The Three Things That
Make Polymers Unique
So you want to know just how and why these polymers, these macromolecules, act differently from small molecules. So we'll tell you. That's just the kind of nice people we are. There are, for those of you who didn't bother to read the title, three ways in which polymers will act differently from small molecules. And the reasons are a little bit more complicated than just "because they're bigger". The three are usually named:
That's all well and good, these fancy names, but what do they mean in reality?Chain Entanglement
Remember now that most polymers are linear polymers; that is, they are molecules whose atoms are joined in a long line to form a huge chain. Now most of the time, but not always, this chain is not stiff and straight, but is flexible. It twists and bends around to form a tangled mess. the chains tend to twist and wrap around each other, so the polymer molecules collectively will form one huge tangled mess.
Now when a polymer is molten, the chains will act like spaghetti tangled up on a plate. If you try to pull out any one strand of spaghetti, it slides right out with no problem. But when polymers are cold and in the solid state, they act more like a ball of string. We're not talking about a new ball of string neatly wrapped up, either. We're talking about that tangled up old ball of string that you've been collecting for years. Trying to pull one strand out of this mess is a little harder. You're more likely to end up making a big knot!
Solid polymers are like this. The chains are all tangled up in
each other and it is difficult to untangle them. This is what
make so many polymers so strong in materials like plastics, paint, elastomers, and composites.
Remember intermolecular forces? If you don't I'll fill you in. All
molecules, both small ones and polymers, interact with each other,
attracting each other through electrostatics. Some molecules are drawn
to each other more than others. Polar molecules stick together better
than nonpolar molecules. For example, water and methane have
similar molecular weights. Methane's weight is sixteen and water's is
eighteen. Methane is a gas at room temperature,
and water is a liquid. This is because water is very polar, polar
enough to stick together as a liquid, while methane is very nonpolar, so
it doesn't stick together very well at all.
As I said, intermolecular forces affect polymers just like small
molecules. But with polymers, these forces are greatly
compounded. The bigger the molecule, the more molecule there is to
exert an intermolecular force. Even when only weak Van der Waals forces are
at play, they can be very strong in binding different polymer chains together.
This is another reason why polymers can be very strong as materials.
Polyethylene, for example is very nonpolar. It
only has Van der Waals forces to play with, but it is so strong it's used
to make bullet proof vests.
One way to put a stop to all this chaotic motion is to make all the kids
join hands when you're walking. This won't be easy, rest
assured, with all the energy they have. But once
you get them to do this, their ability to run around is severely
limited. Of course, their motion will still be chaotic. The chain of
kids will curve and snake this way and that on its way to eat soybean
patties disguised as who knows what. But the motion will be a lot
slower. You see, if one kid gets a notion to just bolt off in one
direction, they can't do it because they will be bogged down by
the weight of all the other kids they're bound to. Sure, the
kid can deviate from the straight path, and make a few other kids next in line do so,
but the deviation will be far less than you'd get if the kids weren't
all linked together.
It's actually the same with molecules. A bunch of small molecules can move
around a lot faster and a lot more chaotically when they're not all tied
to each other. Tie the molecules together in a big long chain and they
slow down, just like kids do when you join them into a chain.
So then how does this make a polymeric material different from a material
made of small molecules? This slow speed of motion makes polymers do
some very unusual things. For one, if you dissolve a polymer in a
solvent, the solution will be a lot more viscous than the pure solvent, or that solvent with the same weight of small molecules as with the polymer.
In fact, measuring this change in viscosity is used to estimate
polymer molecular weight. Click here to find out
how.
So to sum up: because polymers are so big, they tangle up with each other like long pieces of string all mixed up together; the sum of their intermolecular forces is huge; and both those two characteristics mean polymer chains move much more slowly in the melt and in solution.
Summation of Intermolecular Forces
Time Scale of Motion
This is a fancy way of saying polymers move more slowly than small
molecules do. Here's an analogy, for what it's worth:
imagine you are a first grade teacher, and it's time to
go to lunch. Your task is to get your kids from the classroom to the
cafeteria, without losing any of them, and to do so with minimal damage
to the territory you'll have to cover to get there. Keeping
them in line is going to be difficult. Little kids love to run around
every which way, jumping and hollering and bouncing this way and that.
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