Tuesday, October 01, 2013

A New Self Healing Polymer of Interest

Now that I'm above ground again and my belly is against the bench, I have some time to get caught up on the recent flurry of research publications making the news.Tromping around in a mine is fun for a few days, but it's not something I would want to do my entire career. A tip of the hard hat to those that choose otherwise and provided us with most of the naturally occuring elements of the periodic table.

A few weeks ago, Spanish researchers made a splash with their discovery of a elastomer that self-heals at room temperature without the use of catalysts or other activating agents. It makes use of the lability of disulfide bonds. Sulfur-sulfur bonds are well known to be constantly breaking and reforming, so by incorporating them as crosslinking sites, the surfaces of a cut polymer can actively rejoin over time.

While there was a lot of hype surrounding this (calling in a "Terminator polymer" being the least of my issues with the hype), we do need to recognize that polymers that self heal at room temperature have been reported before. I discussed one such example based on poly(vinyl alcohol) not quite a year ago.

There are some significant differences between these two polymers however. The former recovers a whopping 97% of it's mechanical properties within 24 hours, while the latter system after 48 hours had recovered only 80% of its original properties. (Not that either of those 2 data are anything to complain about). The mechanisms for the self-healing are entirely different. The former system reforms covalent bonds, while the latter is based on the interdiffusion of polymer chains. However, the research does show that this new systems does rely on interdiffusion of the polymer chains do a goodly degree. A control polymer made without the disulfide crosslinker was able to recover nearly 50% of its mechanical properties, showing that the disulfide bonds are responsible for only that additional 47 percentage points and not the full 97.

Regardless, this is an exciting revelation, one that is easy to incorporate into polyurethanes and polyureas as was shown here. Incorporation into other polymer systems such as polyamides should also be possible. Acrylates? The free radicals might just take a not-so-innocent liking to the disulfide bonds. Other systems are left for the interested student.

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