Group Application Exercise 7


1. When Styrofoam packing 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. 

Styrofoam is not biodegradable and does not dissolve readily in liquids such as water. This makes it a persistent part of landfill waste. However, Styrofoam dissolves readily in acetone.

When Styrofoam dissolves in acetone, the acetone breaks down the polymer chain and releases the trapped pockets of air. This causes the appearance of the Styrofoam to shrink in size. Hence Styrofoam does not disappear react with acetone, it has simply dissolved in acetone.

After acetone has evaporated, the Styrofoam is left. Since the trapped air has escaped, the Styrofoam is now solid and denser.

Fig 1. Chemical structure of Acetone

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.

LDPE is made from the monomer ethylene. The branching of LDPE keeps the molecular chains from packing tightly into a crystalline form. Thus, LDPE has less tensile strength but greater ductility. Hence LDPE cannot be used in this application as it does not have the required strength

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

HDPE is also made from the monomer ethylene. HDPE is characterized by minimal branching of the polymer chain. Less branching means that linear molecules pack together during crystallization, making HDPE much denser and rigid. Thus HDPE has the required strength. Whereas linear HDPE means that there would be enough flexibility for the application.

3. When you try to stretch a piece of plastic bag, the length of the piece plastic being pulled increases dramatically and the thickness decrease. Does the same thing happen when you pull on a piece of paper? Why or why not? Explain on a molecular level. 
When the piece of plastic is stretched, the strip narrows and “necks down.” The moleculesbecome aligned parallel to each other and in the direction of the pull. This alteration of thethree-dimensional structure is not reversible, and if the pulling continues, the plastic breaks.When the same pulling force is applied to a piece of paper, the paper tears rather thanstretching to any significant extent. The cellulose molecules in paper are held far morerigidly in place, and are not free to become aligned
4. A Teflon ear bone, fallopian tube, or heart valve? A Gore-Tex implant for the face orto repair a hernia? Some polymers are biocompatible and now used to replace or repair bodyparts.
a.List four properties that would be desirable for polymers usedwithinthe human body 
The benefits for polymers intended for use in the body should far outweigh any risks. The two main properties are (1) stable over time of intended use and (2) non-toxic. Other factors to consider are low cost, lack of solubility in body fluids, lack of reactivity in body fluids, and the ease of implantation
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 storethem yourself. From which polymers are contact lenses made? What properties are desirablein these materials? Either a call to an optometrist or a search on the Web may provide some answers. 
Several different types of contact lenses are on the market and each uses a different type of polymer. Polymethyl 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: 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). Manufacturers’ websites are good sources of information. 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).
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?

As mentioned in the previous part, 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.