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Time- and composition- dependent evolution of distinctive microstructures in bitumen Xiaokong Yu, Sergio Granados-Focil, Mingjiang Tao, and Nancy A Burnham Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02467 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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Time- and composition- dependent evolution of distinctive microstructures in bitumen Xiaokong Yu1, Sergio Granados-Focil2, Mingjiang Tao1*, Nancy A. Burnham3* 1. Department of Civil and Environmental Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609 2. Gustaf H. Carlson School of Chemistry, Clark University, 950 Main Street, Worcester, MA, 01610 3. Physics and Biomedical Engineering Departments, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609

ABSTRACT Bitumen’s chemistry often results in complicated intermolecular associations, which is 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 chemicalmicrostructural-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 microns 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 two 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 1 ACS Paragon Plus Environment

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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 towards tuned bitumen performance. Keywords: bitumen/asphalt binder, microstructures, time-dependent evolution, steric hardening, atomic force microscopy (AFM), chemical composition

1

Introduction & 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 polarity, together with traces of metals and other heteroatoms. Bitumen is often separated into fractions including Saturates, Aromatics, Resins, and Asphaltenes, with the combination 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 bitumen’s bulk properties are closely related to its chemical composition and microstructures.18-21 Therefore, the establishment of the chemical-microstructural-mechanical relationships 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 However, chemical compositions of an asphalt binder are contingent on its crude oil origin, petroleumdistillation process, and other physical or chemical treatments (i.e., air-blowing, aging, or chemical modifications). In addition, 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. Bitumen’s complex chemistry and the complicated interactions among its 2 ACS Paragon Plus Environment

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various chemical constituents when subjected to these various influencing factors make it a great challenge to precisely predict bitumen’s microstructures.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 micro-sized features with distinctive shapes.4, 8-9, 12, 23 Starting from the first attempt of investigating bitumen’s microstructures using AFM,8 microstructures of different shapes and sizes from bitumens of different crude oil sources have been reported.2-17 While some asphalt binders display microstructures with fine domains, flake-like 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 thirteen 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 between the AFM morphology and bitumen’s chemical fractions was established. In terms of sample preparation, the authors chose the heat-cast method over the solution-cast approach because they think that the former helps preserve the solid-state structure of bitumen whereas the toluene involved in solution-casting might alter the molecular interactions in bitumen. As AFM is being used more and more frequently for investigation of bitumens’ microstructures and mechanical properties, it is critical to understand whether different sample preparation methods lead to different microstructures of the same bitumen. Film thickness, dependent on the sample preparation methods, also affects development of bitumens’ microstructures. Pauli et al. found that when the solution-cast films were thinner than 1 µm, surface microstructures of the same binder was significantly different; whereas surface microstructures were essentially constant for films thicker than 10 µm.12 Further investigation of how film thickness affects bitumens’ microstructural development would provide insights into establishing standard sample preparation methods for using AFM to study bitumens’ microstructural properties. As the asphalt-coating

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layer in real asphalt pavements is usually about a few microns thick, this study focuses on assessing the effect of sample preparation methods on microstructures of bitumen films with thicknesses in this range. The effect of water exposure at elevated temperature or extended duration on bitumen’s microstructures was studied using AFM and Fourier transform infrared spectroscopy with the goal of understanding the moisture damage mechanism for asphalt binders;3, 7 however the effect of low humidity exposure on bitumen’s microstructures, as occurred when storing the specimen in an ambient environment, is missing. Meanwhile, several studies were dedicated to investigating the chemical origin and mechanism of “beestructures” that appeared in a variety of bitumens.4, 8-9, 12, 24 At first, the “bees” were attributed to the asphaltene fraction in bitumen,8, 24 the most polar component that is soluble in aromatic solvents (i.e., toluene) but insoluble in n-heptane or n-pentane; then the “bee-structures” were associated with the metal contents;9 later Pauli et al. proposed that the “bumble bees” were a result of the interactions between crystallizing paraffin wax and remaining asphalt fractions.12 More recently, dos Santos et al.’s comparison of the microstructures of thin films of the original bitumen and asphaltene-tetracosane mixtures casted from toluene suggested that the undulated pattern in the bee-structures was a result of phase behavior4 and not other physical phenomena such as buckling or wrinkling.3, 5, 25-26 Consistent with bitumens’ temperature-dependent rheology, bitumens’ microstructures are found to be temperature-dependent.2, 12, 27-30 The distinctive microstructures observed at room temperature started to dissolve into the matrix phase as bitumens were heated up and then recover when the samples were cooled down. This phenomenon was more pronounced for bitumens with high wax contents, due to the melting and crystallization of the waxes as a function of temperature. Similar to bitumens’ temperaturedependent microstructures observed by AFM, the mechanism responsible for bitumens’ steric hardening when annealed at room temperature is less well understood, even though it is known that bitumen hardens over a period of hours or days upon cooling from the melt to room temperature.31-32 Through analysis of the reversing and nonreversing heat flow curves from modulated differential scanning calorimetry (MDSC), Masson et al. deduced that bitumens annealed at room temperature would show time-dependent 4 ACS Paragon Plus Environment

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microstructures that are responsible for bitumens’ room-temperature steric hardening.33 Menapace et al. reported that after 24 hours of exposure to air the dispersed phase increased in both the original unaged binders and their corresponding long-term aged samples; however, size expansion in the aged binders was less pronounced than their corresponding unaged binders, which was attributed to a lower mobility of the dispersed phase in the aged samples.28 However, there is still lack of direct observation on how bitumens’ microstructures evolve when annealed at room temperature for a time period longer than 24 hours, which would qualitatively reveal the complicated molecular interactions and chemical compatibility among bitumens’ various components and explain variations in aging susceptibility of bitumens from different crude oil sources and of different chemistry. It is evident from the aforementioned literature review that researchers hold different opinions on appropriate interpretations of bitumens’ microstructures. In addition, various experimental artifacts can be encountered due to the lack of understanding of AFM techniques (e.g., phase inversion,9 inconsistent image processing of the AFM results). In consideration of the importance of relationships between bitumen’s chemistry, microstructures, and its bulk performance, this study aims at addressing some of the main concerns related to AFM characterization of bitumens’ microstructures. The followng critical questions were addressed: how do sample preparation methods affect bitumens’ microstructures? Are microstructures observed by AFM purely surface phenomena? How do bitumens’ microstructures evolve over time when annealed in ambient conditions? What is the significance of bitumens’ time-dependent microstructures? How do chemical composition differences affect bitumens’ morphologies? Among the many factors that affect bitumen’s microstructures, effects of sample preparation methods, film thickness, exposure to low relative humidity, annealing time and heat treatment, and chemical composition (e.g., increasing asphaltene concentration) were carefully investigated in this study. Observations and results from this study will facilitate better understanding of the chemical compatibility among bitumens’ chemical compositions and variation in bitumens’ aging behavior, and bring us a step closer to a more


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complete picture of the chemical-microstructural-mechanical properties relationships for bituminous materials.

2

Materials and Methodology

2.1 Virgin bitumens The three virgin bitumens chosen in this study are denoted as 261643 (PG58-28), ABD (PG 58-10), and AAD (PG58-28), respectively, according to their respective suppliers. The first binder was provided by the Maine Department of Transportation (DOT), and the other two binders were requested from the SHRP Material Research Laboratory. All bitumens were stored in stable ambient conditions prior to any AFM characterization, and aging of these samples is considered to be minimal. Descriptions for the three binders including their crude source, PG grade, chemical composition, and wax content are shown in Table 1. Table 1. Bitumen name, crude source, PG grade, chemical composition, and wax content Sample name

Crude source

261643

Whiting, ME

virgin ABD

b

ABD-A10 b

PG

Asphaltenes (A)

Maltenes (M)

Wax content

grade

(wt.%)

(wt.%)

(wt.%)

-

-

-

58-28 58-10

a

10.0

a

90.0

a

0.81 a

-

10

90

-

-

25

75

-

-

35

65

-

ABD-A50

-

50

50

-

virgin AAD c

58-28 a

24.2 a

75.8 a

1.94 a

-

0

100

-

-

12

88

-

-

24

76

-

-

50

50

-

ABD-A25 ABD-A35

California Valley

AAD-Maltenes AAD-A12 AAD-A24 c AAD-A50

California Coast

a

values taken from SHRP report, and the weight percentage of asphaltenes and maltenes were normalized against the sum of the SARA fractions.34 b the remixed ABD-A10 has the same asphaltenes/maltenes ratio as the virgin ABD. c the remixed AAD-A24 has the same asphaltenes/maltenes ratio as the virgin AAD.


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2.2 Bitumen fractionation and remixing In order to study how bitumen’s chemical composition (i.e., variation in the amount of asphaltene and maltenes in a bitumen) affects its microstructures, virgin bitumens ABD and AAD were firstly separated into their asphaltene (A) and maltene (M) fractions, and the two components were then remixed at different weight ratios to produce artificially remixed bitumens. Fractionation of the two virgin bitumens was conducted according to ASTM standard D4124-09 2010 and detailed procedures can be found in this reference.16 The asphaltenes and maltenes separated from their respective source binders were stored in sealed vials to prevent contamination. A series of derivative binders was obtained by remixing asphaltenes/maltenes (A/M) from their respective binder source (i.e., virgin ABD and virgin AAD) at various mass ratios (i.e., A/M = 10/90, 24/ 76, 50/50, etc. Table 1). To reduce the numbers of variables, cross-mixing of the asphaltenes and maltenes from the two different virgin binders was not attempted in this study. In order to verify whether the virgin binders could be reproduced by the remixing process, ABD-A10 and AAD-A24 were made to mimic the chemical composition of virgin ABD and virgin AAD, respectively. The remixing process was completed as the following: (1) weighing out the right amounts of the asphaltenes and maltenes on a Mettler Toledo scale (AB104-S) with precision of ±0.001 g; (2) first mixing of the weighed asphaltenes and maltenes using toluene (HPLC grade, 35 ml toluene per gram of total mass of solute) in a shallow glass or ceramic container followed by the second mixing using toluene (35 ml toluene per gram of total mass of solute); the solution was manually stirred with a glass stirring rod at a slow speed for ~ 10 mins after adding the toluene to ensure thorough mixing between the asphaltenes and maltenes; and (3) evaporating the toluene in a vacuum oven (20 inch Hg, ~105°C, 24 hours) to obtain the final remixed bitumens, which were stored in sealed containers to avoid contamination prior to AFM measurements.


