Micromorphology of asphalt by polymer and carbon nanotubes

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Micromorphology of asphalt by polymer and carbon nanotubes modified through molecular dynamics simulation and experiments: role of the strengthened interfacial interactions Peng Wang, Fei Zhai, Ze-jiao Dong, Li-zhi Wang, Jian-ping Liao, and Gui-rong Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02909 • Publication Date (Web): 13 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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Micromorphology of asphalt by polymer and carbon nanotubes modified through molecular dynamics simulation and experiments: role of the strengthened interfacial interactions Peng Wang a, Fei Zhai b, Ze–jiao Dong c,*, Li–zhi Wang a, Jian–ping Liaoa, and Gui–rong Lia a

School of Transportation Engineering, Shandong Jianzhu University, Jinan 250101, P. R China

b

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, P. R China

c

School of Transportation Science and Engineering, Harbin Institute of Technology, Harbin 150090, P. R

China

ABSTRACT: Polymer modifiers have been used to improve the performances of asphalt binders in pavement engineering. The modifying effect of polymer on asphalt is largely depended on the morphological characteristics of polymer–modified asphalt. The morphologies of polymer– modified asphalt are composed of polymer–rich phase, asphaltene–rich phase, and the interphase between the two phases. Interfacial interactions importantly contribute to morphology but are commonly overlooked. In this study, carbon nanotubes (CNTs) were selected to improve the interfacial interactions of polymer–modified asphalt. Fluorescence microscope (FM), scanning electron microscope (SEM), micro–Raman spectra (MRS) and molecular dynamics simulation

ACS Paragon Plus Environment

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(MD) were used to capture the characteristics of the interphase and polymer–rich phase. CNTs– polymer–modified asphalt involves stronger intermolecular forces than those in asphalt modified by only styrene–butadiene–styrene (SBS) or CNTs. This discrepancy highlights the intensified interfacial interaction in the former material. Raman peak and MD findings reveal that the C=C of CNTs interacted with the alkanes and aromatic hydrocarbons of asphalt. SBS were entwined or surrounded with CNTs through the π–π conjugation of the benzene rings of the two components. Consequently, synergistic effect enhanced the intermolecular force between SBS and CNTs in the interphase. SEM results indicated that CNTs were enriched in the interphase, enhancing mechanical anchorage between the polymer and asphalt. As a results, CNTs increased the roughness of the interphase and produced a prominent cage construction of polymer–rich phase. Moreover, the observed pullout behaviors of CNTs alleviated interfacial failure. FM images displayed that CNTs enhanced the swelling degree of polymer–rich phase. This effect was realized because CNTs served as a tunnel for transporting saturates, aromatics, and small resin molecules, as shown by MD analysis. This work revealed the importance of the interfacial interactions on the micromorphologies of polymer–modified asphalt. KEYWORDS: Polymer–modified asphalt, Carbon nanotubes, Interfacial interactions, Molecular dynamics simulations, Micro–Raman spectra, Scanning electron microscopy 1. INTRODUCTION Asphalt is an important paving material. However, straight asphalt or base asphalt from crude petroleum exhibits low anticracking and antirutting properties in applications. Therefore, asphalt modification has become a popular method to strengthen base asphalt. The most widely employed asphalt modification method is use of polymers as modifiers, such as styrene–


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butadiene–styrene copolymer (SBS). The road performance of polymer–modified asphalt relies strongly on its micromorphologies 1–5. The “micromorphologies” of polymer–modified asphalt comprise the polymer–rich phase, asphaltene–rich phase, and interphase

6,7

. Although detailed studies on the distribution of

polymer–rich phases are available, the relative importance of the interphase in the asphalt field is scarcely known. An improved understanding of the interphase of polymer–modified asphalt in response to morphologies is required in determining the important relationship between microstructure and macroperformance of asphalt binders. The interphase of composite material is a bridge that links different phases. Polymer– modified asphalt is a multiphase composite 8. Liu proposed the existence of an “interphase region” in the polymer matrix composite 9. The mechanical properties of composite are closely correlated with the changes of interfacial characteristics

10

. The stability of polymer–modified

asphalt is also influenced by the interphase 11. Intermolecular interaction is a primary mechanism that influences the interfacial interaction 12, which is strongly dependent on the chemical affinity between polymer and asphalt components

13

. Chemical affinity is an intermolecular adsorption

effect. Thus, the interphase of polymer–modified asphalt is an adsorption film. However, the molecular weight, density, and chemical nature of polymer molecules totally differ from those of asphalt

