Strategies for elemental mapping from energy-filtered TEM of polymeric materials
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Research Letter
Strategies for elemental mapping from energy-filtered TEM of polymeric materials Brooke Kuei, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA Bernd Kabius, and Jennifer L. Gray, Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA Enrique D. Gomez, Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA; Materials Research Institute, The Pennsylvania State University, University Park, PA 16802, USA; Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802, USA Address all correspondence to Enrique D. Gomez at [email protected] (Received 22 June 2018; accepted 27 July 2018)
Abstract Energy-filtered transmission electron microscopy provides an opportunity to map the nanoscale elemental composition in polymeric systems. Nevertheless, it presents its own set of unique challenges in its application to soft materials. Here, we outline an optimized protocol for elemental mapping in soft materials using sulfur mapping of polymer/fullerene mixtures as an example. Three factors are crucial: (1) focusing at zero-loss, (2) using an objective aperture, and (3) maximizing signal-to-noise and counts for the chosen imaging conditions. Analyzing the corresponding source images, bright field images, and thickness maps can ensure optimum conditions are achieved for elemental mapping of polymers.
Introduction Transmission electron microscopy (TEM) has contributed to many advances in polymer and soft matter science. Some of the earliest applications of TEM for polymers revealed spherulitic crystallization of natural rubber and low-density polyethylene.[1] Later, TEM studies of lamellar crystals of polyethylene led to the folded chain model.[2] These pioneering studies relied on standard bright-field techniques with the aid of gold or chromium shadowing to generate contrast in the TEM, but as the need arose to image more complex systems, such as blends and block copolymers, new modalities for imaging became warranted. Most TEM studies of polymeric materials rely on mass contrast, where the image intensity depends on local mass thickness (thickness multiplied by density). Nevertheless, polymeric materials are generally composed of elements with low atomic numbers where domains have little or no difference in electron density. One strategy has been to use heavy element stains, but staining can perturb the morphology and specificity relies on differences in physical absorption or chemical reactivity, which depend on the details of the system. Alternatively, the image formation process can rely on phase and diffraction contrast.[3] Phase contrast relies on defocus of the objective lens to provide interference between the scattered and unscattered beam based on differences in the mean inner potential, but the contrast enhancement is often modest and resolution is limited due to defocus. If one of the materials is crystalline, d
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