Secondary Ion Mass Spectrometry of Polymers: a ToF SIMS Study of Monodispersed PMMA Standards

Abstract

This paper is concerned with the effects that polymer molecular weight, casting solvent, sonification (with a non-polymer solvent) and annealing have upon the secondary ion mass spectrometry (SIMS) of polymers. Films of a series of monodispersed poly(methyl methacrylate) (PMMA) standards, Mw 2965–1200000, were prepared by solution casting from four solvents (tetrahydrofuran (THF), 2-butanone, toulene and chloroform) onto clean aluminium (Al) substrates. The PMMA films were characterized by high- and unit-mass resolution time-of-flight secondary ion mass spectrometry (ToF-SIMS). To determine the significance of variations seen in ion intensities, one PMMA sample was re-analysed 24 times, and the scatter in the absolute counts and normalized and ratioed ion intensities for specific (termed ‘key’) positive and negative ions were calculated. This provided 95% confidence error bars, which were subsequently employed. The 95% confidence limits were significantly reduced by normalizing or ratioing. Molecular weight was found to have the greatest effect in the SIMS spectra obtained, with the differences being most marked between Mw=2965 and Mw=89100. This was seen in both the total negative ion counts (m/z 31–200) and the key negative ion ratios. An explanation based on the surface concentration of chain ends is presented. Negative ion ratios were shown to be very sensitive to trace amounts of residual solvents. By annealing above the PMMA glass transition temperature Tg solvent-free films were produced from three solvents and these were spectrally indistinguishable. Solvent-free films could not be produced from THF. Residual solvent was identified by high mass resolution ToF-SIMS. Sonification with hexane, a polymer non-solvent, had a considerable effect on the total negative ion counts (m/z 31–200). No concomitant chemical changes were detected, so this effect is thought to arise from a physical perturbation of the surface. © 1997 by John Wiley & Sons, Ltd.