Semi-flexible copolyimides with various alkyl chain lengths (BTDA-ODA/CnMPD PIs) were newly synthesized in N-methyl-2-pyrrolidone from benzophmonetetracarboxylic dianhydride, 4,4'-oxydiphenylene diamine, and 3,5-diaminobenzoyl n-alkanoates. The films were rubbed with varying rubbing densities, and on 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 well as the rubbing density. The results inform that flexible n-alkyl side groups in the copolyimide play a critical role to align LCs on the surface, and their role is strongly dependent on its length. In addition, thermal. optical, and dielectric properties were investigated.