Orientational order induced by a polymer network in the isotropic phase of liquid crystal

We studied the paranematic ordering induced by a polymer network in the isotropic phase of a liquid crystal (LC) that occurs in polymer-stabilized cells with bend configuration of the LC director (π cells) fabricated via photopolymerization of photoreactive monomer RM 82 added in small concentrations (3–5 wt %) to a nematic LC [4-cyano-4′-pentylbiphenyl (5CB)] when low voltage was applied across the cell. The polymer network formed in the nematic phase of the LC consists of fine fibrils that are aligned along the LC director and thus mirror the bend deformation of the LC at the time of polymerization. When heated to temperatures above the nematic-to-isotropic (N-I) phase transition such highly ordered polymer network anchors LC molecules providing ordering of the LC around the fibrils which results in unusually high optical retardation of the cell, $R_{\mathrm{cell}}$. We present a theoretical model that relates $R_{\mathrm{cell}}$ to the degree of order of the fibrils, the anchoring energy of the LC molecules on the surface of the polymer fibrils, and the fibril radius $r_0$. Fitting of the experimental $R_{\mathrm{cell}}\left(T\right)$ curves with the developed model reveals correlation of $r_0$ with the nematic correlation length ${\xi }_0$ which characterizes penetration of the nematic order in the isotropic phase of the LC. Accepting ${\xi }_0$ as a material constant of about 1 nm leads to a very small radius of the fibrils, $r_0\sim 1\mathrm{nm}$, which is also supported by other reported experimental data. High optical retardation and fast electro-optical response of the cells at the temperatures deep into the isotropic phase point toward the enhancement of the polymer-induced paranematic order by a well-oriented layer of LC molecules that are absorbed on the surface of fibrils. Application of high voltage at the isotropic phase temperatures results in high variations of the optical retardation of the cells. Characteristic on and off response times were about 10–100 μs, independent of the cell gap. Combination of large voltage-driven changes of the optical retardation occurring in the low-viscosity isotropic state with switching times that are at least two orders of magnitude shorter than the typical relaxation times of the cells operating in the nematic phase make such polymer-stabilized π cells very promising for application in fast electro-optical switches and light modulators.

Figure 1

π-cell configuration.

Figure 2

Schematic representation of the structure of polymerized π-cells: lateral (a) and longitudinal (b) cross sections. Polymer chains (represented by solid lines) follow the π-bend configuration of the LC director at the time of polymerization (a). Polymer fibrils with the average radius $r_0$ are uniformly distributed in the cell at the average half-distance $D_0$ (b).

Figure 3

The order parameter of LC inside the fibril $S_0$ is approximately equal to the order parameter of the polymer chains $S_{\mathrm{polymer}}$ (set during polymerization in the presence of an external electric field in the nematic phase of the LC). $S_s$ is the order parameter in the isotropic phase of the LC induced by the surfaces of fibrils due to the anchoring of the LC molecules to the fibrils.

Figure 4

$R_{\mathrm{cell}}\left(T\right)$ curves for 4.3-μm-thick cell filled with the mixture of 5CB and 3%, 4%, and 5% of RM 82, respectively, and a 5.3-μm-thick cell filled with the mixture of 5CB and 3.5% of RM 82; all are polymerized at 2.4 V.

Figure 5

Measured (symbols) and modeled (solid lines) $R_{\mathrm{cell}}\left(T\right)$ curves for ${\xi }_0\le 1$ nm.

Figure 6

$R_{\mathrm{cell}}\left(T\right)$ curves for the cell ($d=5.3\mu \mathrm{m}$, $c=3.5\%$) under continuously applied ac field (10 kHz; the rms values of the voltage are shown in the figure).

Figure 7

Transmitted intensity of the cell ($d=5.3\mu \mathrm{m}$, $c=3.5\%$) under crossed polarizers at $T=36.0{}^{\circ }\mathrm{C}$ when subjected to a short pulse of dc voltage $\tau =100\mu \mathrm{s}$, $U=160\mathrm{V}$: experiment (blue points) and fit (red lines). The red lines correspond to the fit of the on and off parts of the curve with a sum of two exponentials with characteristic times ${\tau }_1^{\mathrm{on}}=5.6\mu \mathrm{s}$, ${\tau }_2^{\mathrm{on}}=110\mu \mathrm{s}$, ${\tau }_3^{\mathrm{off}}=8.8\mu \mathrm{s}$, and ${\tau }_2^{\mathrm{off}}=140\mu \mathrm{s}$. The inset shows the response of the cell to a longer dc pulse, $\tau =2\mathrm{ms}$, revealing an additional third characteristic time, ${\tau }_3^{\mathrm{on}}\approx {\tau }_3^{\mathrm{off}}\ge 1\mathrm{ms}$.

Figure 8

Modulation of the light by a polymer-stabilized π-cell ($d=4.3\mu \mathrm{m}$, $c=4\%$) in its isotropic state, 0.5 °C above the N-I phase transition. The cell is driven by sinusoidal ac pulses (black continuous line), with frequency $f=1\mathrm{kHz}$ and rms voltage $U_{\mathrm{rms}}=40\mathrm{V}$. Light transmitted by the cell observed under crossed polarizers (blue data points) is modulated at the double frequency, 2 kHz.