Spontaneous Periodic Deformations in Nonchiral Planar-Aligned Bimesogens with a Nematic-Nematic Transition and a Negative Elastic Constant

Hydrocarbon linked mesogenic dimers are found to exhibit an additional nematic phase below the conventional uniaxial nematic phase as confirmed by x-ray diffraction. The phase produces unusual periodic stripe domains in planar cells. The stripes are found to be parallel to the rubbing direction (in rubbed cells) with a well-defined period equal to double the cell gap. The stripes appear without external electromagnetic field, temperature or thickness gradients, rubbing or hybrid alignment treatments. Simple modeling proposes a negative sign for at least one of the two elastic constants: splay and twist, as a necessary condition for the observed pattern.

Figure 1

Liquid crystalline materials used. $M1$: Cr ($91°\mathrm{C}$) $N_x$ ($109°\mathrm{C}$) $N_u$ ($127°\mathrm{C}$) Iso; $M2$: Cr ($95°\mathrm{C}$) $N_x$ ($137°\mathrm{C}$) $N_u$ ($193°\mathrm{C}$) Iso; $M3$: Cr ($85°\mathrm{C}$) $N_x$ ($123°\mathrm{C}$) $N_u$ ($170°\mathrm{C}$) Iso. $N_x$ is the unusual nematic phase under investigation.

Figure 2

A $7\mu \mathrm{m}$ planar unrubbed cell of $M2$ between crossed polarizers at different temperatures. The length of the white bar is $100\mu \mathrm{m}$ (a) $N_u$ phase, $T=140°\mathrm{C}$, (b) $N_x$ phase, uniform, $T=135°\mathrm{C}$, (c) $N_x$ phase, striped, $T=120°\mathrm{C}$.

Figure 3

(a) Pattern periodicity in different materials as a function of the cell gap. $D=2d$ is an eye-guide line. The insets explain the definition of the pattern period $D$ using (bottom) POM image of $M3$ at $110°\mathrm{C}$ in $9\mu \mathrm{m}$ commercial cell with the rubbing direction rotated by 20° from the polarizer axis and (top) a schematic cross section of the proposed molecular distribution by the $y\mathrm{\text{−}}z$ plane. (b)–(d) Rotation of a $5\mu \mathrm{m}$ planar rubbed cell in crossed polarizers. $M3$, $T=110°\mathrm{C}$. Image height $\sim 40\mu \mathrm{m}$. The angle between the polarizer ($\mathbf{P}$) and the rubbing direction ($\mathbf{R}$) is (b) 0°, (c) 20°, (d) 45°.

Figure 4

Definition of the coordinates and angles of the director distribution. Rubbing direction ($\mathbf{R}$) is parallel to the $x$ axis. $\theta$ and $\phi$ are the polar and the azimuthal angles, respectively.

Figure 5

XRD patterns of samples aligned by $\sim 1\mathrm{T}$ magnetic field on cooling. (a),(b) $M1$, (c),(d) $M3$, (a),(c) $N_u$ phase, (b),(d) $N_x$ phase.