Seeing Polymers in a New Light

Introduction

Many objects made from polystyrene and other polymers exhibit bright, rainbow-like color patterns due to birefringence when viewed between two polarizing filters. This effect makes it possible to “see” areas in the structure of polymers where the molecules are lined up in an orderly, crystalline fashion.

Concepts

• Polymers
• Amorphous vs. crystalline solids
• Polarized light
• Birefringence

Materials

Safety Precautions

The materials used in this demonstration are considered nonhazardous. Please observe all normal laboratory safety guidelines.

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 CD case and polarizing filters may be stored for future use.

Prelab Preparation

1. Mounted Polarizing Filter: To prevent overheating when the filter is placed on the overhead projector stage, mount one of the polarizing filters on a wooden frame made of craft sticks. Tape opposite sides (edges) of the polarizing filter to two craft sticks.
2. Polyethylene “Dumbbells”: Cut several 2 x 8 cm dumbbell-shaped strips out of the polyethylene zipper-lock bag. The strips should be cut parallel to the zipper.
   {12533_Preparation_Figure_1_Plastic “dumbbell”}
3. (Optional) Make copies of the demonstration worksheet for student use.

Procedure

1. Place the mounted polarizing filter on the overhead projector stage. Place the second filter on top of the first and observe how much light is transmitted. Gradually rotate or turn the second polarizing filter relative to the first and observe any changes. Answer Question 1 on the Polymers Worksheet.
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 and gently tape it to the lens of the overhead projector (see Figure 2).
   {12533_Procedure_Figure_2}
3. Place the clear plastic CD case on top of the mounted polarizing filter. Observe and record the interference pattern due to birefringence. Answer Questions 2–5 on the Polymers Worksheet.
4. Remove the CD case and obtain a polyethylene “dumbbell” cut from a zipper-lock bag. Place the polyethylene dumbbell on the polarizer at a 45° angle and observe.
5. Holding the plastic at this 45° angle, stretch it evenly from both ends. Immediately place the stretched dumbbell back on top of the polarizer and observe any changes. Answer Question 6 on the Polymers Worksheet.

Student Worksheet PDF

12533_Student.pdf

Teacher Tips

• All of the materials except the polyethylene dumbbell are reusable—the demonstration may be repeated many times. Use a fresh polyethylene strip for each demonstration.
• Students will have a lot of fun examining a variety of plastic objects under polarized light. Rulers, protractors, plastic cups, clear plastic spoons and forks, deli or salad trays—allow students to get creative. The color patterns are intriguing, if not always easy to explain!
• 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 the how strain will be distributed in plastic models of bridges and other structures.

Answers to Questions

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 must be parallel to each other when the light that comes through is brightest. (b) The slits must be perpendicular to each other when no light passes through and the second filter is “dark.”

2. In the first box below, use colored pencils to sketch and color the birefringence pattern observed for the plastic CD case (Box A).
   {12533_Answers_Figure_5}
3. In the second box, mark off and label the areas of the plastic case in which the polymer molecules are in an amorphous versus a partially crystalline state, respectively.

   See the diagrams in Question 2.

4. Plastic CD cases 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 the third box, draw and label the regions where (a) the polymer flows into the mold and (b) the polymer molecules have been “frozen” into a partially crystalline arrangement.

   See the diagrams in Question 2.

5. Explain why some areas of the CD case 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 the plastic 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. “Stressing” or stretching the polymer causes the molecules to adopt a crystalline arrangement.

Discussion

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 3). Crystalline regions may form when a polymer crystallizes from a melt, 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 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 4). 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 has changed to pass through. 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 overall result is brightly colored bands of different colors in different regions of the plastic.

References

This activity was adapted from Polymers, Flinn ChemTopic™ Labs, Volume 21; Cesa, I., Editor; Flinn Scientific Inc.: Batavia, IL (2006).