For improving the alkaline stability and other properties of aromatic semiblock copolymer [QPE-bl-11a(C1)] membranes containing benzyltrimethylammonium groups, several novel hydrophilic monomers with different side-chain lengths and substitution positions were designed and synthesized for the polymerization. The pendant-type preaminated copolymers PE-bl-11s were quaternized using iodomethane to obtain the target QPE-bl-11s with well-defined chemical structure. In TEM analyses, QPE-bl-11a(C3) and QPE-bl-11a(C5) membranes with propyl and pentyl side-chains, respectively, showed more developed phase-separated morphology with greater hydrophilic domains (ca. 10-20 nm in width) than that of the C1 equivalent. The phase separation was more distinct and larger for the QPE-bl-11a membranes linked with p-phenylene groups in the hydrophilic part than for the QPE-bl-11b membranes with m-phenylene groups. In particular, QPE-bl-11b(C5) membrane exhibited considerably smaller hydrophilic/hydrophobic domains compared to those of the other membranes. After the alkaline stability test in 1 M KOH aqueous solution at 60 °C for 1000 h, the remaining conductivity was better as increasing the side-chain length: 34% for QPE-bl-11a(C1), 54% for QPE-bl-11a(C3), and 72% for QPE-bl-11a(C5) at 60 °C. The results suggest that the pendant alkyl chains could improve the alkaline stability and the main-chain bond position could improve morphology, water utilization, and mechanical properties of QPE-bl-11 membranes. An H2/O2 fuel cell with QPE-bl-11 membrane showed 139 mW cm-2 of the maximum power density at 0.28 A cm-2 of the current density.