Intro. to the New Chain Gang
An Introduction to the Physical Properties of Polymeric Chains
Objective: The objective of this demonstration is to present the characteristics and properties of polymer chains.
Materials and Supplies:
- 3 different colored sets of pop-beads
- 1 single-layer leaf bag
- 1 double-layer leaf bag
- 1 newspaper
- Cellophane tape
- Iridescent plastic film
- 2 Polarizing lenses
- Identify a polymer chain as being similar to a string of pop beads 100 to 200 in number. Each bead represents a monomer. Simply pour these out of a beaker. This models their linkage to each other. Normal polymer chains will have from 100 to 1000 times the number of monomers.
- Show some of the common physical structures of copolymers (polymers with more than one monomer). Separate the chain types into three classes using colored beads to represent each of the monomers (note you will need at least two different colors of beads). Connect the beads before class.
- Model tearing along the chain versus across the chain of the polymer.
a) Use a series of 4-5 short chains of beads (20-30 each strand) held parallel to each other. Insert a pencil between the chains and move it up and down showing the ease of the separation of the chains.
b) Use an inexpensive 1 layer plastic leaf bag. This will allow easy penetration and will rip up the side of the bag (between the chains).
c) Another model that is a natural polymer is a newspaper. No chemical reaction has occurred in pressing the fibers of paper together. Instead, it is a physical pressing out of excess water. Tearing the newspaper vertically gives a neat, clean, straight tear (between the fibers). Tearing across the newspaper gives a ragged tear (across the fibers).
d) Take the 5 strands of pop beads from part A, hold them at their ends, and pull apart. Notice they do not all break in the same position (ragged) and that it is more difficult than simply separating the strands.
e) Tear cellophane tape along the length and width. Observe how cellophane tape tears smoothly and easily along the length of the tape (between the chains). Notice how it is very difficult to tear across the chains (a sharp edge is usually needed to cut the tape).
- Show how cross-linked polymers tend to be much stronger than non cross-linked polymers. Use a two ply garbage bag. Try to stick your finger through the side of the bag. Its strength is the result of at least two layers of polymers in which the chains are oriented at right angles to each other. Using 2 sets of the 5 chains of pop beads used before, hold at right angles to each other and repeat the above (d) procedure.
- Examine some of the optical properties of polymers.
a) Iridescent films are available for exhibiting the effect of multilayers of polymers that have different indices of optical refraction. A rock sample of mica is a perfect model for representing the polymer layering.
b) Optical rotation properties of polymers.
i) Adequate background on how polarizing lenses work is provided in "Chem Matters," April 1984, page 9. An overhead will work very well making a transparency of the pictures shown.
ii) Set up and demonstrate that no light passes through when two of the polarizing lenses are held at 90 degrees to each other. Set the two filters with formed plastic pieces between the two filters. The plastic pieces will rotate the polarized light due to the presence of the polymer chains. This property is used to identify stress points on shape plastics that are pressed out or extruded. Numerous strips of cellophane tape criss-crossed randomly over a piece of overhead transparency will also give an array of colors. The cellophane tape rotates the plane of polarized light.