Time- and Composition-Dependent Evolution of Distinctive Microstructures in Bitumen Xiaokong Yu, † Sergio Granados-Focil, ‡ Mingjiang Tao,* ,† and Nancy A. Burnham* ,§ † Department of Civil and Environmental Engineering and § Physics and Biomedical Engineering Departments, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609, United States ‡ Gustaf H. Carlson School of Chemistry, Clark University, 950 Main Street, Worcester, Massachusetts 01610, United States * S Supporting Information ABSTRACT: 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. 1. INTRODUCTION AND LITERATURE REVIEW Originating from the bottom of the barrel after the petroleum distillation process, bitumen (also called asphalt binder) is a complex mixture of hydrocarbons of different sizes and polarities, together with traces of metals and other heteroatoms. Bitumen is often separated into fractions including saturates, aromatics, resins, and asphaltenes (SARA), with the combina- tion of the first three components called “maltenes”. 1 These different components often interact with each other resulting in rich and diverse microstructures as observed by atomic force microscopy (AFM). 2-17 Researchers have shown that bitu- men’s bulk properties are closely related to its chemical composition and microstructures. 18-21 Therefore, the establish- ment of the chemical-microstructural-mechanical relation- ships for bitumen is very important to the overall performance of asphalt-related applications (e.g., in asphalt pavements bitumen acts as a glue binding all the aggregates together). 18,22 On one hand, chemical compositions of an asphalt binder are contingent on its crude oil origin, petroleum-distillation process, and other physical or chemical treatments (e.g., air- blowing, or chemical modifications). On the other hand, a bitumen’s microstructures are dependent on a variety of factors including its chemical composition, sample preparation method, thermal history, annealing time, aging, and exposure to moisture of different levels. 2-17 Precise prediction of a bitumen's microstructures is complicated by its complex chemical composition and the wide range of time, temperature, thermal history, and moisture-dependent supramolecular interactions among its components . 11,18,23 Debates and conflicting opinions are often seen in the literature regarding the characteristics of bitumen’s microstructures and the chemical origins of the microsized features with distinctive shapes. 4,8,9,12,23 Starting from the first attempt of investigating bitumen’s microstructures using AFM, 8 features of different shapes and sizes from bitumens of different crude oil sources have been reported. 2-17 While some asphalt binders display morphologies with fine domains, flakelike domains, or dendrite structuring, “bee structures” with undulated patterns of several micrometers in length and tens of nanometers in height are also seen in other binders. 9,12-14 A good example of diverse microstructures of bitumens with different crude oil origins is Masson’s work in which 13 SHRP (Strategic Highway Research Program) bitumens were imaged using tapping mode AFM. 9 The authors observed various types of microstructures on surfaces of thin- film binders from different crude sources, but no correlation Received: August 22, 2017 Revised: October 27, 2017 Published: November 21, 2017 Article pubs.acs.org/EF Cite This: Energy Fuels 2018, 32, 67-80 © 2017 American Chemical Society 67 DOI: 10.1021/acs.energyfuels.7b02467 Energy Fuels 2018, 32, 67-80