Unit 3-polymers


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Unit 3-polymers

  1. 1. POLYMERSPolymer is a macro molecule formed by the union of many small molecules.Depending upon the structure, a polymer may be linear or branched.Ex. – CH2 – CH2 – CH2--Si – O – Si – O - Si Polyethylene | Linear CH2- Si – O – Si – O – Branched silicon resin.Depending type of monomer, they may be homopolymer (made up of same monomer) orCo-polymer (made up of different monomers) O OEx. – CH2 – CH2 - n H 2N – (CH2)6 – NH – C – (CH2)4 – C – NH - Homopolymer Nylon 6,6 Polyethylene (Co-polymer)POLYMERISATION: It may be defined as the process of linking or joining together small molecules likemonomers to make large molecules.Basically there are 3 types of polymerizations. 1. Additional polymerization or Chain polymerization 2. Condensation polymerization or Step polymerization 3. Copolymerization 1. Additional polymerization or Chain polymerization: This polymerization yields an exact multiple of basic monomeric molecules. This monomeric molecule contains one or more double bonds. By intermolecular rearrangement of these double bonds makes the molecule bifunctional. In this polymerization process light, heat and pressure or catalyst is used to breakdown the double covalent bonds of monomers. 2. Condensation polymerization or Step polymerization: May be defined as “a reaction occurring between simple polar-group-containing monomers with the formation of polymer and elimination of small molecules like water, HCl, etc.” For example, hexamethylene diamine and adipic acid condense to form a polymer, Nylon6:6. Additional polymerization is a chain reaction converting of a sequence of three steps. Initiation, propagation and termination.a. Initiation step is considered to involve two reactions. The first is the production of free radicals, usually,by the hemolytic dissociation of an initiator (or catalyst) to yield a pair of radicals R’. I 2R’ ………(1) (Initiator) (Free radicals)The second part of initiation under the addition of this radical to the just moment molecule (M) to produce thechain initiating species M1. R + M M1 ……….(2) Free radical monomer moleculePolymer Science (Unit-III) 1 Prepared by B.SRINIVAS
  2. 2. Thus the polymerization of monomer CH2 = CHY taken in the form. H R + CH2 = CHY R- CH2 – C Rb. Propagation step: Consists of the growth of M1 by successive additions of large numbers of monomermolecules according to equation. M1 + M M2 M2 + M M3 M3+ M M4 or in general terms Mn + M Mn + 1c. Termination step: At some time, the propagation polymer chain steps growing and terminates. H H H H - CH2 – C + C – CH2 - CH2 – C – C – CH2 Y Y Y Y Disproportion in which a hydrogen atom of one radical center is transferred to another radical center. Thisresults in the formations of two polymer molecules, are saturated and one unsaturated e.g. H H H H H CH2 – C + C – CH2 CH2 – CH + C = C – Y Y Y YThe two different modes of terminations can be represented in general terms by: M*n + M*m M*n+m (Coupling) * * M n + Mm Mn + Mm (Disproportionation)Co-polymerization: Polymerization involving two different monomers.Ex. Polymerization of butadiene and styrene to gave Buna –S. H CH2 = CH – CH = CH2 + nx CH2 = CH –Ph -C -CH = CH –CH - -CH2 –CH - 1,3-butadiene (75%) Styrene(25%) H x Ph nQ.No.1: Write the differences between a) Additional – condensation polymerization b) Thermo-set-Thermoplastics c) compressions-Injection moulding.a. Difference between additional & condensation polymerization:Additional polymerization Condensation polymerization1. Additional polymerization also known as Chain polymerization 1. Condensation polymerization2. This polymerization yields an exact multiple of basic monomeric also known as Step polymerizationmolecules. This monomeric molecule contains one or more double 2. This polymerization reactionbonds. occurring between simple polar-By intermolecular rearrangement of these double bonds makes the group-containing monomers withmolecule bi-functional. the formation of polymer andIn this polymerization process light, heat and pressure or catalyst is elimination of small moleculesused to breakdown the double covalent bonds of monomers. like water, HCl, etc.”Eg. Polymerization of ethylene at 1500 atm and a temperature 150 Eg. Hexamethylene diamine and– 250 0C in presence of traces of oxygen formation of polyethylene. adipic acid condenses to form a polymer, Nylon6:6.Polymer Science (Unit-III) 2 Prepared by B.SRINIVAS
