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Cryo-Scanning Electron Microscopy: A New Tool for Interpretation of Fracture Studies in Bitumen/Polymer Blends L. Champion-Lapalu,† A. Wilson,‡ G. Fuchs,*,§ D. Martin,† and J.-P. Planche† TotalFinaElf, Centre de Recherche, BP 22, 69360 Solaize, France, CCTR, Department of Biology, University of York, P.O. Box 373, York, U.K. YO10 5YW, and CRRA, Atofina, Centre de Recherche Rhoˆ ne-Alpes, BP 63, 69493 Pierre-Be´ nite Cedex, France Received June 11, 2001. Revised Manuscript Received September 26, 2001
The fracture morphology of bitumen/polymer blends has been characterized using cryo-scanning electron microscopy. This low-temperature technique has extended observations using environmental electron scanning electron microscopy (ESEM) and fluorescence microscopy to a much higher resolving power. Polymer dispersion has been imaged at a submicrometer scale. For each type of bitumen/polymer blend, the characterization of the distribution of the polymer nanoparticles across the fracture surface provides greater understanding of the crack propagation mechanism controlling the fracture properties.
Introduction Polymer additives are well-known to improve the rheological properties of bitumen. The polymer addition allows an increase in the resistance of the binder to permanent deformation at high temperature.1,2 Besides, the fracture properties including critical stress intensity factor (K1C) at low temperature of polymer modified bitumens (PmB’s) were shown to be higher than those of the bitumen base.3,4 To determine the crack propagation mechanism controlling fracture properties, previous studies have focused on establishing the relationship between the fracture properties and the morphology of polymer modified bitumens.4,5 Optical fluorescence microscopy is commonly used to evaluate structure and homogeneity of PmB’s.6,7 Other techniques such as environmental scanning electron microscopy (ESEM), confocal laser scanning microscopy (CLSM),4 and infrared microscopy8 have been used recently with the same objective. In this paper we concentrate on cryo-scanning electron microscopy applied to the observation of PmB’s fracture morphology. * Author to whom all correspondence should be addressed. † TotalFinaElf, Centre de Recherche. ‡ CCTR, Department of Biology, University of York. § CRRA, Atofina, Centre de Recherche Rho ˆ ne-Alpes. (1) Bonnemazzi, F.; Braga ,V.; Corrieri, R.; Giavarini, C.; Sartori, F. Eurasphalt & Eurobitume Congress, Strasbourg, 1996; 17 pp. (2) Shiau, J. M.; Su-Haw, L.; Shu-Jau, L. 5th Symposium RILEM, 1997; pp 129-136. (3) Ponniah, J. E.; Cullen, R. A.; Hesp, S. A. M. American Chemical Society, Orlando, 1996, 41, 1317-1321. (4) Champion, L.; Ge´rard, J.-F.; Planche, J.-P.; Martin, D.; Anderson, D. Eurobitume Workshop ProceedingssLuxembourg, 1999. (5) Champion, L. Ph.D. Thesis. INSA-Lyon University, 1999. (6) Brule´, B.; Brion, Y. Bull. Liaison Lab. Ponts Chausse´ es 1986, 154, 45-52. (7) Dony, A.; Durrieu, F. Bull. Liaison Lab. Ponts Chausse´ es 1990, 158, 57-63. (8) Mouillet, V.; Kister, J.; Martin, D.; Planche, P.; Scramoncin, C.; Saury, C. Bull. Liaison Lab. Ponts Chausse´ es 1999, 220, 13-19.
Electron microscopy has contributed considerably to the development of theories concerning the structure and research of components of these blends on the basis of optical microscopy and applied research results.9-11 Microstructural characterization of bitumen blends using conventional ambient temperature scanning electron microscopy (SEM) has always proved to be of limited success because of the volatility of the bitumen in the vacuum of the microscope observation chamber, its extreme susceptibility to electron beam damage and the ensuing charging of the specimen surface.12 These problems have restricted the resolving power available for probing bitumen microstructure and have thereby limited the magnification. Cryo-preparation equipment for conventional SEM has been available for over two decades and has been applied widely to both biomedical and materials science.13 This innovation has enabled unique SEM observation of rapidly cooled soft or liquid specimens at low-to-medium magnification (50-50000×). Only recently, however, has the coincidental development of high-resolution field emission gun (FEG) scanning electron microscopy, coupled with commercially available low-contamination, high-vacuum cryo-preparation equipment, enabled much higher resolution observation of such difficult and labile specimens. Monitoring of polymer distribution in the final PmB (9) Bukowsha, M. Polimery 1994, 39 (6), 339-344. (10) Bukowsha, M. Polimery 1983, 28 (12), 425-427. (11) Oba, K.; Portl, M. N. Durably Building Mater. Compon.7, Proc. Int. Conf., 7th; Sjoestroem, C., Ed.; Chapman & Hall: New York, 1996. (12) Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Romig, A. D.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning Electron Microscopy and X-ray MicroanalysissA Text for Biologists, Materials Scientists, and Geologists, 2nd ed.; Plenum Press: New York, 1992. (13) Robards, A. W.; Wilson, A. J. Procedure in Electron Microscopy; Robards, A. W., Wilson, A. J, Eds.; John Wiley & Sons: New York, 1998.
