Materials Included In Kit

Additional Materials Required

Prelab Preparation

Activity A
Sodium Alginate, 2%: Measure 4.0 g of sodium alginate into a 250-mL Erlenmeyer flask. Add 200 mL of distilled or deionized water and a stir bar. Stir on a magnetic stirrer for about 1 hour or until the solid dissolves. Allow the solution to sit overnight—the mixture will form a viscous gel.

Activity B
Glue Strips: Pour Elmer’s Glue-All into three disposable (plastic) Petri dish covers until the glue is about 2 mm deep in each cover. Allow the glue to dry for several days so that it becomes completely solid. Using a spatula, pry off a little bit of hardened glue from one side of each Petri dish cover. Peel the rest of the glue out of the Petri dish cover by hand. Cut four 75 x 100 mm strips from each solid circle of hardened glue.

Hot Water Baths: The water temperature should be 50–60 °C. Hot tap water may be used if it is hot enough—replenish the beakers with hot water as needed as the groups rotate through the stations. Alternatively, water may be heated to the appropriate temperature by heating on a hot plate at a medium setting. Do not overheat.

Activity C
Polarizing Filters, Mounted: To rotate the filter when the bottom filter is taped to the overhead projector stage, one polarizing filter should have a wooden splint taped to one edge of the filter for each station.

Plastic “Dumbbells” (Polyethylene): Cut ten 2 x 8 cm dumbbell-shaped pieces out of polyethylene zipper top bags. The strips should be cut parallel to the zipper. A typical 8" x 8" bag should give at least 8 “dumbbells.”

Safety Precautions

The polymers used in this experiment have a variety of consumer and commercial uses and are considered nontoxic. Exercise care when pouring hot water and avoid contact of all solutions with eyes and skin. Wear chemical splash goggles whenever working with chemicals, heat or glassware in the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information. Remind students to wash hands with soap and water before leaving the lab.

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. Solid calcium and copper alginate gels and glue strips may be disposed of in the trash. Excess calcium chloride, copper(II) chloride, sodium alginate and sodium chloride solutions may be disposed of down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• For best results, set up two stations for each activity throughout the lab. This will allow 10 groups of students to rotate through four activity stations in a 50-minute lab period, if needed. A double lab period (two 50-minute class periods) will allow time both for a review of basic polymer principles before lab and for a collaborative class discussion after lab.
• The activities may be completed in any order. Also, since each activity is a self-contained unit, the experiment may be set up with as many or as few of the activities as the teacher desires. Students should need only 7–8 minutes per station—keep the pace fairly brisk to avoid dawdling. Questions in the Observations and Results section may be answered during downtime between stations.
• Prelab preparation is an essential component of lab safety, and it is also critical for success in the lab. (Standing in front of the lab station is not a good time for students to be reading the activity for the first time.) Having students complete the written prelab assignment for this lab will help teachers ensure that students are prepared for and can work safely in the lab.
• The sodium alginate activity may be extended by testing the solubility of the polymer in other metal salt solutions. The polymer will precipitate with most polyvalent metal ions and gives colored gels with transition metal cations such as Fe³⁺ or Co²⁺. Seaweeds have been tested to see if they can be used to treat water by removing metal ion contaminants. Will sodium alginate completely decolorize a solution of copper(II) chloride?
• Disposable plastic Petri dishes are used rather than glass Petri dishes to prepare the hardened glue strips needed for Activity B. The plastic is more flexible and makes it easier to remove the hardened glue circle from the Petri dish cover.
• A flashlight works well as a substitute for an overhead projector. The flashlight may be stood on end or clamped upright.
• Students will have a lot of fun examining many different plastic objects under polarized light in Activity C. Rulers, protractors, plastic cups, clear plastic spoons and forks, deli or salad trays—the list of possible candidates is very long. The color patterns are intriguing, if not always easy to explain!
• The use of PTFE tape in Activity D is not a hazard. Environmental concerns have recently been raised, however, concerning the safety of Teflon nonstick cookware. It has been shown that PTFE releases toxic fumes when it is overheated and burns. The fumes are apparently due to an additive called perfluorooctanoic acid (PFOA), which is used as a processing aid in the manufacture of Teflon. The EPA recommended in January 2006 that companies stop manufacturing and using PFOA, which has been labeled a “likely human carcinogen.” Perfluorooctanoic acid has been found to be a persistent environmental toxin, showing up in approximately 95% of human blood samples that have been tested. Since the tape will not be heated, it does not pose a health risk.

