Surface Hydrophobicity and Liquid Crystal Pretilt Angle in Polymer-Stabilized Liquid Crystal Alignment Systems

Photocurable monomers lauryl acrylate and biphenyl diacrylate were added into the liquid crystal (LC) E7 in certain concentration range. Under a fixed voltage applied, LC cells were treated with UV light in a constant dosage. LC pretilt angle is stabilized during polymerization and varies continuously over the whole range from 0.8° to 90° by simply increasing the monomer concentration. Polymer aggregations were found on the inner surface of LC cells in a rather uniform size around 250 nm in diameter. The size of polymer aggregations was independent of monomer concentration, yet the number of aggregations increased with monomer concentration and polymer aggregations tended to be in chain-like cluster form. Advancing contact angle of water was utilized as index of surface hydrophobicity. Advancing contact angle of inner cell surface increased with increasing monomer concentration due to the formation of polymer layer. LC pretilt angle showed a great dependence on advancing contact angle. That is, LC pretilt angle increased monotonically with increasing advancing contact angle (or surface hydrophobicity). In other words, surface hydrophobicity plays an essential role in pretilt angle control in polymer-stabilized liquid crystal alignment system.