Unusual Alignment of Liquid Crystals on Rubbed Films of Polyimides with Fluorenyl Side Groups

Abstract

We synthesized two polyimides (PIs), poly(4,4‘-(9,9-fluorenyl)diphenylene cyclobutanyltetracarboximide) (CBDA-FDA) and poly(4,4‘-(9,9-fluorenyl)diphenylene pyromellitimide) (PMDA-FDA), and investigated in detail the surface morphology, molecular orientation, and nematic liquid crystal (LC) alignments of rubbed CBDA-FDA and PMDA-FDA films. The rubbed CBDA-FDA film surface was found to contain microgrooves that run parallel to the rubbing direction, but was found to induce LC alignment perpendicular to the rubbing direction. In contrast, the rubbed PMDA-FDA film surface was found to contain unusual meandering microgrooves that run perpendicular to the rubbing direction; this is the first report of such structures on a rubbed PI film. The PMDA-FDA film surface was found to induce LC alignment parallel to the rubbing direction. Even though these two PI films have quite different groove structures and LC alignment behaviors, the polymer main chains in their surfaces lie in the film plane and are preferentially oriented along the rubbing direction, and the fluorenyl side groups lie in the out-of-plane and are preferentially oriented perpendicular to the rubbing direction. These LC alignment, anchoring energy, surface morphology, and polymer segmental orientation results indicate that LC alignments on the surfaces of rubbed PI films are determined by the interplay between the directionally anisotropic interactions of the LC molecules with the oriented polymer main chain segments, the oriented fluorenyl side groups, and the microgrooves. The directionally anisotropic interactions of the LC molecules with the oriented polymer chain segments were found to be much stronger than those with the directionally developed microgrooves. We conclude that CBDA-FDA and PMDA-FDA PIs are promising alignment layer materials for the fabrication of advanced LC display devices.

The effect of phenoxymethyl side groups on the liquid crystal alignment behavior of polystyrene derivatives

“…1,17,20,[31][32][33][45][46][47][48] Figure 7 shows the IR spectra of the rubbed (a) PHP and (b) P4FP films with a rubbing density of 100 on a Si-wafer with the IR monitoring light parallel and perpendicular with respect to the rubbing direction; the C=C (1508 cm -1 ) peaks from the aromatic rings in the side chains are more intense when the polarization of the incident beam is perpendicular to the rubbing direction, indicating that the aromatic side groups are oriented more or less perpendicular with respect to the rubbing direction. Therefore, the perpendicular LC alignment on the rubbed PHP and P4FP films should be arisen from the possible the favorable anisotropic interactions between the LC molecules and the aromatic side groups oriented perpendicular with respect to the rubbing direction.…”

“…The changes of the molecular orientation on the polymer surfaces caused by the rubbing process are confirmed by the FTIR dichroic spectra shown in Figure 7 as well as by the reports of many others. 17,20,[31][32][33] …”

“…17,20 LC Cell Assembly. Twisted nematic (TN) and antiparallel LC cells were fabricated using the unrubbed or rubbed polymer films onto ITO coated glass slides.…”

“…38 The LC alignment direction of the antiparallel LC cells was investigated by measuring the angular dependence of the absorbance of a dichroic dye (disperse blue 1) dissolved in 5CB at a concentration of 1 wt% on the rotation angle of the samples using optical apparatus equipped with a He-Ne laser, a polarizer, and a photodiode detector. 20 The pretilt angle of the antiparallel LC cells was measured by the crystal rotation method. 39 The cell gap was measured before filling the LCs using a spectrophotometer (Ocean Optics Inc., S 2000).…”

“…[1][2][3][4][5][6] The mechanical rubbing of polyimide (PI) surfaces is the most commonly used technique for producing LC alignment layers, 7-21 which can induce stable homogeneous planar LC alignment 19,20 or homeotropic LC alignment. 14,18 The LC alignment properties such as the pretilt angle and electro-optical (E-O) performance on polyimide surfaces are known to be affected by the molecular structure of the polyimide such as the flexibility of the polymer backbone, isomeric structure of the mesogen end groups, spacer length, and spacer conformation in the side chains.…”