The Glass Transition of Polymers

Location:

Concepts Shown:

The glass transition of polymers, Tg

Equipment:

1. A flask of liquid nitrogen

2. Rubber Tubing (or racquetball)

3. Rubber O-Rings

4. Hammer

5. Tongs to hold tubing and ball

Procedure:

Pass the rubber tubing around the class to allow the students to feel its softness. Then place one end of the tubing in the liquid nitrogen.

Warning: Some of the nitrogen may shoot out of the other end. Goggles for the instructor and for the students in the front row should be provided.

When the tubing has been immersed, the liquid nitrogen will begin to bubble. Once the bubbling has ceased, remove the tubing from the nitrogen and hit on the side of the desk to demonstrate that the rubber has, in effect, been 'frozen'. Place the end on the desk and strike with the hammer. The tubing which was previously rubbery will shatter like glass, showing its present brittle properties. Pick a few of the shattered pieces and place them in front of the students. They will see that the pieces return to their rubbery state as they warm up. One can perform the same type of experiment with a racquetball. Cool the ball in the nitrogen and strike with a hammer. If a ball is used, a pair of tongs will be needed for the immersion process.

Science:

As with many materials, polymers that can be cooled sufficiently below their melting temperatures without crystallizing will undergo a glass transition. There is a dramatic change in the properties of the polymer at Tg. For example, there is a sharp increase in the stiffness of an amorphous polymer at T's < Tg. One of the most widely used methods of demonstrating the glass transition and determining Tg is by measuring the specific volume of a polymer sample as a function of temperature. In the regimes above and below the glass transition temperature, there is a linear variation in the specific volume with temperature. But in the vicinity of the Tg there is a change in the curve which occurs over several degrees. The Tg is normally taken at the point at which the extrapolations of the two lines meet. It is found that the lower the cooling rate, the lower is the value of the Tg. This is because a slower cooling rate permits molecular adjustments. This Tg lowering can be seen in the graph below.

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One of the most useful approaches to analyzing the glass transition is to use the concept of free volume. This concept has been used in the analysis of liquids and it can be readily extended to the consideration of the glass transition in polymers. The free volume is the space in a solid or liquid sample which is not occupied by polymer molecules, i.e. the 'empty space' between molecules. In the liquid state it is supposed that the free volume is high and so molecular motion is able to take place relatively easily because the unoccupied volume allows the molecules space to move and so change their conformations freely. It is envisaged that the free volume will be sensitive to the change in temperature and that most of the thermal expansion of the polymer rubber or melt can be accounted for by a change in the free volume. As the temperature of the melt is lowered the free volume will be reduced until eventually there will not be enough room to allow molecular rotation or translation to take place. The temperature at which this happens corresponds to the Tg, as below this temperature the polymer glass is effectively �frozen� (i.e. brittle). Bulkier repeat units increase Tg as they hinder movement. Polar groups such as -Cl, -OH, and -CN also raise the Tg because the polar interactions restrict rotation. Variations in molecular architecture, such as crosslinking and branching restrict chain mobility and thus increase the Tg of the polymer. The table of Tg values below demonstrates these facts.

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Remarks:

It is important for the students to realize that this is more important than just a cool demo. In fact, ignorance of the glass temperatures could have catastrophic results, as in the Challenger explosion. The rubber O-rings that separate the parts of the rocket booster are supposed to ideally 'squish' in order to form a good seal at the joints. However on the day of the Challenger's ill-fated flight, the temperature was lower than the rubber's Tg. Thus the O-rings underwent their glass transition stage. As a result, they turned brittle and could not be compressed to form that seal. When the engines ignited and the Challenger took off, the fire in the boosters leaked through the seals and destroyed the entire shuttle. What happened to the Challenger can effectively be shown by placing a rubbery O-ring in the liquid nitrogen and watching it turn brittle.

References:

Author:
Rahul Pinto
Credits:


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