1. By: Nadia and Tiffany and Kemuelle

2. Covalent Network: Linear Chain

3. Properties of Polythene PropertyLow Density Polyethylene (LDPE) High Density Polyethylene (HDPE) Melting Point115 o C135 o C Crystallinitylow crystallinity (50-60% crystalline) Main chain contains many side chains of 2-4 carbon atoms leading to irregular packing and low crystallinity (amorphous) highly crystalline (>90% crystalline) contains less than 1 side chain per 200 carbon atoms in the main chain leading to long linear chains that result in regular packing and high crystallinity Flexibilitymore flexible than HDPE due to lower crystallinity more rigid than LDPE due to higher crystallinity Strengthnot as strong as HDPE due to irregular packing of polymer chains strong as a result of regular packing of polymer chains

4. PropertyLow Density Polyethylene (LDPE) High Density Polyethylene (HDPE) Heat Resistanceretains toughness & pliabilty over a wide temperature range, but density drops off dramatically above room temperature. useful above 100 o C Chemical Propertieschemically inert Insolvent at room temperature in most solvents. Good resistance to acids and alkalis. Exposure to light and oxygen results in loss of strength and loss of tear resistance. chemically inert

5. PropertyLow Density Polyethylene (LDPE) High Density Polyethylene (HDPE) Schematic diagram Usessandwich bags, cling wrap, car covers, squeeze bottles, liners for tanks and ponds, moisture barriers in construction freezer bags, water pipes, wire and cable insulation, extrusion coating

6. BONDING IN POLYTHENE Polythene consist of very long molecules. Polythene molecules are made by joining up lots of ethene molecules into chains of covalently bound carbon atoms with hydrogens attached. There may be short branches along the main chain, also consisting of carbon chains with attached hydrogens. The molecules are attracted to each other in the solid by van der Waals dispersion forces. By controlling the conditions under which ethene is polymerised, it is possible to control the amount of branching to give two distinct types of polythene.

7. HIGH DENSITY POLYTHENE High density polythene has virtually unbranched chains. The lack of branching allows molecules to lie close together in a regular way which is almost crystalline. Because the molecules lie close together, dispersion forces are more effective, and so the plastic is relatively strong and has a somewhat higher melting point than low density polythene. High density polythene is used for containers for household chemicals like washing-up liquid, for example, or for bowls or buckets.

8. LOW DENSITY POLYTHENE Low density polythene has lots of short branches along the chain. These branches prevent the chains from lying close together in a tidy arrangement. As a result dispersion forces are less and the plastic is weaker and has a lower melting point. Its density is lower, of course, because of the wasted space within the unevenly packed structure. Low density polythene is used for things like plastic bags

9. Low melting point since covalent bonding exists in only one dimension. However the longer a molecule is, the higher its melting point –as length increase more sites exists for intermolecular forces to occur between adjacent chains. Low electrical conductivity – no charged particles (electrons or ions) are present to carry charge. Exist as soft, flexible substances. Why?? Polythene is made of covalently bonded carbon and hydrogen atoms. The chains are usually curled up and folded rather than being stretched out.