say, Tred = 10 K, as shown in the inset of Figure 8. K33 achieves a minimum for Xn = 10 and then rises with a further increase in 7OCB, demonstrating again the influential role of the calamitic component. Similarly, the elastic

⎛ S ⎞2 ⎟ = K i(2) + K i(3)S + K i(4)⎜ ⎝1 − S ⎠ S

K ii 2

(5)

Here, K(2−4) are fitting parameters. The form of the last term i suggests that, for a perfectly ordered system with S = 1, the elastic constants diverge, resulting in the medium becoming nondeformable. Figures 10 and 11 present the S dependence of the ratios K11/S2 and K33/S2, along with fits to eq 5; the best-fit coefficients K(2−4) are given in Table 1. The applicability of eq 5 ii suggests that the theoretical model formulated for calamitic systems is valid for the presently studied soft-bent materials. K11/S2 versus S shows a concave-shaped behavior for all of the materials investigated. On the other hand, the K33/S2 versus S profile is concave for higher values of Xn where the mixture is rich in the rodlike component but changes its behavior to a convex shape for mixtures Xn with n values lower than 89.3%. This behavior is in line with the expectation that, for bent-rich systems, the bending deformation becomes easier with increasing order parameter.

4. SUMMARY In summary, we have performed measurements of dielectric and elastic constants on a binary system comprising a dimer exhibiting the twist-bend nematic phase as well as a regular nematic (N) phase and a rodlike compound showing only the latter type of phase. The NTB phase of the dimer is seen to survive until a high loading of the calamitic compound, perhaps owing to the structural similarity of the two components. Clear signatures of the transition between the two nematic phases are observed in the permittivity data, with the dielectric anisotropy apparently diminishing in the NTB phase. From the thermal variation of the anisotropy, which can be described by Haller’s approximation, the orientational order parameter could be estimated for the pure rodlike compound and mixtures with Xn > 50, and only a small diminution was seen upon adding the dimer. The threshold voltage showed a marked reduction, being more than halved, from the dimer to the calamitic compound. The relative magnitudes of the splay and bend elastic constants changed from K11 < K33 behavior for 7OCB to that often observed for bent-core materials (K11 > K33), even at a high loading factor of 75.2% 7OCB in CB7CB. This is surprising because the bent shape of the dimer is conformationally driven and not due to covalent linkages as in rigid bentcore molecules. Another nontrivial observation is that the convex-shaped thermal profile of K33 is seen not for the pure dimer but for the intermediate concentrations. A possible reason for this puzzling behavior could be that packing becomes more efficient for certain mixtures, with the smaller 7OCB molecules packing along the arms of the CB7CB molecules, thus conforming to the bent shape of the latter F

DOI: 10.1021/acs.jpcb.6b03048 J. Phys. Chem. B XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry B

of the liquid-crystal dimer 1″,7″-bis (4-cyanobiphenyl-4′-yl) heptane: A twist-bend nematic liquid crystal. Phys. Rev. E 2011, 84, 031704. (8) Majumdar, M.; Salamon, P.; Jakli, A.; Gleeson, J. T.; Sprunt, S. Elastic constants and orientational viscosities of a bent-core nematic liquid crystal. Phys. Rev. E 2011, 83, 031701. (9) Gortz, V.; Southern, C.; Roberts, N. W.; Gleeson, H. F.; Goodby, J. W. Unusual properties of a bent-core liquid-crystalline fluid. Soft Matter 2009, 5, 463−471. (10) Kundu, B.; Pratibha, R.; Madhusudana, N. V. Anomalous temperature dependence of elastic constants in the nematic phase of binary mixtures made of rod like and bent-core molecules. Phys. Rev. Lett. 2007, 99, 247802. (11) Sathyanarayana, P.; Mathew, M.; Li, Q.; Sastry, V. S. S.; Kundu, B.; Le, K. V.; Takezoe, H.; Dhara, S. Splay and bend elasticity of a bent core nematic liquid crystal. Phys. Rev. E 2010, 81, 010702. (12) Buka, Á .; Éber, N.; Fodor-Csorba, K.; Jákli, A.; Salamon, P. Physical properties of a bent-core nematic liquid crystal and its mixtures with calamitic molecules. Phase Transitions 2012, 85, 872− 887. (13) Parthasarathi, S.; Shankar Rao, D. S.; Fodor Csorba, K.; Krishna Prasad, S. Viscoelastic behavior of a binary system of strongly polar bent-core and rodlike nematic liquid crystals. J. Phys. Chem. B 2014, 118, 14526−14535. (14) Deuling, H. J. Deformation of nematic liquid crystals in an electric field. Mol. Cryst. Liq. Cryst. 1972, 19, 123−131. (15) Uchida, T.; Takahashi, Y. New method to determine elastic constants of nematic liquid crystal from C-V curve. Mol. Cryst. Liq. Cryst. 1981, 72, 133−137. (16) Madhuri, P. L.; Krishna Prasad, S.; Hiremath, U. S.; Yelamaggad, C. V. Photo-driven giant reduction of the frank elastic constants in a bent-core nematic liquid crystal. Appl. Phys. Lett. 2014, 104, 241111. (17) Maier, W.; Meier, G. Die. Hauptdielektrizitätskonstanten derhomogen geordneten kristallinflüssigen phase des p azoxyanisols. Z. Naturforsch., A: Phys. Sci. 1961, 16, 470−477. (18) Haller, I. V. Thermodynamic and static properties of liquid crystals. Prog. Solid State Chem. 1975, 10, 103−118. (19) Sathyanarayana, P.; Jampani, V. S. R.; Skarabot, M.; Musevic, I.; Le, K. V.; Takezoe, H.; Dhara, S. Viscoelasticity of ambient temperature nematic binary mixtures of bent-core and rod like molecules. Phys. Rev. E 2012, 85, 011702. (20) Kaur, S.; Addis, J.; Greco, C.; Ferrarini, A.; Gortz, V.; Goodby, J. W.; Gleeson, H. F. Understanding the distinctive elastic constants in an oxadiazole bent-core nematic liquid crystal. Phys. Rev. E 2012, 86, 041703. (21) Cachitas, H. M. M. Visco-elastic parameters of a liquid crystal with a nematic-nematic transition. Ph.D. Thesis, Instituto Superior Técnico, Lisboa, Portugal, 2013. (22) Balachandran, R.; Panov, V. P.; Vij, J. K.; Kocot, A.; Tamba, M. G.; Kohlmeier, A.; Mehl, G. H. Elastic properties of bimesogenic liquid crystals. Liq. Cryst. 2013, 40, 681−688. (23) Berreman, D. W.; Meiboom, S. Tensor representation of Oseen−Frank strain energy in uniaxial cholesterics. Phys. Rev. A: At., Mol., Opt. Phys. 1984, 30, 1955−1958.

