But the chains that melt are not the chains that undergo the glass transition. Products By Bayt. Use Our Mobile App. Get Fresh Updates On your job applications, and stay connected. Download Now. Now the exact temperature at which the polymer chains undergo this big change in mobility depends on the structure of the polymer.
To see how a small change in structure can mean a big change in T g, take a look at the difference between poly methyl acrylate and poly methyl methacrylate on the acrylate page. Twistin' the Night Away There is a difference between polymers and snakes that we probably should discuss at this point. An individual snake is not only wiggling around, but actually moving from one side of the room to the other. This is called translational motion. When you walk down the street, presuming you're not like most Americans who never walk anywhere, you are undergoing translational motion.
While polymers are not incapable of such motion, mostly they are not undergoing this kind of motion. But they are still moving around, wiggling this way and that, much like little kids in church. To be sure, by the time we get down to the glass transition temperature, it is already too cold for the polymer molecules, tangled up in each other as they are, to move any distance in one direction.
The motion that allows a polymer way above its glass transition temperature to be pliable is not usually translational motion, but what is known in the business as long-range segmental motion. While the polymer chain as a whole may not be going anywhere, segments of the chain can wiggle around, swing to and fro, and turn like a giant corkscrew. The polymer samples may be thought of as a crowd of people on a dance floor.
While each whole body tends to stay in the same spot, various arms, legs, and whatnot are changing position a lot. When the temperature drops to near or below the T g, for polymers the party's over, and the long-range segmental motion grinds to a halt.
When this long-range motion ceases, the glass transition occurs, and the polymer changes from being soft and pliable to being hard and brittle. See for yourself Now to make sure this is all clear, we made a little movie showing what happens to the polymer chains at the glass transition temperature. Click here to watch it. Want to have some fun? First, get your teacher to bring some liquid nitrogen to class, or maybe some "dry ice" suspended in acetone or methanol. Then put some in a styrofoam cup, and drop in some household objects made from polymers, like rubber bands or plastic wrap.
The liquid nitrogen, being so cold, will cool the objects below their glass transition temperatures. Try to bend your rubber band hold it with a pair of pliers, because you could get frostbite if you try to touch it with your fingers and it will shatter! Neato, huh? The rubber band will shatter because it's below its glass transition temperature.
And sandwich wrap? It will crumple and sound like hard thin plastic, and might even shatter if it's cold enough. Measuring the T g If you want to know how we measure both melting points and T g's, plus latent heats of melting, and changes in heat capacity, now there's a wonderful page to tell you all about a technique called differential scanning calorimetry.
Go visit it! Want to know more about the wonderful glass transition? Read these little segments! Messing Around with the T g The T g vs. Messing Around with the Glass Transition. Sometimes, a polymer has a T g that is higher than we'd like. That's ok, we just put something in it called a plasticizer. This is a small molecule which will get in between the polymer chains, and space them out from each other.
We call this increasing the free volume. When this happens they can slide past each other more easily. When they slide past each other more easily, they can move around at lower temperatures than they would without the plasticizer.
In this way, the T g of a polymer can be lowered, to make a polymer more pliable, and easier to work with. If you're wondering what kind of small molecule we're talking about, here are some that are used as plasticizers:. Have you ever smelled "that new car smell" in some friend's new car? It's not something I smell too often on the money I make, but that smell is the plasticizer evaporating from the plastic parts on the inside of your car.
After many years, if enough of it evaporates, your dashboard will no longer be plasticized. The T g of the polymers in your dashboard will rise above room temperature, and the dashboard will become brittle and crack.
The Glass Transition vs. Melting Keywords: first order transition , heat capacity , second order transition It's tempting to think of the glass transition as a kind of melting of the polymer. But this is an inaccurate way of looking at things. There are a lot of important differences between the glass transition and melting. Like I said earlier, melting is something that happens to a crystalline polymer, while the glass transition happens only to polymers in the amorphous state.
A given polymer will often have both amorphous and crystalline domains within it, so the same sample can often show a melting point and a T g. But the chains that melt are not the chains that undergo the glass transition. There is another big difference between melting and the glass transition.
When you heat a crystalline polymer at a constant rate, the temperature will increase at a constant rate. The heat amount of heat required to raise the temperature of one gram of the polymer one degree Celsius is called the heat capacity.
Now the temperature will continue to increase until the polymer reaches its melting point. When this happens, the temperature will hold steady for awhile, even though you're adding heat to the polymer.
It will hold steady until the polymer has completely melted. Then the temperature of the polymer will begin to increase once again. The temperature rising stops because melting requires energy. All the energy you add to a crystalline polymer at its melting point goes into melting, and none of it goes into raising the temperature.
This heat is called the latent heat of melting. We can use the enthalpy of melting of the polymer to calculate the degree of crystallinity. TG of polymers is the glass transition temperature of a polymer while TM of polymers is the melting temperature of a polymer. The key difference between TG and TM polymers is that the TG describes the conversion of the glassy state into rubbery state whereas the TM describes the conversion of the crystalline state into an amorphous state.
Therefore, as another important difference between TG and TM polymers, we can say that the TG describes the conversion of the phase of matter solid to rubbery phase but, the TM does not describe the conversion of the phase of matter solid to solid. A further difference between TG and TM polymers is that the TG is applicable for amorphous and semi-crystalline polymers while the TM is applicable for semi-crystalline and crystalline polymers. The TG and TM are very important temperature parameters of polymers.
The key difference between TG and TM polymers is that the TG of polymers describes the conversion of glassy state into rubbery state whereas the TM of polymers describes the conversion of crystalline state into an amorphous state.
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