Wednesday, October 17, 2012

Overlooking the Obvious: Self-Healing PVOH Hydrogels

While polymer research is typically done with ever more advanced technology, every once in a while I run across something amazingly simple that makes me wonder how many other what other simplicities are so readily available.

This new research (free access for a very short time period) is about a rather mundane subject - polyvinyl alcohol (PVOH) [1] hydrogels. I say mundane as PVOH hydrogels are more commonly known to school children as "slime" - the result of mixing white glue with borax. The borax crosslinks the chains using their pendant alcohol groups.

The researchers took a slightly different approach to making their gels, one that did not require the addition of any crosslinking agent. They made a very concentrated (35 wt%) PVOH hydrogel by first dispersing it in 95 oC water [2], froze it for an hour, and then warmed it to room temperature, a process that is well-known for forming crystallites that serve as crosslinks for the gel. What is surprising is that these very stiff gels could self-heal upon being cut, but only under certain circumstances. The plot below shows the stress-strain curves for the original material and also how the repaired gels improve their properties over time.
If you've ever played with slime, you are probably wondering what all the fuss is about. Slime will self-repair very quickly as it is not only very soft, but also crosslinks are rather fluid and prone to breaking and reforming. Those are not characteristics of these gels however, as not only are they stiffer, but the crystallites are pretty well frozen into place. What is surprising about these gels is that in fact, higher concentrations of PVOH are necessary for the self-repair features to be present as this figure shows.
What is not as surprising is that the separation time affects the ability to self-repair. The results above were for gels that were rejoined after 5 seconds of separation, but as the plot below shows, leaving the parts separated for an hour reduces the gel's strength by about 20%, and leaving them apart for a day pretty much eliminates all the self-repair characteristics.
This strongly suggests that the surface of the gels is rearranging itself over time, a rearrangement that then prevents the necessary interdiffusion of the PVOH chains. A similar trend, also not surprising is seen as the number of freeze/thaw cycles increases, steps that are well known for increasing the crystallinity of the hydrogels.
Gels with increase crystallinity will also have a harder time interdiffusing their chains after being cut.

Being that these gels are made only from PVOH and water - no crosslinking agents are needed or used - these materials and their simple chemical makeup have an appeal for biomedical applications. But that such unexpected behavior can be found in such simple formulations and processes is what is intriguing. Making these gels is no more complicate than making slime and could be done by college students (or high schoolers if they can handle the long cooling/freezing times and the boiling water.) How many other simple systems with such surprising properties are out there?

[1] I prefer to use PVOH as the abbreviation for this polymer rather than PVA, as PVA could stand for either polyvinyl alcohol or polyvinyl acetate. And it's all the more confusing as polyvinyl alcohol is made from polyvinyl acetate. You can't polymerize vinyl alcohol as vinyl alcohol doesn't exist. Even if you could make it, it would undergo a keto-enol rearrangement to form acetaldehyde. So instead, polyvinyl acetate is hydrolyzed to form PVOH. If you've ever look into ordering PVOH, you will find that the various grades are not only the result of differences in molecular weight, but also differences in degrees of hydrolysis.

[2] PVOH readily dissolve in water at room temperature - too readily - so much so that making a solution with more than a percent or two is a really problem. If you aren't adding the powder very slowly and with large amounts of mixing, you will end up with lumpy snots of undissolved material. Two options to avoid this are to 1) cool the water (adding ice directly is best), or 2) use extremely hot water as the researchers did here. The former option works because it slows down the dissolution rate so that you can evenly disperse the powder and then dissolution occurs as the water slowly heats up, while the second option works because PVOH is insoluble in the extremely hot water, so that again, it can be dispersed first and then slowly dissolved as the water cools. The ice option can work for solutions of about 10 wt% max. I have no idea what the maximum concentration is for the second option. The researchers here were able to make 40 wt% solutions.

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