Abstract
The surfaces of polymer and interfaces between polymer and inorganic particles are of particular importance for the properties of polymers and composites. However, the determination of the properties of surfaces and interfaces poses many challenges due to their extremely small dimensions. Herein, polystyrene and polymethyl methacrylate thin film on silicon wafer was used as a model system for the measurement of the properties of the polymer near free surface and at the polymer-solid interface. Two different methods, i.e., nanoindentation using atomic force microscopy (AFM) and the gold nanoparticle embedding technique, were used for these measurements. The results showed the elastic modulus of PS near the free surface determined by nanoindentation was lower than the bulk value. Based on contact mechanics analysis, nanoparticle embedding also revealed the existence of a lower-modulus, non-glassy layer near the free surface at temperatures below the bulk glass transition temperature (Tg). However, near the polymer-solid interface, the AFM nanoindentation method is not applicable due to the geometry confinement effect. On the other hand, the nanoparticle embedding technique can still correctly reflect the interactions between the polymer and the substrate when compared to the ellipsometry results.
Highlights
Properties near polymer-air surface and polymer–solid interface play a critical role in many technological applications such as thin films, coating, adhesives and polymer-based micro/nanoscale devices and constructers [1,2]
The spring constant and indenter radii of silicon cantilevers used in this study are about 20 N/m and 50 nm, respectively
When the annealing temperature was at 90 °C, the embedded depth of gold nanoparticles was around 3.7 nm, which agreed well with the value reported in literature and this value can be attributed to the thickness of the surface layer with a lower modulus [21]
Summary
Properties near polymer-air surface and polymer–solid interface play a critical role in many technological applications such as thin films, coating, adhesives and polymer-based micro/nanoscale devices and constructers [1,2]. Applying a precise control of the penetration depth at the nanometer scale and a normal load in the range of nanonewtons to piconewtons, the depth profile of elastic modulus of PS films from free surface to bulk was obtained by Miyake et al [17] Their measurements were far away from the polymer-solid interfacial region [18]. This work demonstrated the limitation of nanoindentation in the study of interface properties, and extended the gold nanoparticle embedding method to the measurement of surface/interface modulus, making it capable to investigate the property profile of polymer thin films from the free surface to the polymer-substrate interface
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