Polymers (Teacher)

The Structure and Properties of Polymers monomer polymer

What is a polymer? A long molecule made up from lots of small molecules called monomers.

All the same monomer Monomers all same type (A) A + A + A + A  -A-A-A-A- eg poly(ethene) polychloroethene PVC

Different monomers – to form Copolymers Monomers of two different types A + B A + B + A + B  -A-B-A-B- eg polyamides polyesters

Copolymerisation Copolymerisation occurs when more than one monomer is used. An irregular chain structure will result eg propene/ethene/propene/propene/ethene Why might polymers designers want to design a polymer in this way? (Hint) Intermolecular bonds!

The Structure of Polymers (plastics) Polymers are created by the chemical bonding of many identical units. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the carbon backbone.

The Structure of Polymers (plastics) Polymers that contain primarily carbon and hydrogen are classified as organic polymers. Polypropylene and polystyrene are examples of these. Even though the basic makeup of many polymers is carbon and hydrogen, other elements can also be involved.

The Structure of Polymers (plastics) Oxygen, chlorine, fluorine, nitrogen, silicon, phosphorous and sulfur are other elements that are found in the molecular makeup of polymers. Polyvinyl chloride (PVC) contains chlorine. Nylon contains nitrogen. Teflon contains fluorine. Polyester and polycarbonates contain oxygen.

The Structure of Polymers (plastics) There are also some polymers that, instead of having a carbon backbone, have a silicon or phosphorous backbone and these are considered inorganic polymers.

Natural Polymers Wool, cotton, linen, hair, skin, nails, rubber, and flesh are all naturally occurring polymers Most naturally polymers are made of protein or cellulose

Synthetic Polymers Commonly referred to as plastics – pliable, able to be moulded

The bonding process: When thermoplastic polymers (left) are heated they become flexible. There are no cross-links and the molecules can slide over each other. Thermosetting polymers (right) do not soften when heated because molecules are crosslinked together and remain rigid.

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.

Thermoplastics Thermoplastic polymers keep their plastic properties They soften on heating and then harden again on cooling These polymer molecules consist of long chains which have only weak bonds between the chains

Thermoplastics The bonds between the chains are so weak that they can be broken when the plastic is heated The chains can then move around to form a different shape The weak bonds reform when it is cooled Thermoplastic material keeps its new shape

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?

Thermosetting Thermosetting polymers never soften once they have been moulded. Once set into a shape, that shape cannot be altered These polymer molecules consist of long chains which have many strong chemical bonds between the chains

Thermosetting The bonds between the chains are so strong that they cannot be broken when the plastic is heated This means that the thermosetting material always keeps its shape

Addition Polymerisation When ethene is subjected to high pressure it changes from a gas to a liquid Liquid ethene (still under high pressure) is heated in the presence of a catalyst (O 2 ), an addition reaction takes place. For addition polymerisation to occur, the monomer must have a double C bond.

Addition Polymerisation This bond breaks to allow the long chains to form. Modifying ethene, substituting different functional groups for hydrogen atoms produces other monomers that can be polymerised to make polymers with different properties.

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’

Addition Polymerisation A carbon = carbon double bond is needed in the monomer A monomer is the small molecule that makes up the polymer

Addition Polymerisation The polymer is the only product Involves the opening out of a double bond The conditions of the reaction can alter the properties of the polymer Reaction proceeds by a free radical mechanism Oxygen often used as the initiator

Addition polymerisation

Conditions are high pressure and an oxygen initiator (to provide the initial free radical). Monomer = phenylethene Polymer = poly(phenylethene)

Prediction the repeating unit This is easy, basically open out the double bond.

Review Work through the sample problem page 200 Complete the revision questions page 200 (1 - 2)

Linear polymers with side branches Linear polymers are those in which the main backbone is unbranched. The way in which side branches are arranged on linear polymers (polypropylene) can affect the properties of the polymer.

Linear polymers with side branches Isotactic Same side of the linear polymer Greater effect of dispersion forces therefore high density, rigid and tough and a high softening temp.

Linear polymers with side branches Atactic Irregular points on both sides of the linear polymer Chains of molecules cannot get close together, therefore low density. Soft, waxy – little use

Poly(propene) This varying degree of randomness will affect the strength and melting point of the polymer. The less random, the stronger the polymer and the higher the melting point This is because in a more ordered polymer they chains can get closer together and hence the van der Waal’s forces will be greater.

