URE 6 Bond lowing point
T- PEEL
2. SAMPLE PREPARATION AND PROBE SELECTION
There is no doubt that a good sample and a suitable probe are necessary for a good AFM image. Considering the characteristic of the bitumen material, this passage aims at talking about the sample preparation technique and probe selection principle Previous research has shown that surface of the sample should be smooth with no contamination and have a sufficient thickness.
The AH-70 virgin bitumen and SBS-bitumen used in this study was obtained from Shell Company China. Corresponding physical properties and the test specification are presented in Table 1.
Table 1 characteristics of virgin bitumen
Performances AH-70# SBS-bitumen Test specification
Penetration（25°C 0.1mm）
68 55 ASTM D 5-61
Softening point（°C） 47 83 ASTM D 26-36
Viscosity（135°C, Pa.s） 0.45 2.5 ASTM D 4402
2.1 Sample preparation
At present, conventional sample preparation methods are solvent method and hot drops method on glass or other substrates. However, these methods exist a large amount of imperfections, this paper proposes a new method of sample preparation on the basis of analysis of the existing methods.
2.1.1 Sample prepared by solvent method
The preparation of thin films of bitumen for testing with the AFM followed procedures by Vasconcelos, namely that thin films of the bitumen are cast over a glass microscope slide using a spin coater. Solvent method was used to prepare AFM samples as follows:
1) bitumen dissolved by toluene、trichloroethylene or other solvent 2) application of solution onto clean glass slide or other substrates 3) covered glass slide in sealed chamber
4) casting
5) releasing vacuum seal after casting 6) thin-film asphalt specimen
Sample prepared by solvent method maybe has a good flatness, but the process of spin casting is very complex. In addition, the effect of the solvent and the
concentration to bitumen is uncertain. The study from Southeast university shows that the trichloroethylene maybe damage the chemical structure of the bitumen and change the molecular structure, the image cannot reflect the true morphology of the bitumen.
Fig. 2(a) presented the sample by solvent (toluene) method, and the AFM images were presented (see Fig. 2(b)).
(a)Sample prepared by toluene (b) AFM images (Allen 2010) Figure.2 the sample by solvent and AFM images
According to early researchers report (Loeber et al. 1996), bee-structure appeared in bitumen at the nanoscale regardless of the type of bitumen. Although Fig.
2(b) presented a few bee-structures, some researchers presented nothing and no bee-structure appeared.in bitumen. Maybe, different solvent could have different effects on the microstructure of the bitumen, large or small. Further more research is needed in the future. From this point of view, sample prepared by solvent method is not recommended in this paper.
2.1.2 Sample prepared by hot drops method
Hot drops method was also used to prepare the AFM samples. Firstly, 15-30 mg of bitumen sample was applied to the AFM sample substrate (steel sample disks or the glass slide) with a spatula. Secondly, the sample subsequently heated a period of time at a determined temperature in order to create a thin flat film. Then the samples were thermally conditioned inside an oven followed by cooling under ambient conditions. Fig.3 presented the sample by hot drops method.
Figure.3 sample prepared by hot drops method
Compared to solvent method, hot drops method was much simple and was applied by most AFM researchers. However, the problem is that the shape and the thickness of the sample were not the same, as shown in the Fig.3. AFM sample should be enough thickness in order to avoid the substrate, effect on the microstructure of the bitumen. Take this into consideration, the sample method needed to be improved.
2.1.3 Sample prepared by molding method
To meet the requirements of AFM observation and obtain homogenous sample with enough thickness and no surface contamination, the paper developed a molding method, namely bitumen sample forming in the mold (diameter 10-20 mm and height 2-3 mm). Fig.4 presented the AFM sample by molding method and the mold with cover can avoid the surface contamination and protect the sample surface.
Figure.4 sample prepared by mold tryout
The AFM image of Base bitumen and SBS modified bitumen were presented in Fig. 5 and Fig. 6.
Base bitumen SBS modified bitumen
Figure.5 Two-dimensional AFM images
Base bitumen SBS modified bitumen Figure.6 Three-dimensional AFM images
“Bee-shaped “structure appeared in both base bitumen and the SBS modified bitumen, the test results was similar to Lobber (Loeber et al. 1996) and Wu research.
Therefore, the sample by molding method can guarantee the reproducibility and stability, the image can reflect the true microstructure of the bitumen at the nanoscale.
2.2 Working mode and Probe selection
AFM is a scanning probe technology that allows reveal surface topography and heterogeneity of materials with high spatial resolution. In AFM imaging (see Fig.7), a cantilever with an extremely sharp tip located on its free end is scanned over the
sample surface utilizing a piezoelectric scanner (Fischer et al. 2012).The changes in tip-sample interactions result in deflection of the cantilever which is measured by an optical-lever detection system. In this system a laser beam is focused onto the back side of the cantilever and the reflected beam is detected with a position sensitive photodiode. While scanning, a specific operating parameter is kept constant by a feedback loop between the optical detection system and the piezoelectric scanners.
Measurements are being recorded electronically. The data acquired build up a map of the surface topography, which is representative of variations in the tip sample interaction.
Figure.7 Atomic Force Microscope System
It has been known that AFM has three working modes, namely contact mode、
tapping-mode and non-contact mode. Contact mode is a primary AFM mode, tip and sample are always in perpetual contacting during the raster-scan. The disadvantage is that combination of lateral forces and high normal forces can result in reduced spatial resolution and may damage soft samples due to scraping between the tip and sample.
However, because of the limited to resolution, non-contact mode are now rarely use Compared with contact mode and non-contact mode in air, tapping-mode can achieve higher lateral resolution on most samples (1 nm to 5 nm) and has lower forces and less damage to soft samples imaged in air. Bitumen is a soft material with viscoelastic nature, therefore, tapping mode is recommended for the experiment.
In addition to working mode, probe selection is vital for a good AFM image, especially for bitumen material. Basic principles are presented in Table 2 according to the modulus of the material. The modulus of bitumen is always smaller than 100 MPa, so, a nominal spring constant of AFM probe that was chosen should be less than 5 N/m at ambient temperature. Besides, in order to operate easily, the cantilever material is always antimony (n) doped silicon which has a 40 ± 10 nm aluminium reflective coating on its backside.
Table 2 Basic principles of probe selection
Sample Modulus（E） Probe Nominal Spring
Constant（k）
1MPa＜E＜20 MPa Scan Asyst 0.5 N/m
5MPa＜E＜500 MPa Tap150A, P/N MPP-12120-10
5 N /m 200MPa＜E＜2000 MPa Tap300A (RTESPA),
P/N MPP-11120-10
40 N/m
1GPa＜E＜20 GPa Tap525A,P/N
MPP-13120-10
200 N/m
10GPa＜E＜100 GPa DNISP-HS 350 N/m