Chain Entanglements, Intermolecular Forces and String that Just Gets All Tied Up:
Things to note:
- Nylon or polypropylene strings (1.5-3 mm thick, preferably smooth and of a bright
color) 700 feet total. (Mason line or Seine twine works quite nicely) These will
represent polyethylene chains of varying size.
- One sided Velcro strips (3 inches wide and 5 feet long). These strands correspond to
polymers with greater intermolecular forces such as nylons
- Scissors or a sharp knife
- Meter sticks or other measuring devices
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.
[Insert picture or video clip of the Velcro strands here]
- Cut the strings into the following lengths:
||Length (feet, each)
- Combine the strings of identical lengths into piles. DO NOT INTERMIX THE STRINGS OF
DIFFERENT LENGTHS! Mix each pile well.
- 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.
- 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.
[Insert picture or video clip of the 100,000 molecular weight strings here]
- 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.
[Insert picture & video clip of the 25,000 molecular weight strings here]
- 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.
[Insert picture or video clip of the 5,000 molecular weight strings here]
- 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.
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.