Microscopy 101
Figure 5: Optical micrographs of PEN taken at 18% strain, showing crack density at saturation for ITO/ PEN samples at room temperature, 23°C (a) and at 150°C (b). The micrographs were taken with a green filter to emphasize contrast, and the loading direction was parallel to the scale bar in the micrographs. Sequential still images were captured at various temperatures between 23°C and 250°C using a Linkam TST350 tensile stage with an Olympus BX60 microscope, and CCD camera.
measurements to be taken from recorded images, providing information on changing size, shape, number and location of particles, and points of interest. Tese data can then be corre- lated with changing environmental parameters such as tempera- ture, humidity, vacuum, and pressure, as well as other measured parameters such as tensile or compressive force from Linkam’s MFS tensile stage and shear forces from the CSS450 shear stage. Combining these imaging tools with one of Linkam’s tem-
perature control stages provides a powerful sample character- ization platform across a broad range of applications, including geology, materials science, pharmaceuticals, and semiconduc- tor research.
In Practice With an optimized instrument setup in place, there are
many applications where dynamic imaging can add insight. Four examples of
below: 1. Flexible polymer thin films. Flexible
imaging involving samples are outlined electronic
devices are becoming more widely used, with examples such as foldable phones and rollable LCD panels already commer- cially available, and wearable flexible devices for sensing and energy collection potentially available in the near future. In recent years, a great deal of research has been dedicated to the study of the mechanical failure of brittle thin films on a polymer substrate typically used for such devices. Tensile testing is one method that can be used to reveal the film properties. In recent work researchers at École Polytechnique Fédérale de Laus- anne in Switzerland were able to stretch thin polyethylene naphthalate (PEN) films, adhered to indium tin oxide (ITO) coatings, to strains of 120% of their origi- nal length and temperatures up to 150°C while observing their suitability as flexible semiconductors [3]. Tis allowed simu- lation of the environmental factors that these films should be able to withstand throughout the life of the flexible devices
2020 November •
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in which they will be used. By imaging the samples under strain at an elevated temperature (Figure 5), the adhesion of PEN to the ITO coating was found to be dependent on temperature due to a soſten- ing of the polymer substrate. Tey quan- tified the crack density—indicative of the material failure—using the images taken during the experiment. Te fact that the material was more flexible but still func- tional at 150°C bodes well for its use, but as temperature increased, thermal expan- sion increased resistance and resulted in greater crack density as seen in Figure 5b. 2. Stress tests. Researchers in the Poly-
mer Technology Group at the University of Salerno captured images of isotactic poly- propylene while the material underwent
shear stress in order to study the phenomenon of flow-induced crystallization [4]. Tis is an area of research that has attracted much interest because of an ability to control the morphology of these polymers and inform improved production processes like injection molding and extrusion. A commercial-grade i-PP resin (T30 G from Montell Ferr-
ara, Italy) was used for the experiments, and the observation of morphology and measurement of nucleation density was made with a polarized light microscope (Olympus BH-41) and a shear cell (Linkam CSS450). Images were captured with a color CCD camera. Images from these video sequences (Figure 6) allowed, for the first time, the group to quantitatively determine the crystal nucleation rate under different shear conditions and temperatures. Tey found that nucleation improved under continuous shearing and increased as a function of shear rate. Furthermore, they observed that nucleation density decreases on increasing the crystallization temperature. 3. Cryo-preservation. Red blood cells (RBCs) are an
essential lifesaving resource. Tey are transfused into patients to increase RBC numbers because of acute blood loss, decreased RBC production in bone marrow, or decreased RBC survival, which places an individual at risk. Te normal preservation strategies for collection and storage of RBCs have enabled blood banks to ensure a readily available and safe blood supply.
Figure 6: Micrographs of isotactic polypropylene (iPP) collected during the crystalization test under shear conditions at 140°C and shear rate of 0.11 s−1
at different shearing times. Images were captured with a color CCD camera on a shear cell (Linkam CSS450) under a polarized light microscope (Olympus BH-41). 47
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