Now we've already looked at how the modulus of elasticity and the strength of the various types of polymers, where we were looking at either a bakelite, amorphous, semi-crystalline, or lightly cross-linked. We looked at the modulus of elasticity here at room temperature. Now we want to look at how does the mechanical properties vary as a function of temperature, because we've already seen that specific volume, reciprocal of density, vary substantially as a function of temperature, whether a material is amorphous or semi-crystalline. Here we have a plot of the modulus of elasticity log as a function of temperature for heavily cross-linked material, lightly cross-linked elastomer and amorphous material C. We can see this would be our melting temperature. Now as we cool, the modulus elasticity increases. Then we have change, and then we undergo a very rapid change here and we get to our high strength. The regions where I have this high strength, high rate of change, we're going to refer to that as our glass transition temperature. Heavily cross-linked material, more carbon bond, higher melting temperature, higher strength are lightly cross-linked elastomer. Then we have our amorphous material. A log of the modulus of elasticity as a function of temperature. Easy to extract out the glass transition temperature. When we have this plot of the log of elasticity, there are four important regions. We have the viscous region, rubbery, leathery, and glassy regions. Glassy region tip is denoted by this flat plateau. The temperature is greater than the glass transition temperature. Now we go, if we heat it up, if we recall, glasses are brittle. Now, if I heat it up, I get to more of a leathery compliant a little bit. As I go to lower temperatures, a falling transition through the glass transition temperature into the rubbery transition region. Again, now at this point, the material is very flexible. Then following, when it slopes downward, this becomes the viscous region, meaning I have a very viscous liquid. The following we've increased to more, you're at the melting temperature. If we go the other way from the liquid would cool down, we get a viscous liquid, then it becomes rubbery. Then we go through the glass transition temperature, and then we hit the plateau in the glassy region. One example, think of the tennis ball. We've seen the example. At room temperature, you can just bounce it, bounce it and bounce it, put it in liquid nitrogen, and it freeze, throw it against the floor, it's a glass, it shatters. Same particular case, we've gone through the glass transition temperature all the way into the glassy phase. We look at the change. Here, we're talking about a change of 100-1000. Similar manner, 100-1000. It's in this region where we'll have our high rate change, we'll call our glass transition temperature. We have the glassy, leathery, rubbery, and the viscous regions. Here's a list of various materials looking at their glass transition temperatures. Polyethylene, teflon, polyethylene, polystyrene. In your recitation, you will have activities where you will look at how the glass transition temperature varies with melting temperature as well as room temperature. Here's one example. We want to know where is room temperature relative to the glass transition temperature for this high density polyethylene. In this particular case, the glass transition temperature is going to be minus 120 degrees C. Well, if that's the glass transition temperature, we would say that the melting temperature is 130. That's about 200 degree difference. We're looking for a glass, 200, we'll put 100 there. We'll say room temperature is there. You would think, our trash bags are rubbery. We can flex them and we can pull on them. We'll do just one more example.
