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.
Answer: When acetone dissolves the polymer (polystyrene), gas trapped in the foaming agent is allowed to escape and the polymer collapses on itself. Given that density = mass/volume, when the gas is removed from the polymer, volume decreases and density increases. Hence, the polymer becomes more dense.
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.
- Suggest a reason why branched LDPE cannot be used in this application.
- Offer a molecular level reason for why linear HDPE is successful in this application.
- Branched LDPE cannot be used in this application as it does not have required strength to act as liners of surgical gloves.
- Linear HDPE is successful in this application as a thin liner of it allows sufficient flexibility while providing the required strength for surgical gloves.
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.
Answer: When a piece of plastic is stretched, the strip narrows and “necks down”. The molecules become aligned parallel to each other and in the direction of the pull. This alteration of the 3D structure is irreversible, and if the pulling continues, the plastic breaks. When the same pulling force is applied to a piece of paper, the paper tears instead of stretching. This is because paper is made up of cellulose molecules which form cross-linkers and are held more rigid in place. As such, the molecules are not able to be aligned parallel to each other to be stretched.
Question 4. A Teflon ear bone, a Fallopian tube, or heart valve? A Gore-Tex implant for the face or to repair a hernia? Some polymers are bio-compatible and now used to replace or repair body parts.
- List four properties that would be desirable for polymers used within the human body.
- 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 restore 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.
- 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?
- The benefits for polymers used within the body should far outweigh any risks. Two main properties of the polymers are being stable over time of the intended use and being non-toxic to the body. The polymers used in the body should also not be able to react with the bodily fluids or dissolve in them, and the ease of implantation should be considered.
- There are several different types of contact lenses on the market and each uses a different type of polymer. Polymathy methacrylate (PMMA), one of the earliest polymers used for rigid gas permeable lenses, is structurally similar to Lucite and plexiglas. Silicone-acrylate materials now are more commonly used under trade names such as Kolfocon. Newer rigid gas permeable (RGP) polymers contain fluorine: flour-silocone-acrylate polymers and flour-silicones.
- 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. Due to such properties, the soft lenses tend to be more comfortable.