The glass transition temperature (Tg) or melting temperature (Tm) of polymers are important characteristic properties. How do we measure those thermal transitions or phase transitions? Now, we will discuss a common approach to quantifying Tg and Tm, called differential scanning calorimetry (DSC). DSC is a technique that measures heat required to change the temperature of a sample and a reference of known heat capacity. The direction of heat flow is very important in DSC, as that will tell us whether we have an exothermic or endothermic transition, and this is often a point of confusion when analyzing data from DSC.
To prepare for our discussion of DSC, let’s first review heat flow and how it relates to phase transitions, which is something you first learned in introductory chemistry. You likely saw a plot something like this, which depicts the heat associated with temperature transitions for one mole of H2O:
Notice that while the material undergoes a phase transition, the temperature does not change even though heat is being added. For example, look at ΔHfus in which H2O goes from solid ice to liquid water. We have to add 6 kJ of heat to cause this transition to happen for 1 mole of water (so it is an endothermic phase transition), but the temperature remains constant at 0°C during this phase transition. Once we get liquid water, as we add heat, the temperature goes up linearly; how much heat you have to add to increase the temperature is a function of the material’s heat capacity.
When we do DSC analysis, we are going to be heating our sample to specific temperatures at a specific rate and looking at how much heat is required to get us to that temperature. In a way, it has a lot of similarities to this plot, except instead of tracking absolute values of heat added, we measure the heat flow. If the heat flow goes up, that means we will be adding more heat to the system to raise it to a given temperature, whereas if heat flow decreases, heat is released from the system to the surroundings. Let’s look at some sample data you might collect from DSC and try to interpret it.
Look at the graph in Figure 11.12. The Y axis is important, and identifying which “direction” is endothermic vs exothermic is key. In this class, I will keep the axis the same, as in what is shown in Figure 11.12 (but be wary that you may see examples in other books or papers that define the axis in the opposite direction). Here, an increase in heat flow means that the DSC machine is putting in extra heat to try to raise the sample temperature – and it has to do this, because the material is undergoing an endothermic phase transition or a change in heat capacity. In the plot in Figure 11.12 below, we see that initially the heat flow is constant while the temperature is increasing. This means the sample has a constant heat capacity. Suddenly, we see a spike in heat flow; this spike corresponds to a phase transition. During a phase transition, heat is either input or released, but there is no change in temperature (recall our phase diagram for water?) This is why you see that sudden increase in the DSC trace – the machine has to put in a lot more heat in order to get the temperature to rise above the transition temperature. In this specific example, the peak is pointing upward, which means we are inputting heat, and thus the phase transition must be endothermic. Thus, if this was a polymer sample, this would be the melting temperature (Tm). (Vaporization is also endothermic, but for polymers, it’s unlikely that we would be vaporizing them).
PROBLEM
A DSC trace is shown below. What is happening at the temperature marked?
ANSWER
The polymer is crystallizing. Notice that this peak is pointing down; at the temperature marked (T), heat flow to the sample is decreased, and that’s because the sample is undergoing an exothermic phase transition. Crystallization is the relevant phase transition for polymers.
Another DSC trace is shown below in Figure 11.13. This one looks slightly different than the melting and crystallization curves we saw before. Here, we notice that the heat flow increased, but never came down again! Therefore, this feature cannot correspond to a phase transition, but must be indicative of something else – such as, a change in the heat capacity (ΔCp). This change in heat capacity is correlated to the glass transition temperature (Tg), in which the polymer is going from a glassy solid state to a more viscous state.
Putting it all together could look something like the DSC trace shown below in Figure 11.14:
Not all polymers will have all of these transitions, and there are a number of factors that can affect the DSC trace. For example, some polymers do not crystallize, so keep in mind which curves you might expect or not expect to see depending on the polymer chemistry and skeletal structure. It can often be very difficult to define a specific temperature for these transitions, as the peaks may be quite broad or happen over a range of temperatures. The thermal history of the polymer can also affect the measurements of melting (Tm) and crystallization (Tc) and glass transition (Tc) temperatures, because of hysteresis. Heating rates and cooling rates can also affect these measurements. However, in general, DSC is a very powerful technique that helps us to probe the thermal properties of polymers.