Liquid-crystal alignment on the rubbed film surface of semi-flexible copolyimides containing n-alkyl side groups

Semi-flexible copolyimides with various alkyl chain lengths (BTDA-ODA/CnMPD PIs) were newly synthesized in N-methyl-2-pyrrolidone from benzophmonetetr acarboxylic dianhydride, 4,4'-oxydiphenylene diamine, and 3,5-diaminobenzoy l n-alkanoates. The films were rubbed with varying rubbing densities, and o n the rubbed surface the alignment behavior of a nematic liquid-crystal (LC ) was examined. LCs were always aligned along the rubbing direction either homogeneously or homeotropically, depending on the side chain length as wel l as the rubbing density. The results inform that flexible n-alkyl side gro ups in the copolyimide play a critical role to align LCs on the surface, an d their role is strongly dependent on its length. In addition, thermal. opt ical, and dielectric properties were investigated.