P Conducting polymers information sheet: page 1 of 5. Intrinsically conducting polymers

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1 oductig polymers iformatio sheet Most textbooks idicate that oe of the most importat properties of polymers is that they are electrical isulators they are used for coverig electrical cables, the bodies of electrical plugs ad sockets, ad so o. This is o loger completely true. Over the past few years several polymeric materials have bee produced that coduct electricity ad a rage of applicatios is beig developed. These coductig polymers are of two basic types: itrisically coductig polymers where the polymeric material itself coducts; ad extrisically coductig polymers which are composites where a coductive material such as carbo black is embedded i a o-coductig polymer such as poly(ethee). Itrisically coductig polymers The simplest itrisically coductig polymer is poly(ethye), sometimes called poly(acetylee), (see below) which, despite its ame, is a alkee ot a alkye. It cosists of a hydrocarbo chai with alteratig sigle ad double bods; called a cojugated system. The p-orbitals which form the double bods ca overlap to form a delocalised π system (similar to the oe i bezee). Electros flow through the delocalised system ad so the polymer ca coduct. I fact, additives such as iodie have to be icorporated to maximise the coductivity by esurig that the polymer does exist i the delocalised form rather tha as localised sigle ad double bods. Suitably doped poly(ethye) ca have a coductivity comparable with that of copper provided the material has bee stretched to alig the chais so that they all ru i the same directio. Poly(ethye) has problems for everyday applicatios as it is attacked by oxyge from the air but other more stable polymers with cojugated systems also have coductig properties. There are some examples o the ext page. ethye poly(ethye) Delocalised π-system Poly(ethye) P oductig polymers iformatio sheet: page 1 of 5 POTOOPY

2 ( 2 ) R N Poly(ethye) S Poly(alkylthiophee) Poly(ailie) S Poly(thiophee) N O Poly(fura) Poly(pyrrole) Examples of itrisically coductig polymers The well-kow coductivity of graphite (see below) ca be explaied i the same way. ere there is a two-dimesioal delocalised system coverig a layer of carbo atoms so that graphite coducts well alog the plaes of carbo atoms but poorly at right agles to them. Delocalised π-system Graphite Extrisically coductig polymers Oe type of extrisically coductig polymer cosists of a matrix of poly(ethee) with a percetage of coductig carbo black (a form of powdered graphite) icorporated i it. If the carbo black particles are close eough to be i cotact with oe aother, the material coducts. If the particles are ot i cotact, it is a isulator. This meas that the degree of electrical coductio depeds o temperature. At high temperature, the poly(ethee) matrix expads ad pulls the particles of carbo black away from each other, decreasig the coductivity. At lower temperatures the poly(ethee) cotracts, the carbo black particles are closer ad the material coducts well. This temperature depedece of coductivity leads to the use of this material i self-regulatig heater cable ad PolySwitch* re-settable circuit protectio devices. * PolySwitch is a registered trademark of Raychem orporatio. oductig polymers iformatio sheet: page 2 of 5 P POTOOPY

3 Beat the freeze the IceStop system Ice ca cause a lot of damage burst pipes, slippery walkways, collapsig roofs - all of which ca be preveted by low level heatig. ovetioal heatig circuits have some disadvatages here as they have a costat curret which ca result i hot spots ad eergy wastage. The IceStop system cosists of parallel copper wires embedded i a coductig polymer. arbo graules form coductig pathways betwee the wires resultig i a large umber of miiature parallel circuits. The polymer coducts electricity well ad thus acts as a heater, oly whe it is cold. As the material warms up the poly(ethee) expads, iterruptig some of the coductig pathways ad switchig off the miiature circuits (see below). + Power supply Polymer holds the carbo black particles i place arbo black particles urret-carryig electrodes oductig pathway of carbo black particles eat is geerated oly where electric curret flows through the carbo black pathways IceStop is a registered trademark of Raychem orporatio. P oductig polymers iformatio sheet: page 3 of 5 POTOOPY

