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| Shared Idea |
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| Polymers
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| Polymers (meaning many parts ) are a broad group of compounds made up by linking together simple compounds called monomers (meaning single parts ). There are many natural polymers, such as starch, rubber, cellulose (in paper and plants), hair, silk, cotton, wool, proteins and DNA. However, when most people here this term, they think of synthetic compounds, including plastics. Synthetic polymers include polystyrene, polycarbonate, polyvinyl chloride, polyester, polyurethane, polyethylene and nylon.
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| Polymers vary widely in their properties, depending on the structure of the monomers and how they are linked together. Chemists are able to create polymers that have desired properties to meet specific needs, but this includes a certain amount of trial and error. The creation of successful plastic soda bottles took about 10,000 trials. In many cases, polymers produced in such efforts seem to have no real use. However, children s interest in the odd sets of properties of some polymers has created a side industry. This began with an effort to make synthetic rubber that resulted in a polymer that sat on a shelf for several years and then became popular as Silly Putty! Other such products include Gak, Ooze Balls, Glue Balls, Happy and Sad Balls, Smart Balls and Slime.
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| Showing these chemicals to students can expand their perception of polymers beyond their probable mental image based on the generic term plastics. It is also useful to note the diversity of properties of polymers, from the pliability of polyethylene (Handi-Wrap or Glad Wrap) to the strength and durability of polycarbonate (used in CDs). It is also worth noting that many polymers are cross-linked. That means that the polymer chains are joined to each other. A good analogy is cooking spaghetti, draining it and then allowing it to dry. The spaghetti strands stick to one another, changing its properties.
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| A good pair of simple demonstrations can show the cross-linking of polymers. The first is commonly performed by magicians. It requires a balloon, a long needle or skewer, some glycerin or oil and a bit of practice. Inflate the balloon and knot it, wipe the needle with the lubricant to clean it (the statement to the audience) and then slowly push the needle through the balloon, starting at the least stretched point at the end of the balloon and coming through near the knot. The polymer stretches around the needle, kept tight by the pressure of the enclosed air, and the lubricant helps prevent tearing while decreasing air leakage. When the needle is removed, the balloon is popped, proving that both it and the needle are normal. (It also hides the deflation that would otherwise follow.) A follow-up requires a Ziploc bag, water and a set of sharp pencils. Fill the Ziploc bag most of the way with water and then seal it. Pencils can be pushed horizontally through the bag and the water will not come out. It is displaying the same properties. However, be sure to hold the bag over a sink when the pencils are being removed!
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| Three other interesting demonstrations or hands-on activities deal with the property of cross-linking. A fast demonstration only requires a thick rubber band. Most rubber bands are made of rubber that is cross-linked by sulfur atoms. In the normal form, the molecules in the rubber band are disordered. When the rubber band is stretched, the molecules are arranged in a more orderly pattern. Doing this requires energy. If the rubber band is simply held against the forehead before and after stretching, the change in temperature makes this energy factor very evident. The second is the creation of Gluep (a glue polymer). The materials needed are white glue (e.g., Elmer s), sodium borate (available in Borax), paper cups, wood stirrers (ice cream sticks), water and paper towels. Food coloring is an optional addition. The main component of Elmer s glue is polyvinyl acetate. Sodium borate will cross-link these molecules. By adding a solution of water and sodium borate to the glue in a cup and then stirring, a synthetic rubber can be produced. When it seems to have all joined together, it can be removed from the cup and rolled into a ball. Rolling it on the paper towel should then remove the excess water. You will have a ball that can bounce until it dried out too much. (If the ball is going home or is being done at home, an important warning is that, if it dries on a rug, it will be difficult to remove.) If a little food coloring is added to the glue in the cup at the initial stage, the ball will be colored. This is messy, but children love it!
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| The third demonstration consists of making a cross-linked polymer similar to Slime, a popular cross-linked polymer sold by the Mattel Corporation. This activity requires polyvinyl alcohol and again using sodium borate as the cross-linking agent. Polyvinyl alcohol laundry bags can be obtained. They are made for use in situations such as hospitals, where the desire is to minimize contact with contaminated clothing and laundry. Polyvinyl alcohol laundry bags will dissolve in hot water therefore can be tossed directly into washing machines. In the classroom, a bag can be dissolved with hot water, serving as a demonstration of the use of this polymer. If sodium borate is then added to the resulting solution, cross-linking will occur, offering a second round to the same demonstration!
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| Some superabsorbent polymers create gels that trap or contain water molecules. One such polymer is polyacrylamide, often sold as polyacrylamide crystals. When placed in water, they expand to a far greater size. If there is no dye, they become nearly invisible and have therefore been used by magicians. Gro-Beasts operate on the same basis. Students can watch these crystals disappear in a glass of water and can then feel in and find the invisible gel that has expanded by absorbing a vast amount of water. A similarly interesting polymer is sodium polyacrylate, sometimes sold as Super Slurper. It is the active ingredient in disposable diapers that retains the water. Some can be placed in a beaker (or cup). When water is added, a thick gel is formed that allows the cup to be inverted. However, if some table salt (sodium chloride) is added, the gel breaks down, making it safe to wash it down the drain. This is why there is a limit as to how much urine diapers will hold before failing.
