Demonstrating the Glass Transition of Polylactic Acid with a Rattle

Co-Authored by Dean J. Campbell*, Rhiannon Davids*, Ian Smith,* and Karl Jung*

*Bradley University, Peoria, Illinois

Modern plastic products come in a diverse range of chemical compositions and properties for a wide range of uses. The familiarity of many types of plastics helps with demonstrations of polymer properties. For example, it is safe to assume that people have encountered polymers that are hard, soft, brittle, and flexible. One intriguing property of many polymer systems is the glass transition.^1,2 At temperatures below their glass transition, polymers tend to be harder and more brittle, that is, glassy in behavior. At temperatures above their glass transition, polymers tend to be softer and less brittle, that is, rubbery in behavior. Not every polymer has a glass transition, and for those that do, there is a bit of variation in the specific temperatures of those transitions. Glass transitions can be either above or below room temperature, meaning that some polymers are more glassy and some are more rubbery in behavior at room temperature.

One demonstration of the glass transition involves placing racquetballs into liquid nitrogen. Differential scanning calorimetry of a racquetball sample indicated a glass transition temperature of ~ -65 °C (208 K). One spectacular option for demonstrating the glassy and brittle properties of the cold racquetball is to smash it. Another approach, which requires fewer racquetballs over time, is the racquetball rattle or “jingle bell". This was first described a decade ago,³ but the related video is no longer on YouTube. A new version is shown in Video 1. A racquetball is hung from a wire, has a cut placed in it, and a metal nut inserted. When the ball is shaken at room temperature, the nut in the rubbery ball makes a dull thumping noise. When the rubbery ball is placed in liquid nitrogen at ~ -196 °C (77 K), the ball cools through its glass transition (~ -65 °C, 208 K) to convert to the glassy state. When the ball is shaken at low temperature, the nut in the glassy ball makes a sharp, tinkling noise.

Video 1. Demonstration of a metal nut in a racquetball being cooled with liquid nitrogen to bring it below its glass transition to produce a tinkling noise when shaken. Chem Demos YouTube channel (accessed 3/11/2024).

Another option for demonstrating the glass transition is to crush a cold-embrittled polypropylene cup (PP, glass transition ~ -10 °C, 263 K),² which still requires cold temperatures such as really cold weather, although liquid nitrogen also works well for that.⁴ There are also glass transitions above room temperature that can be demonstrated. Samples of stretched polyethylene terephthalate (PET, glass transition ~73 °C, 346 K)² and stretched polystyrene (PS, glass transition ~90 °C, 363 K)² from food containers will both contract and thicken when heated above their glass transitions.^5,6

Polylactic acid PLA has a glass transition (~55 °C, 363 K)² that is greater than, but closer to, room temperature. Additionally, PLA can be produced from renewable resources such as corn starch and is biodegradable.^7,8 PLA sheets and foams used for products such as cups and containers are touted as green, biodegradable alternatives to petrochemically-derived polymers that last a very long time in landfills. PLA is also a common and relatively inexpensive filament for 3-D printing objects. In recent years, the price of 3-D printers has decreased, and with an increase in personal models, 3-D printing has become more accessible than ever before, especially as secondary, post-secondary institutions, and public institutions, such as libraries and community centers, across the nation continue to add maker spaces and other resources to their campuses. While 3-D printing has moderate safety concerns, with proper preparation, the use of 3-D printing can be used to create safe and accessible demonstrations to make STEM outreach and education more accessible. A recent blog post described how 3-D printing can be used to produce molecular models of a number of carbon allotropes.⁹ Given the increasing use of PLA, its greenness, and the proximity of its glass-transition to room temperature, demonstrations based on PLA-based rattles that are similar in concept to the liquid nitrogen racquetball rattle, are described below.

Demonstrations

One polylactic acid rattle studied is based on a commercially-purchased PLA cup (16 fl oz cold cups from Eco-Products BPC, Boulder, CO). A small metal nut was placed into the cup. When the cup was shaken at room temperature (below its glass transition), the nut in the cup made a pronounced rattling noise. When the cup was placed partway into boiling water using tongs (not enough to let water into the cup), it was heated above its glass transition and contracted. When the heated cup was removed from the water and shaken, the nut in the hot cup initially made a dull thumping noise, but as the cup cooled down through its glass transition, the noise became sharper and louder. Figure 1 shows the cup before and after heating. Video 2 shows this demonstration.

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Figure 1. PLA cup (LEFT) before and (RIGHT) after heating in boiling water.

Video 2. Demonstration of a metal nut in a PLA cup being heated with boiling water to bring it above its glass transition to produce a thumping noise when shaken. Chem Demos YouTube channel (accessed 3/12/2024).

