Type of presentation: Oral

MS-6-O-2276 In situ characterization of polymers in the ESEM

Poelt P.¹, Nachtnebel M.¹, Zankel A.¹

¹Institute for Electron Microscopy and Nanoanalysis, Graz University of Technology, Graz, Austria

peter.poelt@felmi-zfe.at

The fracture behaviour of polymers is, amongst others, strongly dependent on their microstructure and especially also the type, size and distribution of modifier particles. But the conventional stress-strain diagrams resulting from tensile tests are global quantities, integrated over all microscopic processes occurring during the test in the fracture zone. Therefore a direct observation of these processes during the tensile test is impossible.
Yet tensile tests performed in an environmental scanning electron microscope (ESEM) enable the simultaneous recording of both the stress-strain diagram and the crack propagation at the crack tip [1, 2]. Whereas the former is a macroscopic measurement, the microstructures developing at the crack tip and their variation during the tensile test provide direct insight into the impact of the microstructure of the polymer on the fracture behaviour. Because strong stress concentration takes place at the crack tip, structures forming there and processes going on there are decisive for the fracture behaviour of the material.
In case of inorganic filler particles also the local strain distribution at the surface of the specimen and its change during the tensile test can be tracked (Figures 1 and 2). For this purpose generally the skin of the polymer has to be removed. Care has to be taken that no pre-cracks are formed during the polishing. Every pair of particles can be regarded as a micro-extensiometer. As a consequence the resolution of measured strain fields depends on the distances between the filler particles.
But what is happening at the crack tip does not provide full information about the fracture behaviour of the polymer. Especially the distribution of the cracks and their correlation to the distribution of the filler particles is very interesting. To extract this information the full 3D reconstruction of at least part of the sample is necessary. For this aim the tensile test has to be stopped at a predefined force or elongation. Subsequently automated serial sectioning and imaging by use of a microtome mounted in the ESEM can be used [3]. Finally from the resulting stack of images the 3D reconstruction is possible (Figure 3).
Thus a great wealth of information both on the micro- and macroscale can be gained by performing tests in the ESEM. Correlation of all these results should provide greater insight into the fracture behaviour of polymers.

References:
[1] P. Poelt, A. Zankel, M. Gahleitner, H. Herbst, E. Ingolic, C. Grein, Proc. PPS 24, Salerno, Italy (2008).
[2] P. Poelt, A. Zankel, M. Gahleitner, E. Ingolic, C. Grein, Polymer 51, 3203 (2010).
[3] W. Denk, H. Horstmann, PLoS Biol. 2(11), e329 (2004).

The authors want to thank the company BOREALIS for providing the specimens and for support in the discussion of the results.

Fig. 1: ESEM image (low vacuum mode) recorded during a tensile test (v = 0.2 mm/min) of polypropylene modified with glass spheres (image width: 679 µm). The three bright lines mark the distances between particles used to track the change in the local elongation during the test.	Fig. 2: The local elongation (as determined by the particles marked in Figure 1 during a tensile test) as a function of the overall elongation (specimen length: 42 mm).
Fig. 3: 3D representation of part of a crack in EPR (ethylene propylene rubber) modified polypropylene after a tensile test (v = 1 mm/min) stopped at 25% yield (axis labels: µm). The sample got stained with RuO4. Close to the centre and at the top edges EPR particles can be seen.