AE 7

Unit 8- The World of Polymers and Plastics

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.

Although it looks like a chemical reaction due to the large change in size, it is actually a physical reaction of dissolving. The acetone (the solvent) in the nail polish will cause the styrene (the solute) to dissolve. The large change in size is because styrofoam consists largely of air, thus, as the air escapes, only the small amounts of styrene dissolved in acetone are left. When the acetone evaporates, solid styrene is left.

This styrene could have air introduced into it again and become styrofoam afterwards.

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.

a. Suggest a reason why branched LDPE cannot be used in this application.

LDPE is not strong enough to have a very high resistance to being cut.

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

Linear HDPE chains are arranged in a way that they are strong enough for this application. Using a thin layer of HDPE will enable it to be more flexible.

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.

Plastic is a series of long chain polymers. When the piece of plastic is stretched, the molecules are aligned parallel to each other and facing in the direction of the pull. However, when the same pulling force is applied to a piece of paper, the paper tears rather than stretching as the cellulose molecules in paper are held far more rigidly in place, and are not free to become aligned. They are similar to the cross linked polymers and does not stretch.

Question 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

It should be non cytotoxic, stable for a long time of extended use, has low reactivity with body fluids and acceptable elasticity.

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.

Contact lenses are made from hydrogel, it absorbs water so as to transmit oxygen into eyes.

The contact lens is a device that covers part of the cornea, for such a device, it would be desirable if the material used allows oxygen to pass so that extended wearing will not harm the cornea while being able to absorb water ensures hydration of the covered portion of the cornea to a certain degree, increasing the comfort while wearing the lenses.

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.

Hard contact lenses were made of a type of plastic called poly methyl methacrylate (PMMA).

Since oxygen cannot pass through a PMMA contact lens, the only way for this vital element to reach the cornea was for tears to wash underneath the lens with each blink. In order for this blink-induced, tear-pumping action to occur, PMMA lenses had to be made relatively small in size. Also, there had to be a significant gap between the edge of the lens and the surface of the cornea.

These design characteristics made many people very aware of PMMA lenses on their eyes or caused discomfort that made wearing the lenses impossible. In some cases, these features also caused problems with PMMA lenses popping off the eye, especially during sports.

Most GP (soft) lenses incorporate silicone, which makes them more flexible than PMMA, thus the permeability-related design constraints that plague the hard lenses are largely overcome, allowing the soft lenses to actually be designed with comfort in mind.

Source: http://www.allaboutvision.com/contacts/rgps.htm 

Published by

Hilda Ting

Year 2 Chemistry student taking CM8001