Molecular design of some semi-alicyclic polyimides as a route to improve refraction and dielectric properties for liquid crystal display applications

Abstract

The study establishes an adequate monomer combination for achieving the best balance of properties required in obtaining polyimide alignment layers for display devices. The molecular design of some aliphatic/aromatic polyimides was performed by using dianhydride monomers with distinct configurations in terms of rigidity, size, and symmetry. The polyimides based on semi-flexible and nonsymmetric dianhydride moieties present lower refractive index and dielectric constant (<3) than those obtained from symmetric and rigid dianhydride units. This determines faster traveling of visible and microwave radiations as required for liquid crystal alignment purposes. The interactions of samples with the N-(4-Methoxybenzylidene)-4-butylaniline, cyanobiphenyl, and 4-pentyl-4-cyanobiphenyl nematics were assessed. The analyzed polyimides have higher surface tensions than the ones of the liquid crystals, determining a parallel arrangement of the nematic molecules. As the dispersive interactions at the polymer surface are lower, the work of spreading is higher as a result of improved adhesion of liquid crystal with the polyimide alignment layer. The sample containing rigid, symmetric, noncoplanar dianhydride units (PI.4) and the one based on semi-flexible, nonsymmetric dianhydride moieties are the most transparent for visible and microwave radiations, allowing low cohesion interaction at interface with nematics. These aspects recommend the two studied polyimides as candidates for alignment layers.