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Anal. Chem. 1980. 52, 226-232
Static Secondary Ion Mass Spectrometry of Polymer Systems Joseph A. Gardeiia, Jr. and David M. Hercules” Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Poly( alkyl methacrylates) were studied by static secondary ion mass spectrometry (SIMS), to obtain mass spectra of the polymer surface. Results from an homologous series provide a direct fingerprint for each member of the series, unobtainable with standard ionization methods in m a s spectrometry. Resuits also show the characterization of surface impurities, present from processing. Analysis of characteristic mass fragmentation patterns provides a method for examination of the process(es) of secondary ionization and ejection of molecular ions during the SIMS process. Results show the utility of SIMS as an analytical tool to investigate polymer surface structure. A second portion of this report gives preliminary results on the use of SIMS to investigate mechanisms of surface degradation using model reactions: SIMS shows increased surface sensitivity for analysis of reactive effects over ESCA.
Polymers can provide interesting systems for surface analysis. Previously, surface analysis of polymers has been limited almost exclusively to X-ray photoelectron spectroscopy (XPS or ESCA) (1). ESCA provides multielement and chemical state information from examination of core level photoemission lines. However, the technique is not readily exploited for most polymers since the chemical states of carbon, oxygen, and silicon, typical of polymer systems, do not lead to easily resolvable ESCA chemical shifts. Polymers typically exhibit few other elements and chemical states. The semiquantitative data from ESCA results show that photoelectron escape depth, a measure of sampling depth, is sufficient t o average results from several atomic layers, approaching bulk results (2). ESCA does show fingerprint capability for members of an homologous series through valence band examination (2);however the valence band cross sections for soft X-ray ionization are typically small compared to core lines. Therefore, intensities are very low in ESCA for valence bands, and spectra of these regions involve long data acquisition times. ESCA valence band spectra are difficult to deconvolute for comparison with theoretical predictions (3). Ion scattering spectroscopy (ISS) has been used to analyze polymer systems ( 4 , s ) . Quantitative ISS measurements indicate true surface structure since ISS is sensitive to the first atomic layer ( 6 ) ,but chemical information cannot be readily extracted. The power of secondary ion mass spectrometry (SIMS) as an analytical tool is exploited in the present study of polymer surface structure. Several analytical aspects of SIMS make it amenable to analysis of polymer systems. SIMS provides a direct method for obtaining fingerprint mass spectra of nonvolatile solids (7). SIMS trace analytical capabilities can be used to monitor contaminants from processing (S), and its multielement capabilities include detection of hydrogen, unique among surface analytical tools (9). Pattern analysis of molecular ions serves as a method to examine surface bonding and molecular structure (10). SIMS can be used for depth profiling (11) or for estimating surface concentrations semiquantitatively in well defined systems (12). These last two aspects were not examined in this study. 0003-2700/80/0352-0226$01.00/0
Previous work on sputtering of polymers (13,141 has suggested that sputtering causes significant disruption of surface structure. Indeed, it is likely that sputtering at beam currents > 10 pA/cm2 causes significant selective disruption in organic polymer surfaces during typical SIMS analysis times (13). At primary ion energies of 5 kV, even a t beam current densities as low as 0.8 pA/cm2, 20 s of sputtering time is enough to significantly disrupt relatively stable chemical structures such as benzene rings in polystyrene (14). In order to retain analytical information for SIMS examination of polymers, the combination of low primary ion energies (0.5-2 kV) and low primary ion current densities (lo-@ A/cm2) of noble gas ion beams used for so-called “static” SIMS (15)are employed in this study. The utility of static SIMS for analysis of nonvolatile organic species (16-19) has been demonstrated. There is a minimum loss of sample information for these systems that makes SIMS desirable for organic materials. Previous SIMS work on polymers has been limited. Werner (20)has shown the utility of SIMS for Teflon polymers in which the possibilities of structure elucidation are stated. Tantsyrev et al. (21) used a primary “neutral” beam-ion emission technique in which ions were ejected for a series of ethylene fluoropolymers and copolymers. It is not clear if the mechanism for ionization and ejection caused by a “neutral” beam is any different than for SIMS, since results for Teflon by the two are similar. Prager (22)has used reactive gas ions to increase intensities of ejected ions from polymers; however, oxidation of the surface does not allow retention of surface integrity. Muller (23)has examined Teflon in the course of studying insulators. Charge neutralization effects were minimized and representative spectra similar to Werner’s work (20) were reported for positive and negative SIMS. In the present study, we have applied static SIMS to an homologous series of poly(alky1 methacrylates), studied previously by ESCA ( Z ) , to examine SIMS capabilities for polymer analysis. We have investigated such parameters as technique optimization, mechanistic considerations of the SIMS process, structural characterization capabilities, and reaction effects monitoring.
EXPERIMENTAL The polymers examined in this study are an homologous series of poly(alky1methacrylates) where the length and the structure of the alkyl ester group is varied. The structures and physical characteristics of these polymers are shown in Table I. They are commercially available from Scientific Polymer Products Inc., Ontario, N.Y. Poly(laury1 methacrylate) was reclaimed from solution by room temperature evaporation of the solvent (toluene). The polymers were supported on double sided sticky tape (Scotch Brand) for analysis by SIMS on a 3M Model 610 SIMS/525 ISS. Vacuum was maintained by ion pumping and oil diffusion pumping (for noble gas ions) at a base pressure of 5 X IO4 Torr, with cryopanel operation. The system was backfiied to a pressure of
