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Example of XPS

Materials

Three fluoroacrylate homopolymers were synthesized using a homogeneous free-radical solution polymerization.  The homopolymers (Figure 1) included 1,1-dihydroperfluorooctyl acrylate (PFOA), 1,1-dihydroperfluorooctyl methacrylate (PFOMA), 1,1,2,2-tetrahydroperfluorooctyl acrylate (PTAN).  Additionally, three block copolymers of polysterene and PFOA (Figure 1) were synthesized by an iniferter process with the block lengths q = n = 51, q = 34 and n = 113, and q = 29 and n = 103.  The polymers were prepared into a 1 wt% solution in 1,1,2-trichlorotrifluoroethane and spun cast onto silicon substrates followed by an annealing process at various temperatures for varying times.  Unannealed films were also prepared for comparison.

 
Instrumentation

A Physical Electronics Model 5400 spectrometer was used to conduct X-ray photoelectron spectroscopy (XPS). The spectrometer was equipped with a hemispherical capacity analyzer and the specimens were irradiated with X-rays from a monochromatic Al Ka1,2 source - spot size was 1mm in diameter.  The instrument was calibrating by setting the binding energy scale to Au4f7/2 = 84.0 eV, Cu2p3/2 = 932.6 eV, and Ag3d5/2= 368.3 eV.  Additionally, the operating conditions included a test chamber vacuum of 2x10^-8^ Torr, 15 kV, 500-600 W, and 35-40 mA for the Al X-ray source.  Scans for an elemental survey were performed from 0-1000 eV  with a pass energy of 89.5 eV.  Conversely, high-resolution scans were acquired with a pass energy of 17.9 or 35.8 eV for C1s, F1s, and O1s regions.  Three take-off angles, 20, 40, and 70°, were used between the sample and the analyzer.

Results

The results from the XPS measurements were used to determine and confirm the degree of chain branching in the PFOA, PFOMA, and the block copolymer of PFOA and polystyrene. Concentrations of fluorine, carbon, and oxygen were determined with XPS and graphed in Figure 2.  These results indicate that when compared to the acrylate backbone, the fluoro-carbon side chain is preferentially found at the surface.
Reference

Kassis, C.M., Steehler, J.K., Betts, D.E., Gaun, Z., Romack, T.J., DeSimone, J.M., Linton, R.W.  (1996)  "XPS studies of fluorinated acrylate polymers and block copolymers with polystyrene", Macromolecules, Vol. 29 No. 9, pp. 3247-3254.