CM8001

The polymer is dissolved by the acetone, which releases the gas of the foaming agent. A foaming agent is used to make it easier to make foam by reducing the surface tension of a liquid. The loss of the foaming agent causes the polymer to collapse on itself, resulting in it becoming denser. Polymer chains can rearrange into a more condensed structure following the removal of the air bubbles, which are caused by the foaming agent.

The main difference between LDPE and HDPE is their structure. HDPE are more linear than LDPE. Depending on the shape of the polymer, their physical properties may be very different (even when they are composed of the same number of the same molecules).

a. Branched LDPE (Low Density Polyethylene) cannot be used because it cracks easily under stress (fragile). It is soft, stretchy but not very strong. Hence, it is not good to protect against cuts.

b. HDPE (High Density Polyethylene) has little branching and hence has stronger intermolecular forces and tensile strength than LDPE. It is harder, denser an stronger than LDPE.

No, it does not. This is because the weak but extensive dispersion forces in plastics allow the molecules to slip pass one another and ‘stretch’, making the polymer longer with reduced thickness. Dispersion forces are weak Van Der Waals forces of interaction, which can be broken easily. On the other hand, paper is made from cellulose fibres, which have high tensile strength due to the presence of covalent cross-linkages between the glucose polymers. Therefore, the chemical bonds and structures in paper makes paper resistant to stretching.

a. Biocompatibility – material must be biologically compatible and must not elicit local or systemic responses from the living tissues or systems; Non-toxic; Hydrophilicity – material must be ‘water-loving’ so that cells are able to attach onto the polymers (but there are some that must be hydrophobic as well); Porosity – ratio of the pore volume to total volume, must enable substances or molecules to pass through if the body part that it is replacing has those properties; Degradability – if it is used to replace the body part permanently, it must be able to withstand the conditions within the body and not be degraded or broken down, e.g. acidity, alkalinity, enzymes; mechanical properties – changes depending on its use within the body.

b. Contact lenses are made from silicone hydrogel. Some desirable properties involve the materials being soft, durable and transparent. Moreover, it has to have an appropriate refractive index.

c. The ‘hard’ material that is used in contact lenses is Poly methyl methacrylate (PMMA), and it makes it uncomfortable to wear contact lenses as it is rigid. It does not allow air to pass through. On the other hand, the ‘soft’ material is silicone hydrogel, which makes wearing contact lenses comfortable. It is soft, flexible and allows air passage.