Grades: Grades 7-12
Author: Steven J. Wood
Source: This material is based upon work supported by the National Science Foundation under Grant No. EEC-1161732. Original.
Shape memory polymers are an emerging class of polymers that have the capability of changing into a different programmed shape and then back to its original shape. This shape change is usually caused by some outside stimulus like heat, light, magnetism or electricity. Because shape memory polymers can exist in different shapes, they have many potential uses such as self-tying sutures, medical implants and other high tech applications. In this activity, students will transform a homemade rubber band into a shape memory polymer using latex and lauric acid. Students will then be tasked with creating specific shapes with their shape memory polymer as well as creating new shapes of their own.
What should students know as a result of this lesson?
What should the students be able to do as a result of this lesson?
Teacher Demo: Engage students with a teacher demo. The instructor should wear goggles and gloves for this demo.
Using ~ 10.0 mL of liquid latex, add about ~20.0 mL of household vinegar. Take and mix the latex and vinegar together. The latex will coagulate and form a glob. Form the glob of latex into the shape of a rubber ball. Be careful as liquid latex will squirt out if you squeeze too hard. Demonstrate this reaction and discuss how the latex polymer forms.
Ask the students if they know different materials that are made of rubber? (Tires, rubber bands, balloons, shoes, etc.)
(optional) Time permitting it may be of interest to explore how latex is produced and where it comes from.
This short video does a nice job explaining latex production. https://www.youtube.com/watch?v=CKq42J7SaWw
Latex has a special property in that it is a liquid as long as it is alkaline (basic). Ammonia is added to keep the latex in the liquid form. Polymer chains of latex form micelles in a basic solution. When the base is removed, the micelles are destroyed and the polymer chains clump together. This process is called coagulation. The larger the polymer chains become, the more solid the material is.
Show the following video. https://www.youtube.com/watch?v=iWZouwl8g8A
Tell the students they will use this same reaction to make homemade rubber bands by dipping forms into liquid latex and coagulating them with vinegar.
We will then use our homemade rubber band to create a shape memory polymer. A shape memory polymer is a material that has the ability to be programmed into different shapes, but also has the ability to return to its original shape. The process of changing the shaped into a new one is called programming. Students will program their shape memory polymer by placing it in hot water. The process is similar to this video. https://www.youtube.com/watch?v=E_VrPyDJqx4
Students will be tasked with creating a shape memory polymer and programming it into 3 predetermined shapes (square, triangle and coil). Students will then be tasked with programming their own shapes into the shape memory polymer.
This is a video of the shape memory polymers returning back to the original shape. https://youtu.be/ssH-ryD-WBE
Day 1 – Students will follow the procedure as written in the "Creating Shape Memory Polymers" handout to make homemade rubber bands. Students will see how the latex coagulates and forms a solid. Qualitative observations should be made in the student's data table. The goal of day one is that each student makes three homemade rubber bands that will be transformed into shape memory polymers in day 2. Students should also test the elasticity of the homemade rubber band. It should be noted that the strength of the rubber band is less because the polymer is not cured. Curing of latex causes cross linked bonds to form, which makes the polymer stronger and more elastic. Students can clean up the excess latex by coagulating it with vinegar to form a latex ball. Rubber bands should be left out overnight to finish curing.
Day 2- Students will soak their home made rubber bands in lauric acid at 50 degrees C as explained in the "Creating Shape Memory Polymers" handout. The rubber band soaks up the lauric acid. Once the rubber band has absorbed the lauric acid it is ready to be programed into the first shape. Students will dip the rubber band/lauric acid polymer in hot water (around 45-50 degrees C). They will then form the polymer into the shape they desire. The lab handout tasks the students to make three required shapes (square, triangle and coil). These shapes can be made by the students manipulating the shape memory polymer in their hands or using a form of some shape. Once the student has the desired shape, allow the shape memory polymer to cool by either plunging it into cool water or let it cool in air. Students can be creative in making different shapes and exploring ways to manipulate the shape memory polymer. (Example: rubber bands can be cut and letters can be made to spell things out.).
Students should be able to explain the coagulation process of latex into rubber by household vinegar. They should be able to observe the effects of cross linking on the elasticity and strength of the rubber bands they create. Students should also understand that as a polymer chain gets longer, the properties of the material change. Liquid latex has relatively short polymer chains that are not attached. As the latex coagulates, the polymer chains combine and the material becomes more solid and stronger.
Students should also be able to explain what the shape memory cycle is. The shape memory cycle is basically two shapes. A normal shape and a programmed shape. Students should know how to program the shape memory polymer by heating the polymer to above the melting point of lauric acid (~50 degrees C) and locking in the shape by cooling it back down. To return the programmed shape back to the original, the shape memory polymer can be heated to 50 degrees C. Students can also be evaluated by their success in making the three shapes (square, triangle and coil) and returning them back to the original.
Students will use their knowledge of the shape memory cycle to create new shapes and applications for the shape memory polymer. After students have created the three basic shapes (square, triangle and coil), they can be assessed on making new and useful shapes with their shape memory polymer. Students can be given engineering problems such as; "make your shape memory polymer fold up and then unfold." Students can also dream up potential uses for shape memory polymers.
Common Core Standards:
Polymers are long chained molecules made of many repeating units called monomers
Cross linking is connecting polymer chains through side chained bonding.
Shape memory polymers are modern materials that have the ability to exist in multiple shapes depending upon programming.
Programming is using heat, light, electricity, etc. to change the shape of a shape memory polymer.
Coagulation is the process by which latex forms into rubber.
Caution should be used in working with latex as some students have allergic reactions to it. Students should wear non-latex gloves for this activity. Students should also use caution working with lauric acid as it is flammable. Do not use open flames for this activity. Students should wear safety goggles for this activity.
Shape memory polymers are currently used in medical applications such as self-tying sutures and medical implants. They are also used for satellites and other high tech applications. Uses for these polymers are still being developed as they are discovered.
Students can be assessed on their ability to make the proper shapes (square, triangle and coil).
Students can also be assessed on their lab write up. Data collection, conclusions, questions, etc.
Grouping Suggestions: Students should work in pairs.
I used Holden's HX-80 latex, but this procedure would work with other types as well. Just be sure that it has ammonia in it so that it can be coagulated with vinegar.
Stearic acid also works instead of Lauric, but in has a higher melting point, so the lab would need to be run at 70 degrees C instead of 50 degrees C.
Real rubber bands can be used instead of homemade rubber bands, but the soaking time in lauric acid is longer.
The handout requires the students to make three rubber bands. This activity can be shortened by having the students make one or two. Students can reuse the shape memory polymer several times to make different shapes.
Students should be encouraged to use the shape memory polymer to solve some engineering problem. (Like make a folded shape unfold) or (make a self-tying knot).
Pacing/Suggested Time: Introduction/Engagement of the activity (30 minutes); Day 1- Rubber band making (~45 minutes); Day 2 – Shape memory polymers (~45 minutes)