Substrate influence on the surface glass transition temperature of polymers

Enhanced molecular mobility near the free (polymer–air) surface is crucial for advancing organic electronic devices, yet understanding the substrate's impact on the surface glass transition temperature (T_g^surf) as film thickness decreases remains limited. This study explores how polymer films possessing attractive, neutral, and unfavorable polymer–substrate interactions affect T_g^surf. Results show that neutral interactions have no effect, while attractive or unfavorable interactions can increase or decrease T_g^surf by up to ∼37 °C. The onset thickness for this change is smaller for attractive interactions (up to 37 nm) than for unfavorable interactions (>100 nm), supporting the observed broadening of the surface glass transition with attractive interactions. We surmise that segment exchange between surface and subsurface regions introduces disparate dynamic components at the surface. Therefore, attractive interactions causing a sharper change in T_g^surf with film thickness lead to a broader surface glass transition.