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2.3 Sample preparation and microstructure characterization using AFM 2.3.1

Thin-film sample preparation

Thin-film binder samples for AFM characterization were prepared by both heat-cast and solution-cast methods, the two methods that are frequently used by researchers. There is no difference in terms of the substrates used for heat-cast or solution-cast methods. Substrates including micro glass slides, steel plates, or small pieces of silicon wafer can be used for both methods. Substrates were always cleaned using acetone and methanol prior to deposition of the solid form bitumen (for heat-casting) or the bitumentoluene solution (for solution-casting). To make a heat-cast thin-film sample, a bead of bitumen (15-25 mg) was deposited onto a clean substrate and then was heated for ~10 mins on a hot plate or in an oven at 115±5°C, a temperature high enough to make the binder flow but not so high to cause any oxidation of the binder. Once the bitumen was in liquid form, a spatula was used to spread the bitumen to a circular shape of ~1 cm diameter, and the bitumen film was then kept on the hot plate or in the oven for another 10-15 mins to obtain a smooth finish. Lastly the thin film was allowed to cool to room temperature and annealed at ambient for different time durations (i.e., 1 day, 7 days, 21 days, etc.) prior to AFM characterization. Annealing as defined here is storage at room temperature (i.e., 21.2±0.4°C). When assuming bitumen has a density of 1 g/cm3,18 the film thickness can be roughly estimated by the mass-volume relationship (thickness = mass/(density×surface area of the film)). Ellipsometry was also employed to estimate the film thickness yet it failed to provide meaningful measurements due to the high multidispersity of bitumen. Heat-cast films prepared by the aforementioned approach are estimated to have a thickness of 100-200 µm. Thinner heat-cast films can be prepared by depositing a smaller amount of binder and butter-spreading the binder onto a larger surface area on the substrate. To make a solution-cast thin-film sample, bitumen solution was prepared by dissolving a certain amount of bitumen into toluene at various concentrations in order to make thin films of different thicknesses. When depositing the same volume of binder solution onto the substrate, the higher the concentration of 8 ACS Paragon Plus Environment

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the solution, the thicker the binder film is. The solution was left overnight for complete dissolution of the binder. About 30-50 µl of the binder solution was transferred from the stock solution using a Fisherbrand pipette and drop-cast onto a clean substrate. The thin-layer sample along with its substrate was then left under a fume hood with a beaker covered on top until the toluene mostly evaporated and the film became a paste. Then the sample was transferred into a vacuum oven (~105°C, 20 inch Hg) for ~24h such that the toluene residue trapped in the binder film was likely completely removed (the boiling point for toluene under 20 inch Hg is ~100°C, lower than the oven temperature setting). This step is also referred as the first heating (or the initial heating) before annealing of the solution-cast bitumen film. After the first heating, the thin-film bitumen was allowed to cool down to room temperature, and stored at ambient for equilibration for different time durations prior to AFM characterization of its time-dependent morphology. Thickness of solution-cast films can be estimated by the mass (volume of the solution casted on the substrate × concentration of bitumen in the solution) - volume relationship and it varies from a tenth of a micron to ~100 µm. A centrifuge was used when binder films thinner than hundreds of nanometers was necessary. Regardless of their preparation methods, all thin-film binder samples were then stored in a gel box to avoid dust pickup prior to AFM measurements. However, thin films produced from the heat-cast method are generally thicker than the solution-cast ones, especially when the concentration of the binder solution used for solution-casting is low. Figure S1(a & b) (Supporting Information) shows pictures of heat-cast and solution-cast films of the same binder on glass slides, and the solution-cast sample is much thinner than the heat-cast one. On the other hand, when the concentration of the binder solution is high enough (i.e., 200 g/l), solution-cast binder films can be of similar thickness to those produced by the heat-cast method. For example, the thicknesses of both heat-cast and solution-cast films in Figure S1(c & d) (Supporting Information) were estimated to be ~100 µm. To examine how bitumen’s microstructures evolve over time in ambient conditions, solution-cast thinfilm virgin ABD and ABD-A25 after the first heating were annealed at ambient for a few weeks. (Here 9 ACS Paragon Plus Environment

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“ambient condition” refers to 20% relative humidity, 21.2±0.4°C, and in air, as measured in our AFM laboratory. This definition applies throughout this paper.) Morphologies of both bitumens were monitored using AFM at various annealing durations (e.g., 2 days, 9 days, 15 days, 22 days, 32 days, and 39 days). Then the 39-day-old samples were subjected to a second heating in the vacuum oven (80°C, 23 inch Hg, 24 hours), and their morphologies were monitored 0 day, 4 days, and 7 days after the second heating, in order to evaluate changes in bitumens’ microstructures due to the second heating and the time evolution of the microstructures after the second heating. These samples are designated according to their heat treatment (i.e., first heating or second heating) and annealing time (i.e., 1 day, 7 days, 21 days, etc.), as shown in Table 2. Table 2. Designation of bitumen samples used in investigation of their time-dependent microstructures 1st heating

Heat treatment Sample designation

2nd heating

1H-2d

1H-9d

1H-15d

1H-22d

1H-32d

1H-39d

2H-0d

2H-4d

2H-7d

2H-40d

2

9

15

22

32

39

0

4

7

40

2

9

15

22

32

39

40

44

51

91

Annealing duration (days) Accumulative annealing duration (days)

To study the moisture effect on binders’ microstructures, solution-cast virgin ABD and ABD-A25 with thickness of ~100 µm (two samples for each binder) were prepared. The control samples (one for each binder) were stored in an ambient condition, the same as how the majority of the thin-film binders were stored in this study. Moisture-free samples, denoted as the treated group, were stored in the vacuum oven (room temperature, 20 inch Hg). Samples of the same annealing time from both the control and treated groups were imaged by AFM and their microstructures were compared. 2.3.2

AFM characterization techniques

An Asylum Research MFP-3D AFM was used for microscopic characterization (morphologies and phase contrast) of the thin-film binders. All AFM measurements were conducted at room temperature (~ 21.5°C and 20% relative humidity). MikroMasch NSC14 AFM probes (nominal stiffness and resonant frequency


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are 5 N/m and 160 kHz, respectively) and NSC 16 AFM probes (nominal stiffness and resonant frequency are 45 N/m and 190 kHz, respectively) were used. Throughout the AFM morphological characterization, the set point was typically set to be between 0.45 - 0.55, and scan rate was ~ 1 Hz. Bitumen films were imaged in intermittent-contact AFM mode in either attractive imaging (phase above 90°) or repulsive imaging regime (phase below 90°).35 In attractive-imaging mode, the phase angle from the tip-sample interaction can hop above or below 90°, making it impossible to correlate the phase angle with mechanical properties of a sample’s microstructures. On the other hand, repulsive-imaging mode captures the long and short range repulsive tip-sample interactions and the phase image obtained using this mode reveals contrasts in the sample’s viscoelasticity and adhesive energy hysteresis over different domains.36 The phase image reflects qualitative contrast in terms of the mechanical properties among the multiple microstructures in a sample: microstructures of a sample with a larger phase angle are more adhesive, viscous, or compliant; whereas domains with a smaller phase angle are less adhesive, viscous, or compliant. Note that comparison of phase contrast between microstructures acquired from the same scan with the same imaging parameters (e.g., set point, free amplitude) is more reliable than comparison across different scans that are associated with different scanning parameters. The choice of the imaging mode depends on the primary goal of investigation: when the primary interest was to simply evaluate the topographical characteristics of a binder, attractive-imaging mode was used as it does not require frequent adjustment of the setpoint value to get a phase response smaller than 90 degrees; whereas repulsiveimaging mode was preferred when phase (or mechanical property) contrast among the multiple phases of the binder’s microstructures was also of interest. To the authors’ experience, topographic images of the binders in this study were found to be independent of the imaging mode (i.e., attractive imaging vs. repulsive imaging) and cantilever type (i.e., NSC14 vs. NSC16). All AFM images were processed using the Igor Pro software. For consistency, all topographic images were processed with a flatten order of 2 (i.e., 2nd order polynomial line fitting to each scan line) and a XY planefit order of 1 to remove any tilting effect and other artifacts. (The reader should be aware that occasionally line-by-line flattening induces


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horizontally aligned distortions in topographic images with large stand-alone features, as is evident in Figs. 3 and 6.) Grey scales for both topographic and phase images were set appropriately (e.g., either in the same scale or proportionally) to facilitate visual comparison. 3

Results

3.1 Effects of sample preparation methods and film thickness Some researchers prefer the heat-cast method rather than the solution-cast approach when preparing thinfilm bitumen samples for AFM measurements because they are concerned that toluene used in solution casting could possibly modify bitumens’ microstructures.9 However, we found that heat-cast and solutioncast virgin ABD showed similar microstructures when their film thicknesses were comparable. This also applies to virgin AAD (Supporting Information). In addition, we confirmed that variations in film thickness played a more significant role in development of bitumen’s microstructures as compared to the effect of toluene used in solution casting (assuming that toluene was evaporated in the finished solutioncast film). The three dimensional bee-structures seemed to be more prevalent in films thicker than tens of microns and the number of the bee-structures was significantly reduced as the bitumen films became thinner than one micron12 (Supporting Information). Therefore, surface morphologies of the same binder prepared by both heat-cast and solution-cast approaches are similar to each other when the following conditions are satisfied: (1) thickness of the binder film should be sufficient (i.e., a few microns or thicker); (2) toluene should be evaporated when using the solution-cast approach; (3) same annealing durations for both heat-cast and solution-cast films under the same annealing environment; and (4) other experimental conditions (e.g., AFM imaging parameters) were set to be similar. Clarification of variations in microstructures of the same bitumen due to sample preparation would enable more meaningful comparison among microstructures of bitumens from different crude oil origins, or manufacturing processes, or at different aging stages.