4 OLD eat geerated by flowig curret Electrically isulatig layers oductig pathway of carbo black particles urret carryig electrode old polymer + OT Whe heated the polymer expads. This separates the carbo black particles, breakig the coductig pathway arbo black particles separated by expasio + Expaded hot polymer eater cable IceStop cable ca be laid alog water pipes, uder pathways ad alog gutterig to provide low-level, self-regulatig heatig which keeps the eviromet frost-free. It is flexible ad easy to istall, ca be cut to ay legth required ad ca be overlapped or woud roud a pipe. Protectig batteries the PolySwitch device Lithium batteries are used i may small had-held electrical appliaces such as cameras but a curret overload ca lead to overheatig resultig i, at best, damage to the appliace, ad, at worst, the battery explodig. Lithium batteries are also used i telecom systems, audio speakers, fire ad burglar alarms ad persoal computers. A covetioal fuse could provide the required protectio but eeds to be replaced if it blows. A PolySwitch device does the same job but ca be reset, rather tha havig to be replaced, oce the fault has bee rectified. As i IceStop cable, carbo graules form coductig pathways through the polymer ad these pathways are broke if the material becomes too warm. This protects the appliace from curret overload. Why does t the fuse keep resettig itself? Whe the PolySwitch device gets hot, it does ot switch off the curret completely as does a melted wire i a fuse. A very small curret still flows through the device. This is eough to keep it hot. Oce the fault has bee rectified, the PolySwitch device ca be reset by first turig off the power to allow it to cool. oductig polymers iformatio sheet: page 4 of 5 P POTOOPY

5 Questios 1. Explai the term cojugated. 2. The groups attached to the double bods i poly(ethye) ca be either cis or tras. Poly(ethye) exists i two extreme forms all cis ad all tras. Draw the structure of each. You should draw at least five repeatig uits. 3. What is the fuctioal group i poly(ethye)? 4. Why would you expect oxyge to attack poly(ethye)? Suggest a possible product of the reactio. 5. Suggest some advatages which a coductig polymer might have over a coductig metal. Assume that the typical polymer properties are essetially uchaged (except for electrical coductivity). 6. Draw a displayed formula for poly(pyrrole) showig at least three repeatig uits. 7. What is a composite material? What affects its properties? 8. Does the electrical coductio of metals rise or fall as the temperature icreases? (it: thik about supercoductivity.) 9. Look at the formulae of the two polymers below. Predict which oe you might expect to be a itrisic coductor ad explai your choice. a) b) oductig polymers 10. Use a data book to fid the bod legths you would expect for ad for =. What is the carbo carbo distace i bezee? What techique might you use to fid out if poly(ethye) were i the delocalised or o-delocalised form? 11. Describe the way i which electros move i metals ad allow them to coduct electricity. ompare this with the situatio i poly(ethye) ad graphite. Write dow ay similarities ad differeces. P oductig polymers iformatio sheet: page 5 of 5 POTOOPY

Shape memory polymers iformatio sheet These polymers remember the shape ito which they have bee moulded ad will retur to it o getle heatig. They are based o thermoplastic polymers.

