22 April, 2011

Mending polymers with light

We know all too well from daily life that materials made from polymers can be damaged: Tires get punctured, garbage bags rip, plastic eyeglass lenses get scratched. But over the past decade, several ways have been developed to heal polymers. Some methods are autonomous, drawing only on resources within the material. More commonly, though, the repair process is externally activated, typically by heat: When heated above their glass transition or melting temperature, the polymer chains can rearrange, diffuse, and re-entangle. Stuart Rowan (Case Western Reserve University), Christoph Weder (University of Fribourg), and colleagues have now demonstrated a healing mechanism activated by light. The team's method exploits so-called supramolecular polymers, in which noncovalent bonds connect large repeating units. In the new work, the repeating units were elastic, hydrophobic hydrocarbon segments terminated by hydrophilic ligands. Those building blocks, termed macromonomers, were joined together into chainlike structures by metal ions, either Zn2+ or La3+. The chains' hydrophobic and hydrophilic regions phase separated into a tough, cross-linked lamellar pattern. When UV light excited the metal–ligand bonds, most of the absorbed energy went into heating, which locally dissociated and liquefied the macromonomers. Thus freed, the macromonomers could quickly diffuse and mend defects. The researchers found that for some experimental samples, two 30-second bursts of light could completely heal cuts halfway through a thin film and restore the material to its original toughness. (M. Burnworth et al., Nature 472, 334, 2011.)—Richard J. Fitzgerald

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