Three-Dimensional Visualization and Characterization of Polymeric Self-Assemblies by Transmission Electron Microtomography

Abstract

* S Supporting Information CONSPECTUS: Self-assembling structures and their dynamical processes in polymeric systems have been investigated using three-dimensional transmission electron microscopy (3D-TEM). Block copolymers (BCPs) self-assemble into nanoscale periodic structures called microphase-separated structures, a deep understanding of which is important for creating nanomaterials with superior physical properties, such as high-performance membranes with well-defined pore size and high-density data storage media. Because microphase-separated structures have become increasingly complicated with advances in precision polymerization, characterizing these complex morphologies is becoming increasingly difficult. Thus, microscopes capable of obtaining 3D images are required. In this article, we demonstrate that 3D-TEM is an essential tool for studying BCP nanostructures, especially those self-assembled during dynamical processes and under confined conditions. The first example is a dynamical process called order−order transitions (OOTs). Upon changing temperature or pressure or applying an external field, such as a shear flow or electric field, BCP nanostructures transform from one type of structure to another. The OOTs are examined by freezing the specimens in the middle of the OOT and then observing the boundary structures between the preexisting and newly formed nanostructures in three-dimensions. In an OOT between the bicontinuous double gyroid and hexagonally packed cylindrical structures, two different types of epitaxial phase transition paths are found. Interestingly, the paths depend on the direction of the OOT. The second example is BCP self-assemblies under confinement that have been examined by 3D-TEM. A variety of intriguing and very complicated 3D morphologies can be formed even from the BCPs that self-assemble into simple nanostructures, such as lamellar and cylindrical structures in the bulk (in free space). Although 3D-TEM is becoming more frequently used for detailed morphological investigations, it is generally used to study static nanostructures. Although OOTs are dynamical processes, the actual experiment is done in the static state, through a detailed morphological study of a snapshot taken during the OOT. Developing time-dependent nanoscale 3D imaging has become a hot topic. Here, the two main problems preventing the development of in situ electron tomography for polymer materials are addressed. First, the staining protocol often used to enhance contrast for electrons is replaced by a new contrast enhancement based on chemical differences between polymers. In this case, no staining is necessary. Second, a new 3D reconstruction algorithm allows us to obtain a high-contrast, quantitative 3D image from fewer projections than is required for the conventional algorithm to achieve similar contrast, reducing the number of projections and thus the electron beam dose. Combining these two new developments is expected to open new doors to 3D in situ real-time structural observation of polymer materials.

Research Update: Electron beam-based metrology after CMOS

“…Such defects can be successfully resolved using TEM tomography, when a process such as sequential infiltration synthesis (SIS) has increased the atomic number in one phase, increasing the material contrast, and rendering the morphology relatively stable with respect to beam-induced damage. 29,30 However, TEM is not suitable for defect review in a production process, and it is not clear if a suitable combination of operating conditions and signal detection schemes exists to make SEM a viable alternative.…”

Electron Crystallographic Investigation of Crystals on the Mesostructural Scale

“…On the other hand, electron microscopy has shown to be very powerful for solving three-dimensional (3D) structures which provides structural information, not only from reciprocal space but also from real space by imaging. To retrieve the 3D information out of a projection, electron tomography reconstruction in the transmission electron microscope (TEM) or scanning transmission electron microscope (STEM) (Yates et al, 2006; Wikander et al, 2007; Linton et al, 2009; Midgley & Dunin-Borkowski, 2009; Goris et al, 2012, 2013; Saghi & Midgley, 2012; Scott et al, 2012; Van den Broek et al, 2012; Hsueh et al, 2014; Liu et al, 2014; Chu et al, 2015; Ercius et al, 2015; Jinnai et al, 2017; Wang et al, 2017; Wen et al, 2019; Zhou et al, 2019) may apply, from which the targeted 3D information can be obtained by reconstructing from tilt image series with a certain angular step. However, the 3D maps from electron tomography rely on reconstruction algorithms and the tilt range, which is often limited by spatial resolution.…”

About the Art and Science of Visualizing Polymer Morphology using Transmission Electron Microscopy

“…As microphase‐separated structures have become increasingly complicated with the development of controlled polymerization, three‐dimensional imaging has also been of growing importance. Transmission electron microtomography is particularly useful when microphase separation of block copolymers produces interpenetrated three‐dimensional morphologies …”

“…Transmission electron microtomography is particularly useful when microphase separation of block copolymers produces interpenetrated three-dimensional morphologies. [24,25] Besides, atomic force microscopy, which belongs to the family of scanning proximity probe microscopies developed since the late 1990s, has become a widely used technique that complements electron microscopy in studies of polymers. [26] AFM may provide direct real-space access on morphologies on all length scales of hierarchical ordering of polymers, and can be applied in a broad range of experimental conditions, including in liquid media, which renders the technique applicable for insitu studies.…”