6 Shape memory polymers iformatio sheet These polymers remember the shape ito which they have bee moulded ad will retur to it o getle heatig. They are based o thermoplastic polymers. Durig maufacture the polymer is moulded ito a particular shape ad irradiated with β-radiatio. It is the heated, reshaped ad cooled. It does, however, remember the shape which it had whe irradiated ad returs to it whe re-heated. Oe applicatio of this effect is heatshrikable sleeves which are used to hold together budles of wires i car wirig haresses (see below). Bodig withi polymers Applyig a shrikable sleeve ere a cylidrical legth of poly(ethee) sleeve is moulded ito shape so that it has a arrow iteral diameter which firmly holds together a buch of wires. The sleeve is ow irradiated, which causes covalet crossliks betwee the poly(ethee) chais. The tube is ext heated to above its crystallie meltig poit to softe it. The it is stretched ito a larger diameter. This stretches the crossliks. The tube is cooled ad this locks the chais i their stretched positio. Now the large diameter tube ca easily be slipped over a buch of wires. If it is heated (by a hot air gu or blowlamp) above its crystallie meltig poit, the stretched crossliks pull the material back ito the shape it had o irradiatio ad it holds the buch of wires firmly together. Polymers ca be classified as thermoplastic (thermosofteig) or thermosettig. Thermoplastics softe o heatig ad ca be moulded ito a shape which they retai o coolig. They ca be reheated ad moulded idefiitely. They cosist of log chai molecules, each chai beig essetially idepedet of the others. There are o covalet bods betwee the chais. The plastics retai their shape whe cool because of itermolecular forces betwee the chais. I particular there are areas where the the chais lie up i a ordered way so-called areas of crystalliity. If crystallie areas o two adjacet chais lie up, the itermolecular iteractios are particularly strog. This is resposible for much of the stregth of thermoplastics i the solid state. O heatig above the crystallie meltig poit, icreased thermal motio makes the crystallie areas disappear. The polymer softes, the chais become free to move past oe aother ad the plastic ca be moulded. O coolig, ew crystallie areas re-form, which help retai the ew shape (see below). Shape memory polymers iformatio sheet: page 1 of 3 P POTOOPY

7 rystallie areas i a polymer chai Irradiatio Thermosettig plastics have may covalet crossliks betwee the polymer chais which form as the polymer is made. Oce made, the polymer is uaffected by heat (util it begis to bur or decompose). Shape memory plastics have a degree of crosslikig (after irradiatio) which is less tha that of a thermoset but more tha that of a thermosofteig plastic. ow does the irradiatio process produce the cross liks? β-radiatio is a stream of electros each with more tha eough eergy to break covalet bods. β-irradiatio of poly(ethee) breaks some of the bods i the poly(ethee) chais. As carbo ad hydroge atoms have similar electroegativity, the bods ted to break homolytically leavig a free hydroge atom ad a carbo free radical, a carbo atom with a sigle ie upaired electro. Such carbo atoms are extremely reactive ad two close together may form a covalet bod thus pairig up their electros. This forms a crosslik betwee the chais (see below). β-irradiatio The effect of β-irradiatio o poly(ethee) The hydroge atoms, which also have a upaired electro each, ted to come together to form hydroge molecules which escape from the polymer. P Shape memory polymers iformatio sheet: page 2 of 3 POTOOPY

8 Questios 1. What other bods will the β-radiatio break? Suggest what effects this might have o the polymer. 2. What might happe if too may crossliks are formed i the polymer? What effect might this have o its properties? 3. What are the itermolecular forces which operate withi the crystallie regios of a thermosofteig plastic called? ompare the stregth of a sigle itermolecular iteractio with that of a typical covalet bod. Explai why, whe cool, these itermolecular forces have comparable effects to covalet bods ie thermosets ad thermoplastics have comparable stregths. 4. Aother free radical reactio is that of bromie with methae i ultraviolet light. The steps are: 1. Br Br 2Br Br Br + 3 ad Br Br + 3 Br UV 3. Br + Br Br Br Br a) The first step is called iitiatio. Name the other two. b) I this case, there are o hydroge free radicals formed by the reactio Suggest a reaso for this. What does this tell you about the eergy of ultraviolet light compared with that of β-radiatio? c) Which of the three possibilities for step 3 above is most similar to the reactio which occurs i the polymer crosslikig reactio? Explai your choice. 5. Explai the terms homolytically ad electroegativity as used i the passage. Shape memory polymers iformatio sheet: page 3 of 3 P POTOOPY

Homework 7 Due 5 December 2017 The numbers following each question give the approximate percentage of marks allocated to that question.

Name: Homework 7 Due 5 December 2017 The umbers followig each questio give the approximate percetage of marks allocated to that questio. 1. Use the reciprocal metric tesor agai to calculate the agle betwee