Polymers, Polymers, Polymers

Introduction

Four popular polymer demonstrations in one Super Value Kit! Introduce students to the basic structure of polymer molecules and their unusual properties with four fun activities. Demonstrations include the following:

• Part A: Polyurethane Foam
• Part B. Let’s Make Slime!
• Part C. Sodium Polyacrylate
• Part D. Preparation of Nylon

Concepts

• Polymers
• Catalysis
• Superabsorbents
• Osmosis
• Industrial chemistry
• Polymerization
• Condensation polymer

Experiment Overview

Part A. Polyurethane Foam
Simply mix two viscous liquids together and watch as the mixture expands to about 30 times its original volume. The result is a hardened, lightweight polyurethane foam.

Part B. Let’s Make Slime!
A popular demonstration for students, mix polyvinyl alcohol and sodium borate to create slime!

Part C. Sodium Polyacrylate
Water in one cup is poured into an “empty” cup (actually containing sodium polyacrylate) and the water “disappears”!

Part D. Preparation of Nylon
Two solutions are poured together in a beaker. A paper clip is inserted down into the solutions. As the paper clip is withdrawn, almost by magic a very long strand of nylon is pulled from the beaker. A super demonstration to discuss polymer concepts.

Materials

Safety Precautions

Part A of this activity should only be performed in a fume hood or well ventilated area. Avoid breathing any vapors produced and avoid skin contact, as both Part A and Part B may contain skin and tissue irritants. Students should be warned not to ingest the material and to use it only for the purposes intended. Do not allow slime to remain on clothing, upholstery, carpet, or wood surfaces. The slime will stain many surfaces. Clean up any slime as soon as possible. Sodium polyacrylate is non-toxic. However, it is irritating to the eyes and to nasal membranes if inhaled. Hexamethylenediamine/sodium hydroxide solution from part D is toxic by ingestion and is corrosive. Adipoyl chloride/hexane solution is a flammable liquid and is toxic by ingestion and inhalation. Perform this demonstration under a fume hood or in a well-ventilated room. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Do not handle the nylon without wearing gloves unless it has been thoroughly washed. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The disposable cups may be thrown in the trash. Any leftover liquids should be mixed together, allowed to react, and then the solidified polymer may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Slime, polyvinyl alcohol solution, cups, and wooden sticks may be placed in the trash. Leftover sodium borate solution may be saved for later use or rinsed down the drain with water according to Flinn Suggested Disposal Method #26b. Sodium polyacrylate and the gelled material may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Do not put sodium polyacrylate down the sink! The nylon produced may be washed and dried. Dispose of it in the solid trash according to Flinn Suggested Disposal Method #26.

Prelab Preparation

Part C. Sodium Polyacrylate

1. Before the students come into the room, place 0.5 g of sodium polyacrylate in one of the disposable cups.

Procedure

Part A. Polyurethane Foam

1. In a fume hood or well ventilated area, pour approximately 20 mL of liquid Part A in a disposable cup. Do not use glassware! Note: The exact volume is not critical. Add a few drops of food coloring, if desired, and stir.
2. Place approximately 20 mL of liquid Part B in a second disposable cup. Note: The volume of Part B should be approximately equal to that of Part A.
3. Spread a paper towel or newspaper flat on the table and place one of the cups in the center of the paper towel.
4. Pour the contents from the second cup into the cup on the paper towel and stir thoroughly with a stirring rod or tongue depressor until you see the foam beginning to expand. Remove the stirring rod.
5. Observe the foam as it expands to about 30 times its original volume. The cup will get warm, indicating an exothermic reaction. Do not touch the foam until it is completely hardened.

Part B. Let’s Make Slime!

1. Place 50 mL of 4% polyvinyl alcohol solution into a disposable cup. Add a couple drops of food coloring, if desired, and stir with a wooden stick.
2. Pour 5 mL of saturated sodium borate solution (4%) into the cup while stirring (saturated sodium borate is about 4 g per 100 mL of water). The mixture will gel almost immediately but keep stirring until smooth.
3. To observe the properties of slime, knead it into a ball. Hold a small part of the ball and watch it stretch without breaking. Try stretching the slime quickly and see how it will break under these conditions. Slime will also pick up ink from paper. Have the students draw a picture or write their name backwards with a water soluble marker. Press the ball of slime onto the paper for only a split second. The design will be transferred to the slime. (The slime will stick to the paper if left on too long.)
4. The slime will last two days to a week. Store it in an airtight, plastic sandwich bag. When the slime starts to mold, dispose of it in a waste container.

