Page 56 - Plastics News August 2019

TECHNOLOGY

Light-controlled polymers can switch between sturdy and soft

IT researchers have to ligand molecules. Those ratios determine the size of
Mdesigned a polymer the cages. In the new study, the researchers set out to
material that can change design a material that could reversibly switch between
its structure in response to two different-sized cages: one with 24 atoms of palladium
light, converting from a rigid and 48 ligands, and one with three palladium atoms and
substance to a softer one that six ligand molecules. To achieve that, they incorporated
can heal itself when damaged. a light-sensitive molecule called DTE into the ligand. The
<RX FDQ VZLWFK WKH PDWHULDO size of the cages is determined by the angle of bonds that
states back and forth, and a nitrogen molecule on the ligand forms with palladium.
in each of those states, the :KHQ '7( LV H[SRVHG WR XOWUDYLROHW OLJKW LW IRUPV D ULQJ LQ
material acts as though it were the ligand, which increases the size of the angle at which
a completely different material, even though it's made nitrogen can bond to palladium. This makes the clusters
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associate professor of chemistry at MIT, a member of MIT's shine green light on the material, the ring is broken, the
Koch Institute for Integrative Cancer Research and the bond angle becomes smaller, and the smaller clusters re-
Program in Polymers and Soft Matter, and the leader of the IRUP 7KH SURFHVV WDNHV DERXW ÀYH KRXUV WR FRPSOHWH DQG
research team. The material consists of polymers attached the researchers found they could perform the reversal up
to a light-sensitive molecule that can be used to alter the to seven times; with each reversal, a small percentage of
bonds formed within the material. Such materials could the polymers fails to switch back, which eventually causes
be used to coat objects such as cars or satellites, giving WKH PDWHULDO WR IDOO DSDUW :KHQ WKH PDWHULDO LV LQ WKH
them the ability to heal after being damaged, though small-cluster state, it becomes up to 10 times softer and
such applications are still far in the future, Johnson says. more dynamic. This approach overcomes the tradeoff that
The lead author of the paper, which appears in the July 18 usually occurs with self-healing materials, which is that
issue of Nature, is MIT graduate student Yuwei Gu. Other structurally they tend to be relatively weak. In this case,
authors are MIT graduate student Eric Alt, MIT assistant the material can switch between the softer, self-healing
SURIHVVRU RI FKHPLVWU\ $GDP :LOODUG DQG +HQJ :DQJ DQG state and a more rigid state.
Xiaopeng Li of the University of South Florida. Controlled Self-healing materials- In this paper, the researchers used
structure - Many of the properties of polymers, such as the polymer polyethylene glycol (PEG) to make their
their stiffness and their ability to expand, are controlled material, but they say this approach could be used with
by their topology -- how the components of the material any kind of polymer. Potential applications include self-
are arranged. Usually, once a material is formed, its healing materials, although for this approach to be widely
topology cannot be changed reversibly. For example, a used, palladium, a rare and expensive metal, would likely
rubber ball remains elastic and cannot be made brittle KDYH WR EH UHSODFHG E\ D FKHDSHU DOWHUQDWLYH $Q\WKLQJ
without changing its chemical composition. In this paper, made from plastic or rubber, if it could be healed when it
the researchers wanted to create a material that could was damaged, then it wouldn't have to be thrown away.
reversibly switch between two different topological Maybe this approach would provide materials with longer
states, which has not been done before. Johnson and OLIH F\FOHV -RKQVRQ VD\V $QRWKHU SRVVLEOH DSSOLFDWLRQ
his colleagues realized that a type of material they for these materials is drug delivery. Johnson believes it
designed a few years ago, known as polymer metal-organic could be possible to encapsulate drugs inside the larger
cages, or polyMOCs, was a promising candidate for this cages, then expose them to green light to make them
approach. PolyMOCs consist of metal-containing, cage-like open up and release their contents. Applying green light
VWUXFWXUHV MRLQHG WRJHWKHU E\ ÁH[LEOH SRO\PHU OLQNHUV 7KH could enable recapture of the drugs, providing a novel
researchers created these materials by mixing polymers approach to reversible drug delivery. The researchers are
attached to groups called ligands, which can bind to a also working on creating materials that can reversibly
metal atom. Each metal atom -- in this case, palladium switch from a solid state to a liquid state, and on using
-- can form bonds with four ligand molecules, creating light to create patterns of soft and rigid sections within
rigid cage-like clusters with varying ratios of palladium the same material.

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