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Chapter 40
Fluorinated Distyrylbenzene Containing Copolymers for Photoinduced Formation of Anisotropic Materials as Photoalignment Layers for Liquid Crystals Veronika
Strehmel ,
Burkhard Stiller , Bernd
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Strehmel , 5,*
Ananda M . Sarker , and Douglas C . Neckers 5,6
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Institute of Chemistry, Applied Polymer Chemistry, and Physical Chemistry, University of Potsdam, Karl-Liebknecht Strasse 24/25, D-14476 Golm, Germany Institute of Physics, University of Potsdam, Am Neuen Palais 10, D-14415 Potsdam, Germany Department of Polymer Science and Engineering, University of Massachusetts at Amherst, Amherst, MA 01003-4510 Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403 4
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New fluorine containing photoactive polymers that contain the distyrylbenzene chromophore in the main chain have been investigated for use as photoalignment layers. Both photoisomerization and [2+2] photocycloaddition can occur competitively depending on the substitution pattern. The latter in particular results in anisotropic properties if the polymer film is irradiated with linearly polarized light. [2+2] photocycloaddition results in an insoluble material that is useful for efficient alignment of liquid crystals. Extinction difference (ED) diagrams support the result that mainly the [2+2] photocycloaddition is responsible for an increase of dichroism. Particularly the fluorine substitution in the chromophore influences both the photoreactivity of the polymer and the alignment of a liquid crystal in a manufactured cell. The results are compared with a distyrylbenzene copolymer with no fluorine. The dichroism of the liquid crystal obtained using a photoalignment layer with a particular substitution pattern 482
© 2003 American Chemical Society
Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
483 is comparable with that of a commercial cell made of a rubbed polyimide film. AFM measurements of fluorine containing polymers indicate a significant increase of surface roughness after irradiation. In addition, A F M figures show a nanostructural pattern. The results should give new impetus for development of photoalignment layers that could be used to substitute classical rubbed polyimidefilmsin LC devices.
Introduction Photooriented polymer films have become of increased interest for the alignment of liquid crystalline compounds despite the broad distribution of mechanically rubbed polyimide surfaces , and materials that can be aligned either by electric or magnetic fields used commercially. Alignment of liquid crystals has been an important technological problem for the development of flat panel displays. Despite their widely known disadvantages, rubbed polyimide films have been generally used in commercial display applications. These disadvantages include dust formation and static electricity build-up during the rubbing process. Photoalignment layers obtained by photochemical reactions reduce the problems of the commercially rubbed polyimide films. Furthermore, photoalignment layers include the possibility of using light to locally control the orientational direction of liquid crystals. Surface effects are particularly crucial for the alignment of liquid crystals controlled by photoactive polymeric films. Polymers containing side chains of azobenzene, coumarin and cinnamate have been investigated in detail for their use as photoalignment layers . In the case of polymers containing azobenzene chromophores trans-cis photoisomerization is significant for photoorientation. The dichroism of the materials formed after irradiation with linearly polarized light is quite high. However, the thermal stability is often not sufficient enough because the c/s-isomer formed thermally reverts to the trans-lsomer without light. Coumarins and cinnamates are alternative materials because they can form a polymer with anisotropic structures after irradiation with linearly polarized light that does not lose its dichroism due to the formation of crosslinked structures by means of a [2+2] photocycloaddition reaction. It is assumed that the trans-cis photoisomerization contributes to the formation of anisotropic structures in the case of cinnamates and the [2+2] photocycloaddition finally freezes the anisotropy. Coumarins cannot undergo an isomerization reaction but they are useful for the generation of anisotropic structures by means of a [2+2] photocycloaddition by irradiation with linearly polarized light. [2+2] Photocycloaddition has had a long tradition in photoinduced crosslinking in solid materials. Thus it may be attractive for some applications because no additional photoinitiator is 1
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Belfield and Crivello; Photoinitiated Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
484 necessary to start this reaction. However, such materials mostly absorb light (
