Stability enhancement of polystyrene thin films on aminopropyltriethoxysilane ultrathin layer modified surfaces
Abstract-Ultrathin layers (< 2 nm) of 3-aminopropyltriethoxysilane (APTES), one of the most common silane coupling agents, have been found to enhance the adhesion, thus stability, of polystyrene (PS) thin films on modified substrates. This phenomenon is primarily attributed to the active terminal amino group in APTES that can hydrogen bond with the hydrolyzed head groups of the molecule and hydroxyl groups of the substrate to form cyclic structures and a complex loose network under ambient conditions. Upon heating, the hydrogen bond is destroyed, allowing more silanol groups to condense and form siloxane bonds, hence tightening the loose 3-dimensional network. When a PS thin film is deposited on top of a thin APTES layer modified surface and then subjected to a thermal process, some PS chains can diffuse/penetrate slightly into the APTES network as the network is being tightened, embracing these chains within the network. The embraced chains assist in stabilizing the thin film onto the substrate. Stability of the thin film was enhanced for PS/APTES that was thermally pre-treated at approx. 80°C or 120°C prior to annealing of the thin film at higher temperatures. The presence of water at the interface was also found to improve the thin film stability upon annealing. In addition, a stronger suppression of dewetting was noticed for PS having a molecular weight higher than the entanglement weight (Me). When PS thin films were deposited onto a pre-cured highly cross-linked APTES network, no stability enhancement was observed.