Teacher Tips

• Help students build connections between chemistry and food science or nutrition by having them conduct a kitchen or grocery store search for foods containing sodium alginate or other alginate food additives. “Nutrition” may seem an unlikely term to use here, since snack foods are most likely to contain alginates. The exercise, however, may make students question whether fresh fruit or a “fruit roll-up” has better nutritional value!
• To illustrate the phase transition of a polymer at its glass transition temperature, place a polystyrene cup in boiling water. The cup will revert to its pre-processing shape—a flattened disk—after about 15 minutes. The T_g value of polystyrene is 100 °C.
• A particular color will appear in the birefringence pattern of an object when wavelengths of light that are complementary to that color have been subtracted out from the polarized white light that “arrived” at the object. A yellow band, for example, will be visible when blue light is “rotated” as it passes through the object, so that its plane of polarization becomes perpendicular to the “slit” on the second polarizing filter. Removing blue from white light leaves mainly red and green, which is perceived as yellow.
• Birefringence is used by engineers and designers to determine the degree to which a transparent plastic object has been stressed during manufacture or processing. Structural stress is visible at locations where there is a large concentration of colored bands. Engineers also use this tool to map out how strain will be distributed in plastic models of bridges and other structures.

Answers to Prelab Questions

Read the Background material and Procedure for each activity, A–D. Prepare a summary of the polymer used in each activity and a brief, one-sentence description of the test procedure for each polymer.

The polymer in Activity A is sodium alginate, a natural polymer obtained from seaweed. The polymer will be added to calcium chloride and copper(II) chloride solutions to test its solubility.

The polymer in Activity B is polyvinyl acetate, the main ingredient in glue. A dried glue strip will be placed in hot water and then in cold water to see if it can be molded into a new shape.

The polymer in Activity C is polystyrene, which is used to make clear plastic Petri dishes. The polymer will be examined with polarized light to “see” whether there are any crystalline areas in the polymer structure.

The polymer in Activity D is Teflon or polytetrafluoroethylene. A sample of Teflon tape will be stretched in different directions to determine how the polymer molecules “line up” in the solid.

Answers to Questions

Activity A. Sodium Alginate—A Natural Polymer

1. Describe the appearance, form and texture of (a) sodium alginate solution, (b) calcium alginate and (c) copper alginate.
   1. Sodium alginate solution is a colorless, slightly cloudy, thick, viscous gel.
   2. Calcium alginate is a smooth, flexible, semi-solid, translucent, gel-like substance that precipitates from water in the form of—worms! The “worms” have the consistency of fruit roll-ups or gummy candy.
   3. Copper alginate has the same form and texture as calcium alginate, but is pale blue.
2. What causes the calcium and copper products to appear swollen and translucent?

   The calcium and copper alginate polymers are hydrophilic and readily absorb water. Water causes the polymers to swell and to appear translucent.

   What evidence is there that copper ions are incorporated into the alginate polymer?

   The blue color of the insoluble product in Beaker B indicates that copper(II) ions have been incorporated into the polymer.

3. How and why does the appearance of calcium alginate change when it is placed in saturated sodium chloride solution?

   Adding the calcium alginate “worms” to saturated sodium chloride solution causes the worms to break down and disintegrate. The pieces do not completely dissolve however. The sodium chloride solution turns cloudy.

4. Polymer solutions form solid gels when numerous long-chain polymer molecules interact to build a three-dimensional “network.” How do calcium or copper ions bind alginate molecules together to form a network?

   Calcium and copper(II) ions are both divalent. Each +2 metal cation can bind to at least two –CO₂^– groups via ionic bonds. If the two carboxylate groups are on adjacent polymer molecules, then the effect of adding divalent cations is to tie together many different polymer molecules into a large, three-dimensional network. Note: The alginate polymer acts like a giant chelating ligand (similar to EDTA). Each Ca²⁺ ion actually binds to four –CO₂^– groups.

