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The Structure and Properties of Polymers. Also known as Bonding + Properties. Mer – The repeating unit in a polymer chain Monomer – Polymer – Many mer - units along a chain A polymer is an organic material and the backbone of every organic material is a chain of carbon atoms.
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The Structure and Properties of Polymers Also known as Bonding + Properties
Mer – The repeating unit in a polymer chain • Monomer – • Polymer – Many mer -units along a chain • A polymer is an organic material and the backbone of every organic material is a chain of carbon atoms.
What is a polymer? • A long molecule made up from lots of small molecules called • monomers.
A polymer is composed of many simple molecules that are repeating structural units called monomers.
A single polymer molecule may consist of hundreds to a million monomers and may have a linear, branched, or network structure.
Covalent bonds hold the atoms in the polymer molecules together. • secondary bonds then hold groups of polymer chains together to form the polymeric material.
Copolymers are polymers composed of two or more different types of monomers. • A B
Copolymerisation • when more than one monomer is used. • An irregular chain structure will result eg propene/ethene/propene/propene/ethene
The carbon atom has four electrons in the outer shell. • Each of these valence electrons can form a covalent bond to another carbon atom or to a foreign atom.
The key to the polymer structure is that two carbon atoms can have up to three common bonds and still bond with other atoms. • The elements found most frequently in polymers are: • H, F, Cl, Bf, and I with 1 valence electron; • O and S with 2 valence electrons; • C and Si with 4 valence electrons.
Consider the material polyethylene, which is made from ethane gas, C2H6. • Ethane gas has a two carbon atoms in the chain and each of the two carbon atoms share two valence electrons with the other.
Types of polymers • Commodity plastics • PE = Polyethylene • PS = Polystyrene • PP = Polypropylene • PVC = Poly(vinyl chloride) • PET = Poly(ethylene terephthalate) • Specialty or Engineering Plastics • Teflon (PTFE) = Poly(tetrafluoroethylene) • PC = Polycarbonate (Lexan) • Polyesters and Polyamides (Nylon)
All the same monomer • Monomers all same type (A) • A + A + A + A • -A-A-A-A- • eg poly(ethene) polychloroethene PVC
Different monomers • Monomers of two different types A + B • A + B + A + B • -A-B-A-B- • eg polyamides • polyesters
Addition polymerisation • Monomers contain C=C bonds • Double bond opens to (link) bond to next monomer molecule • Chain forms when same basic unit is repeated over and over. • Modern polymers also developed based on alkynes R-C C - R’
Thermoplastics (80%) • No cross links between chains. • Weak attractive forces between chains broken by warming. • Change shape - can be remoulded. • Weak forces reform in new shape when cold.
Thermosets • Extensive cross-linking formed by covalent bonds. • Bonds prevent chains moving relative to each other. • What will the properties of this type of plastic be like?
Longer chains make stronger polymers. • Critical length needed before strength increases. • Hydrocarbon polymers average of 100 repeating units necessary but only 40 for nylons. • Tensile strength measures the forces needed to snap a polymer. • More tangles + more touching!!!
Crystalline polymers • Areas in polymer where chains packed in regular way. • Both amorphous and crystalline areas in same polymer. • Crystalline - regular chain structure - no bulky side groups. • More crystalline polymer - stronger and less flexible.
Cold-drawing • When a polymer is stretched a ‘neck’ forms. • What happens to the chains in the ‘neck’? • Cold drawing is used to increase a polymers’ strength. • Why then do the handles of plastic carrier bags snap if you fill them full of tins of beans?