Lesson Background and Concepts for Teachers
Note: The teacher demonstrations require some advance preparation; see instructions below for details. The two sets of demonstrations show each of the Weissenberg, Barus, Kaye and defying gravity effects. One demo uses a polyvinyl alcohol (PVA) solution and requires a power drill and a glass rod. The other demo requires rubber bands and a large wooden rod.
If time does not permit the demo preparations, show students these online videos instead:
Before starting the lesson, it is helpful for the teacher to read through the
Close Encounters of the Polymer Kind Presentation
prompts and notes on each slide of the PowerPoint® file and through the
Close Encounters Worksheet
to review essential background information to supplement that which is provided below. If time does not permit going through the entire lesson background, read only the slides and present students with the
Close Encounters of the Polymer Kind Lesson Handout for Students
, which provides a full summary of the background information contained in the lesson. Then use either the worksheet or handout as a graded assignment.
Materials List for Demos
Making the PVA Solution
Heat 100 ml of water to 80 °C and add 4 grams PVA..
Stir the solution over heat while mixing until the PVA is dissolved. It is a slow process.
In another beaker, add 20 ml water and 1 gram borax.
Mix the two solutions and allow time to cross-link (overnight if possible).
Add a color dye if desired, to make the demo more colorful.
Discussion with Students
Note: The presentation includes embedded videos on slides 7, 12, 15 and 21. In order to play the videos on each slide, you must download the videos:
Three Viscoelastic Effects
, as well as the PowerPoint® file, and save them in the same folder on your computer. Alternatively, view the videos via YouTube at the URLs provided in the Additional Multimedia Support section.
Set up the presentation to display in the classroom. Hand out the
Close Encounters Worksheet
. Go through the slides, guided by the script provided below and text in the "notes" section of each slide. Embedded assessment questions are provided in italics below; allow discussion time before supplying the answers.
- Today we are going to begin a polymer unit, titled "Close Encounters of the Polymer Kind." The title refers to a popular sci-fi movie about UFOs and is appropriate because—for most of you—this topic is a foreign concept.
- Starting with the basics, we will discuss what a polymer is and introduce two categories of polymers, thermoplastics and thermosets. Next, we will discuss the cool behaviors of thermoplastics and the fundamentals that go into making a good thermoset. Then, we will perform a thermoset activity and, using our knowledge from the lesson, make our own thermosets.
what is a polymer?
(Answer: A polymer is a long chain of repeating chemical units.)
- If we look deeper, we see that polymers are typically separated into two types: thermosets, such as tires and car bumpers, and thermoplastics, such as water bottles, which are often made of polyethylene, as mentioned earlier.
Can anyone tell me the difference between a thermoplastic and thermoset?
(Pause to wait for both terms to be defined if possible; emphasize the "set" portion of the word "thermoset.") Here's a tip: If something is "set," can it be changed? (If students are unable to answer, move forward in the lesson. The answers are on the next two slides.)
- A thermoset is a polymer in which the final shape of the product becomes set, or cured, due to an irreversible chemical reaction.
What is meant by curing?
(Answer: To become tougher or harden.) Curing is a term for the chemical reaction that takes place to form the final 3D network that sets the shape.
What is a covalent bond?
(Answer: A chemical bond that shares electrons.)
- Thermoplastic is a linear polymer whose final shape can be changed through heating the material and melting it so that other shapes can be formed.
What are some typical thermoplastics materials you encounter?
(Answer: Water and soda bottles.)
To understand polymers, physical entanglements are an important concept to know. Imagine a bowl of spaghetti with all the noodles entangled due to their long lengths. This analogy helps us to imagine the physical entanglements at the molecular level.
- Now that we have introduced two categories of polymers, let's talk about some of the cool behaviors of thermoplastics!
Demo 1: The Weissenberg Effect
(In front of the class, have a beaker of water, a beaker of prepared PVA solution and a power drill with a glass rod as the bit.)
(While showing students the glass-bit drill and the beaker of water, ask them to make predictions.)
What do you think will happen when we spin the drill in the beaker of water?
Will the water move towards or away from the rod?
(Expect answers to vary.)
(After students answer, conduct the demonstration by accelerating the glass drill bit in the beaker of water, demonstrating that the water moves
What do you think will happen when we place the rod in the PVA solution?
Will the solution move towards or away from the rod?
(Conduct the experiment using the PVA solution, showing that the PVA solution moves
What did you observe in the Weissenberg demo?
