We investigated the crystalline nanostructures and film morphologies of pentacene films deposited onto a polymer brush organic interlayer in high performance organic field-effect transistors (OFETs). Polymer brushes were grafted onto the oxide substrates by spin-coating and thermal annealing. Pentacene FETs fabricated on top of the polymer brushes showed excellent device performance, with a field-effect mobility of 0.82 cm2 V−1s−1 and an on/off current ratio of 107. These properties were superior to those of devices using typical surface modification techniques, such as octadecyltrichlorosilane (ODTS) and hexamethyldisilazane (HMDS). The improvements in OFET performance appeared to be due to the pentacene layer's crystalline nanostructure and grain interconnectivity, which formed during the submonolayer stage of film growth. This stage of growth is strongly correlated with the surface energy, morphology, and viscoelastic properties of the resulting gate dielectrics. The inclusion of a polymer brush dielectric surface modification is a significant step toward optimizing the nanostructures of organic semiconductors, which are directly linked to device performance enhancement, by engineering the interfaces in OFETs.