CM8001

Acetone dissolves the polymer, allowing gas of the foaming agent to escape. The polymer collapses itself and is more dense because the gas has been removed.

a) LDPE cannot be used because it does not have the required strength.

b) The molecules of HDPE must line up in a way that produces the required strength. Using a thin liner of HDPE allows sufficient flexibility.

When a piece of plastic is pulled, the molecules become aligned parallel to each other in the direction of the pull. This change in 3-dimensional structure is not reversible, and if pulling continues, plastic breaks.

When the same pulling force is applied to paper, the paper tears rather than stretching to any significant extent. This is because the cellulose molecules in paper are held far more rigidly in place and are not free to become aligned.

a) The benefits for polymers intended for use in the body should far outweigh any risks. The 2 main properties are: stable over time of intended use and 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) Several different types of contact lenses are on the market and each uses a different types of polymer. Polymethyl methacrylate (PMMA), one of the earliest polymers used for rigid gas permeable lenses, is structurally similar to Lucite and plexigas. Silicone-acrylate materials now are more commonly used under trade names such as Kolfocon. Newer rigid gas permeable (RGP) polymers contain fluorine: flour-silicone-acrylate polymers and flouro-silicones. Polymacon (38% water) is typical of the polymers used for soft lenses and is a polymer of 2-hydroxymethyl 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 non-toxic, 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) Hard contact lenses are typically made of PMMA, a rigid non-gas permeable plastic. The soft lenses that replaced them are made of silicone, which inflexible and allows oxygen to reach the eye. Because of these properties, the soft lenses trend to be more comfortable.