Thermotropic Biaxial Nematic Phase in Liquid Crystalline Organo-Siloxane Tetrapodes

Infrared absorbance measurements have been carried out on two liquid crystalline organo-siloxane tetrapodes. Results unambiguously show the existence of a biaxial nematic phase below a uniaxial nematic phase. The three components of IR absorbance are used to calculate the various order parameters. On cooling, a weak first-order transition from isotropic to nematic is observed, followed by a second-order phase transition to biaxial nematic where the biaxiality parameters are found to be significantly large. Results are supported by observations from conoscopy and texture. Temperature dependences of the order parameters are well explained by the mean-field model for a biaxial phase.

Figure 1

Molecular structures of tetrapodes: (a) $\mathit{A}$ and (b) $\mathit{B}$ with asymmetric and symmetric mesogens, respectively. (c) Platelet formed by a macromolecule of the tetrapode $\mathit{A}$.

Figure 2

Absorbance components normalized with those of the isotropic phase for $\mathit{A}$: △, $A_X$; ▽, $A_Y$; ◇, $A_Z$ values for its phenyl ring stretching band $1160{\mathrm{c}\mathrm{m}}^{-1}$; □, $A_Y$ and ○, $A_X$ values for the carbonyl stretching band $1738{\mathrm{c}\mathrm{m}}^{-1}$ $A_X$ and $A_Y$ are measured for the homeotropically aligned sample.

Figure 3

Order parameters for the tetrapode $\mathit{A}$: □, $S$; ○, $P$; ▽, $D$; ◇, $C$; solid line, predicted by the mean-field model [24].

Figure 4

Fundamental invariants of the mean-field model: □ and ○, experimentally obtained values of $\delta$ and $\Delta$, respectively; solid lines, fits to the mean-field model [24].

Figure 5

Phase diagram showing the dependence of reduced temperature $1/\beta$ on the biaxiality parameter $\lambda$. Solid and broken lines exhibit phase behavior (see the text) and the vertical dotted lines give corresponding values of $\lambda$ for the two tetrapodes.

Figure 6

Textures (top) and conoscopy (bottom) for a freestanding film of tetrapode $\mathit{A}$: (a) biaxial nematic phase; (b) phase transition between the uniaxial and biaxial nematic phases; (c) uniaxial nematic phase.