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Aging of asphaltic binders investigated with atomic force microscopy

L.M. Rebelo a, J.S. de Sousa a,, A.S. Abreu a, M.P.M.A. Baroni b, A.E.V. Alencar c, S.A. Soares c,J. Mendes Filho a, J.B. Soares d

a Departamento de Fsica, Universidade Federal do Cear, Caixa Postal 6030, Campus do Pici, 60455-760 Fortaleza, Cear, Brazilb Instituto Federal de Educao, Cincia e Tecnologia, 01109-010 So Paulo, Brazilc Departamento de Qumica, Universidade Federal do Cear, Brazild Departamento de Engenharia de Transporte, Universidade Federal do Cear, Brazil

h i g h l i g h t s

The aging of bitumen is investigated with Atomic Force Microscopy.

The aging process cause an increase in size of the asphaltene micelles.

Short term aging induces the formation of fractal-like microstructures.

Stiffness increases half (one) order of magnitude for short (long) term aging.

Viscosity increases half-order of magnitude, mainly during short term aging.

a r t i c l e i n f o

Article history:

Received 20 June 2013

Received in revised form 19 August 2013Accepted 6 September 2013

Available online 25 September 2013

Keywords:

Atomic force microscopy

Aging effects

Asphalt binders

a b s t r a c t

We investigated the short and long term aging of asphalt cement (AC) with different AFM techniques

(topography, phase and friction imaging and nano-indentation experiments). The aging process induces

a growth and nucleation of the asphaltene micelles with a concomitant reduction of the maltene phase,whereas the short term aging induces the formation of fractal-like micellar structures. The friction inves-

tigation shows that the aging processes reduce the binder friction coefficient by 50%, and this reduction

occur predominantly during the short term aging, while the growth of the micelles occur predominantly

during the long term aging. The micro-indentation experiments revealed that the aging processes cause a

stiffening of the AC film (half-order of magnitude for short term aging, and one order of magnitude for

long term aging). The aging process also increased the apparent viscosity of the AC films by half-order

of magnitude.

2013 Elsevier Ltd. All rights reserved.

1. Introduction

Aging of asphalt cement (AC), or bitumen, is a key aspect thatcan lead to premature deterioration of asphalt pavements [1].

ACs are affected by oxygen and ultraviolet radiation, which are

the main factors of the aging process. It occurs primarily during

mixing, but also during compaction and service. The material

undergoes chemical alterations that affect its mechanical proper-

ties, making it more viscous and brittle, thereby interfering in its

behavior under repetitive efforts. Several other factors affect the

aging process of the mixture: (i) asphalt characteristics, (ii) nature

of aggregates, (iii) particle size distribution, and (iv) air void con-

tent. Plant related parameters such as mixing temperatures and

time can also influence mixture performance. Good adhesion be-

tween the asphalt binder and aggregates is also crucial for the con-

struction of durable pavements. One of the most common

problems that reduces the lifetime of pavements is the loss of

adhesion due to infiltration of water between the aggregate andthe binder [2]. Therefore, the microscopy investigation of the adhe-

sion characteristics of asphalt binders is a very important topic of

research to future developments of pavement technology.

One of the greatest difficulties to understand and predict as-

phalt pavement behavior is the high variability among different

AC sources with respect to their chemical composition and micro-

structure [36]. The worldwide increase of traffic volume demands

more resistant pavements, reduced maintenance interventions,

and increased life cycle. Temperature susceptibility is another

important variable. ACs must exhibit good rheological perfor-

mance in a wide range of temperatures, offering flexibility in low

temperatures and rigidity in high temperatures to avoid thermal

and fatigue cracking, and permanent deformations. Although

empirical data and mechanistic approaches provide good indica-

tions of pavement performance, the fact that ACs are mixed with

0016-2361/$ - see front matter 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.fuel.2013.09.018

Corresponding author. Tel.: +55 8533669017.E-mail address:[email protected](J.S. de Sousa).

Fuel 117 (2014) 1525

Contents lists available at ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l

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aggregates, along with the high variability of environmental and

loading conditions to which pavements are subjected, there is a

growing need to understand how the asphalt microstructures

influence the overall behavior of ACs[7,8]and to define strategies

to engineer the asphalt properties to improve its performance. One

common strategy is to modify ACs with polymers (e.g. SBS andEVA) to increase its elasticity response and concomitantly reduce

its viscosity[9,10]. Anti-oxidant additives are also commonly used

to reduce the asphalt susceptibility to oxygen [1114].