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product has now been possible.14 In addition, the physical effects of a particular in-situ chemical process have also been monitored using these techniques. The modified bitumens observed were found to be very electron beam sensitive, and prolonged irradiation gave rise to small cracks in the specimen surface.14 Accordingly, wherever possible, image focusing was performed on an area well remote from the field of interest.
The results presented here deal with some of the BmP’s studied in the first part of our study.5,14 The following samples were investigated: [1] a base nonmodified bitumen (Arabian crude, asphaltenes (9.4%), saturates (9%), aromatics (67.8%), resins (13.7%)), normally used for polymer modification; [2] a physical blend SB made at a lab scale for this study consisting of a base bitumen [1] mixed with 5% of a commercial grade styrene-butadiene (SB) diblock copolymer;
[3] a chemical blend (Styrelf) consisting of the previous physical blend [2] further reacted in situ through a proprietary process;17 the cross-linking reaction induces a finer dispersion of the polymer phase in the bitumen matrix and promotes a better storage stability of the blend and higher elastic properties at both high and low temperature, compare to the physical blend; [4] physical blend SBS consisting of base bitumen [1] + 4% styrene-butadiene-styrene copolymer (SBS); [5] physical blend EVA consisting of base bitumen [1] + 6% ethyl-vinyl-acetate copolymer (EVA). Specimen Preparation. The specimen preparation procedure is presented in Figure 1. After the fracture measurements at -20 °C (see fracture data section), the bitumen samples were kept at low temperature in dry ice. The selected fracture surfaces were subsequently mounted on a pre-cooled cryo-SEM shuttle. The shuttle with the sample was then plunged into a preformed pool of sub-cooled nitrogen14 at -202 °C and transferred into the preparation chamber of a VG Microtech LT 7400 Cryo-Prep module (vacuum ) 10-4 Pa). (Sub-cooled nitrogen is a slush of liquid and solid nitrogen which is readily made by lowering the temperature of liquid nitrogen (-196 °C) to below its boiling point, by placing it under a rotary pump vacuum and sub-cooling it (t ) -202 °C). Because the slush thus formed is below its boiling point, subcooled nitrogen has superior heat-transfer characteristics than the liquid and thus as a cryogen its cooling rate is far faster.) Ice which had formed on the fracture surface was removed by sublimation at -90 °C for 1 min. The specimen was subsequently coated with 4 nm of Pt, at a temperature of -160 °C, from a sputter head using ultrapure argon gas. Finally, the shuttle was transferred to a cold stage at -160 °C of a Philips XL40 FEG-SEM. A VG Microtech anti-contaminator cold shroud (T ) -190 °C) was located about 4 mm above the specimen. The temperature difference between the cold-shroud and the specimen fracture surface (∆t ) -30 °C) ensured minimum deposition of contaminating molecules onto the observation surface. All the observations were performed at -160 °C using an accelerating voltage between 2 and 5 kV. To limit contamination and creep of the original fracture surface morphology, the temperature of the sample was maintained as low as possible during the sample preparation and the time between the fracture measurement and cryo-SEM observations was restricted to less than 12 h. This ensured good reproducibility of sampling and subsequent analysis. During the sample preparation, the low temperature (lower than -90 °C) and the SEM preparation time (15 min) ensures that creep effect does not drastically affect the morphology of the fracture surface obtained at -20 °C. Nevertheless, shrinkage effects may occur and lead to a partial decohesion between bitumen and polymer phases but without any change on the global morphology of the fracture surface. Fracture Data. The fracture test was carried out according to a three point bending beam test method based on the ASTM E399-83 procedure. The specimen was loaded at a crosshead speed of 0.6 mm min-1 until the fracture crack propagated all the way through it. The load was recorded to determine the critical stress intensity factor, KIC. The fracture properties measured at -20 °C for some of the products studied in this paper have been previously published.18 We report here some of these data (Table 3). We have shown that SBS modified physical blend displays higher fracture properties than the EVA copolymer modified blend and the fracture toughness of the EVA blend is slightly higher than the pure bitumen.18
(14) Wilson, A. J; Fuchs, G.; Scramoncin, C.; Martin, D.; Planche, J.-P. Energy and Fuels 2000, 14 (3), 575-584. (15) Brion, Y.; Brule´, B. Rapport LCPC PC-6, 1986; 123 pp. (16) Kraus, G. Rubber Chem. Technol. 1982, 55, 1389-1402.