  3. 3. b. Difference between thermo set and thermoplastics. Plastics are materials that show the property of plasticity and can be moulded into any desired shapeand dimension of articles by the application of heat and pressure. Thermoplastics Thermoset plastics 1.These are processed by addition polymerization. 1. These are proceed by condensation 2.Softens on heating and retaining the same chain Polymerization. on cooling. 2. These are infusible and insoluble 3.They are along chain linear polymers without mass on heating i.e., heat resistance. any branched or cross linked chain. 3. They are branched or cross-linked 4.On repeated heating and cooling, there is no Polymer. 4. Some sort of chemical changes occur change in chemical nature. On heating. 5.These plastics undergo purely physical process. 5.These Plastics undergo physical as well As chemical process. 6.By heating the plastics, thy can be proceed. 6. These plastics cannot be proceeding by 7.Waste thermoplastics can be recovered. heating. 7. Waste thermosetting cannot be recovered.c. Differences between compression and injection moulding techniques: Compression Moulding Injection moulding 1. The plastic ingredient in proper 1. In this, the heated plastic is injected into the Proportions are filled in between the two half mould cavity from where it is cooled and taken portions of the mould. These portions are out. moved relative to each other and by the applying heat and pressure, the part can be manufactured. 2. It is applicable to thermoplastic resins. 2. It is applicable to both thermoplastic and thermosetting plastic resins 3. Moulding is somewhat complicated compared 3. Moulding is often simpler. to compression moulding. 4. It is less expensive. 4. It is expensive 5. Require more operation time. 5. Require less operation time 6. Less production rate. 6. High Production rate. 7. There is no limitation to the design of articles 7. There is limitation to the design of articles to be to be moulded. moulded. 8. High moulding cost. 8. Less moulding cost.Polymer Science (Unit-III) 3 Prepared by B.SRINIVAS
  4. 4. Q. No. 2: a. Define Plastic? Describe plastics compounding & its ingredients. b. Describe various types of (compression, injection, extrusion & transfer ) moulding & fabrication processa. Plastics are the materials that show the property of plasticity and can be moulded into anydesired shape and dimensions by the application of heat and pressure. Plastics having variety of properties arein use in present applications. The properties are low thermal and electrical conductivities, easy to fabricate,low specific gravity etc. The plastics can be fabricated for large number of colours and can be used fordecorative purpose. Plastics can be used to produce complicated shapes and accurate dimensions very cheaplyby moulding process. Plastics are generally used for making automobile parts, goggle, telephones, electricalinstruments, optical instruments, household appliances etc. plastics having high wear resistance properties canbe used for making gears, bearings etc.Merits of Plastics 1. Plastics have good shock absorption capacity compared with steel. 2. Plastics have high abrasion resistance. 3. Plastics are chemically inert. 4. Plastics have high corrosion resistance compared to metals. 5. Mounding, machining, drilling etc. can be easily done on plastic materials. 6. Plastics are light in weight having specific gravity from 1 to 2, 4. 7. Plastics can be made according to the order like hard, soft, rigid, tough, brittle, malleable etc. 8. Fabrication of plastics into desired shape and size is cheap. 9. Plastics are dimensionally stable. 10. Plastics are don’t absorb water. 11. Thermal coefficient of expansion of plastic is low. 12. Excellent outer finish can be obtained on plastic products.Demerits of Plastics 1. Plastics are soft 4. Cost of plastics is high. 2. Plastics have poor ductility. 5. Plastics can deform under load. 3. Resistance to heat is less.Compounding of plastics: Compounding of the plastics may be defined as the mixing of different materials likeplasticizers, fillers of extenders, lubricants, dies and pigments to the thermoplastic and thermosetting plasticsto increase their useful properties like strength, toughness, etc. Resins have plasticity or binding property, butneed other ingredients to be mixed with them for fabrication into useful shapes.Polymer Science (Unit-III) 4 Prepared by B.SRINIVAS
  5. 5. Ingredients used in compounding of plastics Some of the ingredients used in compounding of plastics are i) Resins ii) Plasticizers. iii) Fillers or extenders. iv) Dyes and pigments. v) Lubricants.i). Resins: Resin is the binder, which holds the different constituents together. Thermosetting resins are, usually,supplied as linear-polymers of comparatively low molecular-weight, because at this stage they are fusible andhence, mouldable. The conversion of this fusible form into cross-linked infusible form takes place, duringmoulding itself, in presence of catalysts, etc.ii. Plasticizers Plasticizers are substances added to enhance the plasticity of the material and to reduce thecracking on the surface. Plasticizers are added to the plastics to increase flexibility and toughness. Plasticizersalso increase the flow property of the plastics.Example: Dibutytyle oxalate, Castor oil and Tricresyl phosphateiii). fillers or Extenders Fillers are generally added to thermosetting plastics to increase elasticity and crack resistance. Fillersimprove thermal stability, strength, non combustibility, water resistance, electrical insulation properties andexternal appearance.Example: wood flour, Asbestos, Mica, Cotton, Carbon black, Graphite, Barium sulphate etc.iv) Dyes and pigments These are added to impart the desired colour to the plastics and give decorative effect.v) Lubricants These are added to prevent