14

, this difference highlights the poor chemical affinity between them. Therefore, the

strength of the interphase formed by adsorption in polymer–modified asphalt is insufficiently, and interface enhancement is a suitable route to address this issue. The most effective method of interphase enhancement is by using nanomaterials with high specific surface areas, which provide larger contact zones at the interphase. Among composite materials, carbon nanotubes (CNTs) are regarded as ideal reinforcements to improve the


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interphase because of their excellent mechanical properties 15. Most previous works showed that chemically functionalized CNTs improves the interfacial shear strength between CNTs and polymers

16

. In asphalt modification, Nur-Izzi and Zhang reported the enhanced antiaging

properties of asphalt with nanomaterials

17, 18

. Santagata and Arabami demonstrated improved

antirutting and anticracking properties of asphalt with 0.5 wt.% CNTs19, 20. Our previous work also proved that CNTs enhanced the road performance of SBS modified asphalt 21. However, the micromechanism, especially interphase changes, is unclear. The paper is aimed to investigate the interfacial reinforcing effect of CNTs on polymer– modified asphalt. The most popular polymer SBS was selected. Trans–scale enhancement design and blending techniques were adopted to prepare composite–modified asphalt with CNTs and SBS. “Trans–scale enhancement design” refers to the use of materials with different scales in nanocomposite design. Micromorphological characteristics of asphalt samples with or without CNTs were obtained by observing the interaction of interfacial molecules, interphase morphologies, and polymer–rich phases. For the interaction of interfacial molecules, snapshots of molecular dynamics (MD) were used to capture the distribution of different components in asphalt samples, and micro–Raman spectra (MRS) was adopted to reveal interactions between CNTs and other components. For interphase morphologies, scanning electron microscope (SEM) was applied to observe the interfacial morphology, and MD was used to obtain the interfacial shear stress and pullout energy during the pullout of CNTs from the system. For polymer–rich phase characteristics, the polymer swelling degree was determined by fluorescence microscope (FM) and image digitization. This work provides a guidance to fully understand the relationship between the interphase and micromorphologies of polymer–modified asphalt. 2. MATERIALS, EXPERIMENTAL DETAILS, AND SIMULATION PROCEDURE


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2.1. Material preparation Straight asphalt with a penetration of 70 dmm and linear–type SBS with a molecular weight 110 000 g/mol were used. A multi–walled CNTs containing hydroxyl (OH–) (Fig. 1 d) with an external diameter of 30−50nm, and its length of 10−30µm was selected. Modified asphalt was prepared on a high–shear mixer (Wei Yu Machine Co., Ltd., China) under the following conditions: shearing rate 3000–3500 rpm for 40 min and decreased to 2000 rpm for swelling; swelling time, 30min; and blending temperature between 175 °C and 180 °C in all cases. CNTs and SBS were blended first and then combined with hot asphalt. SBS was crushed into size of 20 mesh to increase the contact area and then mixed with CNTs by mechanical stirring. Finally CNTs adhered onto the SBS surface. The SBS amount was 4.3%, CNTs whereas the CNTs amount ranged from 0.02% to 0.5%. The compatibilizer (furfural extract oil) amount was 5%, and the stabilizer (a type of commercial product) amount was 0.25% by weight of asphalt in all samples. Table 1 Road performance of asphalt samples High–temperature properties Road properties

R3200

R100

Jnr3200

Jnr100

S @– 18℃ (MPa)

Np50 @25℃ 6

Viscosity@135℃

(×10 times)

(Pa·s)

(%)

(%)

(1/kPa)

(1/kPa)

A70#

0.73

60.52

2.574

0.377

302

0.035

0.564

CNTs+A70#

1.03

2.01

2.206

2.106

232

0.078

0.611

SBS+A70#

50.27

60.52

0.491

0.377

209

3.01

1.657

SBS/CNTs+A70#

85.92

89.87

0.111

0.074

227

4.61

1.531

Table 1 summarizes the road performance of control asphalt samples and experimental samples, including base asphalt (A70#), CNTs–modified asphalt (CNTs+A70#), SBS–modified asphalt (SBS+A70#), and composite–modified asphalt with SBS and CNTs (SBS/CNTs+A70#). High–temperature properties were evaluated by multi stress creep recovery (MSCR) in accordance with ASTM D7405–10a, and the obtained percent recovery (R) and non–recoverable compliance (Jnr) were 3200 Pa and 100 Pa respectively. Low–


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temperature properties were determined on bending beam rheometer to obtain the stiffness modulus (S) in according with ASTM D6648. Fatigue life (Np50) was determined by time sweep test proposed in Ref. 22. Workability was evaluated as viscosity at 135 ℃. The data in Table 1 showed that the composite–modified asphalt with SBS and CNTs performed more effectively than the asphalt modified by SBS or CNTs alone. Thus, the addition of CNTs positively affected on the properties of the asphalt samples.

