ar7b00103_si_003.mpg (13.34 MB)
Three-Dimensional Visualization and Characterization of Polymeric Self-Assemblies by Transmission Electron Microtomography
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posted on 2017-05-19, 18:20 authored by Hiroshi Jinnai, Takeshi Higuchi, Xiaodong Zhuge, Akihito Kumamoto, Kees Joost Batenburg, Yuichi IkuharaConspectusSelf-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.
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Keywords
electron beam dose3 D reconstruction algorithm3 D morphologiesmicrophase-separated structuresTransmission Electron Microtomography ConspectusSelf-assembling structuresepitaxial phase transition pathsBCP nanostructures3 D imagecontrastpolymer materials3 D images3 D-TEMtime-dependent nanoscale 3 D imagingdata storage mediaOOT
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