(17) U.S. Patent 4,145,322, and Fr. Patent 7,639,233. (18) Champion-Lapalu, L.; Planche, J.-P.; Martin, D.; Anderson, D.; Ge´rard, J.-F. Eurasphalt and Eurobitume Congress, Barcelone 2000, Technical session 1, p.
Experimental Section Samples Investigated. An important class of polymers used in the formulation of bitumen/polymer blends are styrenebutadiene copolymers. They can be introduced by mixing at 180-200 °C (“physical blend”) or chemically reacted in situ with the bitumen (such as in the Styrelf process). When a polymer compatible with bitumen is mixed with bitumen, the maltenes swell the polymer phase. At this step, we have a two-phase material: a polymer-rich phase swelled by light fractions of the bitumen and a bitumen-rich phase with a lower concentration of light fractions. In the case of the styrene-butadiene copolymer, the swelling may be selective. It has been reported that, in bitumen, swelling of butadiene blocks is more efficient than that in styrene.15 This leads to the promotion of the formation of styrene-rich micelles.16 These products previously studied by cryo-scanning electron microscopy14 have amply demonstrated the power of such microscopical techniques. The neat bitumen used as a base for the modified binders was a 70/100-penetration grade (85 1/10 mm penetration at 25 °C; 46 °C ring-and-ball softening point) obtained from a TotalFinaElf refinery. Its glass transition temperature is -27 °C and the crystallized fractions content is 4.5% as measured by differential scanning calorimetry. Two different types of copolymers were used for modifying the bitumen: (i) a semicrystalline copolymers: ethylene-vinyl acetate copolymer (EVA); (ii) two amorphous copolymers: a diblock styrene-butadiene copolymer (SB) and a radial triblock styrene-butadiene copolymer (SBS). The copolymer characteristics are reported in Tables 1 and 2. Table 1: Physical Properties of the Semi-crystalline Copolymer
copolymer
fraction of the comonomer (wt %)
T°g (°C)
crystallinity rate (%)
EVA
28.4
-20
15.3
Table 2: Physical Properties of the Styrene-Butadiene Copolymers
copolymer
fraction of the polystyrene (wt %)
Tg (PB block/PS block) (°C)
M hw (dalton)
SBS SB
30 12
-90/62 -88/66
135 000 125 000
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Figure 1. Schematic sample preparation of bitumen samples for cryo-scanning (FEG) electron microscopy. Table 3: KIC Values for Different Polymer Modified Bitumens Measured at -20 °C
material
amount of polymer (%)
KIC (kPa m1/2)
Neat bitumen Bitumen/EVA Bitumen/SBS Styrelf (chemical blend)
0 6 4 4
48 ( 9 74 ( 20 107 ( 11 113 ( 21
ResultssDiscussion 1. Base Nonmodified Bitumen Matrix. The fracture surface of the nonmodified bitumen matrix material is generally smooth, homogeneous and featureless.14 2. SB and SBS* Physical Blends. Cryo-scanning electron microscopy of the SB and SBS* physical blends demonstrated that nodules of polymer were evident at the fracture surface. The nodules were scattered throughout the bitumen matrix and their diameter was variable by tens of micrometers (Figure 2 arrows). For the SB physical blend, the bigger nodules are surrounded by a well-defined line indicating that the nodules had been embedded in the surrounding bitumen matrix (Figure 3 arrow). At the test temperature (-20 °C), the bitumen matrix is in the glassy state and the polymer-rich phase is in a more rubbery state. So the crack propagates easily in the bitumen matrix leading to smooth fracture surfaces. When the crack reaches the polymer-rich phase, the propagation is more difficult, creating rougher texture
Figure 2. Evidence of nodules (arrows) in the SB physical blend at low magnification.