2.2. Experiment details 2.2.1 Interphase characterization SEM was used to observe interfacial morphology, and SEM samples were tested with gold coating on Helios Nano Lab 600i. After the specimens were placed on a small aluminum stub, a thin gold coating of 0.6 nm was sputter coated at room temperature by using an electron microscopy science system.

2.2.2 Polymer–rich phase characterization Polymer–rich phase distribution and polymer swelling degree were determined using FM and image digitization. The FM samples were prepared by heat–casted method. Hot asphalt was casted on glass slides at a heating temperature of approximately 160 °C to achieve an enhanced flow. Bitumen–covered sample holders were stored overnight at room temperature before testing. Image digitization was applied using MATLAB to convert FM images into binary images containing only two pixel values, 0 and 1, then to binary black (asphaltene–rich phase) or white (SBS–rich phase). Finally, the area of swollen polymer–rich phase was obtained. The ratio of the swollen polymer–rich phase area to the total area was adopted to determine the swelling degree of the polymer in the modified asphalt samples.

2.2.3 Interaction between CNTs and other components MRS is an effective tool to capture the vibrations of nonpolar bonds of the same atom, such as C=C. The symmetrical skeleton vibration of C=C in the infrared spectrum is difficult to observe. For SBS–modified asphalt with CNTs, the changes in C=C can be used to distinguish the interactions between CNTs and other


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components. Therefore, MRS was selected and conducted on Thermo DXR to capture the interactions between the CNTs and other components. The excitation wavelength was 532 nm, excitation power was 10% of the total power, and the objective magnification was 100 times. A charge–coupled device was used to record and collect the Raman spectra signal.

2.3. Simulation procedure 2.3.1. Input parameters of molecular simulations Asphaltene s

(a)

Aromatics

Resins Saturates

Base asphalt

(b)

(c)

Figure 1. Molecular components of base asphalt (a) and molecular structures of CNTs and SBS in molecular simulations: base asphalt model is composed of asphaltenes, resins, aromatics, and saturates. Panel (b) is multi–walled carbon nanotubes with hydroxyl, and panel (c) is SBS polymer with 2 polymerization degree. A straight asphalt model was composed of saturate, aromatic, resin, and asphaltene (Fig. 1a), as supported by our previous work published in Ref. 23. In our previous work 21, multi–walled CNTs with hydroxyl showed an enhanced modification on asphalt. Thus, the molecular structure of CNTs as seen in Fig. 1b was obtained. Herein, the length of CNTs was 20.96 Å, which was the length in one cell. In a periodic cell, the CNTs length intercepts a section. The CNTs presented an external diameter of 17.667 Å and internal diameter of 4.76 Å,


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and was larger than the molecular size of asphalt components and SBS. The molecular structure of linear–type SBS is presented in Fig. 1c. The interphase models of the SBS–modified asphalt with or without CNTs comprised of the straight asphalt model (Fig. 1a), SBS molecules (Fig. 1b), and CNTs (Fig. 1c). The molecule numbers of SBS and CNTs in these models were based on their corresponding amounts in actual samples.

2.3.3. Simulation details MD is a powerful computational method for recognizing the microscopic characteristics of asphalt binders and selecting suitable raw materials for modification. MD was conducted on Materials Studio 8.0 of Accelrys. We adopted a periodic cell and a COMPASS force field. Here, the periodic cell was used under periodic boundary conditions (Section 1 of the supporting information). In the periodic cell, the molecular distribution repeated periodically. Thus, the snapshot of MD was a repeat unit. MD was followed by steps 1−4 to reach the equilibrium state: (1) energy minimization, (2) annealing stage with a temperature increase from 200 K to 500 K with 5 heating ramps per cycle, (3) geometric optimization to ensure energy minimization, and (4) volume shrinking conditions: T = 298 K, NVT ensemble, total simulation time of 0.5−1.0 ns, time step of 0.5 fs, and Nose−Hoover thermostat; T = 298 K, P = 0.0001 GPa, total simulation time of NPT ensemble of 2−4 ns, time step of 0.5−1.0 fs, Nose−Hoover thermostat, and Andersen barostat.

