Mar., 22 /Los Angeles, CA 90089, USA./A Thermally Re-mendable Cross-Linked Polymeric Material. , Xiangxu Chen, Matheus A. Dam, Kanji Ono, Ajit Mal, Hongbin Shen, Steven R. Nutt, Kevin Sheran, Fred Wudl Science, 2002, 295, 1698-1702.

We have developed a transparent organic polymeric material that can repeatedly mend or "re-mend" itself under mild conditions. The material is a tough solid at room temperature and below with mechanical properties equaling those of commercial epoxy resins. At temperatures above 120°C, approximately 30% (as determined by solid-state nuclear magnetic resonance spectroscopy) of "intermonomer" linkages disconnect but then reconnect upon cooling, This process is fully reversible and can be used to restore a fractured part of the polymer multiple times, and it does not require additional ingredients such as a catalyst, additional monomer, or special surface treatment of the fractured interface.

In past decades, highly cross-linked polymers have been studied widely as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, etc.

In past decades, highly cross-linked polymers have been studied widely as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, etc. . The densely cross-linked structures are the basis of superior mechanical properties such as high modulus, high fracture strength, and solvent resistance. However, these materials are irreversibly damaged by high stresses due to the formation and propagation of cracks.

The latter lead to dangerous loss in the load-carrying capacity of polymeric structural engineering materials . The exploration of re-mending and self-healing of polymeric materials has become increasingly more exciting in the recent past, culminating in a genuinely self-healing material . The hot plate welding and crack healing of thermoplastics, where intermolecular noncovalent interactions (chain entanglements) at the interface are responsible for mending, have been well established. Small molecules-induced crack healing has also been studied for thermoplastics, and a composite of a linear polymer with a thermoset that has crack-healing ability was reported. The concept of self-repair was introduced to heal cracks by embedding hollow fibers that can release repair chemicals when a crack propagates. The most recent report of crack self-healing (autonomic healing) of an epoxy resin consists of a clever use of catalytic network formation of an encapsulated add-monomer, which is held within a capsule embedded in the epoxy matrix. But questions remain concerning the long-term stability of the catalyst and the ability of the material to self-heal multiple times. We describe a truly "re-mending" material consisting of a highly cross-linked, transparent polymer that exhibits multiple cycles of autonomic crack mending with simple, uncatalyzed thermal treatment and forms covalent bonds at the interface of the mended parts. In contrast, in hot plate welding and crack healing of thermoplastics, only intermolecular, noncovalent interactions (chain entanglements, hydrogen bonding, etc.) are responsible for the mend and no new covalent bonds are formed between the mended parts.

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1 Exotic Materials Institute and Department of Chemistry and Biochemistry,
2 Department of Materials Science and Engineering,
3 Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095, USA.
4 Department of Materials Science, University of Southern California, Los Angeles, CA 90089, USA.

* To whom correspondence should be addressed. E-mail: wudl@chem.ucla.edu