Time-of-flight secondary ion mass spectrometric analysis of polymer tertiary structure in Langmuir monolayer films of poly(dimethylsiloxane).

A series of model systems of poly(dimethylsiloxane) (PDMS) of molecular mass 2400 Da and low polydispersity (1.09) were prepared using the Langmuir-Blodgett technique to investigate the effects of tertiary structure on the ion formation probability in time-of-flight secondary ion mass spectrometry (TOF-SIMS). Using data from the measured surface pressure-area isotherm for PDMS ordered monolayer, films were transferred to silver mirror substrates from the various regions in the isotherm where structural changes are observed. Two particular conformations of the polymer are examined here: a linear caterpillar-like structure and a coiled helical structure. Reflection absorption Fourier transform infrared spectroscopic analysis indicates structural changes for the films related to adjustments to the orientation of the polymeric backbone consistent with the formation of two distinct structures. The polymeric changes translate into differences in ion formation probability of fragments in both the low (<250 Da) and high (>1000 Da) mass range. Data are also presented to analyze how tertiary structure may affect the apparent polydispersity index calculated from the TOF-SIMS spectra.