Part C. Sodium Polyacrylate

1. Add approximately 100 mL of distilled or deionized water to the second disposable cup.
2. Tell the students that the water will “disappear” when poured into the other cup. (You can tip the cup forward somewhat to show that it is “empty”; it will be difficult to see the 0.5 g of sodium polyacrylate against the white interior of the disposable cup.)
3. Slowly pour the water into the cup containing sodium polyacrylate. Swirl the cup a bit (give the sodium polyacrylate time to absorb the water).
4. Tip the cup downward slightly to show the students that the water has “disappeared”! (Don’t turn the cup upside down, or you may dump the jelly-like mass on the floor!)

Part D. Preparation of Nylon

1. Add 7 mL of the hexamethylenediamine/sodium hydroxide solution to a small beaker.
2. Slowly add 7 mL of the adipoyl chloride/hexane solution down the side of the beaker. Do not stir or mix the solutions.
3. Note the formation of a white film at the interface of the two solutions.
4. Use a bent paper clip (opened to form a hook) to pull the film from the beaker. Pull slowly until there is no more nylon left. The nylon should be easily pulled from the beaker in the form of strands.
5. Wash the nylon strands by rinsing with water several times.
6. Lay the nylon strands on a paper towel to allow them to dry.

Teacher Tips

• For a fun alternative to the Polyurethane Foam demonstration, place about 35 mL of Part A and Part B in a paper cup, mix, and then pour the mixture into a latex glove. Make sure some of the mixture is in each finger of the glove. Now watch the foam expand and fill the glove. When completely hardened, the glove can be removed (probably not in one piece), if desired. You will have made a “hand” out of the polyurethane foam. The liquid may also be placed in plastic molds.
• Any 50/50 mixture of Part A and Part B may be used, but take into consideration the amount of expansion when measuring out the liquids.
• Acetone may be used to remove any hardened polymer on glassware or on the table.
• Do not touch the foam. It will take about 15 minutes for the surface to firmly set and may contain unreacted material for up to 24 hours. Some people may have allergic reactions to unreacted monomers.
• After you have discussed with students what really happened in the Sodium Polyacrylate demonstration, gradually add granulated salt to the gelled polymer. The addition of sodium chloride will break the “gel” as water leaves the polymer to dilute the salt concentration outside the polymer network. The result will be the apparent “deflation” of the gelled polymer.
• Let your students observe that the two solutions in the Preparation of Nylon demonstration do not mix. They are immiscible.
• Observe the production of a film at the interface of the two solutions and the removal of the film as a long string of nylon.
• Make sure that the nylon is washed several times before it is handled.
• The hexamethylenediamine solution is slightly pink to make it more visible during the demonstration. If the pink has faded, add 1–2 drops of red food coloring.
• Sebacoyl chloride can be used in place of adipoyl chloride in the second solution to produce nylon 6,10.
  
  

Discussion

Part A. Polyurethane Foam
There are many forms of polyurethane such as fibers, coatings, elastomers, flexible foams and rigid foams. The foam in this system is a rigid foam that is used in furniture, packaging, insulation, flotation devices and many other items. Here, a rigid polyurethane foam is produced by mixing equal parts of two liquids, called Part A and Part B. This lightweight foam expands to about thirty times its original liquid volume and will become rigid in about five minutes.

Part A is a viscous cream-colored liquid containing a polyether polyol, a silicone surfactant, and a catalyst. The polyether polyol may be a substance such as polypropylene glycol [HO(C₃H₆O)_nH]. The hydroxyl (–OH) end of the polymer is the reactive site. The silicone surfactant reduces the surface tension between the liquids. The catalyst is a tertiary amine which aids in speeding up the reaction without being chemically changed itself. Part B is a dark brown viscous liquid containing diphenylmethane diisocyanate [(C₆H₅)₂C(NCO)₂] and higher oligomers (dimers, trimers or tetramers) of diisocyanate. When the polyether polyol (Part A) is mixed with the diisocyanate (Part B), an exothermic polymerization reaction occurs, producing polyurethane (Equation 1).