2.3.4. Characteristic parameters The total energy (ETotal) of MD includes the bond energy (EBond) and nonbond energy (ENonbond). Nonbond energy is the intermolecular energy derived from intermolecular force. Intermolecular force mainly involves van der Waals (vdW) and electrostatic Coulomb force (electrostatics), but hydrogen bonding is neglected because the hydroxyl groups or strongly electronegative groups existing in the asphalt are too small to produce a large change in the total energy. Interfacial shear stress (γ) and pullout energy were used to describe the interphase changes of the molecular models during CNTs pullout from the system through MD. Interfacial shear stress is estimated by Eq. 1, as supported by Guo 24. In Eq. 1, A is the contact area of the interphase and △E is the energy difference between the total energy of the composite−modified asphalt and the sum of the energies of the individual


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component (Eq. 2). In this equation, Ecomposite is the total energy of the modified asphalt with SBS and CNTs, ECNTs is the total energy of CNTs only, and ESBS is the total energy of modified asphalt with SBS only.

γ=

∆E 2A

(1)

∆E =EComposite − ( ECNTs + ESBS )

(2) (3)

Epullout =E2 − E1

Pullout energy (Epullout) was adopted to describe CNTs pullout behavior, which was significant in enhancing the interphase. If CNTs were pulled out of the interphase, the interphase failed in most composites. Thus, MD was used to model this process to capture the energy changes during CNTs pullout process. Epullout was determined using Eq. 3, where E1 is the total energy of the system at the No. 1 position and E2 is the total energy of the system at the No. 2 position.

3. RESULTS AND DISCUSSION 3.1. Interaction of interfacial molecules The interphase of composite−modified asphalt was composed of CNTs, SBS, and asphalt components. Interfacial strength was determined by the intermolecular interaction of interfacial molecules. However, such parameter was difficult to obtain directly by experiments in the asphalt field. Thus, MD was employed to investigate the interaction among the CNTs, SBS, and asphalt components, and Raman spectra were recorded to verify the MD results. Table 2. Different energy of molecular models of asphalt samples. ETotal

EBond

ENonbond

EvdW

EElectrostatic

(kcal/mol)

(kcal/mol)

(kcal/mol)

(kcal/mol)

(kcal/mol)

A70#

38689

27807

10882

11502

–620

CNTs+A70#

116277

80108

36169

35286

–883

SBS+A70#

44373

30859

13514

14108

–594

SBS/CNTs+A70#

132384

90296

42088

42903

–815

Molecular models


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Intermolecular interaction included vdW and electrostatic force. In Table 2, the Evdw of the asphalt samples were significantly larger than the EElectrostatic. Thus, the interfacial strength of the asphalt samples mainly depended on vdW force. Compared with different asphalt samples, the composite−modified asphalt (SBS/CNTs+70#) achieved a larger Evdw than that of asphalt modified only by SBS or CNTs. This result indicates a strengthened interfacial interactions in the composite−modified asphalt, consequently yielding superior road performance to those of the other two asphalts in Table 1. A70# CNTs+A70#

1200

1600

Raman Internsity/CPS

(a)

1600

Raman Internsity/CPS

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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(b)

Band D

1200

901

800

1575

400

2355

2893 3321

0 0

500

1000 1500 2000

2500 -1

3000

3500

Band G

SBS+A70# SBS/CNTs(0.02%)+A70# SBS/CNTs(0.2%)+A70# SBS/CNTs(0.5%)+A70#

800 400 0 0

500 1000 1500 2000 2500 3000 3500

Raman shift/cm

Raman shift/cm

-1

Figure 2. Raman spectra of the asphalt samples: panel (a) shows the base asphalt A70# and CNTs−modified asphalt (CNTs+A70#); panel (b) displays the SBS−modified asphalt (SBS+A70#) and composite−modified asphalt with CNTs and SBS (SBS/CNTs+A70#).

No chemical reaction was noted in composite−modified asphalt with SBS and CNTs. Thus, the changes of Raman peak of the asphalt samples with different CNTs amounts were ascribed to the intermolecular force. Here, MRS were performed to capture the intermolecular force changes in asphalt samples (Fig. 2). The composite−modified asphalt with SBS and 0.5% CNTs yielded the largest Raman intensity relative to those of the asphalt modified with CNTs or SBS only. This finding also demonstrated the strong interaction of CNTs, SBS, and asphalt components in composite−modified asphalt with a suitable CNTs amount.


