Page 47 - Plastics News May 2017

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In this paper, the researchers first focused on a type of can improve the material's strength by as much as 600
polymer structure known as a star polymer network. This percent, Johnson says. “This very simple ingenious and
material has two different building blocks: a star with powerful approach, based on slow crosslinker addition,
four identical arms, known as “B4,” and a chain known as diminishes the intramolecular cyclization and significantly
“A2.” Each molecule of A2 attaches to the end of one of increases mechanical properties of polymeric networks,”
the B4 arms. However, during the typical synthesis process, says Krzysztof Matyjaszewski, a professor of chemistry
when everything is mixed together at once, some of the at Carnegie Mellon University who was not involved in
A2 chains end up binding to two of the B4 arms, forming the research. The researchers also tried this technique
a loop. The researchers found that if they added B4 very with four other types of polymer network synthesis
reactions. They were not able to measure the number of
slowly to a solution of A2, each of the B4arms would loops for all of those types of polymers, but they did find
quickly react with a single molecule of A2, so there was similar improvements in the strength of the materials.
less opportunity for A2 to form loops. After a few hours This approach could potentially help to improve the
of slowly adding half of the B4 solution, they added the strength of any material made from a gel or other cross-
second half all at once, and the star-shaped subunits linked polymer, including plastics, membranes for water
joined together to form a cross-linked network. This purification, adhesives made of epoxy, or hydrogels such as
material, the researchers found, had about half as many contact lenses. Johnson’s lab is now working on applying
loops as the same material produced using the traditional this strategy to a variety of materials, including gels used
synthesis process. Depending on how many loops were to grow cells for tissue engineering. The research was
in the original material, this “slow then fast” strategy funded by the National Science Foundation.

47 May 2017 | Plastics News