Photo- and rubbing-alignable brush polyimides bearing three different chromophores

Abstract

Three new photoreactive brush polyimides (PSPIs), each bearing a different type of chromophore (cinnamoyl (CA), 3-(2-furyl)acryloyl (FA), and methacryloyl (MA)) in their bristles (i.e., side groups), are successfully synthesized, and are found to produce good-quality films with smooth surfaces through conventional spin-casting and drying processes. These PSPI polymers are thermally stable up to 320 degrees C. This is the first quantitative investigation of the photoaligning and rubbing-aligning processabilities of PSPI polymer films, and of the abilities of the resultant films to control the orientation and anchoring of liquid-crystal (LC) molecules. The chromophores of both poly(1-cinnamoyloxy-2,4-phenylene hexafluoroisopropylidenediphthalimide) (6F-DAP-CA) and poly(1-3-(2-furyl)acryloyloxy-2,4-phenylene hexafluoroisopropylidenediphthalimide) (6F-DAP-FA) PSPIs are found to undergo photodimerization in thin films and, to a lesser extent, photoisomerization, resulting in insoluble, crosslinked films. The MA chromophores of 6F-DAP-MA PSPI are found to undergo photopolymerization in thin films, which might include photodimerization to a lesser extent, resulting in insoluble, crosslinked films. Thin films of the PSPI polymer chains are found to have excellent unidirectional orientation ability as a result of either photoexposure with linearly polarized UV light (LPUVL) or rubbing. Both the photoaligned and the rubbing-aligned polymer chains in the PSPI films are demonstrated to effectively induce the alignment of nematic LCs along their orientation directors by anisotropic interactions between the preferentially oriented polymer chain segments and the LCs. The contribution to LC alignment of the microgrooves developed in the rubbed films is found to be very low. The anchoring energies of the LCs on the photoaligned film surfaces are comparable to those on the rubbing-aligned film surfaces; the anchoring energies are found to be in the range 0.45-2.25 X 10(-5) J m(-2), and to depend on which film treatment process is used and which chromophore bristle is present. In summary, the new PSPIs reported in this paper are promising LC alignment-layer candidates with rubbing-free processing for the production of advanced LC-display (LCD) devices, including LCD televisions with large display areas.