Surface Characterization of EMA Copolymers Using XPS and ToF-SIMS

Semi-crystalline copolymers of ethylene (CH2=CH2) and methyl acrylate (CH2=CH−CO−O−CH3) (EMAs) are commonly used in many applications where surface properties (such as adhesion, printability, cling and sealing) are critical to polymer performance. Despite the importance of EMAs and their surface properties, the surface compositions of pristine EMAs have not been critically examined. In this work, the surface chemistries of a contiguous series of 11 pristine EMAs of varying MA content (up to 33 wt.%) have for the first time been critically examined using angle-resolved XPS and ToF-SIMS analyses. This examination has revealed the details of EMA surface chemistry, and has provided techniques for EMA identification and quantification. Below 20 wt.% bulk MA, the EMA surface is shown to consist of a thin MA-rich phase over an MA-depleted phase. This surface chemistry changes at MA levels greater than 20 wt.%. At these bulk MA concentrations, the MA is depleted from the surface and enriched in the near-surface regions. Time-of-flight SIMS analyses of these EMAs indicate that there is a great enrichment of EMA backbone (with MA branches intimately attached) associated with this MA depletion. This simultaneous backbone enrichment and MA depletion indicates that the MA-containing polymer has not been depleted from the surface. Instead, the MA branches have simply rotated away from the surface into the polymer. Upon melting, these segregation effects are removed and the surface chemistry becomes equal to that in the bulk. X-ray photoelectron spectra provide a measure of the ester and ether groups associated with MA and oxidized polyethylene, but do not provide positive EMA identification. In contrast, ToF-SIMS spectra provide positive EMA identification and detailed information on the orientation of the polymer. Time-of-flight SIMS and XPS calibrations are developed for quantitative (±4 and 2 wt.% MA, respectively) MA determinations on surfaces (top 10–60 Å). Also, these calibrations are combined with an understanding of the surface structure of EMAs to determine the bulk MA content (±2 wt.% MA) of the corresponding EMA films.