DP – Designer Polymers.

 

DP1 – Producing a perfect copy.

 

·        In medieval times monks had to painstakingly copy religious documents etc. by hand!!

·        Nowadays we can use photocopiers – but how do they work?

·        Depend on a polymer called polyvinyl carbazole

 

 

Structure of vinyl carbazole.

 

·        This polymer is photoconductive i.e. it conducts electricity much better when light shines on it than in the dark.

 

Figure 2 in the large box on page 122 of Chemical Storylines explains how a photocopier works.

 

Something New.

 

·        There are many other designer polymers with useful properties.

·        Poly(1,1-difluoroethene) is a piezoelectric – it generates electricity when bent or twisted. If you wobble it you can produce an electrical signal !  -  If you pass electricity through it, it wobbles!  Uses?

·        Kevlar – is as strong as steel but five times stronger.

·        PEEK – a heat resistant polymer.

·        Polymers designed to dissolve or degrade under certain conditions.

 

Many of the polymers we met in The polymer revolution were discovered by accident, but nowadays as our knowledge and understanding of both the properties of polymers and the polymerisation process increases the creation of new polymers has become more systematic. We can design polymers with the properties we require.

         

          Many of the polymers in this unit are condensation polymers (mainly A-B types) formed from a condensation reaction between two monomers. We start with the invention of a very important condensation polymer – NYLON.

 

 

 

DP2 – In and out of Fashion.

·        Towards the end of the 1970’s Nylon became less popular and people started to prefer the softer feel of natural fibres.

·        Nylon fibres are hydrophobic – they repel water. This means water vapour cannot escape so they feel damp and sweaty to wear.

·        Companies manufacturing nylon faced great losses. They did not want to spend large amounts developing new polymers – their machines were designed to make nylon!

·        ICI’s answer was to redevelop nylon to make it more like natural fibres. They;

·        Slimmed down the thickness of the nylon filament.

·        Added delustrant to reduce the shiny appearance.

·        But major breakthrough came when they developed a process to change the shape and texture of the nylon yarn.

·        They blew air through bundles of fibres and created ‘loops’ along their lengths – thus Tactel was born. The loops gave the material softness and a texture similar to cotton.

·        Further developments have led to fabrics which ‘breathe’ i.e. let water vapour out but do not allow liquid water to get in.

 

The answer to the problem of falling demand for nylon turned out to be not chemical but technological – it involved finding new ways to process and handle existing materials.

 

SLDP4 – Kevlar.

 

·        After invention of Nylon chemists began to understand more about the relationship between a polymers structure and it’s properties.

·        At Du Pont they began to look for a ‘super polymer’ with the heat resistance of asbestos and the stiffness of glass.

·        Aromatic polyamides looked promising because they had

·        Planar aromatic rings giving rigid polymer chains

·        High ratio C:O so requires large amounts of O2 to burn.

·        First e.g. was made from 3-aminobenzoic acid – but wasn’t strong enough – zig-zag nature of chains meant they couldn’t line up well.

·        Straighter chains needed – see fig 15 pg.128 (monomers?)

·        This polymer found to be almost perfect – problem was solubility – the only thing it dissolves in is concn sulphuric acid! – which did not impress the engineers – why not?

·        The impressive properties of the polymer made it worthwhile to invest in expensive plant needed to cope with reagents needed.

 

Why is Kevlar so strong?

·        Kevlar is fire resistant, light and strong. (weight for weight 5x stronger than steel)

·        Used to replace steel cords in car tyres – reducing weight and increasing life of tyre.

·        Kevlar is so strong because of the way the rigid, linear molecules pack together – see fig 16 pg. 129.

·        Chains line up parallel to each other and are held together by hydrogen bonds in sheets.

·        Sheets stack together regularly around the fibre axis to give an almost perfectly ordered structure.

·        Its strength is due to its crystalline structure which is due to the way it is processed – a result of development work done by Du Pont scientists.

 

Developing a market for Kevlar.

·        £400 million was needed to build plant to produce Kevlar!

·        Needed to be sure there was a market for the product.

·        New uses had to be found in addition to tyres!

·        Kevlar ropes now used – 20x strength of steel ropes.

·        Stiffer form of Kevlar used in aeroplane wings – where lightness and strength are a great advantage.

·        Bullet proof vests, and jackets for fencers.

 

SLDP5 – Taking Temperature into account – PEEK

·        In early 1960’s John Rose worked for plastics division of ICI. He was investigating high temp materials.

·        Looked at polymers based on aromatic compounds because these would have high melting points and be resistant to oxidation.

·        Tried many monomers, reactions and conditions etc. and eventually came up with PEEK;

 

 

 

 

 

 

 

 

 

·        PEEK is expensive, but has a wide range of applications which make it worth developing. It can be used for precision articles e.g. cogs that can withstand high temps so can be used for example inside engines.

 

A Clever Idea.

Aeroplane seats, wall panels and lockers etc. can be made out of a polymer called PHA (polyhydroxyamide). When heated PHA loses water and forms PBO – another polymer which is fire resistant. The process does not give off any fumes or smoke – very important in the case of fire in an aeroplane.

 

 

 

 

 

 

 

 

Mixing It.

We could not possibly cover every polymer available in this course and even if we did applications are bound to arise which require polymers with a combination of properties not usually seen by one particular polymer. It is often less expensive to modify existing polymers rather than developing new ones – there are several ways this can be done.

 

·        Laminates – sheets of different polymers stackedtogether.

·        Composites – different polymers mixed to give the correct properties.

·        Polymer alloys – polymers are mixed when molten so mixing occurs at a molecular level.

·        Copolymers – different monomers polymerised together.

·        Use of plasticisers to modify properties of polymer.

 

 

SLDP6 – Poly(ethene) by design.

 

·        In Polymer Revolution we met two forms of poly(ethene) discovered partly by chance (hdpe and ldpe)

·        Ldpe is produced using high pressures which is expensive (hdpe is produced under lower pressures but using organometallic catalysts)

·        Demand for ldpe was growing but building plants to withstand pressures was very expensive.

·        Problem was solved by using a Ziegler-Natta catalyst but changing the feedstock – instead of using pure ethene, small amounts of hex-1-ene are added which produces small side chains along the polymer length.

 

 

 

·        The resulting polymer chains are linear (like hdpe) but do not pack as closely as the chains in hdpe because of the bulky side groups.

·        New polymer called linear low density poly(ethene) or lldpe.

·        It is more flexible than hdpe and has a lower melting point.

·        Its properties can be modified by adding different alkenes and in different amounts. The polymers are widely used and are in great demand.