Questions for CM8001-AE 7
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.
When Styrofoam is immersed in acetone, it dissolves into liquid form, allowing gaseous particles in the foaming agent to escape. Evaporation of the acetone from the solution then reverts the Styrofoam back into solid form. The solid Styrofoam then collapses on itself and becomes denser than before due to the release of gaseous particles.
The release of gaseous particles results in the Styrofoam becoming denser due to closer packing of polystyrene molecules. In the presence of a foaming agent, the gaseous particles disrupts the close packing of polystyrene, thus reducing the density. After the gas molecules are released, the polystyrene molecules are better able to adopt a crystalline structure, resulting in higher density.
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.
Branched LDPE have weaker inter-molecular forces of attraction, thus less energy are required to break the forces holding LDPE molecules together. This results in the lower tensile strength of LDPE compared to HDPE.
b. Offer a molecular level reason for why linear HDPE is successful in this application.
First, linear HDPE have straight polyethene molecules that are aligned in the same orientation. This arrangement increases the inter-molecular forces of attraction between molecules, allowing for high tensile strength.
Second, linear HDPE allows the surgical gloves to be made thin while still having high tensile strength. This thin layer allows for greater precision movement that is needed in surgery.
Third, linear HDPE allows for flexible movement, enabling ease of movement of the fingers during surgery.
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.
When the 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. Furthermore, this alteration of the three-dimensional structure is not reversible and if the pulling continues, the length increases and thickness decreases, causing the plastic to break eventually. When the same pulling force is applied to a piece of paper, the paper tears rather than it stretching to any significant extent. The cellulose molecules in paper are held far more rigidly in place and are not free to become aligned, hence it is unable to stretch like how plastic does.
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.
a. List four properties that would be desirable for polymers used within the human body.
- Non-Toxic to the human body.
- Lack of chemical reaction with body fluids.
- Easy to be implanted.
- Stable over time of intended usage.
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 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.
There are many different types of contact lenses and each of them uses different types of polymer. The first polymer contact lenses became commonly available in the early 1960s and were made from a polymer called poly(methylmethacrylate) (PMMA). Lenses made of PMMA are called hard lenses. PMMA lenses are hard, rigid and not very comfortable. The first soft contact lenses were introduced in 1971. These were made from a polymer called polyacrylamide. This polymer is different from PMMA because it contains nitrogen atoms in its structure (PMMA does not contain nitrogen). Polyacrylamide is similar to the polymers used to make acrylic fibers for fabrics. In 1979, the first rigid gas-permeable lenses (also known as RGPs) became available. These lenses are made from a combination of PMMA, silicones and fluoropolymers. This combination allows oxygen to pass directly through the lens to the eye, which makes the lens more comfortable for the wearer.
The properties of an ideal polymer for contact lenses include:
● Transparent ● Some flexibility ● Low density ● Tough ● Unreactive to chemicals on the eye surface ● Easy to manufacture ● Made from a raw material that is available in abundance ● Easy to mould ● Refractive index suitable for bending light rays ● Hydrophilic (‘water-loving’) ● Lets oxygen gas pass through to the eye surface ● Produces lenses that are easy to insert, remove, clean and store.
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?
PMMA lenses are hard, rigid and not very comfortable; it sometimes takes users many weeks to get used to them. The lenses do not allow oxygen to pass directly to the cornea, which can be damaging to the eye. Users have to put a wetting solution in their eyes before putting the lenses in. Hard lenses are not very popular anymore, even though they give good clarity of vision and are very durable – they can last for years. The first soft contact lenses were introduced in 1971. These were made from a polymer called polyacrylamide. This polymer is different from PMMA because it contains nitrogen atoms in its structure (PMMA does not contain nitrogen). Polyacrylamide is similar to the polymers used to make acrylic fibres for fabrics. When the polyacrylamide chains are cross-linked, the material absorbs water. Substances such as this are called hydrophilic (‘water-loving’). This property makes polyacrylamide a useful material for producing contact lenses. Between 38% and 79% of a soft contact lens is water. This water keeps the lens soft and flexible. However, the high water content also makes the lens more fragile and reduces clarity of vision. Soft lenses are cheaper than hard lenses and this has added to their popularity. In fact, some soft lenses can be used for one day and then discarded.