Gel formation in a mixture of a block copolymer and a nematic liquid crystal

The viscoelastic properties of a binary mixture of a mesogenic side-chain block copolymer in a low molecular weight nematic liquid crystal are studied for mass concentrations ranging from the diluted regime up to a liquid crystalline gel state at about 3%. In the gel state, the system does not flow, exhibits a polydomain structure on a microscopic level, and strongly scatters light. Below the gelation point, the system is homogeneous and behaves like a usual nematic, so the continuum theory of liquid crystals can be applied for interpreting the experimental data. Using the dynamic Fréedericksz transition technique, the dependence of the splay elastic constant and the rotational viscosity on the polymer concentration have been obtained. Comparing the dynamic behavior of block copolymer solutions with the respective homopolymer solutions reveals that, above a mass concentration of 1%, self-assembling of the block copolymer chain segments in clusters occurred, resulting in a gel state at higher concentrations. The effective cluster size is estimated as a function of the concentration, and a scaling-law behavior near the sol-gel transition is confirmed. This technique may serve as an alternative method for determining the gelation point.

Figure 1

(a) Nematic liquid crystal 4-Cyano-(4′-pentyl)biphenyl, 5CB. (b) Cyanobiphenyl functionalized block copolymer. (c) Corresponding homopolymer. The average number of repeating units of each segment is given.

Figure 2

Sketch of the liquid crystal cell and definition of the coordinate system.

Figure 3

Images of cells filled with samples of different concentrations $c$ of the block copolymer taken between crossed polarizers. (a) $c=1.1$%; (b) $c=3.0$%; (c) $c=5.0$%. Note the different scales.

Figure 4

Sketch of the experimental setup.

Figure 5

Voltage dependence of the optical phase difference near the Fréedericksz transition for pure 5CB and for the block copolymer solution with a mass concentration of $1.1$%. The voltage steps are always $\Delta U=2$ mV with different time delays $\Delta t$ between the voltage steps.

Figure 6

Typical time dependencies of (a) the transmitted light intensity $I$ and (b) the phase difference $\delta$ for pure 5CB and for samples with block copolymer concentrations of $1.1$% and of $2.1$%. The solid lines in (b) are fits of Eq. (18) to the experimental data.

Figure 7

Histograms of the ${\tau }_{\text{on}}$ distribution over the sample area (a) for pure 5CB and (b) for a block copolymer concentration of $2.1$%. ${\overline{\tau }}_{\text{on}}$ is the mean value obtained from fitting a normal distribution (solid line) and ${\sigma }_{\text{on}}$ is the respective standard deviation.

Figure 8

As functions of the polymer concentration $c$ are shown (a) the dielectric anisotropy ${\epsilon }_{\text{a}}$, (b) the splay elastic constant $k_{11}$, and (c) the rotational viscosity ${\gamma }_1$. In (b) and (c) and in the following figures, the vertical bars represent the distribution of the values within our samples.

Figure 9

Increase of the rotational viscosity for the homopolymer ($\delta {\gamma }_1^h$) and for the block copolymer ($\delta {\gamma }_1^c$) solutions. The solid line is a linear fit with $\delta {\gamma }_1^h=0.156c$, while the dashed line is a guide to the eye.

Figure 10

Dependence of ${(\delta {\gamma }_1^c/\delta {\gamma }_1^h)}^{1/2}$ and, thus, $R_{\perp }^c/R_{\perp }^h$ on the block copolymer concentration $c$. The solid line is obtained by fitting Eq. (28) to the data above $c=1$%.