Approaches such as X-ray diffraction [15], size-exclusion chro-

matography (SEC) [16] and various microscopy techniques, e.g.

scanning electron microscopy (SEM) [17], transmission electron

[18], (TEM) phase contrast[19], polarized light[20], laser-scanning

[21], fluorescence [22], and atomic force microscopy (AFM) [7], have

been used to investigate asphalt microstructure. The combined use

of different AFM modes and various experimental conditions pro-

vides a basis for a comprehensive examination of the micromechan-

ical properties of asphalt [7]. In particular, AFM can be used to

provide insights into surface topography, phase separation and

mechanical properties such as stiffness, adhesion, viscosity and fric-tion. The AFM technique has been already employed to investigate

the microstructure of bitumen. To mention a few, Pauli et al. used

AFMto characterize bituminous binders and their respective proper-

ties, correlating the surface morphology with the constituents in the

bitumen[23]. Masson et al. used phase-detection AFM to evaluate

bitumen morphology and proposed a system to classify bitumen

into three distinct groups, based on the different domains or visible

phases[5]. Aging effects have also been investigated by AFM. Zhang

et al. found that the overall surface stiffness increased and the bitu-

men surface became more solid-like, but the extent of these changes

was dependent on aging conditions[1].

In this work, several AFM-related techniques such as topogra-

phy, phase and friction imaging, and force-volume (FV) analysis

are employed to investigate the nano-morphology and nano-rheol-

ogy of one type of AC before and after aging. The AC aging was in-

duced with the well known RTFOT [24]and PAV [25] processes,

which simulates short and long term aging, respectively.

2. Materials and methods

The present study makes use of one type of bitumen (50/70

penetration grade), processed by at Lubnor/Petrobrs and pro-

duced in Fazenda Campo Alegre (Esprito Santo, Brazil). Thin Layer

Cromatography (TLC-FID) was used to determine SARA fractions of

the bitumen, which resulted in 12% of saturates, 43% of aromatics;

18% of resins and 28% of asphaltenes. Differential Scanning Calo-

rimetry determined that the wax content in the bitumen is 0.25%.

2.1. Aging process

The rolling thin film oven test (RTFOT) was used to simulate

short-term aging. It measures the effect of heat and air on a moving

film of semi-solid asphaltic binder. A temperature of 163 C and

duration of 85 min is expected to produce aging effects comparable

to average asphalt plant conditions. The pressure aging vessel

(PAV) test simulates long-term aging equivalent to 510 years of

in-service pavements [26]. The PAV method was used to age RTFOT

residues.

2.2. AFM experiments

An AFM (Nanoscope IIIa, Bruker, Santa Barbara, CA, USA) was

used to measure topography, phase, friction and mechanicalproperties of AC films. Samples are measured in both contact and

intermittent (tapping) modes of operation. In tapping mode

(topography and phase), we used rectangular silicon cantilevers

(TESP7, Bruker) with a nominal spring constant of 42 N/m. In con-

tact mode (topography, deflection, friction and FV analysis), we

used V shaped cantilevers (OTR8, Bruker) with a nominal spring

constant of 0.57 N/m. All images were acquired at room tempera-

ture and normal pressure, with a scan rate of 1 Hz and resolution

of 512 512 lines.For most AFM measurements (topography, phase and friction

images), AC was heated and a small amount was deposited on

the center of a 13 mm diameter glass slide. The AC on the coverslip

was heated up to 150 C during 2 min until it became fully spread

on the glass surface thus forming an uniform film. The samples

were cooled at room temperature in a closed chamber to avoid

exposure to contaminants during 24 h. For FV analyses, the sample

preparation was slightly different: the AC was deposited on the

glass slide such that only half of it is covered. This step is essential

for the force curves calibration, which was performed on the ex-

posed hard surface[27].

We have also measured AC properties using the AFM in lateral

force mode (LFM)[28,29]. The schematics of this technique is de-

picted inFig. 1. In this operation mode, the AFM system detectsthe torsion of the cantilever around its axis as the tip scans the

sample surface laterally. The torsion amplitude provides an indi-

rect measure of the friction coefficient between the cantilever tip

and the surface. Therefore, LFM maps offer a convenient method

to identify regions on which friction is higher or lower, which

can be readily correlated with asphalt microstructures.

