学术报告
"Structural analysis of crystalline polymers by electron microscopy and electron diffraction techniques"

Prof. Bernard Lotz
CNRS, Institut Charles Sadron
Strasbourg University, France

时间：2009年8月10日下午14:30   地点：清华大学何添楼406室

清华大学化工系高分子研究所

报告内容:
Crystalline polymers are made of small (several micrometers) and thin (one to
several tens of nanometers) lamellae, which amounts to ≈10-12 grams of material. Electron microscopy is therefore an ideal tool for their investigation, in
spite of the high or very high susceptibility of polymers to electron beam damage.
  Upon crystallization from the bulk, crystalline polymers produce spherulites
with a complex structure. They are made of radial lamellae, frequently twisted
in space, which renders their investigation difficult. However, crystallization from solution or in thin films generates single crystals that are well adapted
for structural investigations. It is also possible to take advantage of
epitaxial crystallization on foreign substrates (usually low molecular weight
organic crystals) to produce thin films with single crystal characteristics, and in which the chains lay in the plane of the film. The combined investigation of single crystals and epitaxially formed films by electron diffraction makes it
possible to access the whole diffraction space of the crystalline polymer, and
therefore to perform a so-called "electron crystallography".
The presentation is intended to be very didactic. It will illustrate several
uses of electron microscopy (imaging mode in bright field and dark field, electron diffraction) in the investigation of the structure and crystallization
process of polymers. Among others, we will develop:
  The structural analysis of metastable phases that cannot be obtained in an oriented, fiber form by classical (usually mechanical) means
  The concept of frustrated packing in crystalline polymers
  The use of dark field imaging to visualize and investigate on crystal-crystal transformations and the growth mechanism of crystalline polymers
  The combined use of epitaxial crystallization, electron diffraction and AFM to observe the structure of polymers with methyl group resolution (≈4?).
  The investigation of the fold structure, i.e. of the amorphous layer that
sandwiches the crystalline core of the lamellae, and in which the chains fold
back and forth to reenter the crystalline core. This folding generates surface
stresses that induce twisted and scrolled lamellar morphologies. Recent work
performed at Tsinghua University is particularly relevant to this topic. Its
impact on our understanding of polymer morphology will be discussed in an
integrated frame.  
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