All posts by June Ming

Ardent lover of books, animals, and a pursuer of arete in ADM, NTU, Singapore.

AE7: The World of Polymers and Plastics

Question 1:

Polymer is dissolved by the acetone, hence the gas of the foaming agent freely escapes. The polymer collapses on itself and is more dense because the gas has been expelled.

Question 2:

a. LDPE cannot be used in this application because the material is not strong enough.

b. The molecules of HDPE must line up according to the way that generates enough strength. Using linear HDPE is successful because it allows sufficient flexibility.

LDPE HDPE

Question 3:

When the piece of plastic bag is stretched, the strip becomes narrow. The molecules are aligned parallel to each other according to the direction of the pull. This is a irreversible alteration of the three-dimensional structure, hence if the pulling persists, the plastic breaks. If this same pulling force is applied to a piece of paper, the paper will tear because it does not have stretching properties. The cellulose molecules in paper are held far more rigidly in place, and are not free to become aligned.

Question 4:

a. 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
  2. non-toxic

Other factors to consider are:

  1. low cost
  2. lack of solubility in body fluids
  3. lack of reactivity in body fluids
  4. ease of implantation

b. 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 plexiglass. Silicone-acrylate materials now are more commonly used under trade names such as Kolfocon. Newer rigid gas permeable (GRP) 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.

The desirable properties in these polymer materials are:

  1. Unreactive to chemicals on the eye surface
  2. Lets oxygen pass through the eye surface
  3. Transparent and moderately flexible
  4. Low density
  5. Easy to manufacture from a raw material that is available in abundance

c. Difference in material used in “hard” and “soft” contact lenses

Hard Soft
Polymethyl methacrylate (PMMA) Polyacrylamide
Does not contain nitrogen Contains nitrogen atoms in its structure
When polyacrylamide chains are cross-linked, the material absorbs water
Hydrophilic (water-loving)

How it affects the ease of wearing of contact lenses:

Hard Soft
Less comfortable because PMMA is a rigid non-gas permeable plastic More comfortable because it is made of silicon and allows oxygen to go through/reach the eye