Comparative study of the anchoring strength of reactive mesogens and industrial polyimides used for the alignment of liquid crystals

We measured the Rapini-Papoular polar anchoring strength coefficient $W$ for $4^{\prime }$-pentyl-4-cyanobiphenyl (5CB) on alignment layers formed by the reactive mesogen photopolymers RM 257, RM 82, and RM 84 [4,$4^{\prime }$-bis(acryloyl)biphenyl] (by Merck). These materials are commonly used for the photostabilization of the liquid crystal (LC) director in the bulk as well as at the surface of the LC layer via the formation of a loose polymer network that captures the director orientation. We developed a method of fabrication of alignment layers from these polymers, and estimated $W$ from the measurements of the optical retardation as a function of applied voltage in uniformly aligned cells. We found that RM 257 yielded $W$ of about $6\times {10}^{-4}\mathrm{J}/{\mathrm{m}}^2$, whereas RM 82 and RM 84 provided anchoring strengths of about $2\times {10}^{-4}$ and $4\times {10}^{-4}\mathrm{J}/{\mathrm{m}}^2$, respectively. Subsequent heating of the sample either destroyed the alignment layer, or substantially decreased $W$ to about $1\times {10}^{-4}\mathrm{J}/{\mathrm{m}}^2$, which was comparable to the anchoring strength of weakly rubbed commercial polyimides.

Figure 1

Director configuration (a) without and (b) with an applied electric field.

Figure 2

T-V curve for 5CB on RM 82 cell No. 2.

Figure 3

$\psi \left(V-V^{*}\right)/{\psi }_0$ vs $\left(V-V^{*}\right)$ curve for 5CB on RM 82, cell No. 2.

Figure 4

$\psi \left(V-V^{*}\right)/{\psi }_0$ vs $\left(V-V^{*}\right)$ curve for 5CB on weakly rubbed PI 2555.

Figure 5

$\psi \left(V-V^{*}\right)/{\psi }_0$ vs $\left(V-V^{*}\right)$ curve for 5CB on strongly rubbed PI 2555 (RM 257 cell No. 1 before heating demonstrates similar behavior).

Figure 6

Numerical simulation of the variation of the slope of the $R\left(V-V^{*}\right)/\left(V_{\mathrm{th}}{R}_0\right)$ vs $\left(V-V^{*}\right)/V_{\mathrm{th}}$ curves due to a small additive error $\delta R$ in the experimentally measured retardation. (a) Very strong anchoring case, with $W=1.3\times {10}^{-3}\mathrm{J}/{\mathrm{m}}^2$ in the usual range for the surface energy on polyimide alignment layers; the slope of the curve is very sensitive to even small retardation errors and can even change its sign, making it impossible to measure the anchoring energy from the slope. (b) Moderately strong anchoring, with $W=0.3\times {10}^{-3}\mathrm{J}/{\mathrm{m}}^2$, similar to the anchoring energy coefficients measured on the photopolymer alignment layers. Although the slope is less sensitive to the retardation errors in this case, high $\delta R$ values can still significantly influence the measured anchoring energies.