In this lecture, we will talk about the glass transition temperature. Why is that important? It's essential knowledge that is needed when processing polymers. Why would you say that? It takes energy to process things. As we said earlier, when we were processing, a lot of times we wanted the material to be soft. It's easy to shape. This lecture, we'll talk about how the mechanical properties vary as a function of temperature. We'll do that in reference to the modulus of elasticity. Let's start off our conversation about looking at the material. How it varies as a function of temperature. Specifically, looking at this specific volume. The specific volume is just one over density. Lets look. If I'm looking at how the specific volume changes as a function of temperature in the liquid. The specific volume goes down as we cool. That shouldn't be right because as I heat up, I get more agitation. Now, for a solid, crystalline solid material, when we cool, there we go from the liquid. There's an isothermal transition where we move the latent heat freezing. We undergo this isothermal transition at the melting temperature. Then we become crystal and solid. Then as we cool, the specific volume continues down this linear path. That's for the crystalline solid. Let's take a glassy material. I cool from the liquid. Now I cross the melting temperature, I change the heat capacity. Then I finally go through another change. As I cool the specific volume changes. Now notice here the liquid does not undergo an isothermal transition. There's a constant rate change. Then finally, when we get to the solid below the glass transition temperature, the glass, the specific volume changes with lowering the temperature. This change here is referred to as the glass transition temperature. At temperatures between the melting temperature and the glass transition temperature the material is soft. This is going to be our working temperature for the glass. That's when we were discussing earlier. The calorie. When we had the glob is soft, we can can roll it into the desired shape and thickness. In-between the glass and the crystalline solid, we can have semi-crystalline material. Let's monitor the change in the specific volume upon cooling for the semi-crystalline. Semi-crystalline, we haven't change. We're going to start cooling it down in a liquid. Then we have an isothermal change in the specific volume. However, we cool and there's a change in the specific volume till we hit the glass transition temperature. Then that rate, as we call, we have another constant rate. For the semi-crystalline similar manner, this would be our working temperature. When we're processing. This is the temperature range where we want to process our material. We have three specific types of polymers presented in this curve. Well, we'll say two. Here we have one that's glassy and then semi-crystalline, and we compare it to a crystalline solid. Again, the crystalline solid we cool from the liquid, undergoes an isothermal reaction, removing the latent heat of freezing, and then it cools along this straight line again, at a constant rate. Liquid specific volume changes upon not freezing. Then the specific volume continues to change at a given rate. However, when we undergo that, or go through the glass transition temperature, it becomes a very stiff, brittle material. Semi-crystalline is in-between crystalline solid in glass. Cool down the latent heat of freezing. We have a rate of change of our specific volume. This range is again, the working temperature. Then when we go through our glass transition temperature, it becomes a brittle solid.
