Comparative study of electrooptic, dielectric, and structural properties of two glassforming antiferroelectric mixtures with a high tilt angle

Vitrification of the antiferroelectric smectic-${\mathrm{C}}_A^{*}$ phase is reported for the orthoconic mixture W-1000 and its new derivative W-356. The crystallization is not observed even upon slow cooling and the cold crystallization on subsequent heating is also absent. Molecular dynamics in the smectic phases of both mixtures is investigated down to 173 K and the fragility parameters are determined from the temperature behavior of the $\alpha$-process. X-ray diffraction is applied to compare the structural parameters of W-356 and W-1000 as well as to study the structural changes during the glass transition of the Sm-${\mathrm{C}}_A^{*}$ phase. The evolution of the smectic layer order within the Sm-${\mathrm{C}}_A^{*}$ glass is reported, while the correlation length of the short-range order in the smectic layers is shown to be almost constant below the glass transition temperature. Electrooptic properties of W-356: spontaneous polarization, tilt angle and switching time are determined and compared with these of W-1000. The observed differences between the properties of W-356 and W-1000 might be explained by the dimer formation of components with the -$\mathrm{C}\equiv \mathrm{N}$ terminal group, present only in the W-356 mixture, and by the different structure of the aromatic molecular core in one of the W-356 components.

Figure 1

Components of the W-1000 and W-356 mixtures and schemes of the molecular arrangement in the smectic-${\mathrm{A}}^{*}$, -${\mathrm{C}}^{*}$, and -${\mathrm{C}}_A^{*}$ phases.

Figure 2

DSC curves registered during cooling and heating for the W-1000 (a) and W-356 (b) mixtures for several chosen rates. The upper and bottom insets show the temperature region in the vicinity of the clearing temperature and the glass transition, respectively, for 2 K/min cooling rate.

Figure 3

POM textures of the smectic phases of W-1000 (a) and W-356 (b) registered for unaligned samples during cooling with 2 K/min rate, and the results of the numerical analysis in TOApy, performed with the “rgb” algorithm. For this option of TOApy, each pixel is decomposed into red, green and blue contributions denoted by numbers between 0 and 255. Then the normalized sum of each contribution over all pixels is plotted vs temperature [32]. The vertical lines denote the phase transition temperatures determined by DSC for the same cooling rate.

Figure 4

Monodomains of W-1000 at 368.5 K (a) and W-356 at 361 K (b) in 120 V/4.9 $\mu \mathrm{m}$ field (rectangular signal, 50 Hz, planar alignment). Each texture shows an area of $2.05\times 1.37\mathrm{mm}$. The arrows denote the positions of the polarizers.

Figure 5

Electrooptic parameters of the W-1000 (open symbols) and W-356 (solid symbols) mixtures: spontaneous polarization $P_s$ and tilt angle $\mathrm{\Theta }$ (a), coupling between $P_s$ and $\mathrm{\Theta }$ in the Sm-${\mathrm{C}}_A^{*}$ phase with fitting results of Eq. (1) (b), switching time ${\tau }_{\text{sw}}$ (c), as well as the Arrhenius plot of the rotational viscosity ${\gamma }_{rot}$ of the Sm-${\mathrm{C}}_A^{*}$ phase (d). The inset in panel (c) shows the ${\tau }_{\text{sw}}$ values close to the $\mathrm{Sm}\text{−}{\mathrm{A}}^{*}/\mathrm{Sm}\text{−}{\mathrm{C}}^{*}$ transition. The legend in panel (b) applies to the whole figure.

Figure 6

Dielectric dispersion (right side) and absorption (left side) of W-356 in the Sm-${\mathrm{A}}^{*}$, Sm-${\mathrm{C}}^{*}$ (a) and Sm-${\mathrm{C}}_A^{*}$ (b), (c) phases. Fitting results of Eq. (3) are marked by lines, separately for each relaxation process (ionic conductivity and electrode capacity parts are omitted). The spectrum for the Sm-${\mathrm{A}}^{*}$ phase at 367 K was registered in the $0.8\mathrm{V}/\mu \mathrm{m}$ bias field, while other presented spectra were collected without the bias field. All spectra were registered on cooling.

Figure 7

Arrhenius plot of the relaxation time (a) and temperature dependence of the dielectric strength (b) of relaxation processes observed in W-1000 (open symbols) and W-356 (solid symbols). Results for the soft mode were obtained in the $0.8\mathrm{V}/\mu \mathrm{m}$ bias field.

Figure 8

X-ray diffraction patterns of W-1000 (a) and W-356 (b) collected on cooling from 390 K to 150 K and subsequent heating to 298 K. The intensity is shown in the logarithmic scale.

Figure 9

Integrated intensities of diffraction peaks arising from the smectic layer order (a), the ratio of integrated intensities of the second- and first-order peaks (b), smectic layer spacing (c), and correlation length of the intralayer short-range order (d) vs temperature determined from the diffraction patterns of W-1000 and W-356. The vertical line in each panel shows the glass transition temperature determined by DSC.

Figure 10

Molecules of the components of the W-1000 and W-356 mixtures, optimized with the DFT-B3LYP/def2TZVPP method in conformations with the lowest conformational energy for considered torsional angles (${\phi }_1\approx {276}^{\circ }, {\phi }_2\approx {180}^{\circ }, {\phi }_1^{\prime }\approx$ 0, ${\phi }_3\approx 303-{306}^{\circ }, {\phi }_4\approx {180}^{\circ }$ or $0^{\circ }$). The arrows indicate the total dipole moment vectors $\vec{\mu }$ and $\varphi$ is an angle between $\vec{\mu }$ and the molecular long axis (shown for 3F5HPhF6 by the dashed line).

Figure 11

Relative conformational energy, with respect to the lowest-energy conformation for each torsional angle and compound, calculated with the PM7 (open symbols) and DFT-B3LYP/def2TZVPP (solid symbols) methods for torsional angles ${\phi }_1, {\phi }_2$ (${\phi }_1^{\prime }$ = ${180}^{\circ } - {\phi }_2$), ${\phi }_3, {\phi }_4$ indicated in FIG. 10.

Figure 12

Possible molecular packing of the components in one smectic layer of the Sm-${\mathrm{C}}_A^{*}$ phase of the W-1000 (a) and W-356 (b) mixtures (in the neighbor layers, molecules are tilted in an opposite direction). Horizontal dashed lines indicate the smectic layer spacing. In the actual W-356 mixture, the concentration of the C1 and C2 components is lower. Although the presentations of structures are schematic, the proportions between the experimental layer spacing and the size of molecular models are preserved. The 3F5HPhF6, 3F7FPhH6, C1, and C2 molecules are marked as blue, black, green and red, respectively (a color figure available in the online version).