Segregation of dioctyl phthalate to the surface of polystyrene films characterized by ToF-SIMS and XPS

Polystyrene (PS)-dioctyl phthalate (DOP) films were prepared by spin coating solutions of a mixture of PS and DOP on Si wafers and characterized by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). ToF-SIMS depth profile results showed that DOP mainly segregated to the top 10 nm of the surface of the PS-DOP film as well as to the interface between the film and the substrate. The normalized intensities of the ions corresponding to DOP on the surface increased as the annealing temperature increased and reached a maximum at the annealing temperature of 60 degrees C, and then decreased as the annealing temperature increased. XPS results indicate that the maximum amount of DOP segregated to the top surface of the PS-DOP film can reach to 39 wt% at the annealing temperature of 60 degrees C. In addition, DOP evaporated at 25 degrees C under ultra-high vacuum conditions; hence, ToF-SIMS and XPS analyses need to be carried out at low temperatures (e.g., -40 degrees C).