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Polymers
Small-angle neutron scattering has provided dramatic evidence to support a Nobel laureate's
predictions about the behavior of polymer chains. |
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Polymers
can be formed the pollution-free way by suspending small molecules (red) in supercritical
carbon dioxide (blue) using detergent-like molecules called surfactants (green)
that cluster into micelles (top) when pressure is changed to increase CO2
density. |
In the 1940s industrial chemists
started synthesizing giant molecules called polymers to make strong, light materials
called plastics. In the past 50 years, they have had a string of successes:
- Tougher, lighter materials used in cars and airplanes
(e.g., the Boeing 757 jet)
- Bulletproof vests to protect police officers
- Synthetic threads for textiles
- The familiar bags of airline peanuts that can
be torn open only at the precut notch
The annual U.S. consumption of polymers exceeds 70 billion pounds, so these materials are the subject of intense
scientific and commercial interest.
A polymer is formed of
long, chainlike molecules that can be precisely oriented for great strength. The
chains are made by bonding together many (poly) smaller molecules (monomers),
which consist of atoms such as hydrogen, carbon, oxygen, and fluorine. Neutrons
have unique properties for studying polymers, such as the ability to "stain" molecules
and make them "visible" via isotopic labeling. During the past two decades, this
technique has provided fundamental insights into the structure of molecules. For
example, small-angle neutron scattering (SANS) has provided dramatic evidence
to support Nobel Laureate Paul Flory's 50-year old prediction that polymer chains
adopt random configurations in the solid state.
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Neutron scattering will help scientists determine the best polymer blends to make high-quality
plastic products.
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When the chemical industry
produces fluoropolymers to make pots and pans (Teflon®)
and to protect carpets against staining (Scotchgard®),
an undesirable by-product is environmental pollution.
Research to develop cleaner ways to make plastics is
under way to develop environmentally friendly processes
that don't use ozone-destroying chlorofluorocarbons
and that could significantly reduce the amount of
contaminated water and toxic waste generated. These
processes make use of supercritical carbon dioxide
(CO2), which is already used
as a nontoxic solvent (e.g., to decaffeinate coffee
beans) and is readily available as a "waste gas" that
can be recycled with no net CO2 addition
to the atmosphere. But, because many polymers don't
dissolve in CO2, ORNL
and UNC have used SANS to determine what makes some
polymers soluble and how to develop emulsifying agents
(detergents) that can suspend CO2-insoluble
materials in solution in much the same way that soap
helps oil dissolve in water. SANS has provided detailed
insight into how solvent pressure may be "tuned" to
dissolve CO2-insoluble materials or cause
them to drop out of solution at the appropriate point
in the process, thus controlling solubility in unique
polymerization, extraction, and cleaning applications.
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Much of the Boeing 757 airplane is made of lightweight plastic. Neutron studies may lead to safer, faster, more energy-efficient aircraft.
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Because of difficulties in commercializing new polymers,
industry has turned increasingly to combining (blending)
existing polymers to optimize the mixture's end-use
properties. Such materials account for approximately
one quarter of the polymer market; this segment is
growing at twice the rate of industrial plastics
as a whole. SANS is the premier technique for investigating
such blends. Using SNS, such experiments can be
performed in seconds, not minutes or hours. In this
way, researchers can quickly determine how well polymers
will mix, how long they should be ground and compressed,
at which temperature they should be melted together
to get the best mixing, and which mixtures will form
the best products. For example, if interest in recycling
plastics picks up, SNS can help scientists understand
which polymers can be melted down and mixed to form
useful polymer blends. Currently, less than 10% of
polymers are recycled, so determining the degree
of compatibility of different components can help
in designing strategies for reprocessing and in evaluating
the usefulness of the resulting material.
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