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3.2 Effect of low humidity Studies reported that when bitumen samples were fully exposed to water (i.e., moisture damage) at either ambient or elevated temperature, distinctive topographic evolution occurred due to an increased concentration of polar constituents at the bitumen surface.3, 7 However, it is not known how low relative humidity (RH) affects bitumens’ microstructural characteristics, as can happen during the storage of the thin-film binder samples in an ambient environment. Our study implies that microstructures of treated virgin ABD and ABD-A25 (i.e., moisture-free) generally resembled those of the control samples (i.e., exposed to RH 50%).3, 7 Minimal microstructural changes for these two binders under low humidity also suggest that low humidity exposure (i.e., < 20%) would not likely affect the adhesion measurement on the bitumen films to a large extent.15 Now that the effects of samples preparation methods and exposure to lower humidity on bitumens’ microstructures are known, the following measurements regarding the effects of annealing conditions and chemical composition on bitumens’ microstructures were all conducted on solution-cast bitumen films with thicknesses of ~100 µm and RH < 20%. 3.3 Effect of annealing time & heat treatment 3.3.1

Time-dependent microstructures of virgin ABD

The effects of annealing time and heat treatment on microstructures of solution-cast virgin ABD (~100 µm thick) are shown in Figure 1 (topographic images) and Figure 2 (the corresponding phase images). Regardless of the annealing time and the second heat treatment, morphologies of virgin ABD from different annealing durations consist of flake-like or dendritic microstructures dispersed in a continuous


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matrix phase, with the dispersed phase 1-2 nm lower in height than its surrounding matrix phase. On the other hand, the size of the dispersed phase changed with the annealing time and the second heat treatment. 1H-2d (i.e., virgin ABD annealed for 2 days after the 1st heating, Figure 1(a)) showed a dispersed phase of irregular shapes that are about 0.5-1 µm in the lateral dimension. The phase image of this sample (Figure 2(a)) revealed that: (1) the dispersed phase had a smaller phase angle than the matrix phase; (2) about one-third of the dispersed phase had what looks to be nucleation centers (i.e., the bright spots in the centers of the dispersed phases in the phase image in Figure 2(a)). The dispersed phases in 1H-9d (Figure 1(b) and Figure 2(b)) evolved to be dendritic structures of ~2 µm in the lateral dimension, one-third of which showed nucleation centers as seen in its phase image. The dimensions of the dispersed phase (in both lateral and z-scale) in 1H-15d (Figure 1(c) and Figure 2(c)) were comparable to that of 1H-9d; however the “branches” of the dendrites in 1H-15d were finer than those in 1H-9d. In addition, an interface between the dispersed phase and the continuous matrix phase appeared in 1H-15d. Microstructural characteristics of 1H-22d (images not shown in Figure 1 and Figure 2), 1H-32d (Figure 1(d) & Figure 2 (d)), and 1H-39d (Figure 1(e) and Figure 2 (e)) were very similar to those of 1H-15d, except that the interface between the dispersed phase and the matrix phase grew relatively larger and became more visible over this time period (especially in the phase images). Such interface between the dispersed phase and the matrix phase observed in solution-cast virgin ABD of 15 days old and older after the initial heating (Figure 1(c - e) and Figure 2(c - e)) resembles the solvation layer (i.e., mostly consisting of the resins in bitumen) in the colloidal model of bitumen;18 however the exact chemical composition of such an interface needs further investigation. When 1H-39d (39 days old after the 1st heating) was subjected to a second heat treatment in the vacuum oven (80°C, 23 inch Hg, 24 hours) and imaged within 4-5 hours of annealing time, the recessed dendritic structures along with the interface were replaced with sub-micron sized quasi-spheres that were 1-2 nm lower than the matrix phase (i.e., 2H-0d in Figure 1 (f) and Figure 2(f)). These spheres are half of the size of the dispersed phase in the 2-day-old sample from the initial heat treatment, and the size difference


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between the two is probably related to the second heat treatment and the annealing time difference between the two (i.e., 2H-0d vs. 1H-2d). Interestingly, monitoring of this sample annealed for 7 more days after the second heating showed that quasi-spheres in 2H-0d (Figure 1(f) and Figure 2(f)) evolved to be dendritic structures (Figure 1(g & h) and Figure 2(g & h)) similar to the dendrites in the 9 day-old and older sample after the first heating (Figure 1(b-e) and Figure 2 (b-e)). For example, microstructural characteristics (in terms of shape and size) of the dispersed phase in the 2H-7d (Figure 1(h) and Figure 2 (h)) were similar to those of the 1H-9d (Figure 1(b) and Figure 2 (b)). In addition, the interface between the dispersed phase and the matrix phase that was only present in the 15 days older sample after the initial heat treatment (Figure 1(c-e) and Figure 2 (c-e)) was also observed in samples over four days old after the second heat treatment. Morphologies of 2H-40d (images not shown) were similar to those of 2H-7d (h), except that the interface in the former was relatively larger than that in the latter. Comparison of microstructures of solution-cast virgin ABD annealed at room temperature over a period of 39 days after its initial heat treatment (Figure 1(a – e) and Figure 2(a – e)) demonstrated that the size of the dispersed phase in this sample increased over time until it reached a stable state about two weeks after the initial heat treatment. The perturbation of the second heat treatment resulted in a size decrease of the dispersed phase at first (Figure 1(f) and Figure 2(f)); however, the dispersed phase grew larger over time (Figure 1(g & h) and Figure 2(g & h)) and evolved to be comparable to microstructures of the sample at younger age after the initial heat treatment (i.e., Figure 1(h) and Figure 2 (h) vs. (b)) but with a slightly faster rate. Virgin ABD’s time-dependent microstructures observed directly by AFM is consistent with the mechanism that Masson et al. inferred from modulated differential scanning calorimetry (MDSC), which was associated with bitumen’s room-temperature steric hardening.33 Another study from the same research group suggested that the asphaltenes and resins probably form a mesophase (i.e., the disperse phases in AFM images) that reach a steady state over 14 days.32 Such a slow ordering process of the mesophase was attributed to resins’ and asphaltenes’ high aromaticity and large molecular size.32 Bitumens’ two-phase system as a result of the phase behavior among bitumens’ complicated chemical


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constituents was also predicted by an attenuated association model in which the dispersed phase consists of mainly asphaltenes but also some amount of aromatics and resins.37 Attribution of the dispersed phase in virgin ABD to its asphaltene and resin fractions can also be justified by the phase contrast between the dispersed phase and its surrounding matrix. In Figure 2, regardless of the annealing time and the second heating in-between, the phase angle over the dispersed phase was always smaller than that over the matrix phase and the phase angle difference between these two domains is about 10 degrees. According to the physics associated with repulsive-imaging mode, the phase difference between the dispersed domain and its surrounding matrix suggests that chemical compositions associated with the dispersed domain had more rigid molecular structures (i.e., asphaltenes & resins of larger size, higher polarity and aromaticity) than the chemical components responsible for the matrix domain. The dispersed phase grew larger when the sample was annealed at room temperature for an extended time period, until it reached a stable state over two weeks upon the completion of the reordering of the molecules among themselves and their surroundings. Such stabilized dendritic structures in solution-cast thin-film virgin ABD did not change much until the second heat treatment made all molecules in the binder active again, resulting in dispersed spherical structures of smaller size due to the increased mobility of all molecules within the bitumen. However, after the second heating as the sample was annealed for longer, the dispersed phase grew to be similar to that in the sample without the second heating (i.e., after the initial heat treatment), demonstrating the re-forming of the dispersed phase in bitumen at room temperature over time. Furthermore, direct observation of bitumen’s time-dependent microstructures at the micro scale and its consistency with bulk room-temperature steric hardening imply that these microstructures seen by AFM were probably not a pure surface phenomenon.23 In addition, regardless of the sample’s age and the second heating the dispersed phase was always about 1-2 nm lower in height than the continuous matrix, and the variation in height difference between these two domains in samples of different annealing times was marginal. The height recession of the dispersed phase relative to the matrix phase might be related to the density or molecular structural differences 16 ACS Paragon Plus Environment

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between the chemical compositions that are associated with the two different domains. Such a small height difference along with the phase contrast between the two domains, on the other hand, could also be an indication that no asphaltene aggregates of large size (i.e., greater than 2 nm38) were formed in this binder and the asphaltenes together with resins were probably well dispersed in the other SARA fractions of virgin ABD, according to the Yen-Mullins model of asphaltenes38 and the colloidal model of bitumen.18-19 3.3.2