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(a)

(b)

Figure 3. Interactions between CNTs and other component: (a) is the interaction between small molecules in asphalt and CNTs, (b) is the interaction between asphaltenes and CNTs.

For investigating the interaction between CNTs and asphalt, the C=C content in the base asphalt was traced. The six–membered ring was abundant, so only a broad peak at 1575 cm–1 was observed in Fig. 2a. However, the peak corresponding to the CNTs+A70# is weak. This result may be ascribed to the diffusion of light components in CNTs. The peak at 901 cm–1 of CNTs+A70# belongs to the shift in respiratory vibration of C–C in benzene ring

25

. This C–C

peak is usually around 990 cm–1. The peak shift to 901 cm–1 can be explained by the π–π conjugation of the benzene rings and the diffusion of small molecules into the CNTs. MD supported this viewpoint; specifically, the benzene rings of the resins were conjugated with the CNTs (Fig. 3a), the aromatics molecules were absorbed into the walls of the CNTs (Fig. 3a), and the long alkyl side chains of the asphaltenes were inserted into the CNTs (Fig. 3b). Therefore, the CNTs interacted with alkanes and aromatic hydrocarbons in asphalt. Table 3. Raman characteristic peaks shift of asphalt samples with different CNTs amount Band types

Band D /cm–1

Band G/cm–1

ID/IG

SBS+A70#

1360.91

1589.27

0.81

SBS/CNTs(0.02%)+A70#

1346.38

1591.95

0.84

SBS/CNTs(0.2%)+A70#

1345.92

1588.07

0.96

SBS/CNTs(0.5%)+A70#

1346.12

1587.86

0.92


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For interactions among CNTs, SBS, and asphalt, SBS−modified asphalt with or without CNTs all produced distinct characteristic peaks at around 1360 cm–1 and 1580 cm–1 (Fig. 2b). The peak at 1360 cm–1 indicated the defect peak (B and D) caused by the defect in CNTs or crystallite size effect 26, 27, which was a double resonance peak of C=C vibration. Meanwhile, the peak at 1580 cm–1 corresponded to the graphite peak (B and G) of the C=C tangential vibration; this peak was the characteristic peak of the plane symmetry vibration of six–member rings

28

.

The molecular structure of SBS polymer also contained C=C and six–member rings; thus, SBS−modified asphalt (SBS+A70#) showed peaks similar to those of the composite−modified asphalt (SBS/CNTs+A70#) in Fig. 2b. The changes in peak position and intensity for D and G reflected the changes in intermolecular interaction 26. For the composite−modified asphalt (SBS/CNTs+A70#), the peak shift of D was insignificant, and the G peak offset to a lower wavenumber as the CNTs amount increased (Table 3). Michel believed that the peak shift of G was proportional to the bending degree of the six–membered ring in carbon materials 29. The peak intensity ratio of D and G (ID/IG) were used to identify the disorder degree of CNTs in the composite. Given the absence of chemical reactions among asphalt, SBS, and CNTs, the changes in ID/IG demonstrated the interactions among all molecules. The high ID/IG of SBS/CNTs+A70# indicated the strong intermolecular interaction derived from the network formed by the self–overlap of the CNTs. Meanwhile, the lower ID/IG of the composite–modified asphalt with 0.5% CNTs than those of the samples with 0.2% CNTs may be ascribed to the self–aggregation of the CNTs. Moreover, the Raman signal of the CNTs+A70# was significantly weaker than that of SBS/CNTs+A70#. Thus, a synergistic effect occurred between SBS and CNTs.


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(a)

(b)

(c)

(d)

Figure 4. Interaction between SBS and CNTs in the composite–modified asphalt as obtained by MD: (a) molecular model snapshot of the composite–modified asphalt after MD, (b) SBS intertwined with CNTs, (c) SBS absorbed into the CNTs, and (d) π–π conjugation between CNTs and SBS.

Fig. 4 provided the synergistic effect between SBS and CNTs. SBS entwined CNTs (Fig. 4b) and surrounded by CNTs or absorbed into the walls of CNTs at the same time (Fig. 4c) in the absence of asphalt. A similar behavior of SBS was observed in the presence of asphalt (Fig. 4a). The synergistic effect between SBS and CNTs benefited from the π–π conjugation of the benzene rings belonging to the two materials. Therefore, the findings from MD showed that the composite–modified asphalt with CNTs exhibited a stronger interfacial interaction than modified asphalt without CNTs. 3.2. Interfacial morphologies of the asphalt samples SEM was used to obtain the interfacial morphologies of asphalt samples (Fig. 5). The interphase in SBS–modified asphalt was an indistinct line in Fig. 5a. The interphase was composed of different molecules, as shown by the MD snapshot in Fig. 6. However, these characteristics are difficult to measure using currently available experimental technology. Thus, MD was used to determine molecular distribution in the interphase. The SBS–modified asphalt showed smooth interphase morphology (Fig. 5a). In the composite–modified asphalt, the CNTs were absorbed into the surface of the SBS, acting as an