the plastics from sticking to the moulds. Example: Oils, Waxes, Soaps etc. Thus the objective of compounding is to improve the properties of the basic resin, such that thefabrication is made easy. b. Fabrication of plastics: Many methods of fabricating plastics into desired shaped articles are employed. This production of plastics is known as fabrication of plastics. The methods, usually depends upon the types of resins used i.e., whether thermosetting or thermoplastic. Different fabrication techniques are described below. Moulding of Plastics Moulding of plastics comprises of forming an article to the desired shape by application of heat and pressure to the moulding compounds in a suitable mould and hardening the material in the mould. The method of moulding depends upon the type of resins used. i) Compression moulding: This method is applied to both thermoplastic and thermosetting resins. The predetermined quantity of plastic Ingredients in proper properties are filled between the two half –pieces of mould which are capable of being moved relative to each other heat and pressure are than applied according to specifications. The containers filled with fluidized plastic. Two halves are closed very slowly. Finally curing is done either by heating or cooling. After curing the moulded article is taken out by opening the mould parts. Polymer Science (Unit-III) 5 Prepared by B.SRINIVAS
  6. 6. ii) Injection moulding:In this method, the moulding plastic powder is fed into a heated cylinder from where it is injected at acontrolled rate into the tightly locked mould by means of a screw arrangement or by a piston plunger. Themould is kept cold to allow the hot plastic to cure and become rigid. When the materials have been curedsufficiently, half of the mould is opened to allow the injection of the finished article without any deformation,etc. Heating is done by oil or electricity.iii) Transfer moulding:In this method, the principle is like injection moulding. The moulding powder is heated in a chamber tobecome plastic. Later it is injected into a mould by plunger working at high pressure through orifice. Due tothis heat is developed and the plastic melts, takes the shape of the mould.d) Extrusion moulding:This process is useful in the preparation of continuous wires with uniform cross section. The heated plastic ispushed into the die with the help of screw conveyor. In the die, the plastic gets cooled due to the exposure toatmosphere and by artificial air jets.Extrusion moulding is used mainly for continuous moulding of thermoplastic materials into articles ofuniform cross section like tubes, rods, strips, insulated electric cables. The thermoplastic ingredients areheated to plastic condition and then pushed by means of a screw conveyor into a die, having the required outershape of the article to the manufactured. Here the plastic mass gets cooled, due to the atmospheric exposure(or artificially by air jets). A long conveyor carries away continuously the cooled product.Polymer Science (Unit-III) 6 Prepared by B.SRINIVAS
  7. 7. Q.No.3: a. Identify the thermosets & thermoplastics among the following: PVC, polyethylene, silicon,polyester fiber, bakelite.b. What is Bakelite? How is it manufactured and mention its uses.c. Describe the manufacture, properties, and uses of Nylon.Answer: a. Thermoplastics: PVC & Polyethylene Thermo-sets plastics: silicon, Polyester fiber & Bakelite b. Bakelite: It is prepared by condensing phenol with formaldehyde in presence of acidic/alkalinecatalyst. The initial reaction results in the formation of O- and P- hydroxyl methyl/phenol which reacts toform linear polymer. During modeling hexamethylene tetramine is added, which converts to insoluble solid ofcross-linked structure Bakelite.Applications: It is used for making electric insulator parts like switches, plugs, switch boards etc. For makingmoulded articles like telephone parts cabinet of radio and television.c. Polyamides: Synthetic fiber forming polyamides are also known as ‘Nylons’.Nylon-6: Preparation: Nylon-6 can be made by self condensation of ε-amino caproic acid. H2N-(CH2)5-COOH -[-NH-(CH2)5-CO-]n- + H2O ε-amino caproic acid. Nylon-6Nylon-6,6: It is prepared by Hexamethylene diamine and Adipic acids are polymerized in 1:1 ratio.Properties:a. The structure of nylons are linear that permits side-by-side alignment. Moreover, the molecular chains are held together by hydrogen bonds. Thus, nylons have high crystalline which imparts high strength, high melting point, elasticity, toughness, abrasion resistance and retention of good mechanical properties upto 1250C.b. They are also sterilisable.c. Since nylons are polar polymers, they have good hydrocarbon resistance. Larger the number of carbon atoms, greater will be ease of processing and hydrocarbon and moisture resistance.Applications:Nylon-6,6 is primarily used for fibers, which find use in making socks, under-garments, carpets.Nylon-6.6 is also used in mechanical engineering for well known applications like gears, bearings, bushes,cams.Nylon-6 is mainly used in manufacture of tyre cord.Nylon6-10 is suitable for monofilaments which are used for bristles, brushes.Polymer Science (Unit-III) 7 Prepared by B.SRINIVAS
