Analysis of base bitumen chemical composition and aging behaviors via atomic force microscopy-based infrared spectroscopy

Conventional atomic force microscopy (AFM) can only capture the surface microstructure and nanomechanical properties of bitumen. As such, AFM cannot directly analyze chemical composition and changes of the surface at the nanoscale. However, state-of-the-art characterization technology has combined AFM with infrared spectroscopy (AFM-IR) to facilitate the analysis of chemical composition on the surface of bitumen at the nanoscale. The present work adopts AFM-IR to collect topography maps, phase maps, functional group distribution maps, and infrared spectra of base bitumen samples before and after aging. Analysis of the topography maps indicates that the bee structures on the surface undergo no apparent change with aging. For surface functional group distributions, the sulfoxide and carbonyl concentrations of each phase on the bitumen surface vary with aging. The present work also proposes a quantitative AFM-IR analysis method based on Fourier transform infrared (FTIR) spectroscopy. The resultant sulfoxide and carbonyl indices measured by AFM-IR and FT-IR spectroscopy exhibit an excellent linear correlation, but the concentrations of carbonyl groups on the surface are greater than that in the bulk material. Finally, the IR spectra and functional group indices of each nanoscale phase on the bitumen surface before and after aging are compared with those obtained for the four fractions of bulk bitumen to investigate the proportions of the four fractions included in each nanoscale phase. The results strongly suggest that the chemical fractions of each phase on the bitumen surface are quite similar and have a higher polarity than that of the bulk material.