Figure: Competition between tendencies towards perpendicular alignment along the brush–solvent meniscus (top) and parallel alignment induced by chain stretching in the brush (bottom).

Polymer Brushes with LC side chains

[Project with → Prof. Diethelm Johannsmann, TU Clausthal]

Liquid crystal (LC) polymer brushes have been suggested as alignment layers for nematic LC’s with adjustable pretilt angles, because situations may be established where the brush tends to align the low molecular weight nematic in a different direction (e.g. perpendicular) than the underlying bare surface. In our research we investigate side chain LC brushes that are synthesized via the „grafting from" procedure. The mesogenic group in the side chain consists of a phenylbenzoate moiety linked to the methacrylate main chain via a flexible spacer. The thickness of the brushes can be controlled by adjusting the monomer concentration during brush growth and by varying the polymerization time. LC brushes with a thickness of up to 200 nm in the dry, solvent-free state have been obtained.

Employing a special rubbing technique it is possible to grow the LC brushes directly into macroscopic monodomains. If this procedure is followed and the appropriate parameters during brush synthesis and cell fabrication are used one can assemble LC cells in which the nematic is oriented at a pretilt angle in respect to the surface.

References
A. Halperin, D.R.M. Williams, Europhys. Lett., 21, 575, 1993.
F. Benmouna, B. Peng, J. Rühe and D. Johannsmann, Liquid Crystals, 26, 1655, 1999
B. Peng, J. Rühe, D. Johannsmann, Adv. Mat. 12, 821, 2000
B. Peng, D. Johannsmann and J. Rühe, Macromolecules 32, 6759, 1999