Effect of polymer network on thermodynamic stability and switching behavior of the smectic-$C_{\alpha }^{*}$ phase

A polymer-stabilized liquid crystal based on $4^{\prime }$-(octyloxy)biphenyl-4-carboxylate 2-fluoro-4-((octyl-2-yloxy)carbonyl)phenyl (D16) and 1,6 hexanediol diacrylate as a monomer was prepared by in situ photopolymerization. The selected antiferroelectric liquid crystal contains a fast-switching smectic $C_{\alpha }^{*}$ phase (${\mathrm{SmC}}_{\alpha }^{*}$), and the influence of the polymer network on the thermodynamic stability of this phase and its switching behavior under applying time-dependent electric field were studied. Using dielectric spectroscopy and polarizing microscopy, the liquid crystal materials were characterized, and subsequently with the use of the reversal current method (RCM) the current response, especially from the ${\mathrm{SmC}}_{\alpha }^{*}$ phase was carefully analyzed. The current response is complex and also depends on the neighboring liquid crystal phases. In the liquid crystal-polymer system, as well as in the liquid crystal-monomer mixture, a significant shift of the temperature range of the ${\mathrm{SmC}}_{\alpha }^{*}$ phase toward lower temperatures was observed; however, the thermodynamic instability related to the transformation to the crystalline phase was also noted and characterized. Because of the fuzzy phase transitions detected in the liquid crystal-polymer system by dielectric spectroscopy and also because of the lack of the characteristic dielectric signature of ${\mathrm{SmC}}_{\alpha }^{*}$ after polymerization, we proposed the use of the RCM, as a complementary one, to identify the ${\mathrm{SmC}}_{\alpha }^{*}$ phase even in such complex materials.

Figure 1

Structural formula of liquid crystal D16 molecule.

Figure 2

Thermal hysteresis of the temperature dependency of the electric permittivity in neat D16 at 440 Hz taken on heating (a) and cooling (b).

Figure 3

Textures recorded for neat D16 in the ${\mathrm{SmC}}_{\alpha }^{*}$ phase at 58 °C (a) and in the ${\mathrm{SmC}}_{\beta }^{*}$ phase at 48 °C (b) with the polarized microscope.

Figure 4

Kinetics of the crystallization process in neat D16 at 40, 46, 48, 52 °C (a) and in PSD16 at 46 °C (after 30 and 50 min of polymerization) in comparison to neat D16 at 46 °C (b).

Figure 5

Thermal hysteresis of the temperature dependencies of the electric permittivity in D16, D16/monomer, and PSD16 on heating (a) and on cooling (b) cycles.

Figure 6

The current response from the ${\mathrm{SmC}}_{\alpha }^{*}$ phase in D16 at 10 Hz (a) and 100 Hz (b), the mixture of D16/HDDA at 100 Hz (c) and PSD16 at 10 Hz (d) recorded with reversal current method (RCM) on cooling cycles; the applied voltage was changed linearly from 40 to −40 V for (a), (b), and (d), and from 15 to −15 V for (c).