5. Calcium alginate is spun into fibers that are used to make gauze-type dressings for burns and other wounds. Suggest some possible advantages of calcium alginate wound dressings compared to other types of materials.

   Calcium alginate dressings absorb water from wound secretions and promote healing. The dressings are also biodegradable. The calcium alginate fibers essentially form gels and are very easy to remove without tearing or injuring the wound. Rinsing the dressing with saline solution (NaCl) washes away the dressing!

Activity B. Molding Glue—Properties of an Amorphous Polymer

1. Describe any changes that were observed when the hardened glue strip was placed in hot water.

   The hardened glue strip quickly became pliable when it was placed in hot water (55 °C). Some glue seemed to come off and dissolve in the water—the water turned slightly cloudy right around the strip.

2. How easy or hard was it to mold the hot glue strip into the desired shape? Did the molded glue strip maintain its new shape after it was placed in ice water?

   It was easy to twist the softened glue strip into a helix. The glue helix hardened immediately when it was placed in cold water (4 °C), and it maintained this new shape after it was removed from the cold water. Some glue did seem to come off in the cold water as well, again based on slight cloudiness around it, and the newly hardened helix was a little sticky. It was rigid, however, and no longer pliable.

3. Write a short paragraph (2–3 sentences) relating the physical properties of glue when it is heated and then cooled to changes in the polymer molecules themselves. Include the following terms: Amorphous polymer, glass transition temperature, glassy versus rubbery solid.

   The hardened glue strip is an amorphous solid. There is no long range order in the way the polymer molecules are arranged. The individual polymer chains are not very flexible at room temperature, so the glue strip is rigid and brittle—a glassy solid. Heating the glue above its glass transition temperature “loosens” the polymer chains so they are more flexible and elastic. The glue strip behaves like a rubbery solid when it is heated, and it can be twisted into a new shape.

4. Most clear plastic flatware items (knives, forks, and spoons) are made from polystyrene. What are the properties of a polystyrene spoon at room temperature? What temperature would be needed to mold a polystyrene spoon into a new shape? Explain.

   The glass transition temperature (T_g) of polystyrene is about 100 °C. At room temperature, below T_g, the polystyrene spoon is a rigid, brittle, “glassy” solid. In order to mold a polystyrene spoon into a new shape, it would have to be heated to 100 °C.

Activity C. Seeing Polymers in a New Light—Polarized Light and Birefringence

1. Describe how the amount of light that is transmitted through two polarizing filters changes when the second filter is rotated. (a) What is the alignment of the “slits” on the two filters when the maximum amount of light passes through? (b) What is the alignment of the slits when all of the light is blocked?

   The amount of light that passes through the second filter varies from light to dark as the second filter is rotated past a stationary first filter. (a) The “slits” on the two filters are parallel to each other when the light that comes through is brightest. (b) The slits are perpendicular to each other when no light passes through and the second filter is “dark.”

2. In the first box below (Box A), use colored pencils to draw and color the birefringence pattern observed for the plastic Petri dish.
   {12608_Answers_Figure_13}
3. In the second box (Box B), mark off and label the areas of the Petri dish in which the polymer molecules are in an amorphous versus a partially crystalline state, respectively.

   See diagram in Question 2.

4. Plastic Petri dishes are manufactured by a process called injection molding—the melted polymer is forced through a narrow nozzle into a mold. The polymer flows into the mold, where it cools and solidifies. The mold then opens and the plastic object is ejected. In Box C, use an arrow to show where the polymer flows into the mold.

   See diagrams in Question 2.

5. Explain why some areas of the Petri dish are dark when viewed between “crossed” polarizing filters.

   When a plastic object is viewed between two crossed or perpendicular polarizing filters, areas that are amorphous will appear dark because there is no net rotation of any of the wavelengths of polarized light. No light passes through the crossed filters, with or without the plastic object between them.

6. How and why does the birefringence pattern of the plastic “dumbbell” change after it has been stretched? Explain how “stress” of this type may orient the polymer molecules.

