Polypropylene and the Cold Snap

Years ago, I put on a pair of snowshoes during a winter cold snap and found that the plastic binder straps broke fairly easily. At first I thought the plastic straps had simply become more brittle with time, but I found that the straps became less brittle when brought inside to warmer temperatures. This indicated that perhaps the polymer used in the straps had a glass transition temperature not far below room temperature. Above the glass transition temperature (Tg), the molecular chains in a polymer have sufficient energy to move around, and polymers tend to be more flexible and rubbery.  Below the glass transition temperature, there is less molecular mobility and polymers tend to be more hard and brittle.^1-3 A quick online search revealed the likely candidate for the snowshoe straps to be polypropylene, with glass transition temperature in roughly the -10°C to -20°C range.¹ After my initial surprise (“Who makes snowshoes that cannot handle the cold?”) I turned to looking at other polypropylene objects to see how they fared in the cold.

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Figure 1: Polypropylene container lids (at left) flexing at room temperature and (at right) after breaking apart in a brittle manner at -23C

I have found that thin-walled polypropylene containers become less flexible and more brittle when the weather gets cold. These are relatively easy to obtain – look for the #5 PP resin code on the polymer. An example of this is shown in Figure 1 and Video 1 below. Polypropylene container lids were flexible at room temperature, but were quite brittle after sitting outside at -23°C.

Video 1: Glass transition in polypropylene container lids (at left) flexing at room temperature and (at right) after breaking apart in a brittle manner at -23°C, Chem Demos YouTube Channel (accessed 1/20/21)

Thin-walled polypropylene cups, like those currently served at McDonald’s restaurants, can also show glass transition temperature responses. The effects are perhaps a little less clear-cut, but squeezing the McDonald’s cups at room temperature and releasing them often allows them to rebound to their original shape. Squeezing the McDonald’s cups at about -17°C often cracks the cup, sometimes in multiple places. In all of these demonstrations, wearing gloves might help prevent heat from your hand from warming the thin polymer containers. They also help keep your hands warm and protect them from sharp plastic edges.

Possible connections to be made in the classroom include other polymers with demonstrable glass transition temperatures, such as Scrub Daddy sponges,² racquetballs,³ and Shrinky-Dinks shrinkable plastic.³ I investigated the acoustic properties of some polypropylene samples above and below their transition temperature. Like other polymers we have investigated,³ polypropylene cups tend to transmit higher frequencies and “ring” more when they are tapped in their glassy state at -20°C than at above the glass transition temperature at +15°C, as shown in Video 2.

Video 2: Cool sound of polypropylene cups, Chem Demos YouTube Channel (accessed 1/20/21)

As a sort of crowdsourced activity I reached out to local teachers and students and challenged them to hear the temperature/sound connection with their own polypropylene containers. Many, but not all, of the students reported that they could hear a difference. I also attempted to investigate the sound produced by the containers by using SpectumView sound spectrum software on a cell phone, again observing some subtle differences above and below the glass transition temperature. In all cases, care must be taken to tap the polypropylene objects consistently between trials for best results.

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Figure 2: A snowy McDonald’s cups testing area.

Still another, more somber, connection is to use glass transition temperature demonstrations to show how temperatures effects on materials to can cause catastrophic failures. The disintegration of the space shuttle Challenger 35 years ago resulted in part from the inability of O-ring polymer material to quickly rebound to its desired shape in cold weather conditions.⁴ The attractive aspect of using polypropylene for discussing glass transition temperatures is that if the weather is just right (some might say really wrong), there is no need to use liquid nitrogen, dry ice, or ovens to access the glass transition temperature. If the weather turns ridiculously cold, do some cool chemistry!   

Safety

Breaking brittle objects can produce pieces of material with sharp edges. Protect your eyes and skin from and flying objects. Cold temperatures can produce frostbite, so wear warm clothes!  

Acknowledgements

This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium.

References

1. Omnexus. Glass Transition Temperature (accessed Feb 2021).
2. Kuntzleman, T. S., Temperature Experiments with the Scrub Daddy Sponge, ChemEd X, Feb 21, 2018. 
3. Campbell, D. J.; Peterson, J. P.; Fitzjarrald, T. J., Spectroscopy of Sound Transmission in Solid Samples, J. Chem. Educ., 2014, 91, 1684-1688.
4. Report of the PRESIDENTIAL COMMISSION on the Space Shuttle Challenger Accident. (accessed Feb 2021)