Consequences of Being Big
Have you ever wondered what it felt like to be as tall as an NBA center? Or as big and muscular as NFL defensive lineman? Sure, they’re bigger than average, and certainly they have skills and abilities that go beyond what a “normal” human being has, but hey! They’re still human and they still fit in a typical airline seat (although just barely).
So what about the relative size of polymer molecules? Are they all that much bigger than small molecules we may be familiar with? And even if they are bigger, so what? After all, a basketball center can’t jump more than maybe twice as high as you can, so can macromolecules be all that different in behavior from small molecule materials like salt, sugar or natural gas? And the answer is… Emphatically, yes!
So let’s talk more about how and why these polymers, these macromolecules, act differently than small molecules. There are three important ways in which the extreme length of a polymer chain compared to its molecular width cause the polymers to behave in unique and useful ways. The three are usually named:
- Chain entanglement
- Summation of intermolecular forces
- Time scale of motion
That’s all well and good, these fancy names, but what do they mean in reality? Can we find some useful analogies (don’t we just love those!) to help us understand these concepts? Sure and by golly! Let’s start with the first of these:
Chain Entanglement
We’ve discussed this in the “Analogies” lesson using the string example. Remember 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 like the strings in a tangled up ball.
Can’t help ourselves- we’re going to throw a couple more analogies at you and see if they help. First, think back to when you were a kid playing with your mom’s jewelry box. Or it could be a friend’s jewelry or someone else entirely. If you never did that as a kid, try to visualize it now, a gedanken experiment. Some women are less tidy than others, and when they take off a necklace, they toss it into their jewelry box. Do this for awhile and the necklaces tend to overlap and get mixed up, especially if you try to find one that you want now.
You start to pull one of the necklace ends out, hoping to separate them one at a time. Drat! The whole mess of necklaces comes out in one big, tangled up mess. In fact, if there many of the really fine chains in there, it could take hours to disentangle them all to get just the one you wanted. With me so far?
Here’s analogy number two, and like all the others, it has some good points that correspond well with a real polymer chain, and some that don’t. This is one that nobody is actually going to do- too expensive, too time consuming and no athletic department would ever let you mess up their stadium this way. The analogy uses bicycle chains. Why? Because it turns out that the relative scale of one link (length versus thickness) is pretty close to that of the two carbon repeat unit of polyethylene: -CH2CH2-. So far so good, but no for the actual relative length.
If we make the bicycle chain as long (versus chain link width) as a polyethylene chain, the bicycle chain would be about three (3!) football fields long. Hard to imagine ever actually making such a chain, but let's press on. You make enough of these really long bike chains to fill up a typical football stadium, just throwing them over the top or having the football team do it for you. Takes a while but once it’s full, you reach in and try to pull one out. Nope. You decide to use a crane, and tug and tug until Bang! the chain breaks. Same for every other chain you try. The entanglements are so many that there's no way you can pull out a single chain.
Combine these two with the string analogy, and you should have a good mental picture of entanglement. Now on to summation of intermolecular forces, which is just a fancy way of saying “friction.”
Summation of Intermolecular Forces
Ever heard of “intermolecular forces?” If not, no problem, 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, which is why it's a gas at room temperature.
As I said, intermolecular forces affect polymers just like small molecules. But with polymers, these forces are greatly compounded because they add up over the entire molecule. 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 nonpolar. It only has Van der Waals forces to play with, but it is so strong it's used to make bullet proof vests.
Why? Simple answer is because it&'s so big that it gets all tangled up. Only partly right. The “rest of the story” has to do with all those weak van der Walls forces that make even the ethylene monomer units want to be close to each other.
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 young 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 can't do it because he 'will be bogged down by the weight of all the other kids to which he 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 get 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. Join the molecules together in a big long chain and they slow down, just like kids do when you make them hold hands.
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 one way to estimate polymer molecular weight. Click here to find out how this is done.
The Consequences of Being Big
Let's get back to those simple linear polymers now. These giant chain-like molecules, because they're so big and because of their shape, act in ways that small molecules don't. There are three main consequences of their “bigness,” some of which took us a long time to figure out, and even longer to figure out what they mean. To find out what they are, take a look at the next lesson. For now, though, it's time for another (you guessed it!) quiz! Click here for your next evaluation quiz.
Now for the last chapter on where polymers are in the world and what they are used for, both natural and synthetic ones: Polymer Applications.
