Crosslinking sites of sulfur and asphalt molecules: A DFT and macroscopic experimental study

Abstract

In SBS-modified asphalt , additive sulfur acts as an anchor for both asphalt and SBS , preventing phase separation. However, the sulfur-asphalt crosslinking mechanism is still largely speculative, with the reactive sites on asphalt and the factors influencing reaction activity yet to be fully defined. Therefore, this study examines the reaction sites and mechanisms of sulfur-asphalt crosslinking through Density Functional Theory (DFT). First, the crosslinking sites on asphalt were classified based on their structural characteristics, and the bond dissociation energies (BDE) at these sites were calculated to predict reactive positions. Next, the adsorption energies of sulfur radicals, the free energy barriers of crosslinking reactions, and the adsorption energies between radicals were computed. This enabled a comprehensive elucidation of the reaction steps involved in sulfur radical crosslinking with asphalt, resulting in the identification of potential crosslinking sites within the asphalt structure. It was determined that sulfur radicals and asphalt molecules undergo three main stages: adsorption, attack, and crosslinking. The attack of sulfur radicals on asphalt molecules, which leads to the formation of asphalt radicals, necessitates overcoming a significant free energy barrier, which represents the critical step in the crosslinking process. In addition, the aromatic ring in asphalt forms a P-π conjugation with α-carbon, lowering the free energy barrier and becoming a key factor in crosslinking. The size and proximity of the aromatic ring were identified as secondary factors. Finally, the DFT calculations were validated through Fourier transform infrared spectroscopy (FTIR) and hydrogen nuclear magnetic resonance (H NMR). This study provides a theoretical foundation for investigating chemically cross-linked SBS-modified asphalt at the molecular level.

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