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. AB

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, C 2 H 6. Ethane gas has a two carbon atoms in the chain and each of the two carbon atoms share two valence electrons with the other.

Biopolymers

Polymer Chains (Thermoplastics and Thermosets)

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 ’

General form and structure of Carbon-based polymers

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?