Balloon / Needle and Experiment

Overview

Balloons are made up of a network of polymer chains, which are flexible. This flexibility is seen in the expansion of the rubber when the balloon is inflated. The polymer that makes the balloon is also porous which explains the deflation of the balloon after a certain period of tie. In this experiment a needle will be pushed through a balloon without popping the balloon.

Materials

1 large balloon

1 18-inch needle

Procedure

    1. Inflate the balloon and tie it off. (Avoid fully inflating the balloon)
    2. Dip the tip of the needle in cooking or mineral oil.
    3. Use a gentle twisting motion to insert the needle into the thick nipple end of the balloon and continue pushing the needle until it emerges on the opposite end.
    4. Poke the balloon to show that is it a regular balloon.

Discussion

Balloons are typically made of latex, which is a type of rubber. The needle being pushed into the balloon is actually sliding between polymer chains instead of tearing the rubber. Balloons are made out of thin sheets of rubber latex which in turn are made from many long intertwined strands of polymer molecules. The rubber is stretchy because of the elasticity of the polymer chains. When

the balloon is blown up, the polymer strands are stretched. The middle area of the balloon stretches more than the tied end and the nipple end (opposite the tie). A needle can be pushed through the ends of the balloon because it will slide between polymer chains instead of tearing the rubber. After the needle is passed through the balloon, small holes will be left behind and the rubber does not make a perfect seal. A sharp, lubricated point pushed through the strands at the side of the balloon will (usually) pop the balloon because the strands are already stretched and will break. Once a tear begins, it enlarges as the air rushes out of the balloon.

There are three conditions, which must be satisfied in order for a molecule to show rubber-like properties.

  1. It must be composed of long-chain molecules possessing freely rotating links.
  2. In a chain like this, the polymer is capable of changing forms based on the various independent vibrations and rotations of the individual atoms that make up the chain. These movements are associated with the thermal motion or heat energy of the atoms. The rotation of the chain's atoms about their bonds leads to many different shapes that the chain can have. Out of all the possible forms that the chain may take the most likely is that of a kinked-up or crumpled chain. Since there is only one formation in which the chain is fully extended, this is very unlikely to occur by chance in a molecule and even more unlikely to occur in all of a group of molecules. If the chain is pulled into in the extended formation by a force pulling on it and the force is then removed, in the course of time, the chain will return to a kinked-up formation. This is what causes the elastic properties of rubber, the tendency of the system to return to an original state after being forcibly deformed. The two ends of the molecule act as if there was some force tending to draw them together.
  3. The forces between the molecules must be weak, as in a liquid.
  4. This is essential if the molecules are to have the freedom of motion and the ability to change their conformations as discussed above. The material must have a loose structure that enables the molecules to rearrange themselves in various ways in response to their thermal agitation.
  5. The molecules must be joined together or 'cross-linked' at certain points along their length. This is what allows the molecules to have weak forces between them as a liquid does, but exist in the solid form. The long-chain molecules of the polymer are joined together at certain points along their length. Each molecule is connected to at least two other molecules so that the assembly of chains acts as a single structure. The cross-linking allows local freedom of motion of the segments of molecules but it prevents bunk slippage of one molecule past another.

By inserted a lubricated needle into the unstretched material, it is possible for it to slip in between the molecules. Inserted it into the stretched part will break cross-linkages and pop the balloon.

References

  1. Mark, J.E., "Rubber Elasticity", Journal of Chemical Education, 58, 898-903 (1981)
  2. American Chemical Society, Operation Chemistry, Polymers Unit, 4 (1994)
  3. Treloar, L.R.G., Introduction to Polymer Science,36-37, 44-46 (1970)