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antenna of the SBS–rich phase to increase mechanical anchorage between SBS and the asphalt (Fig. 5d). CNTs also provided a markedly rougher interphase than that of the SBS–modified asphalt. (a)

(b)

(c)

(e)

(f)

Asphalt-phases

SBS-phase

(d) Asphalt-phases SBS-phase Pull-out behaviors of CNTs

Figure 5. Interphase morphologies of asphalt samples at different scales: (a) to (c) show the SBS–modified asphalt (SBS+A70#), (d) to (f) display the composite–modified asphalt with SBS and CNTs (SBS/CNTs+A70#).

In Fig. 5e, the small antenna connected the small SBS–rich phase and formed nanopores at a large observation scale. Finally, the composite–modified asphalt with CNTs exhibited a more apparent hierarchy than that in samples without CNTs in (Fig. 5f). However, the SBS–rich phase of the SBS–modified asphalt was scattered in Figs. 5b and 5c. Therefore, SEM images demonstrated that the CNTs provided an increasingly rough interphase for the SBS–modified asphalt. This rough interphase constructed a prominent cage of the SBS–rich phase, thereby yielding abundant structural characteristics of the networks of the SBS–rich phase. Meanwhile, CNTs was evidently larger than the molecule of single SBS or asphalt components in the interphase (Fig. 5d). To balance the accuracy with computational efficiency, we adopted CNTs


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with differing size from the actual size during simulation, and larger external and internal diameters of the CNTs in MD than those of the other molecules in the system. Thus, such parameters were reasonable. Moreover, significant pullout behavior was observed in CNTs (Fig. 5f). Pullout behavior is used to describe how CNTs delay interface failure because of the additional time required to pull themselves out from the interphase. Numerous studies reported on the pullout behaviors of CNTs, and proposed that this behavior is mainly responsible for the improved interphase of composite 30, 31

. Thus pullout behaviors of CNTs were simulated by MD. CNTs were directly pulled out

from SBS–modified asphalt along the axial direction with a pulling distance ranging from 0 nm to 4 nm. The MD snapshot is summarized in Fig.6, whereas the changes in Epullout and interfacial shear stress are presented in Fig. 7. (a)

(d) d=1.5nm

(b) d=0.5nm

(e) d=2.0nm

(c) d=1.0nm

Saturates Aromatics Resins

Asphaltenes SBS

CNTs

Figure 6. Pullout simulation of composited modified asphalt with SBS and CNTs at different pulling distance.


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Epullout

(a)

4

10

0 1

(b)

40 Shear stress(MPa)

20 Epullout(×10 kcal/mol)

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2 3 Distance(nm)

4

30 20 10 0 1

2 3 Distance(nm)

4

Figure 7. Pullout energy (a) and interfacial shear stress (b) during the pullout process of CNTs.

In Figs. 6a and 6b, CNTs were surrounded by saturates, aromatics, small resins, and SBS. Asphaltenes were situated far away from the CNTs because of large steric hindrance. When pulling distance was below 0.5 nm, the CNTs moved within the system; thus, the interfacial shear stress and Epullout showed small changes. With increasing pulling distance, the number and contact area of the contacting molecules decreased, resulting in increased interfacial shear stress and Epullout (Fig. 7b). The increase in Epullout with pulling distance showed that the energy barrier was needed to pull out CNTs from the system. When the CNTs were completely pulled out from the system, the required energy decreased to zero. However, the Epullout did decrease to zero immediately. In Fig. 6e, the CNTs were totally pulled out from the system, but the long paraffin branched chain of the asphaltenes remained intertwined with the CNTs. The vdW force was still operational when the pulling distance exceeded 2.0 nm. The range of vdW’ force was 0.31～0.50 nm 32. Therefore, the largest Epullout and interfacial shear stress were achieved at 2.5 nm. Above 2.5 nm, the two indexes slowly decreased with pulling distance. Guo proposed a similar rule of interfacial shear stress and Epullout 24, 33. For the polymer–modified asphalt with cracking in real cases, the pullout behaviors of CNTs consumed the energy produced by the external force and finally alleviated interfacial failure. Moreover, the interaction between the polymer–rich phase and asphaltene–rich phase