  8. 8. Q.No.4: Describe the manufacture, properties, uses of a) polyethylene b) PVC c) Teflon d) PSAnswer: Polyethylene: This can be obtained by the polymerization of ethylene at 1500 atm and a 0temperature 150 – 250 C in presence of traces of oxygen. Polyethylene also known as polyethene. It is prepared by the process of addition polymerization of ethylene.Polyethylene is two types on the basis of density. a. Low density polyethylene(LDPE): Preparation: it is prepared by polymerizing ethylene at high pressures of 1000 to 5000 atmospheres and at 250 0C in the presence of free radical initiator (Oxygen). Properties: LDPE molecules are branched and don’t allow the molecules to pack efficiently. LDPE has a density 0.91 to 0.925 g/cm 3 LDPE Crystalline nature is also low (55%) LDPE is chemically inert and has good chemical resistance. LDPE is non-polar; hence it has excellent electrical insulation properties. LDPE is tough and flexible even at low temperatures. Uses: LDPE is used in following applications: a. Films for general packing and carrier bags. b. Squeeze bottles particularly for detergents. c. Moulded toys & gift articles d. Ink tubes for pens & Mugs.b. High density polyethylene (HDPE):Preparation: it is prepared by polymerizing under 6 -7 atmospheric pressure at 60 – 70 0C in the presence ofZeigler – Natta (TiCl4 + Al(C2H5)3) as a catalyst.Properties: The HDPE molecules are linear and their packing is very easy. HDPE has a density 0.941 to 0.965 g/cm 3 HDPE has excellent chemical resistance. HDPE has excellent electrical insulation properties. HDPE has sufficiently low water and gas permeability. HDPE is free from odour and toxicity. HDPE is stiffer, hard and possesses greater tensile strength.Polymer Science (Unit-III) 8 Prepared by B.SRINIVAS
  9. 9. Applications: HDPE film is used as wrapping material instead of paper for food products. HDPE used for the manufacture of crates, food tubs, industrial containers & over head tanks. HDPE can also be used for domestic water & gas piping. HDPE can also used for milk bottles, house hold chemicals and drug packing.PVC :Poly Vinyl Chloride is obtained by heating a water emulsion of vinyl chloride in presence of a small amountof benzoyl peroxide or hydrogen peroxide in an auto clave under pressure.Vinyl chloride, so needed is generally prepared by treating acetylene at 1 to 1.5 atmospheres with hydrogenchloride at 600C to 800C in the presence of metal chloride as catalyst. CH = CH + HCl CH2 = CH Cl Acetylene Vinyl chlorideProperties: It occurs as a colourless rigid material.It is having high density and low softening point.It is resistant to light, atmospheric oxygen, inorganic acids and alkalis.It is most widely used synthetic plastic.Uses: It is mainly used as cable insulation, leather cloth, packing and toys.It is used for manufacturing of film, sheet and floor covering.PVC pipes are used for carrying corrosive chemicals in petrochemical factories.TEFLON OR Poly tetra fluoro ethylene:Teflon is obtained by polymerization of water-emulsion tetrafluoroethylene, under pressure in presence ofbenzoyl peroxide as catalyst.Properties: Due to the presence of highly electronegative fluorine atoms and the reqular configuration of thepolytetrafluoro ethylene molecule results in very strong attractive forces between the different chains.These strong attractive forces give the material extreame toughness, high softening point, exceptionally highchemical-resistance towards all chemicals, high density, waxy touch, and very low coefficient of friction,extremely good electrical and mechanical properties: It can be machined, punched and drilled. The material,however, has the disadvantage that it cannot be dissolved and cannot exist in a true molten state. Around3500c, it sinters to form very viscous, opaque mass, which can be moulded into certain forms by applying highpressures.Uses: as insulating material for motors, transformers, cables, wires, fittings, etc, and for making gaskets,packing, pump parts, tank linings, chemical-carrying pipes, tubing’s and tanks, etc,; for coating andimpregnating glass fibres, asbestos fibres and cloths; in non-lubricating bearings and non-sticking stop-cocksetc.Polymer Science (Unit-III) 9 Prepared by B.SRINIVAS
  10. 10. Polystyrene:Preparation: Polystyrene is prepared by free radical addition polymerization of styrene in the presence ofbenzoyl peroxide as catalyst.Properties:i. Because of the presence of bulky phenyl groups, packing of PS chains is not efficient and hence it is amorphous polymer. Its specific gravity (1.054) is also low.ii. It has good optical properties like it is transparent polymer allowing high transmission of all wave lengths. Moreover, its high refractive index (1.592) gives it a particularly high ‘brilliance’.iii. Due to chain stiffening effect of benzene ring, PS is hard but brittle. It emits a characteristic metallic sound when dropped.iv. Being non-polar amorphous polymer, its softening temperature (82-1000C) is low. It cannot withstand the temperature of boiling water.v. As it is non-polar so it has low tendency for moisture