   Before the polyethylene “dumbbell” was stretched, it appeared bright white against a black background when it was viewed between crossed polarizing filters. As it was being stretched, a rainbow-like pattern appeared, starting in the middle of the narrow part of the dumbbell. The pattern was very symmetrical in either direction away from the middle. The order of colors in either direction was yellow (in the middle), followed by green, blue, purple, and pink. This pattern then repeated itself.

Activity D. PTFE Tape—The Long and Short of It

1. Describe the appearance, texture, and “feel” of PTFE. Be as specific as possible.

   PTFE tape is a very thin, opaque, white solid. It is smooth, almost like silk, and very slippery. It also feels cool to the touch.

2. Compare the ability of PTFE tape to stretch when it is pulled lengthwise versus widthwise. Does the tape return to its original shape after it has been stretched widthwise?

   It is very difficult to stretch PTFE tape lengthwise. The strip could only be stretched from 15 cm to about 18 cm. Pulling even a little bit harder caused the strip to break. In contrast, it is extremely easy to stretch the tape widthwise. The strip went from 1 cm wide to more than 8 cm wide as the tape was pulled from top to bottom. When the stretched tape was then pulled lengthwise again, it easily returned to its original shape and dimensions.

3. Based on the elasticity of the tape when it is pulled in either direction, which representation below is the best “picture” of how the polymer molecules are arranged? Explain.
   {12608_Answers_Figure_14}

   The second picture is the most accurate representation of how the polymer molecules are arranged in a strip of PTFE tape. PTFE is a crystalline solid, which means the molecules line up in an orderly or crystalline pattern (the molecules are not all jumbled, like in the third picture). Because the tape basically will not stretch in the horizontal direction, the molecules must be lined up horizontally as well. If the molecules are basically extended chains as shown in the second picture, then they cannot stretch any further when the tape is pulled lengthwise. The tape is very elastic when pulled widthwise. This means there is very little force holding the individual polymer molecules together in this direction.

4. Compare the appearance of the torn ends of tape after it has been pulled lengthwise versus widthwise. What is happening to the polymer molecules when the PTFE strip is being stretched lengthwise and the tape breaks?

   The torn ends are ragged or jagged when the tape breaks lengthwise. Thin strings of tape or tendrils are visible at each end. When the tape is stretched widthwise, it becomes very thin, almost transparent, in the middle, until a small hole forms. The hole then gets bigger, but it is very smooth all around.

5. Which are stronger, the forces of attraction between the polymer molecules or the forces of attraction within the polymer chain? Give evidence to support your answer.

   The forces of attraction within a polymer chain are much stronger than those between molecules. It is thus difficult to stretch the tape lengthwise. Doing so puts strain on the strong covalent bonds holding the polymer molecule together. It is really easy to stretch the tape widthwise, which basically involves overcoming only the weak forces between polymer molecules.

References

This experiment has been adapted from Flinn ChemTopic^™ Labs, Volume 21, Polymers; Cesa, I. Ed., Flinn Scientific: Batavia, IL, 2006.

Introduction

Polymers are an indispensable part of life. Natural polymers include a wide range of biological molecules and materials, including DNA, proteins, starch, cellulose and wood. Synthetic polymers or plastics are incredibly useful modern materials. Examples of polymer “products” that could only be imagined more than 50 years ago include cell phones and computers, contact lenses and artificial joints, bike helmets and bulletproof vests. Polymers are large, chain-like molecules composed of multiple repeating units of smaller molecules, called monomers. A typical polymer molecule may be built up from thousands of monomer molecules that have been joined together using chemical reactions. The properties of a polymer depend on the chemical nature of the monomer, the length of the polymer “chain” and how the monomers are joined together. Let’s look at the structures of polymers and their unusual properties.

Concepts

• Natural and synthetic polymers
• Hydrophilic vs. hydrophobic polymers
• Amorphous vs. crystalline solids
• Glass transition temperature
• Polysaccharides
• Hydrophilic polymer gels
• Amorphous polymers
• Amorphous solid
• Polymer crystallinity
• Polarized light
• Birefringence
• Crystalline polymer
• Elastic polymer

Background

Activity A. Sodium Alginate—A Natural Polymer
Sodium alginate is a natural polymer obtained from kelp and seaweed, brown algae belonging to the phylum Phaeophyta. The polymer is a principal component of the cell wall in brown algae, comprising up to 40% of the dry weight of large species such as giant kelp. Worldwide, about 16 million pounds of sodium alginate are produced per year for use in the food, textile, medical, and pharmaceutical industries.