For all types of measurements, three coverslips were prepared

for each sample. For topography, phase and friction images, we ob-

tained three images of different regions for each coverslip in order

to explore various regions of the film at the frequency of 5 Hz. For

the FV analysis, we used five different indentation rates: 0.5 Hz,

5.0 Hz, 10 Hz, 15 Hz and 30 Hz. Each map is composed of 32 32

force curves equally distributed in a region of 50 lm. The largeamount of data points allows a good statistical validation of our

measurements.

3. AFM data analysis

Fig. 2shows the main features observed in conventional force-

distance AFM curves measured in viscoelastic and adhesive sam-

ples like ACs. Several of those curves were obtained with the FV

contact mode in different regions of the AC films in order to deter-

mine the micro-rheological properties of those films. In this sec-

tion, we describe the models used in the analysis of AFM force

curves to extract the following properties: elasticity (Youngs)

modulus[3032], apparent viscosity[33]and adhesiveness.

3.1. Elasticity modulus

AFM force curves exhibit the form d=f(z), whered is the canti-lever deflection andzis the corresponding translation of the piezo-electric actuator. A schematics of a typical deflection-displacement

curve measured in viscoelastic samples is shown Fig. 2. The hyster-

esis in the approach/retract cycle is a consequence of the viscoelas-

tic response of the sample. A maximum deflection of 50 nm is

imposed to avoid excessive indentation. Beyond the contact point

(z0,d0), the actual cantilever deflection is Dd= d d0, where d0 isthe cantilever deflection far away from the sample surface. The

corresponding piezo-actuator displacement is Dz=z d0, where

d0 also represents the piezo displacement for which the cantilevertouches the sample surface. The sample indentation d is obtained

with d= DzDd. The force deflecting the cantilever is obtained

by Hookes law F= kcDd, wherekcis the cantilever spring constant.This force is transmitted to the sample causing an indentation. In

Hertz contact theory, the loaddisplacement relationship for coni-

cal indenters is given by[3032]:

16 L.M. Rebelo et al./ Fuel 117 (2014) 1525

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Fcone2

pE

1 m2tanhd2; 1

whereEandm represent the elasticity modulus and Poissons ratioof the indented material, respectively. The samples are considered

to be virtually incompressible (m= 0.5).h represents the half-open-ing angle of the conical tip. The above expression is used to fit the

experimental (z,d) curves to extract the elasticity modulus E. For

this, one must choose an interval to which the fitting must be ap-

plied, as shown inFig. 2. The detection of the contact point (z0,d0)

is performed as follows: d0can be determined by averaging an arbi-

trarily chosen region of the non-contact portion of the curve. As for

z0, the best solution for the analysis of few curves is to visuallydetermine the contact point. For a large number of curves, the best

strategy is to treat z0 as a fitting parameter as well.

3.2. Apparent viscosity

For viscous samples, the response to the cantilever force is com-

posed of an elastic and a viscous component (FT= Fel+ Fv), such thatthe work done by the cantilever is partially lost by internal friction,

generating an hysteresis in the approach/retraction cycle. The work

difference between approach and retraction (DWT= DWel+ -

DWv= W(app) W(ret)) can be calculated directly from the (z,d)curves as:

DWT kcZ z2z1

dapp

dret

dz: 2

Since DWel= 0, DWT= DWvandDWv is numerically equal to the en-

ergy lost by friction in the cell, and can be calculated by:

DWv

Z z2z1

Fappv Fret

v

dd: 3

Since the elastic force in Hertzian contact theory takes into account

the contact area between indenter and sample, we assume that the

viscous component should also consider the contact area and thus

can be modeled as Fv= gdA/dt, where A= pd2tan2h is the contact

area between the conical indenter and sample, and g is the apparentviscosity. The areaA changes over time due to (i) sample relaxation

and (ii) the movement of the cantilever with respect to the sample.

