Tuesday, December 10, 2013

Describing Polymers with Everyday Items

I used to have 2 simple models (or maybe 3, you decide) that used everyday items to explain what polymers are like to people with formal education in the subject. I picked up a third one yesterday that I really liked. Let me list them all:
  1. /2. The most common image that people use to describe a polymer is a spaghetti noodle, which is fine for starters, but it falls short fairly quickly, even if you expand the length of the noodle to 20 feet or so. A bowl of spaghetti is somewhat better in that the noodles are coiled up somewhat with short segments going here and there randomly (until twirled by the diner's fork). But the analogy fails when it comes to describing entanglements and getting a chain to move past its neighbors since the noodles (when properly cooked and oiled) slip past each other pretty easily. Much like playing pick-up-sticks, you can pull slowly on the end of one noodle and remove it from the bowl without disturbing any other noodle and that is not realistic of polymer dynamics. Polymers are either all moving (molten state) or not (glassy state) [*].
  2. Moving polymers have a fading memory of what they shape there were previously in. The cooperation of their neighbors that I just noted often comes too slowly which leaves this lasting (but fading) impact on the molecule. Dragging a garden hose around is a (poor) example of polymer memory. If I'm watering the flower beds on the east side of the house and want to move the hose to the south side, it's not just a matter of dragging the spray nozzle around to where I want it - the entire hose has to go with it and it remembers where it was previously lying. I may have a free path to the new location for the nozzle but the remainder of the hose may have to cross trees, lawn chairs, toys... and so sometimes you have to do a lot of maneuvering to remove the memory that the hose has. Like polymers, garden hoses have a fading memory of where they previously were. Move them enough and they have no recall at all, although they keep picking up new memories along the way.
  3. The new idea is one of those so obvious ideas that makes you wonder why you didn't think of it earlier, particularly since it fits in so well with the current holiday season: A big hat tip to the redditor thewizardofosmium for this idea: "Does anyone else look at XMas lights and think of polymer chains with a high friction factor due to rigid side chains?" As I noted earlier, it is possible to pull a single spaghetti noodle free from a pile, but try pulling a strand of Christmas lights out of a jumbled pile. It would be easiest (relatively speaking of course) if the lights were little miniatures or LED bulbs. Doing it with C9 bulbs would give you dreams of something quite distinct from "visions of sugar-plums", and icicle lights would be a pleasure that only the Grinch could deliver.
    The larger/bulkier the side chain, the higher the glass transition temperature will be since mobility is already restricted and it is easy to freeze the molecules in their current position with no hope of any further movement relative to each other.
Our lights are already up for this year, but I'm sure to be thinking of this imagery when I take them down. And put them up next year. And take them down...


[*] Let's make life simple and ignore semi-crystalline polymers for now, shall we?

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