(Answer: Polymer goes towards and up the rod; water moves down and away.)
is the instance in which a polymer travels up a rod as it spins, in contrast to say water that moves away from the rod as it spins.
Why does this happen?
By looking more closely at the differences between long chain polymers and water, we can figure out why this is the case.
Polymer chains interact and entangle, similar to a pile of string. For the case of water, the water has a certain affinity to the rod. As the rod spins, the centrifugal effect throws the water molecule away from the rod. For polymers, however, many more interactions with the rod exist per molecule and so much more force is required to remove the polymer from the rod. The chains are also entangled, so as the rod tries to throw the chain off the rod, it has nowhere to go and begins to wrap around the rod. As the chain wraps around the rod, it drags more chains closer to the rod due to entanglements. This explains why polymers come close to the rod.
(Note: In front of the class, have ready a pile of rubber bands, mound of salt and a wooden dowel.)
- Now, let's look at a macroscopic example. In front of me are a pile of rubber bands and a pile of salt. Watch what happens when a wooden dowel is rotated in salt.
(Rotate the wooden dowel in the mound of salt; expect the salt to move away from the dowel.)
What happened in this demo?
(Answer: Salt, which represents water, moved away from the dowel.)
(Then rotate the wooden dowel in the mound of rubber bands; expect the rubber bands to wrap around the dowel.)
(Answer: The rubber bands, which represent the polymer, wrap around the wooden dowel.)
This is an excellent example of what happens at the microscopic level. To put this in more scientific terms, let's learn some vocabulary so we can relate this to the polymer.
Entropy and Enthalpy
Polymers might be described as like people who are in close contact in a small room: they want plenty of room to stretch their arms and legs. Essentially, they want to increase
or create disorder of the current system.
If a person was standing in the room next to an enemy, s/he would be inclined to move; in this case,
exists between the two people. This is analogous to
Polymer chains also experience attraction and repulsion due to
Van der Waals
In the case of the polymer chain, their desire to increase their ability to stretch out is greater than their attraction and some net movement has to take place. Since they cannot move down the rod without running into other polymer chains, the only option is to move up.
- We have a balance between
interactions. Enthalpic interactions are how much you
the person next to you or—in molecular terms—how much a molecule likes the molecule next to it. Entropic interactions are how close you are to the one sitting next to you. The closer you sit to each other, the less entropy (capacity for disorder) you have available and the more uncomfortable you are in terms of sitting on top of each other.
To understand the difference between enthalpic and entropic, let's use this classroom as an example. Everyone wants to have plenty of space so that they can relax, correct (like polymers do)? Yet, at the same time, you want to sit next to your friend, right? Well...
What if your friend gets too close?
(Listen to student answers.) No matter how favorable the enthalpic interactions (that is, how much you like your friend), you still want to keep some minimum amount of space beween you.
What if the person getting close to you is your enemy?
(Listen to student answers.) In essence, the distance at which you begin to feel uncomfortable is greater than if you are friends. You become repulsed by one another.
So how does this relate to polymers?
(Listen to student answers.) Well, polymers are long chains that get
and wrapped around the dowel. As the dowel is rotated, the polymers (rubber bands) pull more and more toward the dowel due to entanglements (decreasing entropy). The polymers need to spread out, but to maximize the amount of space, the polymer can only go up.
Demo 2: Barus & Kaye Effects
The Barus Effect is similar to the Weissenberg Effect in that it can be explained by the balances of the entropic and enthalpic contributions. Think back to the YouTube video of the three effects.
What happened in the Barus Effect?
As the polymer exited the die (mold), it expanded and swelled. Figure 1 shows a general scheme of this effect.
If we consider the classroom analogy again, we can get a good idea of what is happening. You are in a classroom with many students and one of the walls moves in and out. As the wall slowly moves toward you, you are forced to move closer and closer to your classmates until you get become close enough that through either (or both) entropic or enthalpic contributions, you want more space and you begin to push back on the wall. Now imagine the door suddenly opens and you are allowed to leave. You are naturally going to pour out into the hallways and spread out. This is essentially what happens with the Barus effect.
Now imagine that you have a bunch of polymer chains in a confined space and a plunger slowly puts energy into the system forcing the polymer through a die. As it does this, the chains must move in closer together (decreasing entropy) and the chains align. The chains move through the die, while pushing on the walls. Once the chains exit the die, they are free to relax and stretch out again by swelling (increasing entropy).
