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

Abstract

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 (<i>T</i>_T) at which the surface structure of PS was rearranged. The <i>T</i>_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 (<i>T</i>_g) of that PS. Similar <i>T</i>_Ts were observed on PSs with different molecular weights. As the <i>T</i>_T is strongly related to the <i>T</i>_g and dependent on the molecular weight, it is believed that the <i>T</i>_T determined by TOF-SIMS is related to the surface glass transition temperature (<i>T</i>_g^S) measured by other techniques. This suggests that TOF-SIMS combined with PCA can be used to determine the <i>T</i>_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 <i>T</i>_T. This conclusion was further confirmed by contact angle and XPS measurements