reSolution_LNT_No1_en - Leica Microsystems

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INDUSTRY Fig. 1 a: Stretching of impact Copolymer in neutral position Fig. 1 b: Stretching of impact Copolymer in stretched position 18 reSOLUTION Fig. 1 c: Stretching of impact Copolymer in overnight after stretching A tensile stress is then exerted on the dogbone and the material imaged under tensile stress with AFM (MFP-3D Asylum Research). The dogbone was cryotomed in the Leica EM UC6 / FC6 system with a custom-built hemispherical accessory onto which the dogbone is glued allowing for the dogbone mid section to protrude out towards the knife. Cryotoming conditions were at -60C, a diamond knife speed of 0.3-0.6 mm/s and feed of 100-250 nm. Shown in Figure 1a is an AFM tapping mode phase image with approximately dimensions of 5 um x 5 um of a cryotomed impact copolymer. The commercial impact copolymer used for this study is composed of a polypropylene (PP) matrix with micron-sized domains of ethylene-propylene (EP) rubber domains, which further contain ethylene inclusions produced in a serial polymerization reactor. In the image, the PP matrix is observed as the surrounding purple medium, and the EP rubber domain as the large round bright yellow domain in the middle. Within this EP rubber domain there is a further smaller purple inclusion which is composed of ethylene. The color contrast in this phase image is due to a convolution of various mechanical properties where the EP rubber is softer than the surrounding stiffer PP matrix.
Fig. 2 a,b: Large scale image of a topography... and b: phase of a crack in polypropylene matrix Fig. 3 a,b: High resolution image of a topography... and b: of a crack in polypropylene matrix In Figure 1b, this impact copolymer was elongated in the direction of the black arrow by 1.7% (this elongation length is below the yield strain of PP) resulting in the AFM image shown in Figure 1b where the same rubber component is tracked from Figure 1a. Several new features are visible in this new image. First, rips and tears that were present within the rubber in Figure 1a (circled in blue), have now grown and elongated in Figure 1b (also circled in blue). Second, stretch marks (circled in red) between the rubber and the polypropylene matrix have developed at the north and south poles of the rubber domain, indicating a mismatch in Poisson ratios between the EP and PP materials. If the EP domain is stretched mainly along the equatorial line as in the experiment conducted here, then stretch marks would develop mainly at top and bottom of the EP rubber domain as observed in the AFM image in Figure 1b. INDUSTRY NANOTECHNOLOGY 19
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