- These next two effects are a bit more subtle, so let's watch a YouTube video demonstration (
). The first demo is of the Barus effect and causes what is known as "die swell."
What is meant by "die"?
(Listen to student answers.) When you have a thermoplastic, it can be melted and forced through a die (a mold) to give it a particular shape. For example, if I wanted to make polyethylene (PE) fibers, I could melt PE and force it through a small diameter die to get the fiber.
(With students, watch the Die Swell Portion (Barus Effect) of the video.)*
Why do you think the swelling occurs?
(If students do not understand what is meant by swelling, rewind the video to show how the liquid stream is wider than the mouth of the bottle.) Notice how the liquid stream is wider than the mouth of the bottle; this is "swelling." (Give students time to think about this viscoelastic effect; do not provide the answer.)
Let's look at another classroom analogy to explain the Barus effect phenomenon. Let's say one of the walls can move in and out. It moves so far in that everyone is forced to touch each other.
Do you get more or less comfortable as the wall moves in?
(Let students discuss their answers for a couple of minutes.)
Why? What happens to entropy? Is there more or less space?
(Permit students to discuss their answers for a couple of minutes.) Unless you are all extremely good friends, you are probably a little uncomfortable at the point of the wall closing in on you. The entropy (the disorder) of the room has decreased, as less space available for the same number of people.
What if I open the door so you can leave?
(Listen to student answers.) That's right, you would want to pour out, 'swell,' into the hallway and spread out very quickly.
How does this classroom example relate to the video?
As an example, as the plunger (the classroom wall) is pushed in toward us, the molecules (the class) are forced into a confined space, which decreases entropy. As for the liquid, the molecules exit the die (in this case, the bottle) and increase entropy by spreading out, or swelling.
Which has more entropy: a polymer chain that is forced to be straight or a chain that is free to move?
(Answer: A polymer that is forced to be straight has less entropy available to it, since it is confined.) In order to increase the entropy of the system, the polymer spreads out when exiting the die.
Explain to me why water does not swell after exiting the die. (Listen to student explanations.)
Compare the state of entropy in the die and the one after the die. Is it different or roughly the same
(Answer: It is the same since it is so small. A confined water molecule looks very similar to an unconfined molecule of water so it cannot tell the difference.)
Shear Thinning and Shear Thickening
- This next effect is a much debated topic so we are going to present just one theory as to why it happens. Take a moment to watch the video of the Kaye effect. Pay attention to how the material behaves when it hits the surface.
(Watch the Kaye Effect portion of the YouTube video; either review the Weissenberg Effert portion or skip to the Kaye Effect, which starts at 45 seconds.)
What happens during the Kaye Effect?
(Give students time to discuss.) The stream of poured material "skips" off the surface of the mound. One of the theories as to why this happens is called
. To understand this concept, let's consider the "pool of cornstarch and water" experiment, commonly called "oobleck" (see
). The experiment demonstrates
—the exact opposite of what is happening in the Kaye Effect video. In the cornstarch example, the material gets stiffer as you run across it. In fact, the faster you run, the easier it is to cross the material. With shear thinning, the material gets softer as you run across it. The faster you run, the softer the material gets. Think of the floor; although it is solid, you would fall through it if you ran across it too fast if it was shear thinning.
Imagine that you are a long polymer chain. You have had sufficient time to relax and reach a state that minimizes your free energy (maximized entropy). This shape that you would be in would roughly resemble a sphere (think about why water droplets form). As you move about, you collide with other spheres roughly the same size as you, and it takes considerable time for you to go around each other. As stress (energy) is applied to the system, you are disturbed from your little sphere and are forced to stretch out as you collide with each other. You are not allowed time to relax back to the original state (time constraints), and you remain in this stretched state. You and your neighbors become aligned. You are no longer colliding with large spheres. Instead you are just sliding past each other. Your resistance to flow or viscosity is greatly decreased and you can appear to skip off the surface. Figure 2 illustrates this example.
Let's again refer to the classroom analogy. This time, we are running through the halls and a person in the back wants to move to the front. We consider two situations: one in which the person at the back pulls out and runs slowly to the front; and the other in which the person very quickly moves to the front.
In the first case, the student slowly goes through the group of students en route to the front. S/he is able to move around the other students and not disturb them.
