Thermoplastic Homopolymer Condensation Polymer Natural POLYMERS Synthetic Addition Polymer Copolymer Thermosetting
Polymers are very large covalent molecular substances containing tens of thousands of atoms They are formed by the bonding of many small molecules, called monomers, in a process known as polymerisation. Polymers can be classified as being natural or synthetic. Natural polymers: Starch, Cellulose, Glycogen, Proteins, Silk and DNA. Synthetic polymers: Polyethene, Polyester, Polyurethane and Polystyrene.
There are two types of polymerisation processes - addition polymerisation and condensation polymerisation. Polyethene is an example of an addition polymer. Polyester is an example of a condensation polymer. Polymers may be classified on the basis of their thermal behaviour. Plastics that may be repeatedly melted, reshaped and hardened by cooling are thermoplastics. Plastics that do not melt but char when heated are called thermosetting plastics. These plastics must be moulded or shaped during their manufacture.
Most monomers are simple compounds of carbon and so most polymer molecules are built around atoms of carbon. Covalent bonds form between the monomer molecules to produce a polymer molecule. Suitable monomers for addition polymerisation are unsaturated molecules. The double bonds between the carbon atoms react and new covalent bonds are formed, creating long chains.
Most monomers are simple compounds of carbon and so most polymer molecules are built around atoms of carbon. Covalent bonds form between the monomer molecules to produce a polymer molecule. Suitable monomers for addition polymerisation are unsaturated molecules. The double bonds between the carbon atoms react and new covalent bonds are formed, creating long chains.
Polyethene is a thermoplastic polymer, which means it can be heated again and reshaped. This is because the bonds between the non-polar chains are weak dispersion forces. As it is heated, the molecules gain kinetic energy and the polymer chains can slide over one another, breaking the weak dispersion forces. Polyethene is produced in two main forms: Low Density Polyethene (LDPE) and High Density Polyethene (HDPE).
Polyethene is produced in two main forms: Low Density Polyethene (LDPE) and High Density Polyethene (HDPE). LDPE = Highly branched polyethene HDPE = Unbranched and linear polyethene LDPE forms highly branched chains that don t pack together as well and forms a softer, more flexible form of polyethene. HDPE forms unbranched chains of polyethene that can pack together quite closely and is stronger and less flexible than LDPE.
Tensile strength and softening temperature are two important properties of polymers. Tensile strength is a measure of the material s resistance to breaking under tension. The softening point affects the way the polymer can be moulded or extruded. Tensile strength and softening point are affected by: Degree of branching Nature of atoms or group of atoms attached to carbon chain Arrangement of atoms along the chain.
Polystyrene is an addition polymer made from the monomer styrene. The bulky ring (C 6 H 5 ) prevents chains from stacking close together and forming crystalline regions. As a result, polystyrene is amorphous, transparent, has a low density and is quite brittle. The bulky ring group makes it more difficult for the chains to slide over one another.
Polypropene is an addition polymer made from the monomer propene. The spatial arrangement of -CH 3 side groups makes all the difference between having a soft, greasy wax or a tough polymer. This is because their positions affect how closely the chains can pack together. Three different arrangements are possible.
Cross-linking helps to strengthen a thermoset polymer. Thermosets are rigid, brittle and once formed, they cannot be remoulded, as they do not soften on heating. They char rather than soften when heated. When sufficient heat is applied, the covalent bonds break and the thermosetting polymer decomposes without first softening or melting. They are rigid because the chains cannot slip past each other when a force is applied. Melamine is a thermosetting polymer commonly used for making plates, cups and bowls. It is hard, heat resistant and shatterproof.
Occasional cross-linking produces elastomers or elastic memory polymers. Elastomers are materials that will regain their original shape after being stretched or pulled out of shape. They usually have a relatively small number of cross-links that prevent the chains from slipping past each other. Plasticisers add flexibility to polymers. They are small molecules that fit between polymer chains, holding them slightly further apart. This reduces the effectiveness of intermolecular forces, making the polymer easier to soften and more flexible.
Rubber is one material which is cross-linked with sulphur during manufacture to give it elastic properties. The process now known as vulcanisation was discovered by accident in 1843 when Charles Goodyear heated rubber that had been contaminated with sulphur.