Surface modi. cation of the gate insulators is a key factor in improving the performance of organic thin. film transistors (OTFTs). We have developed a novel method for modifying the surface of a gate-insulating polymer containing an unsaturated double bond. A polyimide (PI) gate insulator containing an unsaturated double bond as a side chain was synthesized using the monomers pyromellitic dianhydride (PMDA), 4,4'-methylenedibenzenamine (MDA), and (E)-3,5-diaminobenzyl-3-(biphenyl-4-yl)acrylate (BIP). Octadecanethiol (ODT) was successfully reacted with the unsaturated double bond of this PI gate insulator under solvent-free conditions, thus modifying its surface and making it more hydrophobic. The modified hydrophobic surface strongly affected the initial growth mechanism of pentacene and, subsequently, the performance of the corresponding OTFTs. The devices fabricated with the modified PI gate insulators showed superior performance compared to those prepared with nonmodified insulators and exhibited a carrier mobility of 0.41 cm2/(Vs), an on/off current ratio of about 8.6 x 10(5), and a subthreshold slope of 2.31 V/dec. (C) 2008 Elsevier B. V. All rights reserved.