High-Resolution Imaging of Unstained Polymer Materials

Abstract

Electron microscopy has played an important role in polymer characterization. Traditionally, electron diffraction is used to study crystalline polymers while transmission electron microscopy is used to study microphase separation in stained block copolymers and other multiphase systems. We describe developments that eliminate the barrier between these two approachesit is now possible to image polymer crystals with atomic resolution. The focus of this Review is on high-resolution imaging (30 Å and smaller) of unstained polymers. Recent advances in hardware allow for capturing numerous (as many as 105) low-dose images from an unperturbed specimen; beam damage is a significant barrier to high-resolution electron microscopy of polymers. Machine-learning-based software is then used to sort and average the images to retrieve pristine structural information from a collection of noisy images. Acknowledging the heterogeneity in polymer samples prior to averaging is essential. Molecular conformations in a wide range of amphiphilic block copolymers, polymerized ionic liquids, and conjugated polymers can be gleaned from two-dimensional projections (2D), three-dimensional (3D) tomograms, and four-dimensional (4D) scanning transmission electron microscopy (STEM) data sets where 2D diffraction patterns are taken as a function of position. Some methods such as phase contrast STEM have been used to image closely related materials such as metal–organic frameworks but not polymers. With improvements in hardware and software, such methods may soon be applied to polymers. Our goal is to provide a comprehensive understanding of the strategies toward the high-resolution imaging of radiation sensitive polymer materials at different length scales.

Electron Microscopy Studies of Soft Nanomaterials

“…Later, Keller and coauthors applied both EM and electron diffraction to study the submicroscopic morphology and related crystal orientations in crystalline high polymers such as PA . Since then, electron diffraction techniques, such as SAED, have been applied in complementary to X-ray methods to study the structures of a variety of polymers such as poly­(ethylene oxide), polypropylene, block copolymers like poly­(ethylene oxide)- b -polystyrene, and semiconducting polymers like poly­(3-hexyl-thiophene-2,5-diyl) (P3HT). − The radiation damage of the electron beam is an important factor to consider in the characterization of polymers, which usually requires a case-by-case study considering the temperature, sample thickness, and the different types of polymers. , …”

The Development of iDPC-STEM and Its Application in Electron Beam Sensitive Materials

“…In the field of low-dose characterization, obtaining structural information of a series of electron-beam sensitive materials, such as zeolites [57][58][59][60][61][62][63], metal-organic frameworks [64][65][66][67][68][69][70], biomaterials [71][72][73][74][75][76], and some organic-inorganic hybrid materials is the main goal of the research. How to maintain a sufficient SNR of the image under low dose is the core issue of its development.…”

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope

“…486 Moreover, modern electron detectors and low dose imaging methods help overcome these obstacles. 487 TEM is commonly used to determine nanoscale morphologies, 481,488 3D structures, 175,489 topological defects, 189 and self-assembly mechanisms 86,100,490 of small molecules and polymeric systems.…”

“…Phase contrast TEM, and the use of phase plates, can significantly improve contrast for organic materials . Moreover, modern electron detectors and low dose imaging methods help overcome these obstacles . TEM is commonly used to determine nanoscale morphologies, , 3D structures, , topological defects, and self-assembly mechanisms ,, of small molecules and polymeric systems.…”