In the second case, the student moves too quickly (that is, too sloppily) to skillfully move around the other students without colliding with them. If the rapidly moving student is too quick, the students being bumped into cannot recover from the collisions and return to their normal states of moving. They are disrupted, so to speak.
- Now extend this to polymers. We have established that polymers like to have their own space and curl up in a relaxed configuration. But when they begin to collide with each other more quickly than they can return back to their relaxed configuration, they get stretched out.
Once stretched, do you think it is easier or harder for polymers to slide past each other?
(Give students time to discuss.) It is easier, which results in shear thinning.
- Viscosity is the resistance to flow. If it is difficult for the fluid to flow, it is said that the viscosity is high—or more viscous. A real-world example of this is paint, which is a type of thermoplastic polymer. When you paint something, you do not want the paint to be all runny and slide down the wall. However, you also do not want it to be too thick, making it difficult to apply. As you apply the brush to the wall, you apply force to the paint and it shear thins, making it easy to apply. When the polymer leaves the brush (and the applied force of the brush) it returns to its original state of being somewhat "thick" so as to not run down the wall.
Demo 3: Defying Gravity Effect
- What if I told you that I could defy gravity and remove material from the beaker without the syringe being in the beaker the whole time?
(Perform the demo described in the next paragraph, or watch the video at:
(Demo instructions: Have ready a syringe, a beaker of water and a beaker of PVA Solution. Place the syringe in the beaker of water and begin to slowly draw some out. As you slowly draw the water, ask students:
Do you think I could continue to pull the water from the beaker while removing the syringe from the beaker?
Then conduct the same experiment with the PVA. To do so, insert the tip of the syringe in the solution and begin to pull back on the syringe. While pulling back, remove the syringe tip from the PVA solution. You should be able to pull the PVA solution out while removing the syringe out of the beaker.)
What happened in the demo? Did I defy gravity?
(Answer: Yes; the material could be removed from the beaker without the syringe being in it.)
Why was this possible?
(Note: Entanglements is the answer but hold off on revealing this just yet.)
Now, let's consider another macroscopic example. In front of me, I have a pile of rubber bands. I need to challenge a volunteer to try to pull one rubber band, and only one rubber band, from the middle of the pile. If you disturb any of the other rubber bands, you fail the challenge. Who would like to accept the challenge?
(Let one student try to pull one rubber band free from the pile.)
Could [name] do it?
(Most probably not. If the student is able, mix up the rubber bands again and have him/her try again by picking a rubber band at random. Repeat until it is clear that if you pull one rubber band, you pull others with it. Essentially, as one rubber band is pulled, it drags others with it.)
How does this relate?
(Listen to student explanations.) A similar event happens in the syringe. As one polymer is pulled up, it drags others with it. When the syringe is initially in the solution, it "grabs" onto a bunch of other polymers. Once removed from the solution, those chains continue to drag other chains into the syringe.
Previously we discussed some of the molecular dynamics of thermoplastic systems including how the ability to relax affects a material's properties (think of shear thinning or shear thickening). Let's switch our focus to
. Say we take the thermoplastic system we saw earlier through the Weissenberg Effect demo, but now we anchor the molecule in place every few points within the molecule. Now the molecules cannot move as well. Anchor a few more points, and the material gets stiffer as the molecules lose the ability to move around. A macroscopic (large scale) example of an anchored thermoset is a bridge, as shown in Figure 3. In the first illustration, the bridge has very few connections.
Do you think this bridge would be very stiff or somewhat flexible?
(Answer: It is somewhat flexible.)
Now we take the exact same material, but make a few more connections.
What happens to the stiffness?
(Answer: It goes up.)
If we shrink this to the molecular level, we have a thermoset system with different amounts of molecular "crosslinks," or connections. If we vary the amount of connections, we change the mechanical properties.
Now let's discuss how we can change the number of crosslinks at a molecular level. Figure 4 shows two
—that is, two sites within the molecule that are reactive towards each other. If they react, they can only create a linear chain of connections.
Is the polymer now linear if we have a thousand difunctional molecules (500 A and 500 B) and exchange a single "A" with a single trifunctional molecule (D)?
(Give students time to discuss before moving on to the next questions.)
what happens if we exchange it for two trifunctional molecules?
(Give students time to discuss before moving on to the next questions.)
What if we directly exchange 10 B molecules for 10 D molecules?
(Give students time to discuss.)
What if we completely replace the 500 B molecules with 333 D molecules?
