Reproducibility of Adsorbed Layer Growth Under Different Sample Preparation Conditions and Its Possible Influence on the Physical Aging of Thin Polystyrene Films

We have recently observed an unexpected molecular weight dependence to the physical aging rate of 31 nm thick polystyrene (PS) films supported on silicon not present in bulk films. Thin films made from ultra high MWs ≥ 6,500 kg/mol exhibit a 40% faster aging response at an aging temperature of 40 ^oC than equivalent films made from (merely) high MWs ≤ 3,500 kg/mol. Because the presence of adsorbed layers to substrates has been considered a potential effect on the glassy dynamics in thin polymer films by several recent studies, we began examining the possible influence of chain adsorption on our unexpected physical aging results. We have investigated various experimental factors that influence the value of the adsorbed layer thickness measured h_ads and find that the growth of these adsorbed layers is highly dependent on the sample’s thermal history, both above and below the glass transition temperature Tg. Here, we discuss the reproducibility of h_ads under different sample preparation conditions and address the precision with which such thin h_ads values can be reliably determined using ellipsometry. Our results show that the presence of small or large adsorbed layers do not alter the observed molecular weight dependence to the physical aging behavior in thin PS films.