Time-dependent microstructures of ABD-A25

AFM topographic and phase images of solution-cast ABD-A25 (~100 µm thick) at different annealing durations after both the initial and the second heating are shown in Figure 3 and Figure 4, respectively. Morphology of the 1H-2d ABD-A25 (Figure 3(a)) consists of “bean-shaped” microstructures of ~1 µm in lateral dimension that protrude out of a continuous matrix phase, with a height difference of 10-15 nm between the two domains. The corresponding phase image of 1H-2d (Figure 4(a)) shows that the dispersed phase had a larger phase angle than the matrix phase. Distinct changes occurred when the same sample was annealed for 7 more days (1H-9d in Figure 3(b) & Figure 4(b)): (1) three instead of two domains appeared in the morphology: the matrix phase, the original dispersed phase that stood out of the matrix phase, and a third phase (highlighted in blue ovals) that is also dispersed in the matrix phase (note that the third phase is less visible in the topographical image due to its small height difference from the matrix phase but it is evident in the phase image); (2) the original dispersed phase grew to be a more elliptical shape 1.5-2 µm in the lateral dimension, and over two-thirds of the dispersed phase started to show a recessed center with a lateral dimension of ~0.5 µm and 7-10 nm into the surface (i.e., the center of this phase was 7-10 nm lower than its direct surroundings in height); (3) the third phase is about 1.5-2 µm laterally, with mini bee-structures in the center of this phase; and (4) phase image obtained from AFM repulsive-imaging mode indicated that among the three different phases the original dispersed phase and the matrix phase had the largest and the smallest phase angle, respectively, and the third phase had an intermediate phase angle. 17 ACS Paragon Plus Environment

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Morphology of 1H-15d (Figure 3(c) & Figure 4(c)) was similar to that of 1H-9d in terms of the lateral dimensions for both the original dispersed phase and the third phase, except some further developments: (1) more of the original dispersed phase started to show a recessed center over annealing time; (2) the lateral dimension of the recessed center had increased slightly and the recessed center grew further into the surface (i.e., 12-15 nm lower than the surface); and (3) the third phase was relatively more developed and more visible in the topographic image. Evolution of microstructures of solution-cast ABD-A25 continued for 1H-22d (Figure 3(d) & Figure 4(d)): (1) the recessed center of the dispersed phase propagated further in both the lateral and the vertical (i.e., 15-18 nm into the surface) dimensions even though the overall size of the protruding dispersed phase did not change much (i.e., about 2 µm); and (2) the third phase with the bee-structures in its center became almost invisible in the topographic image (yet still obvious in the phase image). Microstructures of 1H-32d and 1H-39d were about identical (Figure 3(e & f) & Figure 4(e & f)): the original dispersed phase that stood out of the matrix phase now almost fully sunk into the matrix phase (i.e., 19-25 nm below the surface), and the third phase was almost unidentifiable in the topographic image yet apparent in the phase image. Overall, microstructures of solution-cast ABD-A25 reached a stable state over about 30 days when annealed at ambient after the initial heating. When the stabilized 39 day-old ABD-A25 sample was subjected to the second heating (Figure 3(g) & Figure 4(g)), only the “bean-shaped” phase was observed in AFM topographic and phase images whereas the third phase appeared in the 9-day-old and older after the initial heat treatment (Figure 3(b - f) & Figure 4(b - f)) was missing. The “bean-shaped” protruding phase in 2H-0d is one-third of the lateral dimension of that in 1H-2d (Figure 3(a) & Figure 4(a)), and the height difference between the protruding dispersed phase and its surrounding matrix phase in (g) is ~ 5 nm, half of that in (a). Such microstructural differences between (a) and (g) are most likely associated with the second heat treatment and their annealing time differences (i.e., 2 days after the first heating vs. 0 day after the second heating). After the second heating as the sample was annealed at room temperature for 7 more days, its microstructures


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(Figure 3(h) & Figure 4(h)) evolved to be very similar to that in 1H-9d (Figure 3(b) & Figure 4(b)): the original protruding phase (with a recessed center) and the third phase dispersed in a continuous matrix phase, yet the third phase with “bee-structures” was more visible in (h) than in (b), especially in the topographic images. Morphologies of 2H-40d (images not shown) displayed features reminiscent of those in 1H-32d (e): (1) the recessed center of 1-2 µm in the lateral dimension was again developed in the original protruding phase and it sank into the surface 20 nm on average; and (2) the third phase with beestructures was more visible and has a relatively larger lateral dimension in 2H-40d than in 1H-32d (e), which might be due to the combined effects of the aging from the second heating and the cumulative annealing time. AFM morphologies of solution-cast ABD-A25 annealed at room temperature for over 30 days after its initial heat treatment (i.e., the development of the multiple phases) and for about 40 days after the second heat treatment (i.e., the reappearance of the characteristic microstructures) confirmed this binder’s timedependent microstructural evolution, which is possibly correlated with bitumen’s room-temperature steric hardening behavior as observed in the bulk by MDSC.32-33 3.3.3

Area fraction of the dispersed phase(s) in virgin ABD and ABD-A25

Quantitative comparison of the area fraction of the dispersed phase(s) in both virgin ABD and ABD-A25 annealed at room temperature both in the vacuum oven and at ambient for different time durations (Table 2) was conducted using ImageJ software by masking out the dispersed phase(s) in the phase images and then reporting its area fraction out of the total scan area. Note that in virgin ABD the dispersed phase was defined as the flake-like or dendritic microstructures only (and the interface between the dispersed phase and the matrix was not counted according to the default algorithm settings in ImageJ); whereas in ABDA25 both the distinct dispersed phase that protruded out of the surface with a recessed center and the third phase that had limited height difference from the surface were counted for area fraction calculation. Figure 5 plots area fractions of the dispersed phase(s) for both virgin ABD and ABD-A25 after the initial heat treatment (2 days, 9 days, 15 days, 22 days, 32 days, and 39 days) and after the second heat treatment 19 ACS Paragon Plus Environment

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(0 days, 4 days, 7 days, and 40 days), respectively, from the left to the right with increasing annealing durations. The reported area fractions are average values along with standard deviations calculated from three to five different images that were acquired at different locations on bitumen surfaces, except for the data point from the 40-day-old after the second heating where a single value is reported due to time constraints. Figure 5 shows that after the initial heat treatment both virgin ABD and ABD-A25 started with ~ 4.0% dispersed phase when annealed for 2 days at room temperature, and area fractions of the dispersed phase increased with their annealing time at different rates before reaching to their respective steady state values. Area fraction of the dispersed phase in virgin ABD rapidly increased to (10.9±0.5)% after 9 days of annealing at room temperature after the initial heating and then reached a stable value (i.e., the plateau region in the black circles varying between 8.7% to 10.4%) about two weeks after the initial heating. The slight drop of the area fraction values for 15-days older virgin ABD is likely due to the exclusion of the interface area in ImageJ. In contrast, area fraction of the dispersed phases in ABD-A25 had a slower steady increase with increasing annealing time until it reached a seemingly stable value (i.e., the plateau region in the red squares varying between 9.2% to 10.0%) about four weeks after the initial heating. Observation of ABD-A25 annealed for longer than 39 days after the initial heating might be helpful to find the true stable value of the area fraction of the dispersed phases, yet it was beyond the time limit of this particular study. The second heat treatment caused significant changes in microstructures of both bitumens and consequently area fraction of their dispersed phase(s). The dispersed phase(s) in 0-day-old virgin ABD and 0-day-old ABD-A25 after the second heating account to area fractions of (1.7±0.2)% and (0.8±0.1)%, respectively, which is 44% and 22% of the dispersed phase(s) in their respective 2-day-old samples after the initial heating. As a result of the reappearance of their respective characteristic microstructures when the two binders were annealed for longer time after the second heating, area fraction of the dispersed phase(s) for both binders increased at faster rates as compared to those after the initial heating. For both 20 ACS Paragon Plus Environment

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virgin ABD and ABD-A25 shorter annealing durations were required to reach similar area fraction of the dispersed phase(s). For example, for ABD-A25 it only took 7 days of room-temperature annealing to reach an area fraction of (10.1±0.5)% after the second heating as compared to 32 days of roomtemperature annealing after the first heating for an area fraction of (9.2±0.4)%. A chronological overview of area fractions of the dispersed phase(s) after the first heating and the second heating indicates that: (1) both bitumens showed similar trends of the area fractions over annealing durations (i.e., an increase followed by stabilization); (2) such trends were repeatable after the second heating; and (3) virgin ABD stabilized faster than ABD-A25. Such quantitative analysis of area fraction of the dispersed phase(s) in both samples has confirmed the increase and stabilization (at different rates) of their dispersed phase(s) after the initial heat treatment and the reappearance of their microstructures after the second heating (at faster increasing rates as compared to those after the initial heating). Mechanisms that might explain the faster recovery of both bitumens’ microstructures after the second heating could be: physical or chemical aging due to the cumulative annealing time and the second heating or incomplete diffusion during second heating; however, the exact reason for such a phenomenon needs further investigation. These observations are consistent with the above qualitative description of the time-dependent microstructures for both virgin ABD and ABD-A25. Therefore, it lends support to the correlation between the microstructural evolution observed by AFM and the steric hardening mechanism implied by bulk MDSC and DSR measurements.32-33


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(a) 1H-2d

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(b) 1H-9d

(c) 1H-15d

(d) 1H-32d

(g) 2H-4d

(f) 2H-0d

(e) 1H-39d

2 µm

(h) 2H-7d

2 µm Figure 1. Time-evolution (blue arrows, clockwise) of AFM topographic images of solution-cast virgin ABD (film thickness ~ 100 µm) annealed at room temperature for different durations: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-32d, (e) 1H-39d, (f) 2H-0d, (g) 2H-4d, and (h) 2H-7d. AFM cantilever NSC16 and repulsive-imaging mode were used. Scan size and z-scale for all images are 20×20 µm2 and 3 nm, respectively. The red scale bars in each image indicates 2 µm. The dispersed phase in this sample grew and stabilized in two weeks after the initial heating; even though the dispersed phase shrank to a much smaller size right after the second heating, it reappeared as the sample was annealed for four days and longer.