(Give students time to discuss before explaining the exchange of molecules.)
Every A functional group can react with one D functional group—perfect number of connections due to a stoichiometric ratio.
What if we keep going and add 1,000 trifunctional molecules? Are enough difunctional molecules present to react with all of the trifunctional?
(Answer: No, and now you have loose ends again, which causes the material to become flexible again.) This happens through a reaction between an epoxy (difunctional molecule) and an amine-bearing molecule that possesses more than two functional groups (shown in Figure 4). Common examples of epoxy resins are car bumpers or panels and wind turbine blades.
- Thermosets are polymer systems that have gone through a curing reaction and are "set" in their final shapes. A thermoset cannot be reshaped by heating. Thermosets are interesting because you can take a bunch of small reactive molecules, fill up a shape that you like, and then the molecules react, forming permanent bonds that keep the shape of the mold. Because the starting materials are small molecules, they can fill up all the little features of the mold before it reacts (something that can be difficult with the larger molecule thermoplastics).
As a result of this, thermosets often find uses in polymer/fiber composites, where the small molecules fill molds that contain long thin fibers (typically a few microns thick). The long fibers provide most of the strength, but the thermoset keeps the fibers in place and distributes the force among the fibers.
- The top left image is a close-up of a thermoset matrix surrounding fibers in a thermoset/fiber composite. The fibers are typically on the order of 1 micron in diameter so you can see that the thermoset precursors, the small molecules that will eventually react, are useful to fill those small spaces. The Koenigsegg super car (right image) is a really cool example of such an application, in which the entire car body is composed of a thermoset/carbon fiber composite, desired for its low weight and high strength. So how is the thermoset portion made? Can you control the properties of that portion?
- The thermoset materials undergo a chemical reaction in which new covalent bonds are created. Let's take a look at some of the details. To make a linear molecule (thermoplastic), you mix molecule A (red) and molecule B (blue) together to react. One red group reacts with one blue group to make a purple group. No matter how much A and B react, they always create a linear molecule (imagine many A and B molecules are mixed together).
Next, let's consider the foundation for a thermoset system. Here we have a small molecule A and a small molecule D. They react and form molecule E. The more they react, they form this huge branched interconnected 3-D network (unfortunately, we can only represent 2D on the slides). So how do we control the properties?
- Here we have two bridges. All things being equal, you could probably say that the bridge on the bottom would be more mechanically robust than the bridge on the top. The obvious difference is that the number of connections is different. We consider the bridge with more connections to be stronger.
- Now let's apply that to thermoset. We already talked about the basics of the reaction, but how can we control the strength of our bridge? Let's assume that we have 1,000 molecules: 500 are A and 500 are B, and we make changes as shown on the next slide.
- If we exchange 1 A molecule with 1 D molecule (499A + 500B + 1 D), we have a polymer that is mostly linear, but has one branch at the D molecule.
Is this a strong bridge?
- If we exchange 10 A molecules with 10 D molecules (490A + 500B + 10 D), we have a polymer that is mostly linear, but now has 10 crosslinks in it at the D molecules.
Is this a strong bridge?
(Answer: Not really, but it is getting better.)
- What if we exchange the 500 A molecules with 333 D molecules? Every A functional group can react with a D functional group. You have optimized the number of connections by achieving
—or establishing relative quantities.
Is this a strong bridge?
(Answer: Very strong. You have the most number of connections possible.)
- What if you keep adding D molecules? Say you exchange 500 A with 10,000 D molecules.
Does that help your strength? Why or why not?
(Answer: No, it does not help the strength because you are creating loose ends that do not participate in the connections, but add length to the bridge.)
How would you make a flexible chemical bridge?
(Answer: Bridges with fewer chemical connections are more flexible.)
How would you make a strong chemical bridge?
(Answer: Bridges with more chemical connections are stronger.)
Does an optimum ratio exist?
(Answer: Yes, typically stoichiometry leads to superior mechanical properties.)
– Let's consider a real-life example: we have an
-based molecule and
-based molecule. The amine can attack the epoxy forming crosslinks. Each hydrogen on the nitrogen is referred to as an active hydrogen, meaning that it can participate in the reaction. The amine here has six active hydrogens, so it has a functionality of six, while the epoxy has two epoxide groups. Following the same ideas as before, we can vary the properties by varying the ratio of amine and epoxy, which is exactly what we are going to explore in the associated activity.