Microscopy techniques for studying polymer-polymer blends

Abstract: Semiconductors are used in many electronic applications, for example diodes, solar cells and transistors. Typically, semiconductors are inorganic materials, such as silicon and gallium arsenide, but lately more research and development has been devoted to organic semiconductors, for example semiconducting polymers. One of the reasons is that polymers can be customized, to a greater extent than inorganic semiconductors, to create a material with desired properties. Often, two polymers are blended to obtain the desired function, but two polymers do not usually result in an even blend. Instead they tend to separate from each other to varying degrees. The morphology of the blend affects the material properties, for example how efficiently it can convert electricity to light. In this project, thin films consisting of polymer blends were examined using microscopy techniques for the purpose of increasing our understanding of the morphology of such blends. One goal was to investigate whether a technique called correlative light and electron microscopy can be useful for examining the morphology of these films. In correlative light and electron microscopy, a light microscope and an electron microscope are used in the same location in order to be able to correlate the information from the two microscopes. The second goal was to learn about the morphology of the thin films using various microscopy techniques. The polymers used were Super Yellow and poly(ethylene oxide) with large molecular weight. Super Yellow is a semiconducting and light-emitting polymer while poly(ethylene oxide) is an isolating and non-emitting polymer that can crystallize. In the blend films, large, seemingly crystalline structures appeared. The structures could be up to 1 mm in the lateral direction, while the films were only approximately 170 nm thick. These structures could grow after the films had dried and their shapes were similar to those of poly(ethylene oxide) crystals. Consequently, there is reason to believe that it is the poly(ethylene oxide) that makes up the seemingly crystalline structures, but the structures also emitted more light than the rest of the film, and Raman spectroscopy showed that there was Super Yellow in the same location as the crystals. Among the microscopy techniques used, phase contrast microscopy was particularly interesting. This method visualizes differences in optical path length and was useful for studying polymer blends when the polymers have different indices of refraction. Correlating light and electron microscopy showed that there was a pronounced topographical difference between the seemingly crystalline regions and the rest of the thin film. Light microscopy has a limited resolution due to diffraction, but as long as the resolution of the light microscope is sufficient for seeing phase separation, correlative light and electron microscopy turned out to be a good method for studying the morphology of thin films of polymer blends.