Linear polymers with cross links Crosslinks are covalent bonds that can form between polymer chains. If the number of crosslinks is small an elastomer (vulcanised rubber) will result. If the number of crosslinks is large a hard inflexible thermosetting polymer will be produced.

Linear polymers with cross links To make a thermosetting polymer, the linear chains are produced first The cross linking is brought about either by I). heat or II). adding a chemical to react between the lateral functional groups linking the chains together. Araldite is a good example of a two part glue – the 2 nd method of producing a thermosetting polymer.

Condensation Polymers Condensation polymerisation uses monomers that have two functional groups per molecule. These are said to be difunctional. Polymerisation occurs when these monomers react ‘head-to-tail’ to form a new bond that will eventually join the monomers together A small molecule (often water) is eliminated condensation polymerisation simulation

Condensation Polymers Suitable functional groups -NH2amine-OH alcohol O O -C carboxyl -C chloride OH acid Cl

Condensation Polymers Involves 2 monomers that have different functional groups. They also involve the elimination of water or another small molecule. Hence the term condensation polymer. Monomer A + Monomer B  Polymer + small molecule (normally water).

Condensation Polymers Common condensation polymers include polyesters (the ester linkage) and polyamides (the amide linkage as in proteins). May be natural (protein, starch, cotton, wool, silk) or synthetic (viscose, nylon, polyester)

Polyesters The ester linkage is formed between the monomers This is an ester linkage:

Polyesters The OCR example here is terylene, a polymer of benzene-1,4-dicarboxylic acid and ethane-1,2-diol.

Polyamides These involve the linkage of two monomers through the amide linkage as in proteins (e.g. silk) This is an amide linkage:

Nylon 6,6 a polyamide

Kevlar a polyamide

Uses of polyamides The main use of polyesters and polyamides is as fibres in clothing. Most clothing now has a degree of manufactured fibres woven into the natural material (such as cotton). This gives the material more desirable characteristics, such as stretchiness, and better washability. Don’t forget that proteins are also polyamides, you must know how the linkage works with natural polymers such as proteins.

Review Complete the revision questions page 202 (3 – 5) What would life be without polymers? PVC, Teflon, Polystyrene – check table 9.1 page 198

Addition Polymers Ethene can be polymerised to produce both low and high density polyethene (commonly known as polyethylene) Low density polyethene (LDPE) produced with high temp and high pressure – long side chains – low density (eg. plastic bags) Soft, flexible and translucent with a waxy surface that repels water.

Addition Polymers High Density Polyethene (HDPE) produced with lower temp and pressure – very few short branches – dispersion forces more effective – high density (eg. plastic bottles) Rigid, stronger and more opaque than LDPE Slightly flexible, waxy surface that repels water

Addition Polymers Rubber is an addition polymer that occurs naturally The monomer in natural rubber is isoprene. It polymerises to form long chains. Molecular formula (C 5 H 8 ) n Rubber still contains double bonds which can be attacked by oxygen and can perish (unlike polythene)

Addition Polymers Rubber – not elastic – long chains straighten out when stretched and remain this way Susceptible to temperature changes – brittle when cold and sticky when hot.

Addition Polymers Vulcanisation improved durability and elasticity of rubber. The linear chains are cross linked using heat and sulfur

Condensation Polymers Nylon Can be extruded when molten to form fibres or sheets of strong, durable and elastic material Its invention had a great impact on the textile and clothing industries.

Condensation Polymers Nylon 6 : 6 Nylon is a linear chain containing up to 100 repeated units. The name nylon 6 : 6 refers to the existence of 6 carbon atoms on each of the units

Condensation Polymers PET plastic (Polyethene terephthalate). Soft drink bottles An example of a polyester Note the removal of H 2 O (condensation polymer)

Polymer Selection Due to their versatility, polymers can be produced for almost any imagined purpose. A huge range of polymers exist today and are used for many different applications. Their versatility has made them one of them one of the most useful classes of substances that we rely on in today’s society. This versatility can be attributed to the many different ways that they can be modified

Recycling Most plastics are produced from crude oil, coal or gas. Many of them are not biodegradable and have become a visible part of our environmental litter.

Recycling

Review Polymers on line multiple choice Polymers quiz on line Complete the multiple choice questions pages 212, 213 (1 – 12) Complete the review questions 3, 6, 7, 9, 11, 13, 16, 19, 21