Bitumen’s
chemistry often results in complicated intermolecular
associations, which are manifested by the diverse microstructures
as observed by atomic force microscopy (AFM). These microstructures
largely contribute to bitumens’ bulk mechanical properties;
therefore, it is essential to understand the chemical–microstructural–mechanical
relationships for optimal design of bitumen-related applications.
However, the complex nature of bitumen and the various influencing
factors often lead to practical challenges in investigation of bitumens’
microstructures and their chemical origins. This study aims at addressing
some of the main concerns related to AFM characterization of bitumens’
microstructures, namely the dependence of bitumens’ microstructures
on such factors as sample preparation methods, annealing conditions
and durations, and chemical composition. Our results suggest that
microstructures of bitumen films of a few micrometers or thicker (i.e.,
the thickness of the asphalt-coating layer over the aggregates in
asphalt concrete) were comparable regardless of their sample preparation
methods, provided that toluene was likely completely removed. Additionally,
bitumens annealed at room temperature for over 2 months showed time-dependent
microstructures, which correlate well with bitumens’ room-temperature
steric hardening behavior as verified by other researchers using modulated
differential scanning calorimetry. Microstructures of the bitumen
films stabilized after different annealing durations depending on
the dimensions of the molecular structures and the complexity of the
molecular interactions among the multiple phases in each bitumen.
Distinctive microstructures were observed for remixed bitumens with
increasing asphaltene concentrations. Consistency between our observations
and other relevant literature suggests that microstructures observed
by AFM are probably not just a surface phenomenon. The above findings
provide deeper insights into the establishment of the complicated
chemical–mechanical relationships for bitumen that pave the
path toward tuned bitumen performance.
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