Influence of End Group and Molecular Weight on Polybutadiene Fingerprint Secondary Ion Mass Spectra

Abstract

Polybutadiene samples of different molecular weight have been synthesized by anionic polymerization as initiated by sec-butyllithium with low polydispersity and a major content of 1,2-vinyl units. They have been analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) in order to investigate the sensitivity of this method toward the sec-butyl end group and toward the molecular weight. The SIMS spectra show the characteristic fragment of the end group, C4H9+ at m/z = 57, whose the peak intensity is strongly dependent on the polymer molecular weight, as is the case for almost all the fragment intensities. A model consistent with the peak intensity variations is used to give some new insights into the fragmentation mechanism at the end groups and within the main chain. Moreover, the analysis of the end group fragment allows Mn to be readily determined up to Mn = 4 × 104 from, for example, the Y(53)/Y(57) intensity ratio where Y(53) is the intensity of the deprotonated repeat unit ([M − H]+). Other Mn calibration methods have also been used and are discussed in terms of their accuracy and physical meaning.