Assuming that the main contribution is due to the latter, one canmake the following simplification dA/dt= (dA/dz)(dz/dt) = vzdA/dz,

where vz is the velocity of the piezo movement. This approximation

leads to an analytical form ofDWvthat can be used to calculate the

apparent viscosity directly from (z,d) curves as:

g kc

ptan2hvz

Rz2z1d

app d

retdz

d22 d21

app d22 d

21

ret: 4

3.3. Adhesiveness

A typical force curve measured in a material exhibiting adhe-

siveness to the AFM tip is shown in Fig. 2. During cantilever retrac-

tion, the cantilever undergoes a negative deflection, until its out-pulling force exceeds the adhesion forces, losing contact with the

sample. The point at which the adhesion force between the probe

and the sample is maximum corresponds to the minimum deflec-

tion point (Fig. 2). Using these general characteristics, the adhe-

siveness of bitumen can be deduced qualitatively in two different

ways: (i) the maximum force of adhesion between the probe and

the sample, and (ii) calculating the work of adhesion forces. The

work is obtained by calculating the area of the deflection-displace-

ment curve whose deflection is negative, i.e., the portion corre-

sponding to the adhesion forces.

4. Results

4.1. Topography and phase images analysis

Fig. 3shows representative images (topography and phase in

tapping mode) of each sample studied in this work. Both unaged

Fig. 1. Schematics of the lateral force microscopy (LFM). The cantilever scans the

surface perpendicularly to its axis. The friction between the cantilever tip and

surface induces a torsion around the cantilever axis. The friction force is measured

by tracking the lateral movement of the reflected laser beam in the photodetector.

The larger the torsion, the larger is the lateral movement of the laser beam which

leads to larger voltage reading of the photodetector.

Fig. 2. Schematics of a typical AFM deflection-displacement curve obtained from

asphalt cements, exhibiting hysteresis between approach and retract curves, and

adhesiveness. The fitting of these curves between a given interval of deflection

[d1, d2] with Hertz model provides an estimate of the sample elasticity moduli E

[3032]. From thehysteresis, weemployedthe methodof Ref. [33] to determinethe

apparent viscosity. The strength of the adhesive forces is qualitatively determined

by means of thework done by thecantilever in theregionof the negative cantilever

deflection.

L.M. Rebelo et al. / Fuel 117 (2014) 1525 17

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and aged samples show that the AC surface is composed of do-

mains of asphaltenes (micelles) in a sea of hydrocarbons (maltene).

We did not observe the presence ofbee structures in our samplesbecause the amount of wax in the bitumen used in this work is

negligible[5].

The striking differences between unaged and aged samples are

the following. (i) The topography images of aged samples display

nearly the same features of the phase images, while in unaged

samples the topography image shows a nearly flat surface, exhib-

iting only small dark regions in the center of the asphaltene mi-celles. These dark spots are similar to the so called sal phase

present in some types of bitumen investigated by Masson et al.

[5]. In fact, those micelles can only be observed in phase images

for unaged ACs. (ii) The phase contrast between asphaltene do-

mains and the hydrocarbon sea is inverted between unaged and

aged samples. The brighter the image, the larger is the phase lag

difference between the input sinusoidal cantilever signal and its

response. The phase images in Fig. 3clearly shows that the phase

lag in the asphaltene domains is smaller compared to the hydro-

carbon sea in unaged samples, while this trend is inverted in aged

samples. Disregarding complicated adhesive effects, it is well ac-

cepted that regions displaying small phase angles in tapping mode

images exhibit enhanced storage moduli, while brighter regionsexhibit enhanced loss moduli. In that sense, considering that most

of the volume of the AC film is composed of hydrocarbon sea in

both unaged and aged samples, unaged samples should exhibit

Fig. 3. Topography (left panels) and phase images (right panels) of a unaged (ab), RTFOT aged (cd) and PAV aged (ef) AC films. The lateral dimensions of the images are

10lm 10 lm. The scale of topography images is 20 nm and for phase image is 10.


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whereN(L) is the number of squares of size L needed to cover the

image [38,37,36]. The box-counting dimension is quite similar to

Hausdorff dimension, which considers the number N(r) of balls of

radius rnedeed to cover the object. When ris small, N(r) is large,

i.e., as r approaches zero, 1/rd

is larger, where d is the Hausdorffdimension [38,37,39]. Both methods result in the same value for

many shapes, but there are some exceptions[38,37]. Fig. 6shows

reconstructed versions of Fig. 5(d) with both box-counting and

Hausdorff methods. In this particular case, we obtained a fractal

dimension of 1.9007 and 1.8738, respectively. Several other images

were reconstructed which leaded to Df values ranging between

[1.8271 1.9127] with both methods.

