High field-effect mobilities from diblock copolymers of regioregular poly(3-hexylthiophene) and poly(methyl acrylate) (PMA) are reported. The mobilities are achieved at several PMA contents provided the SiO2 surface is treated with octyltrichlorosilane. The mobility decreases with increasing PMA content when the SiO2 surface is untreated. These results show the importance of self-assembly at the interface for good FET performance in bottom-contact FET devices.