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(a) 1H-2d

(b) 1H-9d

(c) 1H-15d

(d) 1H-32d

(g) 2H-4d

(f) 2H-0d

(e) 1H-39d

2 µm

(h) 2H-7d

2 µm

Figure 2. Time-evolution (clockwise) of solution-cast virgin ABD seen in AFM phase images that correspond to the topographic images in Figure 1: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-32d, (e) 1H-39d, (f) 2H-0d, (g) 2H-4d, and (h) 2H-7d. AFM cantilever NSC16 was used. AFM cantilever NSC16 and repulsive-imaging mode (phase angles smaller than 90°) were used. Scan size and z-scale for all images are 20×20 µm2 and 10°, respectively. The red scale bars in each image indicates 2 µm. The time-evolution of microstructures of virgin ABD seen in the topographic images is consistent with the trend in the phase images. Furthermore, phase contrast suggests that the dispersed phase is either stiffer or less lossy than the continuous matrix phase, indicating that the chemical compositions over these two domains are probably different. 23 ACS Paragon Plus Environment

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(a) 1H-2d

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(b) 1H-9d

(c) 1H-15d

(d) 1H-22d

(g) 2H-0d

(f) 1H-39d

(e) 1H-32d

3 µm

(h) 2H-7d

3 µm Figure 3. Time-evolution (blue arrows, clockwise) of AFM topographic images of solution-cast ABD-A25 (film thickness ~ 100 µm) annealed at room temperature for different durations: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-22d, (e) 1H-32d, (f) 1H-39d, (g) 2H-0d, and (h) 2H-7d. Scan size and z-scale for all images are 30 µm and 15 nm, respectively. The red scale bars in each image indicates 3 µm. The blue ellipses highlight representative microstructures of the third phase that are less visible in the topographic images yet evident in the phase image (Figure 4). AFM cantilever NSC16 and repulsive-imaging mode were used. Evolution of the microstructures of ABD-A25 as a function of annealing time and the second heating is similar to that of virgin ABD (Figure 1& Figure 2); however, it took about a month for the microstructures in ABD-A25 to stabilize, twice that for virgin ABD. 24 ACS Paragon Plus Environment

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(a) 1H-2d

(b) 1H-9d

(c) 1H-15d

(d) 1H-22d

(g) 2H-0d

(f) 1H-39d

(e) 1H-32d

3 µm

(h) 2H-7d

3 µm

Figure 4. Time-evolution (clockwise) of solution-cast ABD-A25 as seen in AFM phase images that correspond to the topographic images in Figure 3: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-22d, (e) 1H-32d, (f) 1H-39d, (g) 2H-0d, and (h) 2H-7d. AFM cantilever NSC16 was used. All phase images were acquired using repulsive-imaging mode with phase angles smaller than 90°. Scan size and z-scale for all images are 30 µm and 10°, respectively. The red scale bars in each image indicates 3 µm. The blue ellipses highlight the corresponding microstructures of the third phase, which is less visible in the topographic images in Figure 4.


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Virgin ABD vs. ABD_A25 15.0

Area fraction of the dispersed phases (%)

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Virgin ABD ABD_A25 12.0

1H-22d 1H-32d 1H-39d 1H-15d

9.0

2H-7d 2H-40d 2H-4d

1H-9d

6.0

3.0 1H-2d 2H-0d

0.0 0

12

24

36

48

60

72

84

96

Accumulative annealing duration (days) Figure 5. Area fractions of the dispersed phase(s) in solution-cast virgin ABD (black circles) and ABDA25 (red squares) annealed at room temperature as a function of accumulative annealing duration (samples are designated in sequence from left to right: 1H-2d, 1H-9d, 1H-15d, 1H-22d, 1H-32d, 1H-39d, 2H-0d, 2H-7d, and 2H-40d, Table 2). Area fractions are average values from three to five different images acquired at different locations on bitumen surfaces, except for the last data point (i.e., 2H-40d) due to time constraints. Dispersed phase(s) for both bitumens annealed for 2 days after the initial heating account for ~4.0 %, and gradually reached their corresponding stable values at different rates after the first heating (i.e., ~2 weeks for virgin ABD vs. ~ 4 weeks for ABD-A25). Area fractions of the dispersed phase(s) for both bitumens dropped right after the second heating due to the melting of the molecular structures of the dispersed phase(s), but they grew back at faster rates as compared to that after the initial heat treatment. The time-dependent microstructures of virgin ABD and ABD-A25 after the initial heating and the recovery of their respective dispersed phase(s) after the second heating indicate the correlation between the microstructural evolution observed by AFM and steric hardening mechanism studied by bulk MDSC and DSR.32-33


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3.4 Effect of chemical composition Morphologies of asphalt binders are very dependent on their chemical compositions, with evidence from the significantly different microstructures in virgin ABD and ABD-A25 as discussed above and as well as observations by other researchers.6, 11-12, 39 In this section, the effect of chemical composition on microstructures of ABD-based and AAD-based binders was further studied by examining morphologies of remixed bitumens with increasing asphaltene concentration (Table 1). Figure 6 shows the AFM topographical images of one-month-old solution-cast virgin ABD, ABD-A10, ABD-A25, ABD-A35, and ABD-A50 (Table 1). All samples were ~ 100 µm thick and were annealed in ambient conditions. Since microstructures of these samples were found to be time-dependent, here the stabilized microstructures are presented. Virgin ABD and its replica (ABD-A10, with an asphaltene/maltene weight ratio of 10/90, the same as virgin ABD) displayed similar microstructures, with micron-sized dendritic microstructures dispersed in a continuous matrix phase. However, virgin ABD had a larger quantity of dendrites than ABD-A10 and the dendritic structures in the former were also more complicated than those in the latter. Microstructural differences between virgin ABD and its replica might be due to chemical composition modification during the processes of binder fractionation and binder remixing. The morphology of ABD-A25 in Figure 6 (c) exhibited the same characteristics as discussed in Section 3.3.2: a continuous matrix phase, a primary dispersed phase with recessed centers, and the third phase that is also dispersed in the matrix phase yet less visible in the topographical image. Both ABD-A35 and ABD-A50 showed dispersed phases in nearly circular shapes of 1-1.5 µm in diameter; however, the quantity of this dispersed phase in ABD-A50 is less than that in ABD-A35. Phase images of the five binders (virgin ABD, ABD-A10, ABD-A25, ABD-A35, and ABD-A50, obtained in repulsive-imaging mode, images not shown) indicated that phase contrast between the dispersed phase and its surrounding matrix phase was inverted for binders with an asphaltene concentration below or above 25 wt. %. Further characterization on the bulk mechanical and thermal properties of these samples and their correlation with AFM observations on microstructures of these bitumens suggest that 25 wt. %


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of asphaltene is likely a critical concentration that alters ABD-based binders from sol-bitumen to gelbitumen.40 Figure 7 shows morphologies of 3-day-old heat-cast AAD-based binders (with thicknesses 100 µm or above) with increasing asphaltene concentration (including AAD-Maltenes, AAD-A12, AAD-A24, and AAD-A50, Table 1). Our observations indicate that microstructures of AAD-based binders were relatively time-independent (i.e., the 3-day-old AAD-A12 displayed microstructures very similar to those in the 30-day-old AAD-A12), which is different from the time-dependent behavior of microstructures of ABD-based bitumens. One possible explanation for difference in the time-dependency of microstructures between AAD-based and ABD-based binders is variation in their chemical miscibility: the maltenes and asphaltenes in virgin AAD and virgin ABD probably have different molecular structures; maltenes and asphaltenes in AAD-based bitumens might be less compatible than those in ABD-based bitumens; the less compatible the system is, the less interactions among the molecules, the quicker the microstructures stabilize. This hypothesis can be further verified through measuring the solubility parameters of the asphaltenes and maltenes separated from virgin AAD and virgin ABD,37 which will be conducted in our future work. As shown in Figure 7, the phase image of AAD-Maltenes (with pure maltenes and no asphaltenes in this sample) reveals three distinctive domains: the continuous matrix phase that had the largest phase angle (i.e., the most predominant peak in the phase histogram), and the edge and the center of the dispersed phase that correspond to the two smaller phase angles in the phase histogram. Morphology of AAD-A12 showed characteristics similar to that in AAD-Maltenes, except that the dispersed phase grew larger in size and the phase differences among the three different domains became smaller in AAD-A12. When the asphaltene concentration increased to 24 wt. %, the dispersed phase in AAD-A24 again grew larger in size, and bee-structures appeared in the center of the dispersed phase with a continuously reduced phase contrast among the three phases. In addition, as a replica of virgin AAD (Figure S3), microstructures of AAD-A24 were very similar to those in virgin AAD, except the presence of the discontinuous domain 28 ACS Paragon Plus Environment

