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Spectra from Beamson and Briggs (“The XPS of Polymers Database”) offer the opportunity to examine more complex polymer data in the context of known synthetic models. Actually, it is sufficient to set the parameter value outside the constraint range currently defined for the parameter. E.g., if it is required to set the FWHM value to 1.1, the constraint interval should be entered with the value “1.1,1.1”. To constraint a parameter so that it does not adjust during an optimization step, set the constraint interval to have the same value as the fixed value. Further input is required to make sense of the new synthetic model. The PAA stoichiometry is still doubtful but the essential positions for a pure PAA envelope have more or less appeared. Two additional peaks appear in the model and further more three of the peaks look like they may have something to do with pure PAA. If it is assumed that the FWHM for a C 1s photoelectron peak is 1.1 eV (only a guess), then applying peaks with said constraint results in a peak fit shown in Figure 3. Figure 2: An initial fit to the C 1s spectra shown in Figure 1.
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The three-peak model suggests further synthetic components are required before the sample can be fully understood.Ĭopyright © 2011 Casa Software Ltd. For example, the FWHM are much bigger than would typically be expected from C 1s profiles the given instrumental resolution. No constraints have been applied and the result is a reasonable fit to the experimental data but the fitting parameters are not readily open to chemical interpretation. Figure 2 shows a synthetic model for this data envelope where three GL peaks have been added, then fitted using a Marquardt-Levenberg optimization algorithm. If Gaussian/Lorentzian (GL) line-shapes are added in an arbitrary way, the curve fit yields little information about the sample other than to say that it deviates from the published data for PAA and therefore demonstrates the presence of additional chemistry at the surface. The high-resolution spectrum in Figure 1 derives from poly (acrylic acid) PAA reacted with inorganic material (or partially reacted) in an acid-base reaction.
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A typical C 1s envelope (Figure 1) includes structure that offers chemical information about a sample, but without some initial starting point it is difficult to construct an appropriate model for such a data-envelope.įigure 1: C 1s high-resolution spectrum taken from a polymer-based sample using a VG ESCALAB 220i at University College London, UK. Curve Fitting and Polymers The availability of data from well-characterized samples such as those offered by Beamson and Briggs owe much to the popularity of XPS as a tool for understanding the chemistry of polymers.