Thin polymer films exhibit glass transition temperatures (Tgs) that are depressed from the bulk material Tg because of increased degrees of chain motion. Using such films as gate dielectrics in organic thin-film transistors provides a sensitive new probe of polymer surface motional characteristics at the buried semiconductor-dielectric interface. Researchers at Northwestern University's Materials Research Science & Engineering Center find that pentacene thin-film transistor mobility depends dramatically on the pentacene growth temperature on the polymer dielectric. Abrupt discontinuities in pentecene film mobility and microstructure as a function of growth temperature can be correlated with a surface Tg which lies substantially below the thin film Tg, is independent of dielectric film thickness, and is strongly dependent on the dielectric surface functionalization. Understanding polymer dielectric surface properties is crucial to the rational design and fabrication of maximum performance organic thin-film transistors.