Soft and stretchy polymers can be hardened by dynamically incorporating new components into the polymer chain.

Jean-Marie Lehn at the University of Strasbourg, France, and colleagues at Mitsui Chemicals, Chiba, Japan, converted a soft polymer into a hard one by adding rigid monomer molecules into the linear polymer chain.

The soft polymer is made up of monomers that are linked together by reversible covalent bonds, called acylhydrazone bonds.  These acylhydrazone links can undergo bond exchange and, providing the functionality is correct, they can incorporate new monomers into the polymer backbone.  The dynamic nature of the bond exchange introduces the molecules into the polymer chain in a random fashion.

To get the polymers to change their mechanical properties Lehn's team simply heated up an acidic solution of the soft and stretchy polymer mixed with rigid monomer molecules. A hard transparent polymer formed when the solvent was removed.

The researchers used NMR spectroscopy to show that the new monomers had been incorporated in the polymer backbone rather than as a side chain.  

'This is useful methodology for modification of the mechanical properties of polymers, giving access to smart and adaptive dynamic materials,' said Lehn.

Alison Stoddart