Three Things That Make Polymers Different

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.

Summation of Intermolecular Forces

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.

Time Scale of Motion

This is a fancy way of saying polymers move more slowly than small molecules do. 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 to the cafeteria. 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. One way to put a stop to all this chaotic motion is to make all the kids join hands when you're walking them to lunch. This won't be easy rest assured, as there's always going to be a lot of little boys who are too macho to hold the hands of the girls next to them in line, and some who are too insecure in their manhood to hold anyone's hand. 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, he or she can't do it because he or she will be bogged down by the weight of all the other kids to which he or she is bound. Sure, the kid can deviate from the straight path, and make a few other kids do so, but the deviation will be far less than you'd bet if the kids weren't all linked together.

It's the same way 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. In fact, measuring this change in viscosity is used to estimate polymer molecular weight. Click here to find out how.

Return to Level Three Directory
Return to Macrogalleria Directory