Evolution Profiles of Styrene Oligomers Observed by Thermal Desorption/Pyrolysis-Direct Analysis in Real Time (DART)-Mass Spectrometry

Abstract

Thermal desorption/pyrolysis (TDP)/Direct Analysis in Real Time (DART)-MS has recently been developed to characterize polymers by observing the gaseous products evolved from polymer samples during programmed heating. Because less care has been paid for the differences between volatile intacting oligomers and oligomeric pyrolysis products generated from high-molecular weight components, evolution profiles of intact oligomers and oligomeric pyrolysis products generated from polystyrene (PS) with well-defined molecular structure were observed by TDP/DART-MS as a fundamental investigation. Volatilization of the intact oligomers up to about 1000 Da started at around 200 °C. At higher temperature above about 270 °C, oligomeric pyrolysis products having an original butyl end group started to volatilize, and then above about 300 °C, evolution of oligomeric pyrolysis products originating from internal main chain by multiple chain cleavages was observed. Chain cleavage mainly proceeded oxidatively, resulting in the formation of oligomeric products having a carbonyl or aldehyde end. The relative intensities of the oligomeric pyrolysis products with the original end group was inversely proportional to the number-average molecular weight (M_n) of the sample. This result suggested that if a calibration curve can be made, M_n of the PS would be rapidly determined by TDP/DART-MS. The combination of mass spectra and evolution profiles observed by TDP/DART-MS can provide useful information concerning the molecular characterization of polymers.