During the course of the polymerization reaction, a small amount of water reacts with some of the diisocyanate. A decomposition reaction occurs and produces carbon dioxide gas, thus causing the solution to foam and expand in volume. Pores in the mixture are created from the gas; these pores are visible when looking at the rigid substance. The multifunctionality of both reactants leads to a high degree of crosslinking in the polymer, causing it to become rigid within minutes (Equation 2). Part B. Let’s Make Slime!
Polyvinyl alcohol (PVA) is the world’s largest volume, synthetic, water soluble polymer. PVA is nonhazardous and is used in many adhesives, films and elastomers. Its most popular use in schools is in the preparation of “slime.” Slime, which was first invented and marketed as a toy, is a semi-rigid, aqueous gel. It is made by combining an aqueous solution of polyvinyl alcohol (PVA) with borax (sodium borate). Polyvinyl alcohol, the main ingredient in slime, is a nonhazardous, hydrophilic (“water-loving”) polymer that is used as an adhesive and as a coating for textiles and paper. The structure of the repeating unit in polyvinyl alcohol is shown in Figure 1. The numerous –OH groups in PVA form strong hydrogen bonds with water molecules or neighboring polymer molecules. 

Its molecular weight can range from 25,000 to 300,000. Part C. Sodium Polyacrylate
Sodium polyacrylate is an example of a superabsorbent polymer. Superabsorbents operate on the principle of osmosis: the passage of water through a membrane permeable only to the water. Here, osmotic pressure results from the difference in sodium ion concentration between the inside of the polymer and the solution in which it is immersed. This osmotic pressure forces water into the solid polymer lattice in an attempt to equilibrate sodium ion concentration inside and outside the polymer. The electrolyte concentration of the water will effect the osmotic pressure, subsequently affecting the amount of water absorbed by the polymer. For example, sodium polyacrylate will absorb approximately 800 times its own weight in distilled water, but will only absorb about 300 times its own weight in tap water, due to the high ion concentration of tap water. Sodium polyacrylate is manufactured by the free-radical polymerization of a mixture of sodium acrylate and acrylic acid, and a cross linker such as trimethylol propanetriacrylate: Sodium polyacrylate is the main ingredient in high-absorbency diapers. (It can absorb about 30 times its own weight in urine). It is also commonly used in alkaline batteries, feminine hygiene products, nursery potting soil, water beds and as a fuel filtration material to remove moisture from automobile and jet fuels.

Part D. Preparation of Nylon
Nylon is a generic name for a family of polyamide polymers. Polyamides are condensation polymers obtained in the reaction of an organic acid with an amine. During a condensation reaction, a molecule of water is also formed as a byproduct (Equation 3). In order to obtain a polymer, the organic compounds must be difunctional, that is, they must contain a reactive functional group at each end of the molecule. Nylon was discovered in 1935 by W. H. Carothers at the DuPont Company. At the time of the discovery, there was a tremendous demand for natural fibers and many scientists were trying to develop synthetic fibers that could be mass-produced from inexpensive materials. It was quickly commercialized and played an important role in World War II clothing and parachutes.

There are many different types of nylon polymers which differ in the number or arrangement of carbon atoms in the repeating units. The first synthetic polyamide was Nylon 6,6, which is made by reacting adipic acid or one of its derivatives with hexamethylenediamine (Equation 2). Note that both monomers are difunctional, that is, they have two functional groups. This is one of the requirements for the synthesis of a condensation polymer. Nylon 6,6 is still the most common synthetic polyamide in use today. Nylon is a thermoplastic and can be molded into shapes or extruded into a fiber. Nylon fibers are stronger and more elastic than silk and are relatively insensitive to moisture and mildew. Nylon is used in many commercial products such as hosiery, athletic apparel, bristles for toothbrushes, rugs and carpets, sails, parachutes and some artificial turfs.

References

Buchholz, F. L. J. Chem. Ed. 1996, 73, 512–515.

Rosato, D. V. Rosato’s Plastics Encyclopedia and Dictionary; Hanser: New York, 1993; pp 318–320, 572.

Shakashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin: Madison, 1983; Vol. 1, pp 216–218.

Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin: Madison, 1989; Vol. 3, p 326.