Evidence of Enhanced Mobility at the Free Surface of Supported Polymer Films by in Situ Variable-Temperature Time-of-Flight-Secondary Ion Mass Spectrometry

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectra of polystyrene (PS) films supported on silicon wafers were obtained at temperatures ranging from room temperature to 100 °C. Principal component analysis (PCA) of the TOF-SIMS data revealed a transition temperature (T_T) at which the surface structure of PS was rearranged. The T_T of a 120-nm thick PS (weight-average molecular weight of 3 000 g/mol) thin film was determined to be about 36 °C, which is approximately 30 °C lower than the bulk glass transition temperature (T_g) of that PS. Similar T_Ts were observed on PSs with different molecular weights. As the T_T is strongly related to the T_g and dependent on the molecular weight, it is believed that the T_T determined by TOF-SIMS is related to the surface glass transition temperature (T_g^S) measured by other techniques. This suggests that TOF-SIMS combined with PCA can be used to determine the T_g^S of polymer films. Furthermore, the detailed PCA analyses indicate that the phenyl groups of PS tended to move away from the surface at temperatures above T_T. This conclusion was further confirmed by contact angle and XPS measurements.