Sodium alginate is a polysaccharide composed of thousands of oxidized sugar “units” joined together to form an ionic polymer. The repeating units are six-membered rings containing negatively charged –CO₂^– groups. The C-1 carbon atom of one ring is connected via an oxygen atom to the C-4 carbon atom of the next ring in the polymer chain (see Figure 1).

The presence of ionic –CO₂^– side chains, as well as numerous –OH groups, make this natural polymer hydrophilic or “waterloving.” The polymer readily absorbs water and will swell up in contact with water to form a gel. The resulting gel is thick, viscous and smooth. Sodium alginate is used as a “thickening agent” in many processed foods, including ice cream, yogurt, cheese products, cake mixes and artificial fruit snacks. The nontoxic food additive absorbs water, helps to emulsify oil and water components and gives foods a smooth texture. Replacing the sodium ions in sodium alginate with calcium ions gives an insoluble product, calcium alginate, which has interesting medical and pharmaceutical uses.

Activity B. Molding Glue—Properties of an Amorphous Polymer
The enormous size of polymer molecules and their chain-like structures give polymers unique and interesting properties. Most polymers are amorphous solids—there is no long-range order or symmetry in the way the molecules are arranged in the solid state. An individual polymer molecule or chain is best described as a “random coil,” with no definite shape. Because of the size and flexibility of the polymer chains, the molecules in an amorphous polymer are also highly entangled—jumbled, scrambled, knotted, twisted, etc. (see Figure 2). The flexibility of polymer chains depends on temperature. Below a certain temperature, called the glass transition temperature, or T_g, the molecules do not move relative to one another, and the polymer is a rigid, brittle solid (a glass). When the polymer is heated above T_g, the polymer chains become more flexible and can begin to slide past one another. The polymer then behaves as a rubbery solid that is easily stretched or deformed. The value of T_g for a polymer depends on its chemical structure (see Figure 3). The T_g value of polyethylene, for example, is –125 °C—very little thermal energy is needed for rotation around the C–C single bonds. In polystyrene, there is a bulky aromatic ring attached to every other carbon atom. The stiffness of this group makes the polymer less flexible, and increases the T_g value to 100 °C. The T_g value for polyvinyl acetate, the main ingredient in white glue, is about 28 °C (very close to room temperature). The vast majority of familiar plastics are thermoplastic polymers—they soften when heated above T_g, and return to their original condition when cooled. This property makes it possible to mold polymers into useful shapes and objects.

Activity C. Seeing Polymers in a New Light—Polarized Light and Birefringence
Most polymers are amorphous solids—there is no long-range order in the way the polymer molecules are arranged. Within an amorphous polymer, however, there may be crystalline regions where the polymer molecules line up in an orderly fashion (see Figure 4). Crystalline regions may form when a polymer crystallizes from its molten state, when a hot polymer is forced through a narrow opening during injection molding, or when a polymer is “stressed” by stretching or bending an object. Using polarized light makes it possible to “see” areas in the polymer structure where the molecules are lined up in an orderly, crystalline fashion. Normal light is said to be unpolarized—the properties of the light beam are the same in all directions. Passing light through a polarizer, such as a Polaroid^® lens or filter, converts light to polarized light, in which all of the wave vibrations lie in a single plane. The filter may be thought of as possessing “slits”—only the light that is vibrating in a single plane will pass through the polarizer. If two polarizing filters are placed in the path of normal light, the amount of light that is transmitted will depend on how the filters are aligned. If the slits on the second filter (called the analyzer) are lined up parallel to the slits in the first filter (called the polarizer), the polarized light will pass through both filters. If the slits are perpendicular, no light will pass through the analyzer (see Figure 5). Many objects made from polystyrene exhibit bright, rainbow-like color patterns when viewed between two polarizing filters. If the two filters are “crossed” (the analyzer is at a right angle to the polarizer), regions in the polymer that are amorphous will appear dark. Semi-crystalline regions in the polymer will appear as brightly colored areas. This effect, called birefringence, occurs when polarized light that enters the polymer is split into two perpendicular components. The two perpendicular wave components travel at different speeds when they encounter polymer molecules arranged in an ordered (crystalline) manner. The light that passes through these areas of the polymer is still polarized, but the angle of polarization has changed. (The polymer “rotates” the plane of polarized light.) The analyzer will absorb all of the light whose polarization did not change, but will allow light whose polarization angle has changed to pass through the analyzer. The amount of rotation depends on the wavelength (color) of light, the degree of crystallinity of the polymer molecules, and the thickness of the polymer. The wavelength (color) that is rotated by the correct amount will be visible through the analyzer. The overall result is brightly colored bands of different colors in different regions of the plastic.