absorption. Moreover, it has good electrical insulation characteristics.vi. It has reasonable chemical resistance but mediocre oil resistance.Applications: Polystyrene is used for making: audio cassettes, containers for talcum powder, house-wares(small jars & storage containers), bottle caps, combs and brush handles. Q.No.5: a. Write a short note on Natural rubbers and to discuss vulcanization? b. Write the preparation, properties & applications of buna-s, butyl rubber & Thiokol rubberNatural Rubber: Rubbers also known as Elastomers, they are high polymers, which have elasticproperties in excess of 300%.Natural rubbers consist of basic material latex, which is a dispersion of isoprene. During the treatment, theseisoprene molecules polymerize to form, long-coiled chains of cis-polyisoprene. Natural rubber is made fromthe saps of a wide range of plants like Hevea brasillians and guayule. Latex: is a milky white fluid that oozes out from the plant Hevea brasillians when a cut is made on the steamof the plant.The latex is diluted with water. Then acetic or formic acid is added [1kg of acid per 200kgs of latex] toprepare coagulum. This is processed to give wither crepe rubber or smoked rubber.Vulcanization: Vulcanization discovered by Charles Goodyear in 1839. It consists of heating the raw rubber at 100 – 1400C with sulphur. The combine chemically at thedouble bonds of different rubber spring and provides cross-linking between the chains. This cross-linkingduring vulcanization brings about a stiffening of the rubber by anchoring and consequently preventingintermolecular movement of rubber springs. The amount of sulphur added determines the extent of stiffness ofvulcanized rubber. For example, ordinary rubber (say for battery case) may contain as much as 30% sulphur.Advantages of vulcanization: i. The tensile strength increase. ii. Vulcanized rubber has excellent resilience. iii. It has boarder useful temperature range (-40 to 1000C) iv. It has better resistance to moisture, oxidation and abrasion. v. It is resistance to organic solvents like CCl4, Benzene petrol etc. vi. It has only slight thickness. vii. It has low elasticity.Polymer Science (Unit-III) 10 Prepared by B.SRINIVAS
  11. 11. Buna – S or STYRENE RUBBER:Buna-S rubber is probably the most important type of synthetic rubber, which is produced bycopolymerization of butadiene (about 75% by weight) and styrene (25% by weight).nCH2 = CH – CH = CH2 + n CH2 = CH –Ph -(-H2C -CH = CH –CH --CH2 –CH – Ph-)n- 1,3-butadiene (75%) Styrene(25%)Properties: Styrene rubber resembles natural rubber in processing characteristics as well as quality offinished products.It possesses high abrasion-resistance, high load-bearing capacity and resilience. However, it gets readilyoxidized, especially in presence of traces of ozone present in the atmosphere. Moreover, it swells in oils andsolvents. It can be vulcanized in the same way as natural rubber either by sulphur or sulphur monochloride(S2Cl2). However, It requires less sulphur, but more accelerators for vulcanization.Uses: Mainly used for the manufacture of motor tyres. Other uses of these elastomers are floor tiles, shoesoles, gaskets, foot-wear components, wire and cable insulations, carpet backing, adhesives, tank-linings, etc.Butyl Rubber (GR-I or Polyisobutylene):Preparation: Butyl rubber is prepared by the aluminum chloride initiated cationic co-polymerization ofisobutene with small amount (0.5 to 2.5%) of isoprene.Properties: Butyl rubber has following characteristics: a. Under normal conditions it is amorphous but it crystallizes on stretching. b. It is quite resistant to oxidation due to low degree of unsaturation. c. Due to very low unsaturation, it can be vulcanized but it cannot be hardened much. d. It can be degraded by heat or light to sticky low-molecular weight products so stabilization is must. e. Butyl rubber is soluble in benzene but has excellent resistance to polar solvents like alcohol, acetone. f. Compared to natural rubber it possesses outstanding low permeability to air and other gases.Applications: Butyl rubber used for a. Insulation of high voltage wires and cables.Polymer Science (Unit-III) 11 Prepared by B.SRINIVAS
  12. 12. b. Inner tubes of automobile tyres. c. Conveyor belts for food and other materials. d. Lining of tanks and Hoses.Thiokol rubber: This also called as polysulphide rubber (or Gr-P). It can be prepared by the condensationpolymerization of sodium polysulphide (Na2Sx) and ethylene dichloride.It is used for the - i. Manufacture of oils hoses, chemically resistant tubing and engine gaskets; ii. Diaphragms and seals in contact with solvents and iii. Printing rolls, iv. Containers for transporting solvents and v. Solid propellant fuels for rockets, etc. Q.No. 6: a. Write a short note on conducting polymers & mention its applications? b. Describe preparation, conduction properties & applications of Polyactylene & polyanillinea. Conducting polymers: Those polymers which conduct electricity are Conducting polymers. Theconduction of polymers may be due to unsaturation or due to the presence of externally added ingredients inthem. The conducting polymers can classify in the following way.Conducting polymers divided in to two types. 