Chain Entanglements, Intermolecular Forces and String that Just Gets All Tied Up:
Materials: Things to note:

The ratio of the length to the diameter of the strings has been related to the molecular weight of the polymer based on some simple trigonometric calculations from on C-C and C-H bond lengths and angles.

  1. Cut the strings into the following lengths:
    Sample Number Length (feet, each) Representative Molecular
    Weight of Polymer
    1 2 100 each 100,000
    2 8 25 each 25,000
    3 50 5 each 5,000
  2. Combine the strings of identical lengths into piles. DO NOT INTERMIX THE STRINGS OF DIFFERENT LENGTHS! Mix each pile well.
  3. Carefully cut the Velcro strips into one-quarter inch wide strips. Do not cut from the length of the strands. Mash them all up together.
  4. Remember that each string represents a polymer chain of a certain molecular weight. Examine the first pile of string. Even though the pile has just two strings total, the mass is quite large and will quickly become entangled. Pick one strand from the top of the pile and try to pull it out. At this molecular weight of polymer, the chain entanglement is so great that removal of one strand from the mass is impossible. The video seen below demonstrates this degree of entanglement.
  5. [Insert picture or video clip of the 100,000 molecular weight strings here]
  6. Notice how the second pile of string looks almost as large as the first. (Remember that the polymer chains represented here are of 25,000 amu, one quarter the molecular weight of the first pile.) Try to pull one string out of this pile. The string should get a little further out, but it is still almost impossible to pull one out completely. The entanglement of the polymer chains again prevents chain pullout.
  7. [Insert picture & video clip of the 25,000 molecular weight strings here]
  8. Now look at pile number three. These strings are considerably shorter than in the first two piles, so the polymer is of considerably lower molecular weight. I bet you can guess what comes next: That's Right! Try to pull one string out of the mass. Surprisingly (or perhaps not), the string pulls out cleanly and quickly. This polymer is of a low enough molecular weight that the chain entanglements are few.
  9. [Insert picture or video clip of the 5,000 molecular weight strings here]
  10. Now examine the Velcro ball. The chains of this polymer are of the same length as the third pile above. Try to pull one of the chains out of the ball with the same amount of force used above. Really tough, isn't it? The adhesive parts of the Velcro strands represent very strong intermolecular forces such as hydrogen bonding and ionic interactions. The chains are actually more difficult to separate than chains of polyethylene (a polymer with low intermolecular forces) of considerably higher molecular weight. The increased intermolecular forces provide for much stronger chain adhesion.
[Insert picture or video clip of the Velcro strands here]


If you want to quiz the students about the level of entanglements present in some of these samples, don't tell them what the approximate molecular weights are. Give them the possible weights and turn them loose on the strings. The level of entanglements of the strings should be their clue to assigning the molecular weights of the materials.