Modified PPTAs were made by introducing ionic, propanesulfonate groups and nonionic, propyl groups. Ionic groups caused polyelectrolyte behaviour for the modified PPTA in a polar solvent and propyl groups improved solubility of the modified polymer. Modified PPTA having small amounts of side groups (less than 40 mol% in total) showed birefringence, reflecting rigidity of polymer chains. In contrast, modified PPTA having a very high percentage of side groups (nearly 100 mol%), which may be needed for solubility in common, polar, organic solvents, seems to assume more disordered structures. Ionic groups are also responsible for miscibility enhancement of the modified PPTA with a polar polymer, poly(4-vinylpyridine), due to increased interactions through ion-dipole interactions. Propyl groups may make an additional contribution to miscibility enhancement through increased flexibility of PPTA molecules.