MOLECULAR-WEIGHT DISTRIBUTIONS OF POLYMERS USING TIME-OF-FLIGHT SECONDARY-ION MASS-SPECTROMETRY

Molecular weight distributions of well-defined polymers (e.g., polystyrenes, perfluorinated polyethers, etc.) were obtained from time-of-flight secondary-ion mass spectra. Number and weight average molecular weights calculated from the spectra are reproducible (relative standard deviation = 0.5-8%) and in good agreement (ca. +/- 10%) with values determined by conventional techniques (e.g., gel permeation chromatography, nuclear magnetic resonance spectroscopy, etc.). The mass resolution Ml DELTA-M = 1000) and detection efficiency of the instrument may bias molecular weight distributions toward low values, but this can be corrected by using high-resolution spectra and calibrating the detector. Disappearance cross sections of oligomers provide a measure of the effect of sputtering on the accuracy of molecular weight distributions; they are a function of polymer type, and increase with oligomer size. Number average molecular weights of polystyrenes (MW = 1000-5000) are altered by approximately 1-5% due to sputtering due the time required to obtain a spectrum.