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was affected by the interphase, whereas the distribution and swelling degree of the polymer–rich phase determined the stability of polymer–modified asphalt. Thus, the characteristics of the polymer–rich phase before and after interfacial change were examined. 3.3. Characterization of the polymer–rich phase of the asphalt samples The distribution and swelling degree of the SBS–rich phase were obtained by FM images. Asphalt and CNTs exhibited no fluorescence (Figs. 8a and 8b). SBS showed observable fluorescence corresponding to the bright yellow dots in Figs. 8c to 8f. The distribution characteristics of the SBS–rich phase differed from those of the samples with and without CNTs. When the CNTs amount was below 0.5%, the SBS–rich phase was scattered, and the CNTs size was not uniform. When the CNTs amount was equal to or more than 0.5%, a clear network appeared, and the network structures became dense as the CNTs amount increased. SEM images showed similar results (Figs. 5c and 5f). Therefore, CNTs improved the network structure of the SBS–rich phase, and conferred improved road performance to the SBS–modified asphalt (Table 1). The swelling degree of SBS increased with CNTs amount (Fig. 9a); this results demonstrates that the CNTs was beneficial to the diffusion of light components in SBS–rich phase. Figs. 9b and 9c showed that the CNTs functioned as a tunnel for transporting saturates, aromatics, and small resin molecules, whereas asphaltenes remained outside the tunnel. The CNTs are hollow nanotube, and the friction of tube wall is almost zero. Lado reported that small molecules diffused rapidly at this confined diameter 34. Zhou ascribed this transport property to the hollow structure of CNTs 35. Moreover, CNTs enriched the surface area of SBS and connected the SBS–rich phase and asphaltene–rich phases, as shown by the SEM images. The CNTs provided numerous communicating passages to transport light components of asphalt. SBS


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swelling was ascribed to the existence of light component. Therefore, increased CNTs amount resulted in enhanced SBS swelling. (a)

(b)

(d) 0.2%CNTs

(e) 0.5%CNTs

(c) 0%CNTs

(f) 0.8%CNTs

Figure 8. Polymer–rich phase distribution characteristics of asphalt samples under FM: panel (a) corresponds to A70#, panel (b) displays CNTs+A70#, panel (c) shows the SBS–modified asphalt, and panel (d) to (f) reveal composite–modified asphalt with SBS and 0.2%, 0.5%,and 0.8% CNTs, respectively. 14 Polymer swelling degree(%)

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(b)

(a)

12

(c)

10 8 6 4 0.0

0.2

0.4 0.6 0.8 CNTs amount(%)

1.0

Figure 9. Small molecule transport effect of CNTs: panel (a) is the polymer swelling degree of the composite– modified asphalt with increasing CNTs amount under FM, panel (b) is the light component diffusion in CNTs by MD, and panel (c) is the asphaltene behavior in asphalt samples by MD.


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(a)

(b)

Reinforcing of CNTs Intertwining of CNTs

Element

(c)

Element

Figure 10. CNTs intertwining and reinforcing effect on SBS: CNTs intertwined with each other in panels (a), CNTs reinforced SBS in panels (b).

Interestingly, the SBS–rich phase in asphalt samples containing high CNTs amounts was smaller than those with less CNTs, and the SBS-rich phase appears elongated in one and not the others. This might be ascribed to two reasons. First, the CNTs formed a loose network. SEM images found that some CNTs intertwined with each other and were enriched on the SBS surface, while other CNTs reinforced SBS (Fig. 10). CNTs classified as 1D nanomaterials, with nanoscale and microscale length 36. Thus, the intertwined CNTs formed a loose network structure that hindered the movement of SBS molecules around CNTs. Consequently, the size of SBS– phase decreased with increasing CNTs amount (Figs. 9e and 9f). Meanwhile, high CNTs amount induced mutual overlapping among CNTs, resulting in denser SBS network of SBS/CNTs+A70# were denser than that of the SBS+A70#. Second, SBS aggregated along the CNTs (Fig. 5c) and resulted in a certain orientation of the SBS–rich phase (Figs. 9e and 9f). Thus, the SBS–rich phase appeared elongated in the samples with 0.5% and 0.8% CNTs, whereas that with low CNTs amounts did not appear the same. Again, the interaction of interfacial molecules played an important role in the distribution and swelling degree of the polymer–rich phase in asphalt. 4. CONCLUSION