1. Intrinsic conducting polymers 2. Extrinsic conducting polymersIntrinsic conducting polymers: These polymers are characterized by intensive conjugation of double bonds intheir structure i.e. the backbone of the polymer. Again intrinsic conducting polymers are two types. a. Conducting polymers having conjugation b. Doped conducting polymersConducting polymers having conjugation: Such polymers having conjugated double bonds in the backbonepossess their conductivity due to pi electrons. In pi bonding the overlapping of the orbital is lateral over theentire backbone resulting in the formation of valence bands and conducting bands which were separated by asignificant Fermi energy gap. The electrical conductivity takes place only after thermal or photolyticactivation of the electrons, which give them sufficient energy to jump the gap and reach into conduction band.Doped conducting polymers: The conducting polymers having pi electrons in their backbone can easily beoxidized or reduced because they possess low ionization potential and high electron affinities. Hence theirconductance can be increased by introducing a positive charge or negative charge on polymer backbone byPolymer Science (Unit-III) 12 Prepared by B.SRINIVAS
  13. 13. oxidation or reduction. This process is similar to semiconductor technology and is called doping. Doping is again two types. 1. Creating a positive site on polymer backbone called p-doping 2. Creating a negative site on polymer backbone called n-doping P - Doping: p-doping is done by oxidation of a conducting polymer like polyacetylene with a lewice acid or iodine vapour. This type of doping also known as oxidative doping. During oxidation process the removal of pi electrons from polymer backbone lead to the formation of a delocalized radical ion called polaron having a hole in between valence band and conducting band. The second oxidation of the polaron results in two positive charge carriers in each chain called bipolaron, which are mobile because of delocalization. These delocalized charge carriers are responsible for conductance when placed in electric field. n-doping: n-doping is carried out by reduction process by the addition of an electron to polymer backbone by using reducing agents like sodium naphthalide. Formation of polaron, bipolaron takes place in two steps, followed by recombination of radicals, which yields two charge carriers on the polyacetylene chain responsible for conduction. The electron added to polyacetylene by reductive doping does not go into the conducting band but goes into an intermediate electronic state within the band gap of radical anion (polaron). Bipolaron contains electrons in the energy levels residing in the band gap. The Bianion lowers its energy by segregating into two negative solitrons at the midgap energy levels. The presence of holes in the band gap allows facile jumps of electrons from valence band into the conduction band. This leads to the generation of conduction pathways. As a consequence the conductivity increases significantly. In general doping increases the surface conductivity of a polymer to a large extent. Polyanilines exist in several oxidation states as far as electrical conductivities are concerned varying from 10- 11 S/cm to >105 S/cm only one form called emeraldine salt is electrically conducting. The flexible dark blue Applications of conducting polymers:a. In rechargeable batteries: These batteries are small in size, longer lasting and can produce current density up to 50mA/cm2. Move over, these rechargeable batteries have ecological advantage as they do not involve heavy metals so they do not appear to have any serious toxicological problems.b. In analytical sensors: Conducting polymers are also used for making sensors for pH, O2, NOx, SO2, NH3 and glucose.c. For making Ion-exchangers: Membranes made up of them can show boundary layer effects with selective permeability for ions, gases, etc.Hence,they are useful for ion-exchangers and controlled release of drugs. Diffusion of the drug from the polymer matrix permits a continuous and controlled Polymer Science (Unit-III) 13 Prepared by B.SRINIVAS
  14. 14. release. With this, regular injection or oral ingestion of the drug is no0 longer needed. This technique has been used for the slow release of birth control drugs and it has potential to be used for cancer chemotherapy.d. In electro-chromic displays and optical filters: ICP’s can absorb visible light to give coloured products so can useful for electro-chromic displays and optical filters. Thus, conducting polymers can be used as electro-chromic materials.e. In electronics: Photo structural lacquers based on ICP’s are useful for electron beam lithography, LED’s and data storage. Emaraldine base form polyaniline is used as a resist for lithography. Its solution in NMP (1-methyl-2-pyrrolidine) mixed with triphenylsulphonium hexafluroantimonate has been used to spin coat thin films on quartz and silicon wafers. This blue film on exposure to UV radiation of 240 nm turns green, characteristic of conducting state of polyaniline. Conducting