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that had the largest phase angle in the phase image of AAD-A24. Such morphological difference is probably a result of chemical composition modification from the processes of binder fractionation and binder remixing. The remixed sample with equal mass of asphaltene and maltene (i.e., AAD-A50) exhibited mainly two phases with less evident phase contrast between the two domains. Overall, an increase in asphaltene concentration in AAD-based binders resulted in a size increase of the dispersed phase (consequently the dispersed phase became more connected) and a reduced phase contrast among the multiple phases in these binders. Differences in microstructures and phase contrast among the multiple domains in AAD-based samples with increasing asphaltene concentrations are likely related to variations in their bulk mechanical and thermal properties.40


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(b)

(a)

(c)

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(d)

(e)

Figure 6. AFM topographic images of solution-cast (a) virgin ABD, (b) ABD-A10, (c) ABD-A25, (d) ABD-A35, and (e) ABD-A50. All samples were ~100 µm thick. In order to provide good contrasts for microstructures in each sample, color scales for (a & b) and (c-e) are set to be 5 nm and 15 nm, respectively (shown at the far right of each row). All samples were annealed in ambient conditions (in air, ~20% humidity, 21.2±0.4°C) and were imaged at a month old (i.e., microstructures were most likely stabilized). AFM cantilever NSC16 was used for (c) and NSC14 for the rest of the samples. As asphaltene concentration increased, ABD-based binders showed distinctive morphologies: virgin ABD and its replica (ABDA10) showed similar dendritic microstructures yet with different quantities and complexity; ABD-A25 displayed a primary dispersed phase with recessed centers and a secondary dispersed phase that was less visible (highlighted in blue ovals) than the primary dispersed phase; both ABD-A35 and ABD-A50 exhibited microstructures of somewhat round shape that stood out of the matrix phase to different extent.


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Figure 7. AFM topographic (1st row) and phase (2nd row) images, and histograms of the phase image (3rd row) of heat-cast AAD-Maltenes (1st column), AAD-A12 (2nd column), AAD-A24 (3rd column), AAD-A50 (4th column). All samples were ~100 µm or thicker. Greyscale for all topographic images is 20 nm. Phase images for AAD-Maltenes and AAD-A12 have a greyscale of 30°, and those for AAD-A24 and AAD-A50 of a greyscale of 15°. All samples were annealed in ambient conditions (in air, ~20% humidity, 21.2±0.4°C) and were imaged 3 days after the first heating. AFM cantilever NSC16 probe and repulsive-imaging mode were used. Different from time-dependent microstructures for ABD-based binders, microstructures of AAD-based binders annealed at ambient were found to be relatively time-independent. Such a difference might be related to variation in chemical miscibility of the asphaltenes and maltenes in these two binders that have different crude oil origins. 31 ACS Paragon Plus Environment

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4

Discussions

Studies have shown that bitumens’ bulk rheological and thermal properties are related to their microstructures.12-13, 18, 23, 32 Here in this study we want to discuss how our observations on bitumens’ distinctive microstructures and the time-evolution of these microstructures provide additional insight into the complicated chemical-microstructural-mechanical relationships for asphalt binders. Before diving into the discussions, one needs to be reminded that the intriguing microstructures of different characteristics are indeed a result of the complicated molecular interactions within a bitumen. Virgin ABD and ABD-A25 have the same sources of asphaltenes (A) and maltenes (M) but different weight ratios of A/M (i.e., 10/90 for virgin ABD vs. 25/75 for ABD-A25). Both binders displayed timedependent microstructures when annealed either in a vacuum oven at room temperature or at ambient over time; however microstructures of ABD-A25 reached a stable state over 30 days, about twice of that for virgin ABD. The difference in the time period required for microstructures of these two binders to reach their stable state is probably associated with variation in molecular interactions of the multiple phases in these two binders, as a result of their asphaltene concentration difference. For example, both the primary dispersed phase (that protruded out of the surface 10-15 nm) and its transformation (microstructures with recessed centers that sank into the surface 10-20 nm) in ABD-A25 had a much larger z-scale than the dispersed phase in virgin ABD (1-2 nm lower in relative to the surface). This suggests that molecular structures of the primary dispersed phase in ABD-A25 are more complicated and probably of larger size (i.e., asphaltenes in ABD-A25 are more aggregated due to its higher concentration38) than the molecules associated with the dispersed phase in virgin ABD. Presence of the third phase with bee-structures in its center and the very limited height difference from the matrix phase in ABD-A25 is also a sign of more complicated molecular interactions within this binder than those within virgin ABD. Overall, a higher concentration of asphaltenes in ABD-A25 probably led to the formation of larger asphaltene aggregates and consequently a longer time for the microstructures to stabilize as compared to virgin ABD. These findings agree with the lower mobility of the dispersed phase


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in aged binders observed by Menapace et al.28 and the bulk MDSC observations in those binders with higher concentration of asphaltenes need a longer time to complete their room-temperature steric hardening process.33 Quantitative comparison of area fraction of the dispersed phase for virgin ABD and ABD-A25 provides further support for the above hypothesis. For instance, over the entire observation period after both the initial and the second heating (~ 80 days), the dispersed phase in virgin ABD had a maximum area fraction of (11.6±0.5)%, which is close to a 10 wt.% of the asphaltene loading in this binder. Considering the small height difference between the dispersed phase and the matrix in virgin ABD, the asphaltenes in this binder are probably assembled in a flat, 2D molecular structure (instead of large 3D asphaltene aggregates), making it possible to correlate the surface area fraction of the dispersed phase directly with the bulk asphaltene concentration in virgin ABD. In contrast, the dispersed phases in ABD-A25 eventually grew 10-20 nm into the surface once stable, and the surface area fraction of the dispersed phase (i.e., 10%-11%) calculated from ImageJ did not consider any asphaltenes developed underneath the surface. Considering the 3D microstructures of the dispersed phases in ABD-A25, it would be more reasonable to calculate the volume fraction (instead of the area fraction) of the dispersed phases and compare it against the bulk asphaltene loading in this binder.11-12 Optical microscopy and optical scattering are also potential tools to probe the bulk microstructures and therefore volume fraction of the dispersed phases in bitumens.29 When considering the effect of chemical composition (i.e., increasing asphaltene concentration) on microstructures of ABD-based samples, height difference and phase contrast between the dispersed phase and the matrix phase were inverted for binders with asphaltene concentration below or above 25 wt. %, which is likely a critical concentration that alters ABD-based binders from sol-type bitumen to gel-type bitumen.18-19, 40 As for AAD-based binders, microstructural changes (i.e., more and more connected dispersed phase, reduced phase contrast among the multiple phases in the microstructures) with increasing asphaltene concentrations are correlated with variations in their bulk thermal and mechanical properties.19, 33 ACS Paragon Plus Environment

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40

In addition, microstructural differences (to different extents) between the source binders (virgin ABD

and virgin AAD) and their replicas (ABD-A10 and AAD-A24) are reflected in discrepancies in their glass transition temperatures and rheological properties.40 5

Conclusions

The complex chemistry of bitumen and complicated molecular interactions among the various chemical compositions within bitumen result in bitumen’s diverse and rich microstructures. Characteristics and evolution of bitumen’s microstructures are dependent on many factors including sample preparation methods, film thickness, exposure to moisture of different levels, annealing time and heat treatment, and chemical composition. Although researchers have been studying bitumens’ microstructures and their relationship with bitumen’s bulk properties in the past twenty years, this paper addresses some of the major concerns and debates regarding the effects of some major influencing factors on bitumen’s microstructures from a more comprehensive point of view. Firstly, the effect of sample preparation methods was found to be insignificant when the film thicknesses of both heat-cast and solution-cast approaches were sufficient and comparable (i.e., > tens of microns) provided that toluene in solution-cast films was likely completely removed. This conclusion is supported by the similar microstructures between films of the same binder produced from the both heat-cast and solution-cast methods. Microstructures of the same binder of different thicknesses were clearly different: the thicker the film is, the more developed the microstructures are (i.e., three-dimensional); microstructures on thinner films grew larger in lateral dimensions as compared to those on a thicker film. Secondly, exposure of bitumen films to relative humidity lower than 20% at room temperature did not modify bitumen’s microstructure to an evident extent during the one-month observation period. This observation justifies the act of storing bitumen films in ambient conditions (with humidity < 20%) for general research purposes when a nitrogen chamber is not accessible.


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Thirdly, both virgin ABD and ABD-A25 showed time-dependent microstructures, which reached their corresponding stable states over ~ two weeks and ~ four weeks, respectively. Such time-dependent microstructural evolution over a two-month observation period was directly observed by AFM for the first time and is correlated with bitumen’s room-temperature steric hardening behavior studied by bulk MDSC and DSR by other researchers for other bitumens.32-33 Differences in the time periods required for microstructures of virgin ABD and ABD-A25 to stabilize is probably a result of their asphaltene concentration difference (i.e., 10 wt. % for virgin ABD vs. 25 wt. % for ABD-A25). The higher concentration of asphaltenes in ABD-A25 probably led to the formation of larger asphaltene aggregates and consequently a longer time for the microstructures to stabilize as compared to virgin ABD. In addition, microstructures of ABD-based and AAD-based binders with increasing asphaltene concentrations exhibited distinct characteristics, which will be shown to be related to variations in these binders’ bulk properties.40 Future work can focus on applying MDSC for investigation of the steric hardening behavior and employing optical scattering for characterization of the bulk microstructures of the binders used in this study so that a direct link between bitumen’s bulk physical properties and microscopic morphologies can be established. Investigation into this will bring us one step closer to the full picture of chemicalmicrostructural-mechanical properties relationships for asphalt binders and therefore enable optimal design of bitumen-related applications.

