Friday, March 18, 2011

Mental Models

Fellow blogger Derek Lowe, a pharma chemist, has an interesting post about the mental models that he carries around in his head for drugs interacting with proteins. He even mentions sounds, which goes a lot further than I do. Take a look at it, as even with all the medicinal applications, there still are some fun ideas.

What's my model for polymers? Probably similar to yours, but I keep tacking on new aspects over time. Everyone has the long chain. Without that, you're not a polymer person - it's that simple. Go long or don't go at all.

But here's some of my additions and why they are there. First, the chain is always moving. Whether from Brownian motion or stress relaxation, it's always on the go. Even with an end (or two!) tethered to a surface, it's wriggling like an earthworm. Or maybe the snake pit that Indiana Jones despised so much.Even in solids, you still have localized motions of the crankshaft type, all of which are needed to explain diffusion of water or oxygen through packaging materials. These additions were added when I started studying rheology.

Another addition are defects. These are a little harder to imagine in great details because the nature of it varies greatly from one polymer to another. Some examples
  • chain ends and the gaps they create
  • endgroups
  • improperly incorporated monomer in the chain (such as head-to-head addition instead of head-to-tail)
  • incorporation of the catalyst, initiators, ...
  • crystal defects, and
  • tie molecules in semi-crystalline polymers
These defects are usually ignored or downplayed, but in my mind they are critical. As the saying goes, a chain is only as strong as its weakest link. These defects are the weakest link. Theoretically, HDPE should rival diamond in its modulus, but it never does and that is because of defects. As was discussed back in November, defects were proposed as the initiation site for degradative oxidation (a big reason that I think that the proposal has legs). And I am sure that defects are the source of this patternthat Duke researcher found occurring when polymer breakdown upon exposure to high voltage. If a polymer were a continuous and pure substance, it would be tough to force an initial break, but with defects always present, initiation is much easier. These ideas were an early addition to my model, when my dissertation involved crystallization of HDPE, tie molecules...

I'm sure there are other additions, but they are comparatively minor. Please feel free to suggest what you carry around, as new additions can be good.

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