E. Thermal Analysis – To study the enthalpy changes associated with heating, annealing, crystallizing, and a wide variety of responses of the system to temperature, including polymerization, degradation, or other chemical changes.

Those technologies include: Differential Scanning Calorimetry; Thermogravimetric analysis; thermomechanical analysis; electrical thermal analysis; effluent gas analysis.

Differential scanning calorimetry (DSC) is a technique we use to study what happens to polymers when they're heated (thermal transitions of a polymer). The melting of a crystalline polymer is one example. The glass transition is also a thermal transition.

There is a certain temperature (different for each polymer) called the glass transition temperature, or Tg for short. When the polymer is cooled below this temperature, it becomes hard and brittle, like glass. Some polymers are used above their glass transition temperatures, and some are used below.

For example:

Hard plastics like polystrene and poly(methyl methacrylate), are used below their glass transition temperatures; that is in their glassy state. Their Tg's are well above room temperature, both at around 100 ^oC.

Rubber elastomers like polyisoprene and polyisobutylene, are used above their Tg's, that is, in the rubbery state, where they are soft and flexible.

However, don’t confuse the glass transition with melting.

The principal of DSC instrument:  

The two pans are heated at the same rate, usually around 10 ^oC per minute.

The Glass Transition Temperature

After a certain temperature, our plot will shift upward suddenly, like this:

This means more heat flows are on the polymer pan because there is an increase in the heat capacity of the polymer. This happens because the polymer has just gone through the glass transition. Polymers have a higher heat capacity above the glass transition temperature than they do below it. Because of this change in heat capacity that occurs at the glass transition, we can use DSC to measure a polymer's glass transition temperature. The change doesn't occur suddenly, but takes place over a temperature range. We usually just take the middle of the incline to be the Tg.

Crystallization

Above the glass transition, when they reach the right temperature, they will have gained enough energy to move into ordered arrangements, which we call crystals.

When polymers fall into these crystalline arrangements, they give off heat. When this heat is dumped out, the computer-controlled heater gives less heat to keep the temperature of the sample pan rising. You can see this drop in the heat flow as a big dip in the plot of heat flow versus temperature:

The temperature at the lowest point of the dip is usually considered to be the polymer's crystallization temperature, or T_c. Also, we can measure the area of the dip, and that will tell us the latent energy of crystallization for the polymer. But most importantly, this dip tells us that the polymer can in fact crystallize. If you analyzed a 100% amorphous polymer, like atactic polystyrene, you wouldn't get one of these dips, because such materials don't crystallize.

Also, because the polymer gives off heat when it crystallizes, we call crystallization an exothermic transition.

Melting

If we keep heating our polymer past its T_c, eventually we'll reach another thermal transition, which is called melting. The chains come out of their ordered arrangements, and begin to move around freely.

Melting is a first order transition. This means that when you reach the melting temperature, the polymer's temperature won't rise until all the crystals have melted. This means that the heater under the sample pan is going to have to put a lot of heat into the polymer in order to both melt the crystals and keep the temperature rising at the same rate as that of the reference pan. This extra heat flow during melting shows up as a big peak on our DSC plot, like this:

The temperature at the top of the peak is the polymer's melting temperature, T_m. Because we have to add energy to the polymer to make it melt, we call melting an endothermic transition.

Putting It All Together

Of course, not everything you see here will be on every DSC plot. The crystallization dip and the melting peak will only show up for polymers that can form crystals. Completely amorphous polymers (polymers don’t crystallize) won't show any crystallization, or any melting either. But polymers with both crystalline and amorphous domains, will show all the features you see above.

Because there is a change in heat capacity, but there is no latent heat involved with the glass transition, we call the glass transition a second order transition. Transitions like melting and crystallization, which do have latent heats, are called first order transitions.

Other Thermal Methods:

Thermogravimetric Analysis (TGA)

A sensitive balance is used to follow the weight change of the sample as a function of temperature.  It is used to study the thermal stability and decomposition temperature of the polymer.

Thermomechanical Analysis (TMA) – Measuring the mechanical response of a polymer as the temperature is changed.

F. Stress-Strain Properties in Tension.

Tensile strength- Tensile strength measures how difficult it is to break a substance when stress is applied to pull it apart. Tensile strength generally increases with molecular weight. An Instron grips a sample and pulls it apart. The instron creates a plot of where the y axis is the force exerted between the two grips, and the x axis is the separation distance between the two clamps. Usually, the electronics cause the Instron to increase the distance between the two clamps (pulling) at a constant rate, and the force or "strength" required is measured.

• A- Stress at the Yield Point: At this point the load (i.e., the force of the tensile pull) is sufficient to cause polymer chains to slip past each other, and yield occurs.
• B- Elongation at the Yield Point: This is how far you can stretch the polymer before yield failure takes place.
• C- Stress at Failure: At this level of stress the polymer molecules can no longer maintain cohesive integrity and the sample breaks.
• D- Elongation at Failure: This is how much the sample can stretch before failure occurs. Elongation Failure is often reported as follows: a 10 cm specimen that breaks at 15 cm is reported as a sample with 50% elongation. For this test you take a sample and clamp it with 10 cm between the two clamps, and then pull. Thus, you would need a sample with a length of 10 cm plus twice whatever length is required to go between the clamps.
• The slope of the curve is stiffness

Homework:

7. Draw typical DSC for a crystalline polymer, showing the glass transition, crystallization, crystalline melting.

8. Which of the stress-strain curves in Figure 9-8 would you expect to find for a polymer useful for a) a gear in a machine. B) a garden hose    c) the packing material to cushion delicate instruments   f) a fiber useful for rope.