Supporting Information The supporting information include: pictures of bitumen films prepared by heat-cast and solution-cast methods, effects of sample preparation (i.e., sample preparation methods and film thickness) on bitumens’ microstructures, and effect of low humidity on bitumens’ microstructures.


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References 1. Corbett, L. W., Composition of asphalt based on generic fractionation, using solvent deasphaltening, elution-adsorption chromatography, and densimetric characterization. Analytical Chemistry 1969, 41 (4), 576-579. 2. Das, P. K.; Kringos, N.; Wallqvist, V.; Birgisson, B., Micromechanical investigation of phase separation in bitumen by combining atomic force microscopy with differential scanning calorimetry results. Road Materials and Pavement Design 2013, 14 (sup1), 25-37. 3. dos Santos, S.; Partl, M. N.; Poulikakos, L. D., Newly observed effects of water on the microstructures of bitumen surface. Construction and Building Materials 2014, 71, 618-627. 4. dos Santos, S.; Poulikakos, L. D.; Partl, M. N., Crystalline structures in tetracosane–asphaltene films. RSC Advances 2016, 6 (47), 41561-41567. 5. Fischer, H.; Stadler, H.; Erina, N., Quantitative temperature‐depending mapping of mechanical properties of bitumen at the nanoscale using the AFM operated with PeakForce TappingTM mode. Journal of microscopy 2013, 250 (3), 210-217. 6. Hung, A. M.; Fini, E. H., AFM study of asphalt binder “bee” structures: origin, mechanical fracture, topological evolution, and experimental artifacts. RSC Advances 2015, 5 (117), 96972-96982. 7. Hung, A. M.; Goodwin, A.; Fini, E. H., Effects of water exposure on bitumen surface microstructure. Construction and Building Materials 2017, 135, 682-688. 8. Loeber, L.; Sutton, O.; Morel, J.; Valleton, J. M.; Muller, G., New direct observations of asphalts and asphalt binders by scanning electron microscopy and atomic force microscopy. Journal of Microscopy 1996, 182 (1), 32-39. 9. Masson, J. F.; Leblond, V.; Margeson, J., Bitumen morphologies by phase‐detection atomic force microscopy. Journal of Microscopy 2006, 221 (1), 17-29. 10. Nahar, S.; Dillingh, B.; Erkens, S.; Schmets, A. J.; Fischer, H.; Scarpas, A.; Schitter, G., Is atomic force microscopy suited as tool for fast screening of bituminous materials? An inter-laboratory comparison study. Citeseer: 2013. 11. Pauli, A.; Branthaver, J.; Robertson, R.; Grimes, W.; Eggleston, C., Atomic force microscopy investigation of SHRP asphalts: Heavy oil and resid compatibility and stability. Preprints-American Chemical Society. Division of Petroleum Chemistry 2001, 46 (2), 104-110. 12. Pauli, A.; Grimes, R.; Beemer, A.; Turner, T.; Branthaver, J., Morphology of asphalts, asphalt fractions and model wax-doped asphalts studied by atomic force microscopy. International Journal of Pavement Engineering 2011, 12 (4), 291-309. 13. Qin, Q.; Farrar, M. J.; Pauli, A. T.; Adams, J. J., Morphology, thermal analysis and rheology of Sasobit modified warm mix asphalt binders. Fuel 2014, 115, 416-425. 14. Sourty, E.; Tamminga, A.; Michels, M.; VELLINGA, W. P.; Meijer, H., The microstructure of petroleum vacuum residue films for bituminous concrete: a microscopy approach. Journal of microscopy 2011, 241 (2), 132-146. 15. Yu, X.; Burnham, N. A.; Mallick, R. B.; Tao, M., A systematic AFM-based method to measure adhesion differences between micron-sized domains in asphalt binders. Fuel 2013, 113, 443-447. 16. Yu, X.; Zaumanis, M.; Dos Santos, S.; Poulikakos, L. D., Rheological, microscopic, and chemical characterization of the rejuvenating effect on asphalt binders. Fuel 2014, 135, 162-171. 17. Zhang, H.; Wang, H.; Yu, J., Effect of aging on morphology of organo‐montmorillonite modified bitumen by atomic force microscopy. Journal of microscopy 2011, 242 (1), 37-45. 18. Lesueur, D., The colloidal structure of bitumen: consequences on the rheology and on the mechanisms of bitumen modification. Advances in colloid and interface science 2009, 145 (1), 42-82. 19. Loeber, L.; Muller, G.; Morel, J.; Sutton, O., Bitumen in colloid science: a chemical, structural and rheological approach. Fuel 1998, 77 (13), 1443-1450. 20. Michalica, P.; Kazatchkov, I. B.; Stastna, J.; Zanzotto, L., Relationship between chemical and rheological properties of two asphalts of different origins. Fuel 2008, 87 (15), 3247-3253. 36 ACS Paragon Plus Environment

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21. Sultana, S.; Bhasin, A., Effect of chemical composition on rheology and mechanical properties of asphalt binder. Construction and Building Materials 2014, 72, 293-300. 22. Hofko, B.; Eberhardsteiner, L.; Füssl, J.; Grothe, H.; Handle, F.; Hospodka, M.; Grossegger, D.; Nahar, S.; Schmets, A.; Scarpas, A., Impact of maltene and asphaltene fraction on mechanical behavior and microstructure of bitumen. Materials and Structures 2016, 49 (3), 829-841. 23. Yu, X.; Burnham, N. A.; Tao, M., Surface microstructure of bitumen characterized by atomic force microscopy. Advances in colloid and interface science 2015, 218, 17-33. 24. Jäger, A.; Lackner, R.; Eisenmenger-Sittner, C.; Blab, R., Identification of four material phases in bitumen by atomic force microscopy. Road Materials and Pavement Design 2004, 5 (sup1), 9-24. 25. Lyne, Å. L.; Wallqvist, V.; Rutland, M. W.; Claesson, P.; Birgisson, B., Surface wrinkling: the phenomenon causing bees in bitumen. Journal of Materials Science 2013, 48 (20), 6970-6976. 26. Pauli, T.; Grimes, W.; Beiswenger, J.; Schmets, A. J., Surface structuring of wax in complex media. Journal of Materials in Civil Engineering 2014, 27 (8), C4014001. 27. MASSON, J. F.; Leblond, V.; Margeson, J.; BUNDALO‐PERC, S., Low‐temperature bitumen stiffness and viscous paraffinic nano‐and micro‐domains by cryogenic AFM and PDM. Journal of microscopy 2007, 227 (3), 191-202. 28. Menapace, I.; Masad, E.; Bhasin, A.; Little, D., Microstructural properties of warm mix asphalt before and after laboratory-simulated long-term ageing. Road Materials and Pavement Design 2015, 16 (sup1), 2-20. 29. Ramm, A.; Sakib, N.; Bhasin, A.; Downer, M., Optical characterization of temperature‐and composition‐dependent microstructure in asphalt binders. Journal of microscopy 2016, 262 (3), 216225. 30. Soenen, H.; Besamusca, J.; Fischer, H. R.; Poulikakos, L. D.; Planche, J.-P.; Das, P. K.; Kringos, N.; Grenfell, J. R.; Lu, X.; Chailleux, E., Laboratory investigation of bitumen based on round robin DSC and AFM tests. Materials and structures 2014, 47 (7), 1205-1220. 31. Traxler, R.; Schweyer, H., Increase in viscosity of asphalts with time. Proc of Am Soc for Testing Materials 1936, 36, 544-551. 32. Masson, J.; Collins, P.; Polomark, G., Steric hardening and the ordering of asphaltenes in bitumen. Energy & fuels 2005, 19 (1), 120-122. 33. Masson, J.; Polomark, G.; Collins, P., Time-dependent microstructure of bitumen and its fractions by modulated differential scanning calorimetry. Energy & fuels 2002, 16 (2), 470-476. 34. Jones, D. R. SHRP materials reference library: asphalt cements: a concise date compilation; 1993. 35. Garcıa, R.; Perez, R., Dynamic atomic force microscopy methods. Surface science reports 2002, 47 (6), 197-301. 36. Garcia, R.; Gómez, C.; Martinez, N.; Patil, S.; Dietz, C.; Magerle, R., Identification of nanoscale dissipation processes by dynamic atomic force microscopy. Physical review letters 2006, 97 (1), 016103. 37. Painter, P.; Veytsman, B.; Youtcheff, J., Phase behavior of bituminous materials. Energy & Fuels 2015, 29 (11), 7048-7057. 38. Akbarzadeh, K.; Hammami, A.; Kharrat, A.; Zhang, D.; Allenson, S.; Creek, J.; Kabir, S.; Jamaluddin, A.; Marshall, A. G.; Rodgers, R. P., Asphaltenes—problematic but rich in potential. Oilfield Review 2007, 19 (2), 22-43. 39. Allen, R. G.; Little, D. N.; Bhasin, A.; Glover, C. J., The effects of chemical composition on asphalt microstructure and their association to pavement performance. International Journal of Pavement Engineering 2014, 15 (1), 9-22. 40. Xiaokong Yu, N. A. B., Sergio Granados-Focil, Mingjiang Tao, The chemical-microstructuralmechanical relationships of asphalt binders.


