Glassy relaxation in a de Vries smectic liquid crystal consisting of bent-core molecules

We report experimental investigations of a liquid crystal comprising thiophene-based achiral bent-core banana shaped molecules. The compound exhibits the following phase sequence on cooling: Isotropic (517.4 K), $N$ (514.9 K), de Vries SmA (402 K), SmC. Practically no layer contraction was observed across the SmA to SmC transition, confirming the “de Vries” nature of the SmA phase. Interestingly, the crystallization does not occur on cooling the sample, unlike most other liquid crystals. Instead, the SmC phase undergoes a glass transition at 271 K even at a slow cooling rate. The dielectric spectroscopy studies carried out on the sample reveal the presence of a dielectric mode whose relaxation process is of the Cole-Cole type. The relaxation frequency of the mode was found to drop rapidly with decreasing temperature, confirming the glassy behavior. The variation of relaxation frequency with temperature follows the Vogel-Fulcher-Tammann equation, indicating the fragile glassy nature of the sample. This report identifies a bent-core liquid crystal exhibiting a “de Vries” SmA phase and glassy behavior at lower temperatures.

Figure 1

Schematic representation of the molecular arrangement in conventional SmA, SmC, and de Vries SmA phases comprising calamitic molecules. In the dSmA phase, the molecules are on average tilted away from the layer normal by a significantly large angle with their long axes randomly distributed on a cone according to the diffuse cone model proposed by de Vries [7].

Figure 2

(a) Molecular structure of the compound BTCN8. (b) DSC thermogram of the compound BTCN8 with heating and cooling rate of 5 K/min. The inset demonstrates the existence of a small-range nematic phase. The sample does not crystallize on cooling to room temperature. The mesomorphic properties of the sample were retained for several weeks, and the melting transition was observed only on the first heating. (c) The temperature variation of the optical transmittance through a planar-aligned sample of thickness $5\mu \mathrm{m}$ kept between crossed polarizers while cooling from the isotropic phase. The optical transmittance clearly detects the following phase transition sequence: Isotropic (523.5 K), N (521.5 K), dSmA (402 K), SmC. The inset reveals the existence of a nematic phase with a small range of temperature. All observed phases are enantiotropic.

Figure 3

(a) XRD intensity profile for the compound BTCN8 in dSmA and SmC phases at temperatures 473 K and 393 K, respectively. (b) The temperature variation of the ratio $d/d_{ac}$ and tilt angle $\theta$ were obtained from the XRD data and estimated molecular length. The layer thickness remains almost unchanged across the dSmA-SmC transition, showing the de Vries nature of the SmA phase. The vertical dotted line indicates the dSmA to the SmC transition temperature. Across the transition depicted by the pink shaded region, the layer contraction is only about 0.17%. The error bars are smaller than the size of the symbols.

Figure 4

POM texture of homeotropically aligned thin sample kept between a clean glass plate and a cover slip at (a) 522 K, (b) 518 K, and (c) 388 K and that of the planar-aligned sample of thickness $5\mu \mathrm{m}$ at (d) 522 K, (e) 508 K, and (f) 333 K. The POM textures were taken under crossed polarizers conditions while cooling the samples from their isotropic phase. $R$ indicates the rubbing direction, and the crossed arrows denote the positions of the polarizers. The scale bar represents a length of $50\mu \mathrm{m}$.

Figure 5

(a) Variation of optical tilt angle ${\theta }_{\mathrm{opt}}$ as a function of ($T_{ac}-T$) in the SmC phase. The solid line shows the fit to the experimental data using Eq. (1). The inset of (a) shows the oppositely tilted domains in the SmC phase with the layer normal denoted by unit vector $\widehat{k}$. In both domains, $\widehat{k}$ are parallel, but the optic axes are tilted on the opposite side of $\widehat{k}$, giving rise to the optical contrast between crossed polarizers. The white and red arrows represent the direction of the layer normal $\widehat{k}$ and the optic axis, respectively, in the two domains. (b) Temperature variation of the effective birefringence ($\mathrm{\Delta }n$) of a planar-aligned sample while cooling from the isotropic phase. The inset of (b) shows the magnified view of the data in the short temperature range of the nematic phase.

Figure 6

Variation of the effective dielectric constant (${\epsilon }_{\mathrm{eff}}$) of the compound BTCN8 in a planar-aligned LC cell of sample thickness $5\mu \mathrm{m}$ as a function of temperature. The data clearly detect the various phase transitions and are in agreement with the DSC and POM observations. The dielectric constant's low value indicates the absence of spontaneous polarization in the sample. The inset shows the magnified view of the marked region.

Figure 7

Variation of the imaginary part of the dielectric constant (${\epsilon }^{\prime \prime }$) as a function of frequency at different temperatures. (a) In the high-temperature range from 423 K to 395 K with a temperature step of 4 K. (b) In the low-temperature range from 393 K to 303 K with a temperature step of 10 K. The peaks in the data represent various dielectric relaxation processes.

Figure 8

(a) Frequency variation of ${\epsilon }^{\prime \prime }$ at 333 K along with fitted curve (solid line) using Eq. (4). The deviation of the fitted curve in the higher frequency region is due to the overlap of the relaxation peak in the MHz range associated with ITO coating. (b) Relaxation frequency of mode $M_1$ as a function of inverse temperature. The solid curve represents the fit to the experimental data using VFT Eq. (5), indicating the non-Arrhenius behavior of the liquid.

Figure 9

Linear variation of ‘$T{[d(ln\tau )/d(1/T)]}^{-1/2}$’ as a function of temperature showing two dynamical regimes separated by the crossover temperature $T_B=370K$. The slope and ordinate of the linear fit in the lower (higher) temperature region are 0.028 and $-6.51$ (0.123 and $-41.65$), respectively.

Figure 10

DSC thermogram of the compound BTCN8 at different rates while cooling from the isotropic phase and subsequent heating. A step change in the curve at a temperature of 271 K is indicative of glass transition. The sample does not crystallize over a few weeks.

Figure 11

Schematic representation for the arrangement of bent-core molecules in a layer of the dSmA phase (a) without electric field and (b) in the presence of the field applied parallel to the smectic layer. The change in the molecular distribution on the application of field gives rise to collective mode $M_1$ observed in our sample.