Activity D. PTFE Tape—The Long and Short of It
Polytetrafluoroethylene, or PTFE, is a synthetic, high molecular weight polymer made by reacting tetrafluoroethylene, F₂C=CF₂, at high pressure in the presence of a catalyst. The structure of PTFE consists of long chains of carbon atoms with two fluorine atoms attached to each carbon atom [–CF₂–CF₂–]. The presence of fluorine atoms gives PTFE unusual characteristics and a range of specialty uses and applications. Carbon–fluorine bonds are exceptionally strong and stable. The strength of the C–F bonds gives PTFE excellent chemical, thermal and electrical resistance. PTFE does not react with even very reactive chemicals, and it is stable over a wide temperature range, from as low as –200 °C to as high as 300 °C.

Teflon^®, a registered trade name for PTFE, is listed in the Guiness Book of World Records as the world’s most slippery substance. Technically, this means that PTFE has the lowest coefficient of friction of any known materialm, warranting the slogan: Nothing sticks to Teflon! The material is both hydrophobic and oleophobic—it repels both water and oil. One explanation for these properties is the electronegativity of fluorine. Fluorine is the most electronegative element and thus does not want to interact or share its electrons with any other substance. It repels them all! PTFE is also biologically inert, because bacteria won’t stick to it either. In addition to its most familiar use in nonstick cookware, PTFE has many important industrial and commercial applications. PTFE is used to make specialty lab and medical equipment, as an insulating material for wires and cables, to make stain-resistant fabrics and textiles and as an additive for automotive products and finishes.

PTFE is a white, opaque, crystalline polymer. Teflon tape is readily available in hardware stores. It is used by plumbers to form a watertight seal on threaded pipes.

Experiment Overview

The purpose of this “activity-stations lab” is to investigate the properties of polymers and to relate these properties to their structures. There are four activity stations set up around the lab. Each activity focuses on a different polymer and is a self-contained unit, complete with background information and discussion questions.

1. Sodium Alginate—A Natural Polymer
2. Molding Glue—Properties of an Amorphous Polymer
3. Seeing Polymers in a New Light—Polarized Light and Birefringence
4. PTFE Tape—The Long and Short of It

Materials

Prelab Questions

Read the Background material and Procedure for each activity, A–D. Prepare a summary of the polymer used in each activity and a brief, one-sentence description of the test procedure for each polymer.

Safety Precautions

The polymers used in this experiment have a variety of consumer and commercial uses and are considered nontoxic. Exercise care when pouring hot water and avoid contact of all solutions with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

Activity A. Sodium Alginate—A Natural Polymer

1. Label three 100-mL beakers A, B and C. Add about 50 mL of the appropriate solution to Beakers A, B and C.
   {12608_Procedure_Table_1}
2. Draw up one pipet-full of sodium alginate solution into a clean Beral-type pipet. Slowly squeeze the pipet bulb and add the sodium alginate solution in one continuous stream into the calcium chloride solution in Beaker A.
3. Repeat step 2, adding sodium alginate into the copper(II) chloride solution in Beaker B.
4. Wait about 1–2 minutes to allow products to form. Using clean forceps, gently lift some of the polymer products out of the solutions in Beakers A and B to observe the appearance of calcium and copper alginate, respectively. Answer Questions 1 and 2 in the Observations and Results section.
5. Using forceps, remove about half of the calcium alginate from Beaker A and add the polymer to the saturated sodium chloride solution in Beaker C.
6. Stir the mixture in Beaker C for 2–3 minutes. Observe any changes in the appearance of the polymer and the solution, and record observations (Question 3).
7. Discard the contents of Beakers A, B and C into the waste beaker. Rinse each beaker with distilled or deionized water, and leave the clean beakers for the next team.
8. Answer the remaining questions in the Observations and Results section.