lines as small as 0.5 m can be obtained by using this technique. b. 1. Poly acetylene: The conjugated polymer with simplest chemical structure is poly acetylene. Polymerisation of acetylene over Ziegler Natta catalysts gives poly acetylene which in its used form with increasing temperature gets transformed to more stable trans-form. This polymer is infusible, insoluble and becomes brittle on exposure to air. The conductivity of poly acetylene is magnified by doping. Exposure of the film to dry ammonia gives a polymer with conducvity of 103 Scm-1, controlled addition of P-doping agents like AgF5, Br2, I2 or HClO4 could move to still higher conductivities. Conducting mechanism in poly acetylene: The semi conducting poly acetylene (CH2)n has a typical carbon – carbon back bone structure. The localized electrons in ‘s’ bonds form the back bone of the polymer chain and dominate the medicinal properties, while the electrons in the π bonds are delocalized along the chain and responsible for the electrical and optical properties of a conjugated polymer. The σ bonds form completely filled low lying energy bonds that have larger energy gas than the π bond electrons. Before passing current, the electrons can flow along the molecule and one or more electrons have to be removed or inserted. In presence of an electric field, the electrons constituting π –bonds can move along the molecular chain. The conductivity of the polymeric material, containing many chains of polymers will be limited by the fact that the electrons have to jump from one molecule to the next. Hence, the chains have to be packed in an ordered row. Doping of poly acetylene: poly acetylene possesses alternate single and double bonds that give rise to mobile –electrons when doped, i.e. become anisotropic metallic conductors. There are two types of doping, oxidation or reduction. 1. oxidation with halogen- (P-doping) 2. Reduction with alkali metal – (n-doping) 2. Poly-aniline: Among the conducting polymers, poly-aniline posses unique properties. Poly-aniline is conjugated polymer and is reactive. Poly-aniline is considered as an organic metal. Its specific conductivity is 55 cm-1. Poly-aniline is transparent in thin layers. In conducting state it is green. It turns red under reducing conditions and blue under oxidising or basic ones. Poly-aniline is a stable conducting polymer. It has wide range of conductivity. It shows multi-colour electro- chromism and chemical sensitivity. It can be synthesised chemically or electro-chemically as a bulk powder or film. Polymer Science (Unit-III) 14 Prepared by B.SRINIVAS
  15. 15. It is one of the oldest conductive polymers known. It was prepared by the anodic oxidation of aniline inH2SO4. Its conductivity is due to conjugated p-bond system formed by the overlapping of carbon p-orbital andalternating C-C bond lengths extending over large number of recurring monomer units. In poly-aniline pzorbitals of nitrogen and carbon rings are also part of the conjugated system.Conjugated double bonds permit the electron mobility throughout the molecule due to delocalised electrons.In addition, it has a conjugated double bond structure, i.e. benzenoid ring between the quininoid imines andthe benzenoid amine structure.Preparation of poly-aniline: poly-aniline is prepared by the redox polymerization of aniline in protonic andaqueous solution in the presence of ammonium per di sulphate as oxidant. It can be regarded as conductingpolymer under certain stimulating conditions like UV light, heat or addition of a suitable dopant to thepolymer.Doping of poly-aniline: it can be made conductive by p-type (oxidation) doping or n-type (reduction) dopingof polymer. In undoped state, it is a poor semi conductor. On doping with dopant para-hydroxy benzenesulphonic acid, its conductivity is increased by a factor of 10 dm -1/cm or more and forms polaron/ bipolaronstructure. The conductivity σ of a conducting poly-aniline is related to number of charge carriers ‘n’ and theirmobility µ . Therefore, σ = neµ Properties of poly-aniline: Due to the presence of extended π –bond system of conjugated polymers, it ishighly susceptible to chemical and electrochemical oxidation or reduction. As a result, the electrical andoptical properties of the polymer are altered.Electronically conducting polymers are extensively conjugated molecules and possess specially delocalisedband line structure.Disadvantages: 1. Poly-aniline decomposes prior to melting. Hence, it is difficult to process. 2. It is insoluble in common solvents except strong acids and N-methyl prolidone. 3. It has poor mechanical properties.Applications: 1. Poly-aniline is used for corrosion protection, sensors, smart windows, printed circuit boards, conductive fabrics and conductive pipes for explosives. Q.No. 7: Write a short note on Liquid crystal polymersLiquid crystal polymers (LCP)Liquid crystals are materials that behave in some ways like solid and in some ways like liquids. StephanieKwolek, a chemist, working at Dupont, had made the first known polymer liquid crystal solution. Kwolek hadinvented a new polymer based on the polyamide, named Kevlar.The macromolecule with the above structure produces very strong fibers. When this polymer dissolved intetra-methyl urea (II) and calcium chloride, the polymer molecules behave strongly. The Kevlar molecularwhich were long, straight, and stiff, lined up like logs floating down a river, because of the strange opalescentlook of the solution.Polymer Science (Unit-III) 15 Prepared by B.SRINIVAS
