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Glass Transition

This is the biggest and most important one to remember. The more flexible the backbone chain is, the better the polymer will move, and the lower its Tg will be. Let's look at some examples. The most dramatic one is that of silicones. Let's take a look at one called polydimethylsiloxane.

This backbone is so flexible that polydimethylsiloxane has a Tg way down at -127 oC! This chain is so flexible that it's a liquid at room temperature, and it's even used to thicken shampoos and conditioners.

Now we'll look at another extreme, poly(phenylene sulfone).

This polymer's backbone is just plain stiff. It's so rigid that it doesn't have a Tg! You can heat this thing to over 500 oC and it will still stay in the glassy state. It will decompose from all the heat before it lets itself undergo a glass transition! In order to make a polymer that's at all processable we have to put some flexible groups in the backbone chain. Ether groups work nicely.

Polymers like this are called poly(ether sulfones), and those flexible ether groups bring the Tg of this one down to a more manageable 190 oC.

Pendant Groups Part I:
Fish Hooks and Boat Anchors

Pendant groups have a big effect on chain mobility. Even a small pendant group can act as a fish hook that will catch on any nearby molecule when the polymer chain tries to move like corkscrew. Pendant groups also catch on each other when chains try to slither past each other.

One of the best pendant groups for getting a high Tg is the big bulky adamantyl group. An adamantyl group is derived from a compound called adamantane.


Click on the adamantane to see it in 3-D!

A big group like this does more than just act like a hook that catches on nearby molecules and keeps the polymer from moving. It's a downright boat anchor. Not only does it get caught on nearby polymer chains, its sheer mass is such a load for its polymer chain to move that it makes the polymer chain move much more slowly. To see how much this affects the Tg, just take a look at two poly(ether ketones), one with an adamantane pendant group and one without.

The Tg of the polymer on the top is already decent at 119 oC, but the adamantyl group raises even higher, to 225 oC.

Pendant Groups Part II:
Elbow Room

But big bulky pendant groups can lower the Tg, too. You see, the big pendant groups limit how closely the polymer chains can pack together. The further they are from each other, the more easily they can move around. This lowers the Tg, in the same way a plasticizer does. The fancy way to say that there is more room between the polymer chains is to say there is more free volume in the polymer. The more free volume, the lower the Tg generally. We can see this with a series of methacrylate polymers:

You can see a big drop each time we make that pendant alkyl chain one carbon longer. We start out at 120 oC for poly(methyl methacrylate), but by the time we get to poly(butyl methacrylate) the Tg has dropped to only 20oC, pretty close to room temperature.