A surfactant
template guides the self-assembly of functional polymer structures in an
aqueous solution.
Image credit: Oak Ridge National Laboratory, U.S. Dept. of Energy;
image by Youngkyu
Han and Renee Manning.
(October 6, 2015) The
efficiency of solar cells depends on precise engineering of polymers that
assemble into films 1,000 times thinner than a human hair.
Today, formation of that polymer assembly requires solvents
that can harm the environment, but scientists at the Department of Energy’s Oak
Ridge National Laboratory have found a “greener” way to control the assembly of
photovoltaic polymers in water using a surfactant— a detergent-like molecule—as
a template. Their findings are reported in Nanoscale, a journal of the Royal
Society of Chemistry.
“Self-assembly of polymers using surfactants provides huge
potential in fabricating nanostructures with molecular-level controllability,”
said senior author Changwoo Do, a researcher at ORNL’s Spallation Neutron
Source (SNS).
The researchers used three DOE Office of Science User
Facilities—the Center for Nanophase Materials Sciences (CNMS) and SNS at ORNL
and the Advanced Photon Source (APS) at Argonne National Laboratory—to
synthesize and characterize the polymers.
“Scattering of neutrons and X-rays is a perfect method to
investigate these structures,” said Do.
The study demonstrates the value of tracking molecular
dynamics with both neutrons and optical probes.
ORNL’s in situ
multimodal test chamber tracks molecular dynamics in solutions as well as
solids.
Image credit - Oak
Ridge National Laboratory, U.S. Dept. of Energy; photographer Ilia Ivanov
“We would like to create very specific polymer stacking in
solution and translate that into thin films where flawless, defect-free polymer
assemblies would enable fast transport of electric charges for photovoltaic
applications,” said Ilia Ivanov, a researcher at CNMS and a corresponding
author with Do. “We demonstrated that this can be accomplished through
understanding of kinetic and thermodynamic mechanisms controlling the polymer
aggregation.”
The accomplishment creates molecular building blocks for the
design of optoelectronic and sensory materials. It entailed design of a
semiconducting polymer with a hydrophobic (“water-fearing”) backbone and
hydrophilic (“water-loving”) side chains. The water-soluble side-chains could
allow “green” processing if the effort produced a polymer that could
self-assemble into an organic photovoltaic material. The researchers added the
polymer to an aqueous solution containing a surfactant molecule that also has
hydrophobic and hydrophilic ends. Depending on temperature and concentration,
the surfactant self-assembles into different templates that guide the polymer
to pack into different nanoscale shapes—hexagons, spherical micelles and
sheets.