I’ve got a piece of electrophoresis equipment with two large parallel plate electrodes. One plate is supported by what looks like a big slab of PMMA (e.g. plexiglass or acrylic). That slab has been regularly developing a warp - last year I contacted the vendor and got a replacement slab, but now they just want to sell me their New! Improved! Version!
So now I’m trying to figure out what’s causing the slab to warp, and what I can do to either fix it or prevent it in the future. I doubt it’s heating – the total power is usually less than 10 W, and at the end of an electrophoresis run nothing is even perceptibly warm. My first suspect is the 20% methanol used in electrophoresis buffer. When I look up various chemical compatibility tables, all say that long term methanol exposure causes some “swelling or softening” in PMMA.
How is methanol causing PMMA to swell? Is methanol just being absorbed into the PMMA, or is it causing some other chemical reaction? If it’s just absorbed, can it be reversed by evaporating the methanol with heat or vacuum?
I can’t address your question specifically, but I’ve had some similar experience with polymers and solvents. My best guess is, yes, the methanol is being absorbed by the PMMA. It may be interculating into the polymer molecules. Also, I can fathom that it could be flushing out some non-polymerized monomers. (this is kind of a way to quantify ‘gel content’ in polymers).
if you heat it up under vacuum I can see you may remove some solvent and it might return closer to the original form, but I doubt it would be exactly as it was.
That’s…complicated. Permeability of low MW alcohol solvents in polymers like PMMA - and the resulting damage they can cause - is poorly understood and appears to involve a non-Fickian diffusion model. (Even modeling these systems is non-trivial and there’s much hand-wringing in the literature.)
In a nutshell, PMMA exposure to low MW alcohols appear to lead to localized microfractures (crazing) in the polymer structure. Over time this leads to shifts in the mechanical stability and, as you noticed, sagging. Given time and the right conditions (very hand-wavy, here), microfractures can grow and lead to material failure.
This is fundamentally a physical/structural change in the alignment of the PMMA network - driving off the solvent isn’t going to fix the problem. One could limit the damage by immediately removing and washing the EP apparatus after every gel (which has probably zero chance of happening in any normal lab ;)).
Transparent plastics (PMMA, PC, PS) and low MW alcohols are just not a great combination - especially over the medium-long term (days to weeks). Opaque plastics can be better, but even there one must read carefully.
Thanks, answers like that are what keep me around these boards!
I do clean it every time I use it… problem is that some of the buffer inevitably gets between the electrode and its backing, and it’s a minor pain in the ass to remove the electrode. It’s attached with super delicate nylon screws whose heads twist off if they even think a screwdriver is near.
Of course, a hundred-pack of suitable thumbscrews is a hell of a lot cheaper than replacing the electrode backing plate on a regular basis…
A minor update, for anyone who might stumble across this thread searching “PMMA + methanol” in future ages:
After sitting through a pitch for a whole new $$$$ piece of equipment, the vendor rep gave a free replacement for the backing plate. (Your sales pitch is less than convincing when it relies on the fact that your old and busted equipment is a badly designed POS. Especially when I can replace the damaged plate once per year for the next few decades for less than the cost of your supposed New Hotness.)
Meanwhile, on a whim and the hope that I could drive off some absorbed methanol I tossed the warped plate in a 60° C incubator. After several weeks, the plate is nearly flat. If there are any “microfractures” they’re too small to see on a good stereomicroscope. The warp is still visible but minor enough that the plate flattens with a little pressure, and it should also be minor enough that my squishy filter paper - gel - filter paper sandwiches should make good contact with each electrode. The old, mostly better backing plate will be kept as a backup.
For the new plate, I got thumbscrews to attach the electrode to the backing plate. Now when using buffers containing methanol, we’ll take off the electrode plate and rinse everything thoroughly. That should prolong the life a fair amount. At least, long enough for me to get my PhD and pass along a little lab folklore about how not to do electrophoresis.