Application Exercise 7

Q1. 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 agent.

Dissolving the polymer in acetone allows the gas of the foaming agent to escape. After the removal of gas, the polymer collapses on itself and becomes denser.

Q2. 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.

It does not have the required strength.

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

The molecules of HDPE must line up in a way that produces the required strength. Using a thin liner of HDPE allows sufficient flexibility.

Q3. 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.

Stretching of the plastic bag cause the plastic strip to narrow and “neck down”. The alignment of molecules cause them to be parallel to each other and in the direction of the pull. This alteration of the 3D structure is irreversible, and would cause the plastic to break if the pulling continues. 

When the same pulling force is applied to a piece of paper, the paper will tear instead.  This is because the cellulose molecules in paper are held far more rigidly in place and hence unable to be moved to align.

Q4. 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. stable over time of intended use

2. non-toxic

3. low cost

4. lack of solubility and reactivity in body fluids

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 search on the Web may provide some answers.

There are several different types of contact lenses on the market which uses different types of polymer. Polymethyl methacrylate (PMMA) is one of the earliest polymers used for rigid gas permeable lenses. Currently, Kolfocon is the commonly used trade name for silicone-acrylate materials. Newer rigid gas permeable (RGP) polymers contain fluorine: fluoro-silicone-acrylate polymers and fluoro-silicones. Polymacon (38% water) is typical of the polymers used for soft lenses and is a polymer of 2-hydroxyethyl methacrylate (HEMA). Other methacrylates include hioxifilcon(48% water) and methafilcon (55% water) or even lidofilcon (70% water). These materials have desirable properties such as being non-toxic, permeable to oxygen, comfortable to wear, and inexpensive. Other important quality include the ability to conform to the eye shape and easily cleaned.

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?

Hard contact lenses are typically made of PMMA, a rigid non-gas permeable plastic. The soft contact lenses that replaced them are made of silicone, which is flexible and allows oxygen to reach the eye. Because of these properties, the soft lenses tend to be more comfortable