Another version of the polylactic acid rattle was 3-D printed. A 2.5 cm wide and 2.5 cm high hollow cylinder with 0.3 cm thick walls was 3-D printed from PLA. A hole was drilled into the cylinder and an ⅛ inch ball bearing was dropped inside. The hole was sealed with adhesive (E6000, Eclectic Products, Pineville, LA) which was also smeared over part of the outside of the cylinder to help seal it, as some printing errors may cause 3-D objects to have small holes. The PLA cylinder was glued to a plastic tube handle (an old bubble tea straw). A similar rattle was constructed based on a polypropylene egg, Figure 2.

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Figure 2. Rattles constructed from (LEFT) a 3-D printed PLA cylinder and (RIGHT) a PP egg, each contain a metal ball, sealed, and glued to a plastic tube handle.

When the rattles were shaken at room temperature, the balls in the rattles made pronounced rattling noises. When the PLA cylinder was placed into boiling water, it was heated above its glass transition. The walls of the cylinder warped in the heat, but not nearly to the same extent as the PLA cup. When the heated cylinder was removed from the water and shaken, the ball in the hot cylinder initially made a dull thumping noise, but as the cylinder cooled down through its glass transition, the noise became sharper and louder. In contrast, the PP egg, which was already above its glass transition, did not show the same change in rattling noise when removed from the boiling water. Video 3 shows this demonstration.

Video 3. Demonstration of a metal ball in a 3-D printed PLA cylinder rattle being heated with boiling water to bring it above its glass transition to produce less noise when shaken. A similar rattle based on a polypropylene egg is shown for comparison. Chem Demos YouTube channel (accessed 3/12/2024).

Conclusions

Each of the two PLA-based demonstrations have their advantages and disadvantages. The cup-based demonstration uses cups that can be purchased and show a dramatic change upon their first use, but they can be difficult to hold in boiling water. Tongs are suggested to hold the cup for this demo, but the demonstration must be carefully monitored to prevent the softened, deforming cup from filling with boiling water. Also, the cups are extensively and irreversibly deformed during that first use. They may still be used to show changes in rattling sound, but they again must be carefully monitored to prevent the softened, deforming cup from filling with boiling water. The 3-D printed rattle-based demonstration can deform less in boiling water, and can be reused more effectively, but spectators will not see the dramatic changes that can occur in the shape of the PLA cup when it is heated. The 3-D printed rattle also requires a little more effort and equipment to assemble, but the cylinder is not a terribly sophisticated object to 3-D print. The adhesive coating to seal the rattles does not appear to be biodegradable, but the handle could be made of renewable materials such as wood (like a chopstick). Long handles are best for use in boiling water. The use of PLA handles that were 3-D printed with the rest of the rattles were explored, but they required more extensive printing that consumed more PLA and sometimes softened in hot water. 3-D printers can use other polymeric materials besides PLA, such as acrylonitrile-butadiene-styrene (ABS, glass transition ~90 °C, 363 K) and polyethylene terephthalate glycol (PETG, glass transition ~79 °C, 352 K).^2,7 Though both have glass transitions that could possibly be reached at boiling water temperatures, they are petrochemically-derived.^7,8 ABS printing is also well known to release VOCs and can cause eye and respiratory irritation with poor ventilation.^7,8  

With proper preparation, the demonstration of polymer glass transitions using PLA presents limited safety and waste concerns, which allows great flexibility for where this demonstration can be showcased. For the general public, this demonstrates chemical properties seen in everyday objects, so that they can better understand the world around them, and for school-age students in the classroom, this demonstration can be tied into discussions to fit standards for a variety of age ranges. The reversibility of glass transition temperatures can be tied into 2-PS1-4, Construct an argument with evidence that some changes caused by heating or cooling can be reversed and some cannot, for very young students, whereas a discussion on how the glass transition temperature changes the plastic allows for diving deeper into standard MS-PS1-4, Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed, for adolescent students. If given the time or opportunity this demonstration could be used to give an introduction to materials chemistry transitioning into further discussion on how properties such as glass transition temperatures dictate what materials we use for different purposes. The green merits of these demonstrations can also be a classroom topic, for example, by discussing the life cycle of components for the rattles and the energy consumed by heating or cooling the demonstrations.

Safety 

When working with 3-D printers, make sure that the printer is free from obstructions and in good condition to prevent fires caused by the heated elements of the printer. In addition, make sure the room is well-ventilated, to allow fumes that may occur while printing to safely leave the room. Caution should be used when working with boiling water, especially in the presence of spectators. Heat-resistant hand protection is recommended. Shaking rattles removed right from boiling water carries a risk of splashing around very hot water. 

Acknowledgments We thank Edward Flint for assistance with the 3D printing. 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. The material contained in this document is based upon work supported by a National Aeronautics and Space Administration (NASA) grant or cooperative agreement. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of NASA. This work was supported through a NASA grant awarded to the Illinois/NASA Space Grant Consortium.

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