Several studies report that asphaltenes suffer precipitation andform fractal-like structures during phase separation process

[4042]. Using X-ray scattering at low angles (SAXS), several

authors suggest that clustered asphaltene structures in solvents

Fig. 5. Phase images of RTFOT aged AC. Images (A), (B), (C) and (D) have the respective lateral dimensions: 5, 10, 25 and 50lm. Image (D) exhibits an interface between a

region composed of micelles and a region composed of fractal structures.

Fig. 6. Reconstruction of phase image ofFig. 5(D) with both Hausdorff and box-counting methods. The fractal dimension of the micelle structure is 1.8738 and 1.9007,

respectively.


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(e.g. toluene and bitumen) are consistent with structures

exhibiting with fractal dimension Df 2 [4345]. Besides that,Raghunathan et al. determined the Hausdorff fractal dimension

of asphaltene polymers as ranging between 1.6 and 2.0 [46].

Fig. 7. Friction maps of (a) pure, (b) RTFOT aged AC, and (c) PAV aged AC. The vertical scale (in Volts) measures the cantilever torsion. All maps were measured with a lateral

scanning frequency of 8 Hz.

0.3 0.2 0.1 0 0.1 0.2 0.30

0.5

1

1.5

2x 10

5

COUNT

PURE

0.3 0.2 0.1 0 0.1 0.2 0.30

0.5

1

1.5

2x 10

5

COUNT

RTFOT

0.3 0.2 0.1 0 0.1 0.2 0.30

0.5

1

1.5

2

2.5x 10

5

COUNT

PAV

VOLTAGE (V)

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

PURE RTFOT PAV

negative peak

positive peak

VOLTAGE (V)

VOLTAGE (V)

(a) (b)

(c) (d)

Fig. 8. Histograms constructed from the friction images of (a) pure, (b) RTFOT aged AC, and (c) PAV aged AC. Those histograms were fitted with a double gaussian curves

(black lines). Ther2 values of each dataset are r2pure 0:999; r2RTFOT 0:987, andr

2PAV 0:996. (d) Average torsion of the cantilever (in Volts) in the maltene phase (blue) and in

the micelles (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


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Therefore we can conclude that our fractal analysis of aged ACs isin agreement with other results in the literature.

4.3. Friction analysis

Fig. 7shows representative friction images of pure, RTFOT and

PAV aged samples obtained with the lateral force mode of the

AFM. The images represent the raw AFM friction data without

any enhancement. The horizontal lines in the top region of the

maps are due to the fast lateral scanning which is necessary to

avoid adhesion of the tip. The friction maps show some similarities

with the phase images discussed previously: (i) the micelle struc-

tures are readily visible in this AFM mode, (ii) the size of the mi-

celles increase with aging, and (iii) the image contrast of the

pure AC is larger compared to the ones of aged samples. Sincethe contrast is the difference between the lowest and highest can-

tilever torsion (measured in Volts), we conclude that the friction

coefficient between the cantilever tip and sample is larger for un-

aged samples. The similar image contrast between aged samplesreveal that they have similar friction coefficients. The voltage scale

of the images span from negative to positive values, corresponding

to the torsion experienced by the cantilever which is recorded by

the photodetector on its left and right quadrants, respectively.

The more distant the value is from zero (untorsioned cantilever),

the highest the friction between tip and sample surface. Based on

the images ofFig. 7, we clearly see that positive voltages are asso-

ciated to the surface of the micelles, while negative voltages repre-

sent the surface of maltene phase.

In our friction analyses we produced 27 maps (nine maps for

each sample type). From those images we constructed friction his-

tograms to provide insights about the overall effect of aging on the

friction coefficient of ACs. These data are shown inFig. 8. The his-

tograms are composed of two broad peaks: a peak at negative volt-ages corresponding to the overall friction response of the maltene

phase, and a peak at positive voltages describing the friction distri-

bution of the micelles. The histogram of pure AC is composed of

0 0.2 0.4 0.6 0.8 1 1.20

50

100

150

200

COUNT

SLOPE

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.2 0.4 0.6 0.8 1 1.20

20

40

60

80

100

120

140

COUNT

SLOPE

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.2 0.4 0.6 0.8 1 1.20

50

100

150

200

250

COUNT

SLOPE

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

PURE AC10Hz

RTFOT AC

10Hz

PAV AC10Hz

(a) (b)

(c) (d)

(e) (f)

Fig. 9. Slope maps of pure (a), RTFOT (c) and PAV AC films (e). Their respective histograms are shown in (b), (d) and (f).


