Neutrons make structural changes in molecular brushes visible

Date:
October 29, 2020
Source:
Technical University of Munich (TUM)
Summary:
They look like microscopic bottle brushes: Polymers with a backbone
and tufts of side arms. This molecular design gives them unusual
abilities: For example, they can bind active agents and release
them again when the temperature changes. With the help of neutrons,
a research team has now succeeded to unveil the changes in the
internal structure in course of the process.



FULL STORY ==========================================================================
They look like microscopic bottle brushes: Polymers with a backbone and
tufts of side arms. This molecular design gives them unusual abilities:
For example, they can bind active agents and release them again when the temperature changes. With the help of neutrons, a research team from
the Technical University of Munich (TUM) has now succeeded to unveil
the changes in the internal structure in course of the process.


==========================================================================
"The structure of the bottle-brush polymers, which are only nanometers
in size, cannot be investigated using classical optical methods: It
can be seen that an aqueous solution containing these polymers becomes
turbid at a certain temperature. But why this is the case, and how the
backbone and the side arms stretch out into in the water or contract,
has not yet been clarified," reports Prof. Christine Papadakis.

There is a simple reason why scientists would like to know more about
the inner life of bottle-brush polymers: The fluffy molecules, which
consist of different polymer chains and abruptly change their solubility
in water at a certain temperature, are promising candidates for a variety
of applications.

For example, they could be used as catalysts to accelerate chemical
reactions, as molecular switches to open or close tiny valves, or
as transport media for medical drugs -- the molecular brushes could
thus bring pharmaceuticals to a center of inflammation and, because
the temperature is elevated there, release them directly at the site
of action.

However, the basic prerequisite for using the brush molecules is that
their behavior can be programmed: Theoretically, chemists can use a
combination of water-soluble and water-insoluble building blocks to
determine precisely at what temperature the polymers clump together
and the liquid in which they were just dissolved becomes cloudy. "In
practice, however, you have to know exactly how and under what conditions
the structure of the polymers changes if you want to design smart brush molecules," explains Papadakis.

Neutrons reveal their molecular inner life Together with her team in
the Soft Matter Physics Group at the Technical University of Munich,
she has now been able to visualize for the first time the changes that bottle-brush polymers with arms made of two different types of building
blocks undergo when the temperature reaches the cloud point.



==========================================================================
The scientists used neutron radiation from the Research Neutron Source
Heinz Maier-Leibnitz (FRM II) on the campus Garching in a special
instrument for small angle neutron scattering, which is operated by the Forschungszentrum Ju"lich This method is particularly well suited for
the investigation because neutrons are electrically neutral and therefore penetrate matter easily. There they are scattered by the atomic nuclei,
which results in detailed information about the brush molecules. In
combination with modern cryo electron microscopy, a detailed understanding
of these molecules could be obtained.

When brushes clump together The thermoresponsive brush molecules studied
by Papadakis' team were synthesized by chemists from the National Hellenic Research Foundation in Greece and the Technische Universita"t Dresden, respectively.

In the first step, the samples were dissolved in water, then gradually
heated up to the cloud point and irradiated with neutrons. A detector
monitored the scattered radiation. From the scattering signal, the
researchers were able to deduce the structural changes.

Depending on the structure of the polymers, water molecules split-off
already before the cloud point was reached. At the cloud point itself,
the molecular structure of the polymers collapsed. What remained were water-insoluble polymer coils, which formed loose or compact clusters
depending on the residual water content.

"The results will help to develop bottle-brush polymers suitable for
practical use," the physicist is convinced. "If you know exactly how
polymers change at the cloud point, you can optimize their chemical
structure for different applications."

========================================================================== Story Source: Materials provided by
Technical_University_of_Munich_(TUM). Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Jia-Jhen Kang, Kaltrina Shehu, Clemens Sachse, Florian A. Jung,
Chia-Hsin
Ko, Lester C. Barnsley, Rainer Jordan, Christine M. Papadakis. A
molecular brush with thermoresponsive poly(2-ethyl-2-oxazoline)
side chains: a structural investigation. Colloid and Polymer
Science, 2020; DOI: 10.1007/s00396-020-04704-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201029115830.htm

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