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Time- and composition- dependent evolution of distinctive microstructures in bitumen Xiaokong Yu1, Sergio Granados-Focil2, Mingjiang Tao1*, Nancy A. Burnham3* 1. Department of Civil and Environmental Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609 2. Gustaf H. Carlson School of Chemistry, Clark University, 950 Main Street, Worcester, MA, 01610 3. Physics and Biomedical Engineering Departments, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609

Table 1. Bitumen name, crude source, PG grade, chemical composition, and wax content PG

Asphaltenes (A)

Maltenes (M)

Wax content

grade

(wt.%)

(wt.%)

(wt.%)

58-28

-

-

-

58-10 a

10.0 a

90.0 a

0.81 a

-

10

90

-

-

25

75

-

-

35

65

-

ABD-A50

-

50

50

-

virgin AAD c

58-28 a

24.2 a

75.8 a

1.94 a

-

0

100

-

-

12

88

-

-

24

76

-

-

50

50

-

Sample name

Crude source

261643

Whiting, ME

virgin ABD b ABD-A10 b ABD-A25

California Valley

ABD-A35

AAD-Maltenes California

AAD-A12 AAD-A24 AAD-A50

c

Coast

a

values taken from SHRP report, and the weight percentage of asphaltenes and maltenes were normalized against the sum of the SARA fractions.34 b the remixed ABD-A10 has the same asphaltenes/maltenes ratio as the virgin ABD. c the remixed AAD-A24 has the same asphaltenes/maltenes ratio as the virgin AAD.


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Table 2. Designation of bitumen samples used in investigation of their time-dependent microstructures 1st heating

Heat treatment Sample designation

2nd heating

1H-2d

1H-9d

1H-15d

1H-22d

1H-32d

1H-39d

2H-0d

2H-4d

2H-7d

2H-40d

2

9

15

22

32

39

0

4

7

40

2

9

15

22

32

39

40

44

51

91

Annealing duration (days) Accumulative annealing duration (days)


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(a) 1H-2d

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(b) 1H-9d

(c) 1H-15d

(d) 1H-32d

(g) 2H-4d

(f) 2H-0d

(e) 1H-39d

2 µm

(h) 2H-7d

2 µm Figure 1. Time-evolution (blue arrows, clockwise) of AFM topographic images of solution-cast virgin ABD (film thickness ~ 100 µm) annealed at room temperature for different durations: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-32d, (e) 1H-39d, (f) 2H-0d, (g) 2H-4d, and (h) 2H-7d. AFM cantilever NSC16 and repulsive-imaging mode were used. Scan size and z-scale for all images are 20×20 µm2 and 3 nm, respectively. The red scale bars in each image indicates 2 µm. The dispersed phase in this sample grew and stabilized in two weeks after the initial heating; even though the dispersed phase shrank to a much smaller size right after the second heating, it reappeared as the sample was annealed for four days and longer.


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(a) 1H-2d

(b) 1H-9d

(c) 1H-15d

(d) 1H-32d

(g) 2H-4d

(f) 2H-0d

(e) 1H-39d

2 µm

(h) 2H-7d

2 µm

Figure 2. Time-evolution (clockwise) of solution-cast virgin ABD seen in AFM phase images that correspond to the topographic images in Figure 1: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-32d, (e) 1H-39d, (f) 2H-0d, (g) 2H-4d, and (h) 2H-7d. AFM cantilever NSC16 was used. AFM cantilever NSC16 and repulsive-imaging mode (phase angles smaller than 90°) were used. Scan size and z-scale for all images are 20×20 µm2 and 10°, respectively. The red scale bars in each image indicates 2 µm. The time-evolution of microstructures of virgin ABD seen in the topographic images is consistent with the trend in the phase images. Furthermore, phase contrast suggests that the dispersed phase is either stiffer or less lossy than the continuous matrix phase, indicating that the chemical compositions over these two domains are probably different. 4 ACS Paragon Plus Environment

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(a) 1H-2d

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(b) 1H-9d

(c) 1H-15d

(d) 1H-22d

(g) 2H-0d

(f) 1H-39d

(e) 1H-32d

3 µm

(h) 2H-7d

3 µm Figure 3. Time-evolution (blue arrows, clockwise) of AFM topographic images of solution-cast ABD-A25 (film thickness ~ 100 µm) annealed at room temperature for different durations: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-22d, (e) 1H-32d, (f) 1H-39d, (g) 2H-0d, and (h) 2H-7d. Scan size and z-scale for all images are 30 µm and 15 nm, respectively. The red scale bars in each image indicates 3 µm. The blue ellipses highlight representative microstructures of the third phase that are less visible in the topographic images yet evident in the phase image (Figure 4). AFM cantilever NSC16 and repulsive-imaging mode were used. Evolution of the microstructures of ABD-A25 as a function of annealing time and the second heating is similar to that of virgin ABD (Figure 1& Figure 2); however, it took about a month for the microstructures in ABD-A25 to stabilize, twice that for virgin ABD. 5 ACS Paragon Plus Environment

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Energy & Fuels

(a) 1H-2d

(b) 1H-9d

(c) 1H-15d

(d) 1H-22d

(g) 2H-0d

(f) 1H-39d

(e) 1H-32d

3 µm

(h) 2H-7d

3 µm

Figure 4. Time-evolution (clockwise) of solution-cast ABD-A25 as seen in AFM phase images that correspond to the topographic images in Figure 3: (a) 1H-2d, (b) 1H-9d, (c) 1H-15d, (d) 1H-22d, (e) 1H-32d, (f) 1H-39d, (g) 2H-0d, and (h) 2H-7d. AFM cantilever NSC16 was used. All phase images were acquired using repulsive-imaging mode with phase angles smaller than 90°. Scan size and z-scale for all images are 30 µm and 10°, respectively. The red scale bars in each image indicates 3 µm. The blue ellipses highlight the corresponding microstructures of the third phase, which is less visible in the topographic images in Figure 4.


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Virgin ABD vs. ABD_A25 15.0

Area fraction of the dispersed phases (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Virgin ABD ABD_A25 12.0

1H-22d 1H-32d 1H-39d 1H-15d

9.0

2H-7d 2H-40d 2H-4d

1H-9d

6.0

3.0 1H-2d 2H-0d

0.0 0

12

24

36

48

60

72

84

96

Accumulative annealing duration (days) Figure 5. Area fractions of the dispersed phase(s) in solution-cast virgin ABD (black circles) and ABDA25 (red squares) annealed at room temperature as a function of accumulative annealing duration (samples are designated in sequence from left to right: 1H-2d, 1H-9d, 1H-15d, 1H-22d, 1H-32d, 1H-39d, 2H-0d, 2H-7d, and 2H-40d, Table 2). Area fractions are average values from three to five different images acquired at different locations on bitumen surfaces, except for the last data point (i.e., 2H-40d) due to time constraints. Dispersed phase(s) for both bitumens annealed for 2 days after the initial heating account for ~4.0 %, and gradually reached their corresponding stable values at different rates after the first heating (i.e., ~2 weeks for virgin ABD vs. ~ 4 weeks for ABD-A25). Area fractions of the dispersed phase(s) for both bitumens dropped right after the second heating due to the melting of the molecular structures of the dispersed phase(s), but they grew back at faster rates as compared to that after the initial heat treatment. The time-dependent microstructures of virgin ABD and ABD-A25 after the initial heating and the recovery of their respective dispersed phase(s) after the second heating indicate the correlation between the microstructural evolution observed by AFM and steric hardening mechanism studied by bulk MDSC and DSR.32-33


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Energy & Fuels

(b)

(a)

(c)

(d)

(e)

Figure 6. AFM topographic images of solution-cast (a) virgin ABD, (b) ABD-A10, (c) ABD-A25, (d) ABD-A35, and (e) ABD-A50. All samples were ~100 µm thick. In order to provide good contrasts for microstructures in each sample, color scales for (a & b) and (c-e) are set to be 5 nm and 15 nm, respectively (shown at the far right of each row). All samples were annealed in ambient conditions (in air, ~20% humidity, 21.2±0.4°C) and were imaged at a month old (i.e., microstructures were most likely stabilized). AFM cantilever NSC16 was used for (c) and NSC14 for the rest of the samples. As asphaltene concentration increased, ABD-based binders showed distinctive morphologies: virgin ABD and its replica (ABDA10) showed similar dendritic microstructures yet with different quantities and complexity; ABD-A25 displayed a primary dispersed phase with recessed centers and a secondary dispersed phase that was less visible (highlighted in blue ovals) than the primary dispersed phase; both ABD-A35 and ABD-A50 exhibited microstructures of somewhat round shape that stood out of the matrix phase to different extent.


Energy & Fuels

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Figure 7. AFM topographic (1st row) and phase (2nd row) images, and histograms of the phase image (3rd row) of heat-cast AAD-Maltenes (1st column), AAD-A12 (2nd column), AAD-A24 (3rd column), AAD-A50 (4th column). All samples were ~100 µm or thicker. Greyscale for all topographic images is 20 nm. Phase images for AAD-Maltenes and AAD-A12 have a greyscale of 30°, and those for AAD-A24 and AAD-A50 of a greyscale of 15°. All samples were annealed in ambient conditions (in air, ~20% humidity, 21.2±0.4°C) and were imaged 3 days after the first heating. AFM cantilever NSC16 probe and repulsive-imaging mode were used. Different from time-dependent microstructures for ABD-based binders, microstructures of AAD-based binders annealed at ambient were found to be relatively time-independent. Such a difference might be related to variation in chemical miscibility of the asphaltenes and maltenes in these two binders that have different crude oil origins. 9 ACS Paragon Plus Environment

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Time- and Composition-Dependent Evolution of ... - ACS Publications

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