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