Nanofabrication based on block copolymer self-assembly is one of the most promising methods for producing nanopatterns with a sub-10 nm feature size. Although polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) has been widely studied as a suitable template for nanofabrication, the insufficient incompatibility between the PS and PMMA blocks makes it difficult to achieve a microphase separation with the domain spacing of less than ca. 20 nm. We now present a simple and efficient method for the post-polymerization modification of PS-b-PMMA for effectively increasing the incompatibility between the two blocks, so that microphase separation can be achieved even at a low degree of polymerization. The ester-amide exchange reactions of PS-b-PMMA with primary and secondary amines were performed to introduce a small number of methacrylamides into the PMMA block to increase its hydrophilicity. The results of small-angle X-ray scattering measurements performed on bulk samples showed that the modified PS-b-PMMA self-assembled to form a lamellar phase even at the extremely low molecular weight of 8.5 kg mol(-1) due to the increased incompatibility between the blocks. The smallest domain spacing of the modified PS-bPMMA observed in this study was 11.1 nm. That is to say, the smallest feature size was 5.6 nm. The modified PS-b-PMMA thin film showed well-aligned line patterns via the graphoepitaxy directed self-assembly process, demonstrating that these materials obtained from PS-b-PMMA have the potential to be used for sub-10 nm nanofabrication applications.