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| Another simple demonstration can show the difference between Handi-Wrap (polyethylene) and Saran Wrap (a copolymer of two monomers vinyl chloride and vinyldiene chloride). Place something with a strong odor, such as vanilla or fresh-cut onion, in two beakers and then use these two polymers to cover them. Pass them around the room. It will be easy to identify which beaker is covered with the polyethylene. It is more porous, allowing the odor to escape. Saran Wrap costs more because it is far less permeable, keeping odors in and oxygen out, thereby working better for long-term storage.
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| The Locktite Corporation makes polymers that bond materials together. One of their chemists designed a means of applying the polymer to a screw contained in small beads so they can sit on a shell until they are going to be used. The beads are compressed and burst open, releasing the polymer, when the screw enters a nut. The absence of air allows the polymer to bond the metal screw and nut so that the nut cannot work its way loose.
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| Kevlar is another interesting polymer. It is in the waterproof envelopes that will not tear. It is also used in bulletproof vests. If you can obtain Kevlar string, an interesting demonstration can be done comparing it to normal string. In addition to the two types of string, you need a piece of two-by-four and two smaller pieces of wood. Wrap each end of the string around the two smaller pieces of wood with a couple of yards of string remaining between them and rest the two-by-four on the middle of the string. Have one volunteer stand on the two-by-four while holding someone else s hand for stability. Have two other volunteers try to lift the person by raising the string-wrapped wood. Repeat this with the Kevlar. The string should break, but such will not be the case with the Kevlar.
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| Pluronics are a special group of polymers. In the middle is a polymer of propylene oxide (1,2-epoxypropane) and at both ends are polymers of ethylene oxide (1,2-epoxyethane). That is, the middle is polypropylene oxide and the ends are polyethylene oxide. These synthetic polymers are widely used as surfactants, emulsifiers and stabilizers in pharmaceutical drug delivery. One version, produced by BASF, has a special property. It is a liquid at room temperature. When warmed, it becomes a gel, but if the temperature continues to increase, it is once again liquified.
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| What most people call Styrofoam is actually expanded polystyrene foam. (Styrofoam is a brand name for expanded polystyrene foam that is used for household insulation.) The foam peanuts used in shipping are actually produced and shipped in a much smaller form. They contain a chemical that undergoes expansion when heated with steam, allowing air to diffuse into the molecular structure. Four heatings are required to bring it to the final size, which is 150 times the original volume. (As a note, Chlorofluorocarbons (CFCs) are no longer used for this purpose. What is now commonly used is n-pentane.) On the average, these packing peanuts get reused seven times. Polystyrene is also very recyclable. (McDonald s was recycling their polystyrene until the public s conventional wisdom led them to stop using it.)
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| An easy demonstration is to use some acetone and then add either expanded polystyrene or foam peanuts. They will dissolve in the solvent, allowing them to shrink in size. (It reminds me of the shrinking of the Wicked Witch in The Wizard of Oz.) This polystyrene can then be placed into a mold to create new objects, such as Christmas ornaments or doorstops. A related demonstration is showing how Ecofoam dissolves in water. This starch-based polymer has replaced a large portion of the expanded polystyrene packing peanuts. It breaks down when wet and the resulting starch can be absorbed in the natural food chain. One other interesting demonstration can be done with a clean polystyrene food container and a toaster oven. The containers are made by heating the polystyrene, exerting pressure and then allowing it to cool. It then retains the new shape. However, it also remembers its original shape. If placed in a toaster oven and heated with no exerted pressure, it will return to its original shape. This usually fascinates viewers both children and adults!
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| Recyclable plastics are identified by initials and code numbers written within the recycling symbol. PET (1) is polyethylene terephthalate, which is a polyester. HDPE (2) is high density polyethylene. PVC (3) is polyvinyl chloride. LDPE (4) is low density polyethylene. PP (5) is polypropylene. PS (6) is polystyrene. The number 7 refers to any other recyclable plastics. Which of these that are recycled in any system unfortunately varies from community to community, making the program a bit more difficult, but everyone should recycle as much as is possible.
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| To gain a better understanding of polymers and polymerization (the creation of polymers from monomers), a molecular model kit can be used. Have students make three or four models of a desired monomer. The monomers all contain a carbon-carbon double bond. Some good examples are ethylene or ethylene (CH2=CH2), vinyl chloride or chloroethene (CH2=CHCl) and propylene or propene (CH2=CHCH3). When polymerized, the first creates polyethylene, the second creates polyvinyl chloride and the third creates polypropylene. To create the polymer, the double bonds should be opened, replacing them with single bonds that are then used to connect the monomers in sequence. If structural formulas are created, there should be an open bond at each end, as the model is only showing a section of the polymer. For example, the structural formula for three linked molecules of ethylene would be CH2 CH2 CH2 CH2 CH2 CH2 .
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| NOTE: Sodium borate (borax) is toxic by ingestion. While it is able to be obtained in the supermarket as a detergent, it is important to follow proper safety precautions with students when using it in the classroom.
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