Nanophase segregation in binary mixtures of a bent-core and a rodlike liquid-crystal molecule

We studied mixtures of the achiral bent-core mesogen NOBOW 1,3-phenylene bis[4-(4-9-alkoxyphenyliminonetyl)benzoates] (P-9-O-PIMB) and the small, rodlike liquid crystal 8CB using high-resolution synchrotron x-ray diffraction, freeze fracture transmission electron microscopy, and differential scanning calorimetry. NOBOW and 8CB mix in an isotropic state at high temperatures but phase separate at lower temperatures when NOBOW transforms into the B4 phase and forms chiral helical nanofilaments. In pure NOBOW, the nanofilaments are close packed but at moderate 8CB concentrations, they are separated by nanosized gaps filled by 8CB. At higher concentrations of 8CB, macroscopic phase separation occurs.

Figure 1

DSC scans of NOBOW-8CB mixtures ($c=10\mathrm{\%}$, 30%, 50%, 70%, and 89%), pure 8CB, and pure NOBOW obtained on cooling. Dashed lines 1, 2, and 3 mark the pure 8CB $\text{Sm}A$-N transition, the pure 8CB $I\text{−}N$ transition, and the pure NOBOW B4 glass transition, respectively. Dashed line 4 marks the prealignment of 8CB into the nematic phase [12]. As the 8CB concentration increases, the transition temperature of the I-B2 transition and the I-B4 transition decreases due to freezing point depression. The nematic range is observed to broaden slightly as the NOBOW concentration increases (see Fig. 2 for an enlarged view).

Figure 2

DSC results of NOBOW-8CB mixtures near room temperature. Dashed lines mark the pure 8CB $N\text{−}\text{Sm}A$ and the pure 8CB $I\text{−}N$ transitions, respectively. The transitions in the mixtures are marked with arrows. As the 8CB concentration decreases, the $I\text{−}N$ transition temperature increases while the $N\text{−}\text{Sm}A$ transition temperature decreases, slightly broadening the 8CB nematic range.

Figure 3

FFTEM image of the $c=75\mathrm{\%}$ NOBOW-8CB mixture in the Bx phase. B4 NOBOW forms chiral helical nanofilaments (inset), which develop into an inhomogeneous porous network structure. The sample was first kept in the Bx phase before rapid quenching in liquid propane.

Figure 4

(Color online) X-ray scattering results for NOBOW-8CB mixtures. (a) The Bx phase in a $c=89\mathrm{\%}$ mixture at $25°\text{C}$ shows the wide B4 NOBOW peak (in red) and the sharp, nanoconfined $\text{Sm}A$ 8CB peak (in blue). The resolution-limited $\text{Sm}A$ peak of pure bulk 8CB (in black), sharper than the nanoconfined $\text{Sm}A$ 8CB peak, is also shown for comparison. Its peak position is shifted leftwards by $0.01{\text{Å}}^{-1}$ to help the eye. (b) Layer peak position vs temperature at $c=50\mathrm{\%}$, 70%, and 89%. The peak observed near $q=0.13{\text{Å}}^{-1}$ at high temperatures comes from B4 NOBOW. Two peaks are observed near $q=0.2{\text{Å}}^{-1}$ and $q=0.13{\text{Å}}^{-1}$ at lower temperatures, suggesting the phase coexistence of B4 NOBOW and $\text{Sm}A$ 8CB. As $c$ increases, $T_{\text{I-B}4}$ (arrows 1, 2, and 3 indicate the I-B4 transition for $c=50\mathrm{\%}$, 70%, and 89%, respectively) decreases due to freezing point depression.

Figure 5

(Color online) Scattering wave vector $q$ and FWHM of smectic layering peaks vs temperature for different 8CB concentration in NOBOW-8CB mixtures. The $q$’s are essentially no different from those seen in pure bulk $\text{Sm}A$ 8CB and pure bulk B4 NOBOW, respectively. However, as $c$ increases, the FWHM of the $\text{Sm}A$ 8CB peak decreases and the FWHM of the B4 NOBOW peak increases, exhibiting a similarity to LC in confined geometries.

Figure 6

(Color online) $\text{Sm}A$ 8CB peak for (a) $c=50\mathrm{\%}$, (b) 75%, and (c) 89%. The raw data were fit with two subpeaks: (1) the blue curve corresponding to the bulk 8CB and (2) the magenta curve corresponding to the nanoconfined 8CB. For $c=50\mathrm{\%}$, it was impossible to include in any component of the bulk 8CB in the fit. As $c$ increases, the bulk 8CB signal grows, as evidenced by the area under the blue curve.

Figure 7

(Color online) (a) Smectic correlation length $\xi$ of the nanoconfined $\text{Sm}A$ 8CB and the 2D triangular lattice parameter a of model B4 nanofilament array vs 8CB concentration. Inset schematically shows our model in which 8CB exists in both bulk form and nanoconfined form with NOBOW. The 2D triangular lattice parameter increases as the 8CB concentration increases due to the dilution of B4 nanofilaments by the addition of 8CB. The correlation length of the nanoconfined $\text{Sm}A$ 8CB increases as the 8CB concentration increases because the space among B4 nanofilaments increases. (b) Bulk form 8CB percentage vs 8CB concentration. At low 8CB concentration, there is no macroscopic phase separation. At $c>50\mathrm{\%}$, macroscopic phase separation occurs and the bulk form 8CB component increases as $c$ increases, reaching 22% at $c=89\mathrm{\%}$.