Synthesis and properties of photoalignable aromatic polyesters containing phenylenediacrylate units in their backbones and n-alkyl moieties in their side groups

A series of poly [oxy(4-n-alkyl-3,5-benzoate)oxy-1,4-phenylenediacryloyll s (PPDA-CnBZ polymers) with high molecular weights was synthesized. These polymers exhibit excellent solubility in some common organic solvents and produce good quality films using conventional spin-casting and drying processes. The polymers are thermally stable up to 357-362 degreesC in a nitrogen atmosphere; their glass transition temperatures are greater than 121 degreesC. The photoreactions and photoalignments of the polymers were investigated using ultraviolet-visible and infrared spectroscopy, and their liquid crystal (LC) alignment properties were examined. The phenylenediacrylate (PDA) chromophores in the polyesters were found to mainly undergo photocyclization upon ultraviolet light irradiation. Irradiation of the polyester films with linearly polarized ultraviolet light (LPUVL) induces preferential orientation of the polymer main chains, while the unreacted PDA chromophores are aligned along the direction perpendicular to the electric vector of the LPUVL. All the films irradiated with LPUVL were found to align LCs in a direction perpendicular to the electric vector of the LPUVL. Moreover, these LC alignments persisted even on irradiated films annealed at temperatures up to 210 degreesC, which is much higher than the glass transition temperatures of the polyesters. These LC alignment characteristics are due to the anisotropic interactions of the LC molecules with the oriented polymer chains and with the unreacted PDA chromophores. LC alignments on the polyester film surfaces have homeotropic to homogeneous characteristics, depending on the length of the n-alkyl side group, providing strong evidence that the n-alkyl side groups of the polyesters play a critical role in determining the pretilt angles of the LCs. The LC pretilt angles were also found to be influenced by the thermal annealing history of the irradiated films. In summary, the excellent properties of the PPDA-CnBZ polymers make them promising candidate materials for use as LC alignment layers in advanced LC display devices. (C) 2004 Wiley Periodicals, lnc.