Control of Anchoring of Nematic Fluids at Polymer Surfaces Created by in Situ Photopolymerization Jian Zhou, † David M. Collard, ‡ Jung O. Park, †,§ and Mohan Srinivasarao* ,†,‡,§ School of Polymer, Textile & Fiber Engineering, School of Chemistry and Biochemistry, and Center for AdVanced Research in Optical Microscopy (CAROM), Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: September 29, 2004; In Final Form: February 18, 2005 In situ photopolymerization of alkyl acrylate monomers in the presence of a nematic fluid provides a cellular matrix of liquid crystalline droplets in which the chemical structure of the encapsulating polymer exerts control over the alignment (anchoring) of the liquid crystalline molecules. Control is obtained by variation of the alkyl side chains and through copolymerization of two dissimilar monofunctional acrylates. For example, among a series of poly(methylheptyl acrylate)s, the 1-methylheptyl analogue prefers planar anchoring of a nematic (TL205) over the temperature range studied. However, the polymers of other methylheptyl side chains display a homeotropic-to-planar anchoring thermal transition temperature similar to that of the n-heptyl analogue. Copolymerization of two monofunctional acrylates with opposing tendencies of aligning liquid crystal leads to tunability of anchoring behavior over a wide temperature range. The broad anchoring transitions we observed provide a way of achieving highly tilted anchoring. Introduction A liquid crystalline (LC) phase in contact with an interface exhibits preferential alignment (so-called “anchoring”), in which the director of the LC phase can be parallel (“planar” anchoring, P), perpendicular (“homeotropic” anchoring, H), or tilted at an intermediate angle (tilted anchoring) to the interface. 1 Anchoring plays an important role in determining the electrooptical properties of liquid crystal-based devices. 2 Numerous techniques have been used to modify surfaces to control anchoring in LC devices, such as mechanical rubbing, 3,4 oblique evaporation of inorganic materials (e.g., SiO x ), 5,6 photoalignment, 7,8 deposition of surfactants 9,10 and self-assembled monolayers, 11-13 grafting of polymer brushes, 14 and use of micropatterned surfaces, 15-17 etc. In most commercial LC display (LCD) devices, mechanical rubbing is used to provide a strong and unidirectional planar anchoring. Most of these techniques require a separate surface treatment step in the fabrication of LC devices. It would be an advantage to develop methods to obtain a desired alignment on a surface produced in situ, for example, by photopolymerization- induced phase separation. This process has been used to prepare LC-polymer composite films with a variety of morphologies: droplets-in-matrix morphology within typical polymer-dispersed liquid crystal (PDLC) films, 18,19 cellular morphology, 20,21 and parallel layer morphology. 22 In addition to their ease of manufacture, these LC-polymer composite films are compatible with methods used to make flexible LC displays. 23,24 However, the relationships between the alignment properties of nematic fluids and the encapsulating polymer surfaces are still poorly understood and need to be systematically explored. We have previously shown that the anchoring behavior of nematic fluids in a PDLC film can be controlled by the length 20 and branching 21 of the side chains attached to the backbone of the polyacrylate matrix. In this paper, we demonstrate: (i) how the copolym- erization of two monomers with opposing tendencies of aligning nematic fluids leads to the ability to tune the homeotropic-to- planar (H-to-P) anchoring transition temperature (T t ) over a large range of temperatures, and (ii) why continuous H-to-P anchoring transitions occur at some copolymer surfaces, which are in contrast to sharp anchoring transition at homopolymer sur- faces. 20,21 These results not only provide a convenient way of controlling anchoring in LC-based devices by tailoring polymer surfaces but also contribute to understanding the molecular origins of interfacial properties of nematic fluids. LC alignment with a high pretilt angle to a substrate is usually desired in many LCD devices (for example, twisted nematic devices) because it prevents the formation of defect by inhibiting the bulk LC director from realigning in two opposite directions during a switching-off process and also helps to reduce threshold voltage. 4,25 However, a highly tilted anchoring is not commonly obtained without special surface treatments. 1 The techniques to achieve a high pretilt angle include tangentially evaporating SiO x materials to glass, rubbing polyimides with homeotropic aligning tendency, 26 deposition of two chemicals with competing anchor- ing tendencies, 27 using inhomogeneous surfaces, 28 or use of a continuous homeotropic-to-planar transition. 29 Here we report a highly tilted anchoring which can be readily obtained in situ by tuning the composition of the copolymer with which the nematic phase is in contact. Experimental Section Materials. The nematic fluid we used, TL205 (EM Indus- tries), is a mixture of chlorinated bi- and ter-phenyls with aliphatic tails of 2 to 5 carbons. This fluid displays a nematic phase from -20 to +87 °C, thereby allowing the exploration of the anchoring behavior over a temperature range greater than is possible with a single-component nematic. 20 2-Ethylhexyl acrylate (2-EHxA), n-hexyl acrylate (C6A), n-octyl acrylate (C8A), n-decyl acrylate (C10A) and 1,1,1-trimethylol propane * Corresponding author. E-mail: mohan@ptfe.gatech.edu. † School of Polymer, Textile & Fiber Engineering. ‡ School of Chemistry and Biochemistry. § Center for Advanced Research in Optical Microscopy. 8838 J. Phys. Chem. B 2005, 109, 8838-8844 10.1021/jp040635a CCC: $30.25 © 2005 American Chemical Society Published on Web 04/07/2005