Activity B. Molding Glue—Properties of an Amorphous Polymer

1. Carefully pour hot tap water (50–60 °C) into one disposable Petri dish, and ice water (0–5 °C) into a second Petri dish. The Petri dishes should be about half-full.
2. Using forceps, place a hardened glue strip into the warm water in the Petri dish and hold it there for two seconds.
3. Remove the glue strip from the warm water and quickly twist the strip into a helical shape using your fingers.
4. When it has the desired shape, dip the “glue helix” into the ice water in the second Petri dish and hold it there for two seconds.
5. Remove the helix from the ice water and place on paper towels to dry. Rinse hands with water and dry.
6. Pour the hot and cold water from the Petri dishes into the waste beaker. Rinse the Petri dishes with distilled water and dry the dishes using paper towels.
7. Describe observations and answer the follow-up questions in the Observations and Results section.

Activity C. Seeing Polymers in a New Light—Polarized Light and Birefringence

1. Tape the unmounted polarizing filter on the overhead projector stage. Place the mounted filter on top of the first and observe how much light is transmitted. Gradually rotate or turn the mounted polarizing filter relative to the first and observe any changes. Answer Question 1 in the Observations and Results section.
2. Determine the alignment of the second polarizing filter that will block all of the light coming through the first filter. Hold the second filter in this “crossed” alignment (see Figure 6).
   {12608_Procedure_Figure_6}
3. Place a clear plastic Petri dish lid on top of the taped polarizing filter. Move the mounted polarizing filter over the Petri dish lid in the “crossed” alignment position. Observe and record the birefringence pattern. Answer Questions 2–5.
4. Remove the Petri dish lid and obtain a polyethylene “dumbbell” cut from a zipper-lock bag. Place the polyethylene strip on the polarizer at a 45° angle. Move the mounted polarizing filter over the strip in the “crossed” alignment position and observe.
5. Holding the plastic “dumbbell” at this 45° angle, stretch it evenly from both ends. Immediately place the plastic back on top of the polarizer and observe any changes. Answer Question 6.

Activity D. PTFE Tape—The Long and Short of It

1. Cut a piece of PTFE tape 15–20 cm long.
2. Grip the tape from the middle of each end horizontally as shown and carefully pull the strip lengthwise (see Figure 7).
   {12608_Procedure_Figure_7}
3. Gently hold the top and bottom edges of the strip, and carefully pull the strip widthwise just a small amount (see Figure 8).
   {12608_Procedure_Figure_8}
4. Repeat steps 2 and 3 with the same strip of tape.
5. Carefully stretch the tape lengthwise until the strip breaks and observe the torn or broken ends of the tape.
6. Cut a fresh piece of PTFE tape about 5 cm long. Slowly stretch the tape widthwise until it breaks and observe the torn ends of the tape.
7. Cut a new piece of PTFE tape about 8 cm long and place the strip on a paper towel.
8. Using a permanent marker, gently write your name or a short greeting on the tape. To avoid pulling and tearing the tape, form the letters with connected dots as shown in Figure 9.
   {12608_Procedure_Figure_9}
9. Fold a short piece of transparent tape onto each end of the PTFE tape (see Figure 9). Gently stretch the PTFE tape widthwise (from top to bottom) to distort the message written on the tape. Be careful not to tear the tape.
10. Hold the taped ends of the PTFE strip and carefully pull the tape lengthwise. What happens to the message? Can this process be repeated?
11. Answer Questions 1–5 in the Observations and Results section.

Student Worksheet PDF

12608_Student1.pdf