molecules and consequently lowering the K33 values. Lowering the temperature stiffens not only the terminal chain of 7OCB, making it a better rod, but also the spacer chain of CB7CB, making it realize a better bent shape. The combination leads to a better coupling between the bending at the local scale and the bend deformation of the director, causing K33 to be significantly lowered. Experiments currently being planned to vary the dimensional difference between the rodlike and dimer components to investigate the behavior especially of the bend elastic constant are expected to shed more light on this feature. The elastic isotropy points might also be interesting from the viewpoint of investigating electroconvective phenomena.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b03048. Detailed synthetic procedure and characterization of CB7CB using NMR spectroscopy and DSC, POM textures, POM texture showing striped pattern in the NTB phase of pure CB7CB and the mixture X49.6, and intensity versus pixel number extracted from POM images of the two materials (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 91-80-23084218. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S.P. thanks Mr. Y. Venkata Jayasurya, TIFR, Mumbai, India, for his valuable help in MATLAB programming. C.V.Y. thanks SERB, DST, Government of India, for financial support under Project SR/S1/OC-04/2012.

■

REFERENCES

(1) Dozov, I. On the spontaneous symmetry breaking in the mesophases of achiral banana-shaped molecules. Europhys. Lett. 2001, 56, 247−253. (2) Panov, V. P.; Nagaraj, M.; Vij, J. K.; Panarin, Y. P.; Kohlmeier, A.; Tamba, M. G.; Lewis, R. A.; Mehl, G. H. Spontaneous periodic deformations in nonchiral planar aligned bimesogens with a nematicnematic transition and a negative elastic constant. Phys. Rev. Lett. 2010, 105, 167801. (3) Cestari, M.; Frezza, E.; Ferrarini, A.; Luckhurst, G. R. Crucial role of molecular curvature for the bend elastic and flexoelectric properties of liquid crystals: mesogenic dimers as a case study. J. Mater. Chem. 2011, 21, 12303−12308. (4) Chen, D.; Porada, J. H.; Hooper, J. B.; Klittnick, A.; Shen, Y. M.; Tuchband, R.; Korblova, E.; Bedrov, D.; Walba, D. M.; Glaser, M. A.; et al. Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 15931−15936. (5) Henderson, P. A.; Imrie, C. T. Methylene-linked liquid crystal dimers and the twist-bend nematic phase. Liq. Cryst. 2011, 38, 1407− 1414. (6) Pelzl, G.; Weissflog, W. Mesophase Behaviour at the Borderline between Calamitic and “Banana-shaped” Mesogens. In Thermotropic Liquid Crystals Recent Advances; Ramamoorthy, A., Ed.; Springer: Dordrecht, The Netherlands, 2010; Chapter 1, pp 1−58. (7) Cestari, M.; Diez-Berart, S.; Dunmur, D. A.; Ferrarini, A.; de la Fuente, M. R.; Jackson, D. J. B.; Lopez, D. O.; Luckhurst, G. R.; PerezJubindo, M. A.; Richardson, R. M.; et al. Phase behavior and properties G

DOI: 10.1021/acs.jpcb.6b03048 J. Phys. Chem. B XXXX, XXX, XXX−XXX