Nano-scale mechanical properties testing on polymer thin films via AFM indentation

The nanoconfinement effects in polymer thin film have been a major interest in past decades. It was observed by using various experimental methods that the glass transition temperature (Tg) differs from bulk material value at both polymer/substrate interfaces and free surfaces, which indicates the altered polymer molecular structure under confinements. The local elastic moduli of polymer thin films near interfaces and free surfaces also differ from that of the bulk but those differences have been hard to quantify. In this study, an AFM nanomechanical testing-based method was developed to study the elastic modulus change at both polymer interfaces and free surfaces. The new method is capable of providing the local modulus mapping with nanometer resolution inside submicron spatial range. Therefore the full gradient of modulus change can be revealed for the first time with ultrahigh resolution on both sides of a supported film, which facilitates the further understanding of polymer confinement effects. The experiments have been performed on PMMA films spin coated on silicon substrates. The results showed that, on the interface side where the polymer interacts with a surface, PMMA modulus starts from 1.5 times of the bulk material value, and declines exponentially to the bulk value in 50 nm range. On the free surface side, the modulus starts around half of the bulk value, then rapidly increases to normal in just 10 nm distance