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two symmetric peaks and the cantilever torsion spans from0.3V

to + 0.3 V. The histogram of RTFOT also exhibit two symmetric

peaks, but the cantilever torsion spans from 0.2 V to +0.2 V. As

for the PAV histogram, the peaks are no longer symmetric, with

the peak at positive voltages being nearly twice as large as com-

pared to the negative peak. Besides that, the cantilever torsion

spans from0.2 V to +0.2 V. From these observations, it is possible

to conclude that the aging processes affect the (i) friction coeffi-

cient of the samples, and the (ii) relative composition of the mi-

celles and maltene phase. The first observation can be better

visualised by fitting all histograms with a double gaussian curve

and tracking the center of each peak. This is shown in Fig. 8(d).The average cantilever torsion in the micelles reduces from

+0.1 V in pure AC to +0.05 V in RTFOT aged AC, which represents

a 50% of reduction of the friction coefficient. Besides that, the

counts of data points remains nearly the same, meaning that the

size of the micelles grows only slightly in the short term aging.

From RTFOT to PAV, the cantilever torsion remains unchanged

(+0.05 V), but the count of data points increases strongly. This

means that the long term aging does not noticeably change the

friction coefficient, but strongly increases the percentage of the mi-

celles at the expense of a reduction in the maltene phase. As for the

evolution of the maltene phase with aging, the cantilever torsion

reduces from 0.1 V to 0.05 V in the short term aging, while

the long term aging induces a nearly zero torsion in the cantilever.

We remark that the presence of image artefacts due to fast scan-ning do not change the above analysis because their only effect

is an small increase in the mean width of the peaks in the histo-

grams ofFig. 8.

4.4. Microrheology analysis

The slope of the force curve is a dimensionless quantity that

provides quantitative information about the mechanical nature of

the sample. In our case, it indicates if the sample is more or less

deformable. A map representative of each sample and their respec-

tive slope histograms of approach curves is shown inFig. 9. These

histograms show mainly two peaks in their distributions. The left

peak corresponds to the slope on the AC surface. The right peak

centered around slope 1 (indicating an infinitely rigid surface)corresponds to the region of the substrate used for calibration.

The maps inFig. 9reveals that the aging process increases thestiffness of AC films. For example, the slope on top of the unaged

AC film ranges between 0.2 0.6, and its average slope is around

0.3. For the RTFOT aged AC, the main slope contribution ranges be-

tween 0.5 0.7, and its average slope is around 0.6. For the PAV

aged AC, the main slope contribution ranges between 0.5 0.8,

and its average slope is approximately 0.7. The maps also show

that films of unaged and PAV aged ACs exhibit a pretty uniform

slope distribution, while the RTFOT aged film exhibits slope inho-

mogeneities in the surface. This qualitatively agrees with the fact

that different AC components may contribute differently for the

macroscopic rheological properties of the material. We remark that

the micellar structures shown in Figs. 37 could not be spatially re-

solved in the slope maps because of the reduced resolution

(32 32) and low maximum force employed in the FVmeasurements.

We have also mapped the elasticity modulus E and apparentviscosityg over the AC films (three different locations in three dif-

10

9

8

7

6

5

log10

[E(P

a)]

4 6 8

1

2 4 6 8

10

2 4

FREQUENCY (Hz)

PURE AC

PAV AC

RTFOT AC

8

7

6

5

4

log10

[(P

a.s

)]

4 6 8

1

2 4 6 8

10

2 4

FREQUENCY (Hz)

PURE AC

PAV AC

RTFOT AC

5

4

3

2

1

0

ADHESIONWORK

(fJ)

4 6 8

1

2 4 6 8

10

2 4

FREQUENCY (Hz)

PURE AC

PAV AC

RTFOT AC

60

50

40

30

20

10

ADHESIONFMIN(nN)

4 6 8

1

2 4 6 8

10

2 4

FREQUENCY (Hz)

PURE AC

PAV AC

RTFOT AC

(a) (b)

(c) (d)

Fig. 10. Average elasticity modulus E(a) and apparent viscosity g (b) as a function of the vertical scan frequency (which is proportional to the indentation speed) of unagedand aged AC films. The dispersion bars were determining by averaging nine maps (with resolution of 32 32 data points) for each sample. Average adhesiveness of AC films

estimated with two different methods: (c) work done by the adhesive forces, and (d) minimum adhesive force.


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