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1. When Styrofoam packing peanuts are immersed in acetone, 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.
Answer: Styrofoam is composed of 98% air. When polystyrene dissolves in acetone, the air in the foam is released, thus compacting the Styrofoam into a solid which is much denser.
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. Branched LDPE are softer and more flexible but have lower tensile strength, thus, during a surgery, it may break easily exposing surgeons to viruses or bacteria present.
b. The polymer chains in HDPE, on the other hand, are more linear. They pack closer together, resulting in greater intermolecular forces and a more “crystalline” structure. HDPE has greater tensile strength than LDPE.
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: No it does not happen to paper. For plastic bags, the molecules are packed together with high degree of branching, thus creating a less strong intermolecular forces which results in lower tensile strength and increased ductility. This happens in the opposite manner for paper whereby paper has a low ductility characteristic.
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.
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 differences in properties affect the ease of wearing of contact lenses?
Answer:
a. The properties are (1) stable over time of intended use, (2) non-toxic, (3) inert and (4) light. Other factors to consider are low cost, lack of solubility in body fluids, and the ease of implantation.
b. Polymethyl methacrylate (PMMA), one of the earliest polymers used for rigid gas permeable lenses. Silicone-acrylate materials now are more commonly used under trade names such as Kolfocon. 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.
c. 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.