The mechanical stability of polymeric nanostructures is critical to the processing, assembly and performance of numerous existing and emerging technologies. A key predictor of mechanical stability is the elastic modulus. However, a significant reduction in modulus has been reported for thin films and nanostructures when the thickness or size of the polymer material decreases below a critical length scale. Routes to mitigate or even eliminate this reduction in modulus, and thus enhancing the mechanically stability of polymeric nanostructures, would be extremely valuable. Here, two routes to modulate the mechanical properties of polymers at the nanoscale are described. Exposure to ultraviolet light and ozone (UVO) crosslinks the near surface region of high molecular weight PS films, rendering the elastic modulus independent of thickness. However, UVO cannot eliminate the decrease in modulus of low molecular mass PS or PMMA due to limited reaction depth and photodegradation, respectively. Alternatively, the thickness dependence of the elastic modulus of both PS and PMMA can be eliminated by addition of dioctyl phthalate (DOP) at 5 mass %. Furthermore, an increase in modulus is observed for film < 30 nm with 5 mass % DOP in comparison to neat PS. Although DOP acts as a plasticizer for both PS and PMMA in the bulk, it appears that DOP acts as an antiplasticizer at the nanoscale. By maintaining or even increasing the elastic modulus of polymers at the nanoscale, these methods could lead to improved stability of polymeric nanostructures and devices.
Citation: ACS Nano
Pub Type: Journals
wrinkling, confinement, thin film, modulus, crosslinking, antiplasticizer