This paper describes the ester-amide exchange reaction for the facile and efficient post-polymerization modification of polystyrene-<i>block</i>-poly(methyl methacrylate) (PS-<i>b</i>-PMMA) to produce high-χ/low-N PS-b-PMMA analogues that exhibit a microphase separation with features on a sub-10 nm scale. Various amines were used for the ester-amide exchange reaction to introduce a small number of methacrylamide units into the PMMA block of the parent PS-<i>b</i>- PMMA. A small-angle X-ray scattering analysis revealed that a tiny amount of the methacrylamide units led to a significant increase in the incompatibility between the blocks. Consequently, we obtained a lamellar microphaseseparated structure with a domain-spacing as small as 11.1 nm through the simple post-polymerization modification of low-molecular-weight PS-<i>b</i>-PMMA. More importantly, directed self-assembly for the modified PS-<i>b</i>-PMMAs was demonstrated using topographically prepatterned silicon substrates with a PS brush layer, thus highlighting the practical utility of the proposed method in next generation nanolithographic applications.