CM8001

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

a) LDPE cannot be used in this application because it does not have the required strength as strength of HDPE is greater than LDPE.
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 you try to stretch a piece of plastic bag, the strip nip arrows and “necks down”.
– Molecules align parallel to each other and in the direction of the pull.
– This alteration of the 3D structure is not reversible, if the pulling continues, the plastic breaks.

When same force is applied on a piece of paper, the paper tears rather than stretch.
– Cellulose molecules in paper are held more rigidly in place, and are not free to become aligned.

a) 1) Must be sufficiently strong until surrounding tissue has healed
2) Does not invoke inflammatory or toxic response
3) To be metabolised in the body after fufililing its purpose, leaving no trace
4) Must demonstrate acceptable shelf life and be easily sterilised

b) Contact lenses are made from poly(methylmethacrylate) (PMMA), silicon-based polymers, polyacrylamide and similar polymers.
Properties that are desireable in these materials:
1) transparent
2) flexibility
3) Low density
4) unreactivity to chemicals on the eye surface
5) tough
6) easy to manufacture
7) raw materials is in abundance
8) easy to mould
9) suitable refractive index
10) hydrophilic
11) allows oxygen gas pass through to the eye surface
12) produces lenses that are easy to insert, remove, clean and store

c) Hard lenses are made from PMMA
Soft lenses are made from polyacrylamide and silicon-based polymers
Polyacrylamide contains nitrogen atoms, when the polyacrylamide chains are cross-linked, the material absorbs water which keeps the lens soft and flexible and more easy to wear.
PMMA does not contain nitrogen atoms. They are hard and rigid which makes it less comfortable to wear since the lenses do not allow oxygen to pass directly to the cornea.