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The unknown correlations among the chemical components, micromorphology, and mechanical properties of asphalts remain undetermined after more than 50 years of extensive research. This relationship was clearly understandable through microscopic observation techniques, associating the microstructure of polymer–modified asphalt with its properties. However, the function of the interphase, an important microstructure in polymer–modified asphalt has not been fully understood. In this paper, the role of the interphase in the micromorphologies of polymer–modified asphalt was investigated. CNTs were used to regulate the interphase. The interaction of interfacial molecules, interfacial morphologies, and polymer– rich phase characterization of the polymer–modified asphalt with or without CNTs were analyzed. Composite–modified asphalt with CNTs showed a the larger intermolecular force than asphalt modified by only SBS or CNTs. This result demonstrates the strengthened interfacial interaction in the composite–modified asphalt. The C=C of CNTs interacted with the alkanes and aromatic hydrocarbons of asphalt. A synergistic effect occurred between SBS and the CNTs, benefitting from the π–π conjugation of the benzene rings belonging the two materials. Consequently, SBS was intertwined with CNTs and surrounded by CNTs or absorbed into CNTs. In composited–modified asphalt, the CNTs were absorbed into the surface of the SBS as shown by SEM. In this case, CNTs acted as an antenna of the SBS–rich phase to increase mechanical anchorage between SBS and asphalt component. CNTs provided a roughened interphase for the SBS–modified asphalt, which constructed a prominent cage of the SBS–rich phase, wielding abundant structural characteristics of the networks of the SBS–rich phase. The interaction between the polymer–rich phase and asphaltene–rich phase was affected by the interphase. The swelling degree of the polymer–rich phase increased with CNTs amount


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because CNTs were beneficial to the diffusion of light components from the asphaltene–rich phase to the SBS–rich phase. The CNTs acted as a frictionless tunnel to transport saturates, aromatics, and small resin molecules. Meanwhile, CNTs overlapped to form a loose network, hindering the movement of SBS molecules around CNTs. Finally, the size of the SBS–phase decreased with increasing CNTs amount increased. Furthermore, the SBS aggregated along the CNTs, leading to a certain orientation of the SBS–rich phase. CNTs played a positive role in the micromorphologies of polymer–modified asphalt, and enhanced the asphalt’s road performance. This work revealed the important contribution of interfacial interactions to the micromorphologies of polymer–modified asphalt. In the future, new methods characterizing the mechanical properties of interphases in polymer–modified asphalt should be explored. ASSOCIATED CONTENT Supporting Information. Section 1, sketch map of periodic boundary conditions. Section 2, SEM images of base asphalt and CNTs. Section 3, SEM images of SBS polymer and the blends of SBS polymer and CNTs. Section 4, molecular models of SBS modified asphalt. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E–mail: [email protected]; [email protected] Present Addresses


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a

Peng Wang: School of Transportation Engineering, Shandong Jianzhu University, Fengming Road 1000#,

Lingang Developing District, Jinan City, Shandong Province, CHINA 250101; Tel: +86–18560027486; Fax: +86–0531–86361807; E–mail: [email protected], [email protected]

Notes The authors declare on competing financial interest. ACKNOWLEDGMENT This work was sponsored by Shandong Province Natural Science Foundation ZR2016EEP07, China National Natural Science Foundation No.51278159 and No.51478154. ZR2016EEP07 was aimed at interface enhancement mechanism. No.51278159 and No.51478154 were aimed at numerical simulation in micro–meso scale. REFERENCES 1. Fischer, H. R.; Dillingh, E. C., On the investigation of the bulk microstructure of bitumen – introducing two new techniques. Fuel 2014, 118, 365–368. 2. Allen, R. G.; Little, D. N.; Bhasin, A.; Glover, A. 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. 3. Redelius, P.; Soenen, H., Relation between bitumen chemistry and performance. Fuel 2015, 140, 34–43. 4. Soenen, H.; Lu, X.; Redelius, P., The morphology of bitumen–SBS blends by UV microscopy. Road Materials and Pavement Design 2008, 9, (1), 97–110. 5. Hernández, G.; Medina, E. M.; Sánchez, R.; Mendoza, A. M., Thermomechanical and rheological asphalt modification using styrene−butadiene triblock copolymers with different microstructure. Energy & Fuels 2006, 20, (6), 2623–2626. 6. Collins, P.; Masson, J.; Polomark, G., Ordering and steric–hardening in SBS–modified bitumen. Energy & Fuels 2006, 20, (3), 1266–1268.


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Micromorphology of asphalt by polymer and carbon nanotubes

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