No abstract

A mixture of two polyamic acids, one having an alkyl side chain and ordinarily used for vertical liquid crystal alignment, and the other without a side chanin and ordinarily used for planar alignment, is deposited on two substrates and baked at high temperatures. When the resulting cell is filled with the liquid crystal pentyl cyanobiphenyl, it is found that the pretilt angle θ 0 is a function of the baking temperature, and can be controlled continuously over the range 0 • ≤ θ 0. 90 • .

Reactive mesogens are low-molecular-weight liquid-crystalline monomers that can be photopolymerized in order to form well-organized, high-molecular-weight structures. [1,2] The order and alignment of the molecules imposed by internal fields originating from cholesteric molecule interactions, [1] external forces originating from surfaces, [1] external fields, [3] shear flow, and solvents [4] can be covalently captured by a photopolymerization process.[5] Reactive mesogen films have been utilized for a plethora of applications including reflective color-filter arrays, [6] broadband reflective polarizers, [7] viewing-angle-enhancement technology for liquid-crystal displays, [8] responsive microstructures, [9][10][11] polymer microelectromechanical systems, [12] and on the fundamental side, the stabilization and subsequent investigations of low-molecular-weight liquidcrystal ordering templates.[13]A desired property in polymer systems is systematic and accurate control over molecular order in three dimensions. [5] The ability to tailor molecular order to such an extent would enable a wide variety of novel thin films for optical and electrical devices. Reactive mesogen materials come close to meeting this goal-molecular order and orientation can be controlled using a variety of methods that successfully manipulate low-molar-mass liquid crystals. These alignments can then be indefinitely captured through photopolymerization. [2,5] Reactive mesogens are often composed of a three-ring mesogenic unit with reactive side groups. The presence of reactive groups plays a minor role in the mesomorphic properties of the material, including the effect of lowering their phase transition temperatures by destabilizing the mesophases. The central ring of the mesogenic group can also be substituted with a methyl group in order to reduce the phase-transition temperatures.[2] In order to induce helicoidal order the side group can be substituted with a chiral methyl group [2] or the central ring can be exchanged by a chiral sorbitol moiety.[14]The reactive mesogen used to fabricate the thermally activated topological features described here was a nematic difunctional liquid-crystalline acrylate with a methyl group on the central ring of the mesogenic core doped with a chiral reactive mesogen additive. The use of these materials allows for accurate control of the molecular order in multiple dimensions, and each of the various liquid-crystalline states can be fixed by the polymerization process. [2,7,15] In reactive mesogen materials, the ordering of the molecules in the polymerized liquid-crystal network is closely reminiscent of the molecules in their monomeric state. The basic thermal-expansion behavior of planar aligned reactive mesogens has previously been reported; there is an anisotropy in their thermal-expansion behavior because of their liquid crystallinity. [12,16] The thermal expansion of the reactive mesogen is substantially lower along the long molecular axis than normal to it, owing to the fact that most of the covalent bonds...