SECONDARY-ION MASS-SPECTROMETRY OF POLYMERS - A TOF SIMS STUDY OF MONODISPERSED PMMA STANDARDS

This paper is concerned with the effects that polymer molecular weight , casting solvent, sonification (with a nonpolymer solvent) and anneal ing have open the secondary ion mass spectrometry (SIMS) of polymers. Films of a series of monodispersed poly(methyl methacrylate) (PMMA) st andards, M(w) 2965-1200000, were prepared by solution casting from fou r solvents (tetrahydrofuran (THF), 2-butanone, toulene and chloroform) onto clean aluminium (Al) substrates. The PMMA films were characteriz ed by high- and unit-mass resolution time-off-light 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 int ensities for specific (termed 'key') positive and negative ions were c alculated. This provided 95% confidence error bars, which were subsequ ently employed The 95% confidence limits were significantly reduced by normalizing or ratioing. Molecular weight was found to have the great est effect in the SIMS spectra obtained, with the differences being mo st marked between M(w) = 2965 and M(w) = 89100. This was seen in both the total negative ion counts (m/z 31-200) and the key negative ion ra tios. An explanation based on the surface concentration of chain ends is presented. Negative ion ratios were shown to be very sensitive to t race amounts of residual solvents. By annealing above the PMMA glass t ransition temperature T-g solvent-free films were produced from three solvents and these were spectrally indistinguishable. Solvent-free fil ms 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 ef fect is thought to arise from a physical perturbation of the surface. (C) 1997 by John Wiley & Sons, Ltd.