Time-of-flight secondary ion mass spectrometry of polymer materials

Abstract

Secondary ion formation from polystyrene standards with well-defined molecular weight distributions between 103 and 106 amu has systematically been investigated. Maximum yields of cationized fragments and oligomers are obtained from silver substrates covered by about one monolayer of polymer material. Intact oligomers of polystyrene are detected in the mass range up to 10 000 amu. Disappearance cross sections of sputtered molecules are correlated to the size of the desorbed species and increase almost linearly with mass. Transformation probabilities of cationized oligomers decrease by about three orders of magnitude between 1000 and 10 000 amu. Number and weight average molecular weights (Mn , Mw ) determined from peak intensities of intact polymer molecules compare well with data from gel permeation chromatography. A correction of Mn and Mw for decreasing transformation probabilities is necessary for broad molecular weight distributions. The formation of high molecular weight fragment ions can be explained by a statistical main chain scission. For polystyrenes with Mn >10 000 amu the emission of fragment ions in the mass range between 500 and 5000 amu seems to be independent of the chain length of the original polymer.