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.)
Answer: Acetone dissolves the polymer which enables the gas of the foaming agent to escape. Since the gas has been removed, the polymer collapses on itself and become more dense.
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.
b. Offer a molecular level reason for why linear HDPE is successful in this application.
Answer: a) LDPE cannot be used in this application due to the insufficient strength it possesses.
Answer: b) The molecules of HDPE must be arranged in a way that will give the required strength. In order to have the sufficient flexibility, a thin liner of HDPE is used.
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.
Answer: The paper tears rather than stretching to any significant extent when the same pulling force is applied. The cellulose molecules in paper are being held more rigidly in place as compared to the cellulose molecules in plastic bag, and are not free to become aligned.
4. 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.
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.
c. What is the difference in the material used in “hard” and “soft” contact lenses? How do the difference in properties affect the ease of wearing contact lenses?
Ans: a) The four properties are: (1) stable over time of intended use, (2) non-toxic, (3) low cost and (4) the ease of implantation.
Ans: b) Several different types of contact lenses are on the market and each uses a different type of polymer. These include Polymethyl methacrylate (PMMA), Silicone-acrylate materials, newer rigid gas permeable (RGP) polymers, Polymacon.
Desirable properties include being nontoxic, permeable to oxygen, comfortable to wear, and inexpensive. Also desirable is the ability to conform to the shape of the eye and to be easily cleaned (if not disposable).
Ans: c) Hard contact lenses are typically made of PMMA whereas soft contact lenses are made of silicone, which is flexible and allows oxygen to reach the eye. The soft lenses tend to be more comfortable as compared to the hard contact lenses.