of the surface. We assume that the well-defined surrounding of the nodules indicates the interface between these two different fracture behavior regions. While the presence of nodules was first suspected by CLSM and ESEM,4,5 the detailed characterization of the polymer nanoparticles has been achieved by application of high-resolution cryo-preparation for FEG/SEM.14 A complex honeycomb structure of the polymer nodules is revealed: the walls of the honeycomb are composed of tiny nanoparticles of polymer, only tens of nanometers in diameter (Figure 4). (The high contrast of polymer nanoparticles may be surprising. The difference in secondary electron yield between polymer and bitumen
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Figure 3. Honeycomb structure of a polymer nodule in SB physical blend sample. Note also the line surrounding the polymer nodule.
Figure 5. Evidence of nodules in the SBS physical blend at low (a) and high magnification (b). Figure 4. As Figure 3. High magnification detail of the nodule surface morphology.
cannot be attributed solely to differences in composition or density (bitumen, polymer). On images presenting the highest contrast between polymer and bitumen, the polymer nanoparticles may be slightly enrobed with a very thin layer of ice coming from the water residual pressure in the microscope.) These observations are in agreement with the structure of the polymer nodules as determined by cryo-scanning electron microscopy.14 Since swelling of butadiene blocks is more efficient that styrene, one may evoke that the spherical micelles of tens of nanometers may correspond to polystyrenerich phase. As previously mentioned,14 the high contrast of these colloids may also be attributed to the formation of ice nanoparticules on the fracture surface during the preparation. Cryo-FEG/SEM images indicate that the fracture follows the interface between the polymer phase and the bitumen matrix. The fracture mechanism involved in this type of composite material is called “crack deviation”. The crack bypasses the polymer-rich nodules and the rupture occurs at the interface, meaning that the adhesion between the two phases is weak. For the SBS* physical blend, cryo-FEG/SEM shows that the polymer nodules are plastically deformed before being pulled-out (Figure 5). This mechanism contributes to the improvement of the fracture properties and can be explained by a better adhesion between the two phases of the blend compared to the SB-based blend.
Figure 6. Chemical blend fracture morphology demonstrating even distribution of nanoparticles throughout the bitumen matrix.
3. Chemical Blend. The large polymer nodules which characterize the physical blend sample were almost totally absent in the reacted modified bitumen. The polymer was found exclusively in the form of 50100 nm nanoparticles which were distributed homogeneously throughout the nonmodified bitumen matrix (Figures 6 and 7). 4. Physical Blend EVA. Cryo-scanning electron microscopy images of the EVA physical blend show that polymer-rich nodules are dispersed in the bitumen-rich matrix (Figures 8 and 9).
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Figure 7. As Figure 6. Higher magnification information showing the plastic deformation of the polymer nanoparticles.
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Figure 9. Characterization of the “nodules” surface morphology in the EVA physical blend.
Conclusion and Perspectives
Figure 8. “Nodules” in the EVA physical blend.
As previously mentioned (sub-section 2), the polymerrich nodules are well defined by a line on the fracture surface and more defined features appear within this inner area. This line may correspond to the interface between the glassy phase (bitumen) and the rubbery polymer-rich phase. The fracture faces of the EVA binder exhibit particle pull-out with a slight plastic deformation in the bitumen matrix. For this blend, the crack by-passes the polymer domains and the fracture mainly occurs at the interface between the two phases. As a consequence, the fracture mechanism is governed mainly by the poor adhesion between the polymer-rich domains and the bitumen matrix.
Reproducible high-resolution images of polymer particles within fracture faces morphology and information on their distribution within polymer modified bitumens have been obtained. This was due to the enhanced gun/ electron beam brightness of the SEM field emission source and the fidelity of the cryo-preparation equipment employed. The distribution of the polymer nanoparticles in the fracture surface has been determined at a submicrometer scale for both physical and chemical blends using cryo-SEM. The fracture morphologies characterization for various types of BmP’s lead us to propose, for each case, a fracture mechanism. For the SBS* physical blend, the polymer nodules are plastically deformed before being pulled-out. This mechanism contributes to improve the fracture properties and can be explained by a better adhesion and compatibility between the two phases of the blend compared to the SB-based blend. For the EVA blend, the crack by-passes the polymer domains and the fracture occurs at the interface between the two phases. As a consequence, the fracture mechanism is governed mainly by the poor adhesion between the polymer-rich domains and the bitumen matrix. EF010122S
