| Popis: |
In the past, liquid crystal alignment has been achieved through a surface alignment technique. Before the liquid crystal (LC) cell is fabricated, the two substrates are coated with a thin film of polymer which is then rubbed with a nylon cloth. The process is difficult to control, leads to the formation of electrostatic charge at the film surface and can induce only one type of alignment within a LC cell. A surface-alignment technique has been developed in which a polymer coated substrate is prepared not through a rubbing process but by a linear photopolymerisation (LPP) reaction. A substrate is coated with a thin film of a photoactive polymer. Illumination with plane polarised UV light causes the polymer to undergo a side-chain cross-linking reaction. The plane polarised nature of the UV light produces an anisotropic distribution of un-reacted side-chains. It is thought that these residual side-chains induce alignment in a liquid crystal phase. Using glass, ITO-coated glass and quartz substrates, we have fabricated LC cells by sandwiching a nematic phase between (i) two LPP treated substrates; or (ii) one LPP treated substrate and one conventional rubbed polymer (RP) treated substrate. The relative aligning strength of the LPP and RP treated substrates were studied qualitatively using optical microscopy and quantitatively using an optical transmission experiment and dielectric relaxation spectroscopy. It was found that in terms of both azimuthal and zenithal aligning strength, the RP treated substrates were more effective at aligning a nematic phase than the LPP treated surfaces. Liquid crystal alignment using a photoactive substrate leads to the possibility of patterning a LC cell with pixels that appear optically different when viewed between polarizers. Patterned LC cells could have enormous and wide-ranging applications in the display, electrooptical element and information storage industries. We have shown that high-density, controllable patterning can be achieved in LC cells incorporating a LPP treated surface. |
