Question 1

When Styrofoam packing peanuts are immersed in acetone (the primary component in some nail-polish removers), they dissolve. If the acetone is allowed to evaporate, a solid remains. The solid still consists of Styrofoam, but now it is solid and much denser. Explain. Hint: Remember that Styrofoam is made with foaming agents.

Styrofoam is made up of polystyrene foam. When Styrofoam dissolve in acetone, the gas of the foaming agent escape. However the polymer is not dissolving. The polymer collapses on itself results as a solid.

Question 2

Consider Spectra, Allied-Signal Corporation’s HDPE fiber, used as liners for surgical gloves. Although the Spectra liner has a very high resistance to being cut, the polymer allows a surgeon to maintain a delicate sense of touch. The interesting thing is that Spectra is linear HDPE, which is usually associated with being rigid and not very flexible.

a. Suggest a reason why branched LDPE cannot be used in this application.

Branched LDPE cannot be used as, because it has more branching than linear HDPE, its intermolecular forces are weaker, its resistance to tension is four times lower, and it is also three times worser in compressive strength.

b. Offer a molecular level reason for why linear HDPE is successful in this application.

HDPE has less branching and thus, its intermolecular forces are stronger. Its molecules are more tightly packed and more crystalline, so its density is higher.  This makes it have four times higher tensile strength and three times better in compressive strength. It also fulfills FDA requirements.  

Question 3

When you try to stretch a piece of plastic bag, the length of the piece of plastic being pulled increases dramatically and the thickness decreases. Does the same thing happen when you pull on a piece of paper? Why or why not? Explain on a molecular level.

Plastic bag is commonly made from polyethylene, a material containing long chain molecules that are randomly dispersed. When a piece of plastic bag is stretched, the molecules move and becoming aligned in the direction of the pull. A “neck” would be formed, where there is sufficient stretching leading to a nearly complete alignment of molecules. The polyethylene becomes crystalline and this increases the strength of the plastic bag since crystalline polymer is stronger than the non-crystalline materials. This alteration is not reversible and if the piece of plastic bag is continue to be pulled, the plastic would break.

The same thing or similar concept would not happen on a piece of paper. Paper is made up of cellulose fibers. The paper molecules are held rigidly in place. They are not free to become aligned. Hence, as the same pulling force is applied to the paper, the paper tears instead of stretching.

Question 4

A Teflon ear bone, fallopian tube, or heart valve? A Gore-Tex implant for the face or to repair a hernia? Some polymers are biocompatible and now used to replace or repair body parts.

a. List four properties that would be desirable for polymers used within the human body.

1. Non-toxic
2. Stable and durable
3. Capable of controlled rates of degeneration
4. Capable of maintaining good mechanical integrity

b. Other polymers may be used outside your body, but in close contact with it. For example, no surgeon is needed for you to use your contact lenses – you insert, remove, clean, and store them yourself. From which polymers are contact lenses made? What properties are desirable in these materials? Either a call to an optometrist or a research on the Web may provide some answers.

Hard contact lenses are made of poly(methylmethacrylate) (PMMA). Soft contact lenses are made of polyacrylamide. Some desirable properties include transparency, unreactive to chemicals on eye surface, easy to mould and low density.

c. What is the difference in the material used in “hard” and “soft” contact lenses? How do the differences in properties affect the ease of wearing of contact lenses?

Polyacrylamide contains nitrogen atoms as compared to PMMA. When the polyacrylamide chains are cross-linked, the material absorbs water. This water keeps the lens soft and flexible resulting in an increase in ease of wearing of lenses for users.