  16. 16. This is unusual because normally molecules in a solution or a pure liquid are not arranged in any orderlyfashion. Molecules of solids materials are arranged in orderly fashion called ‘crystals’.Since Kevlar solution is a liquid, but its molecules are orderly arranged, the solution is called a ‘liquidcrystals’. Liquid crystals can be classified in to 2 types. 1. Liquid crystalline in polymers may occur by dissolving a polymer in a solvent, which are called lypo- tropic liquid crystal polymers. Eg. Kevlar. 2. Liquid crystalline in polymer may occur by heating a polymer above its glass transition temperature or melting transition point, which are called thermo-tropic liquid crystal polymers. Eg. VectraProperties of liquid crystal polymers: 1. These polymers are capable of forming regions of highly ordered structure while in liquid phase. The degree of order is somewhat less than that of a regular solid crystal. 2. The liquid crystal polymers have high mechanical strength at high temperatures. 3. These liquid crystal polymers possess extreme chemical resistance. 4. They possess inherent flame retardancy and good weather-ability. 5. They can be easily fabricated into a variety of forms. 6. LCP can be welded. The lines created by welding are the weak points in the resulting product. 7. LCP has high Z-axis co-efficient of thermal expansion. 8. LCP resist stress cracking the presence of most chemicals at elevated temperatures.Applications of LCP:LCP are some times called ‘super polymers’. Their wide range of exceptional properties and case ofprocessing make them design for many demanding applications. 1. LCP thermoplastic fibers possess exceptional strength and residity, suitable for industrial, electronic and also space applications as well as high performance ropes and tennis rackets. 2. LCP finds extensive applications as coatings, composites, and additives. 3. The electrical motor components are made from LCP. 4. LCP finds its applications in electronic industry as LED’s and SMT components. 5. LCP ha 6. s an interesting application like information storage media.Polymer Science (Unit-III) 16 Prepared by B.SRINIVAS
  17. 17. Q.No.8: Define fibers, classify & write preparation, properties & applications of polyester.Fibers: Fibers are a class of materials that are continuous filaments or discrete elongated pieces. They arehighly crystalline form of polymers. They are biologically very important in both plants and animals. Theli8gments etc. that hold the tissue material are basically fibers. Fibers are used for making textiles, utilities,ropes, strings, etc.Fibers are classified in to two types depending on their origin. 1. Natural fibers 2. Synthetic fibersNatural fibers: Natural fibers include those produced by plants, animals and geological material. They areenvironment friendly and biodegradable. They are: a. Vegetable fibers: They are basically cellulose material and include cotton, jute, etc. plant fibers found application by even early man and are used for making textiles, ropes, mats, paper and bags etc. dietary fiber is an essential component of our food and its deficiency could result in cancer. b. Wood fibers: The strength of a plant is due to presence of wood fiber. Wood pulp is used in making paper and wood fibers like jute are used for making bags. c. Animal fibers: They are largely made of protein. Pure silk, wool, hair are animal fibers. We move our li8mbs using fibers present in them. Spider silk is used for making special bullet proof jackets. d. Mineral fibers: Asbestos is a typical example of mineral fiber. Mica and other minerals are also used as fibers. Synthetic fibers: this type fibers can be produced in large quantities and are cheaper than some of the natural fibers like pure silk. Polyamide nylons, polyesters, PVC, phenol-formaldehyde resins, polyethylene are often used for making textiles.POLYESTER This category of polymers has ester linkages in the main chain. It takes 18% of market share ofsynthetic polymers. Terylene is a polyester fiber made from ethylene glycol and terephthalicacid. Terephtalic acid requiredfor the manufacture of Terylene is produced by the catalytic atmospheric oxidation of p-xylene.Polymer Science (Unit-III) 17 Prepared by B.SRINIVAS
  18. 18. Properties: This occurs as a colourless rigid substance. This is highly resistant to mineral and organic acidsbut is less resistant to alkalis. This is hydrophobic in nature. This has high melting point due to presence ofaromatic ring.Uses: It is mostly used for making synthetic fibers. It can be blended with wool, cotton for better use andwrinkle resistance. Other application of polyethylene terephthalate film is in electrical insulation. Q.No. 9: Write a short note on Fiber reinforced plastics.Fiber reinforced plastics (FRP)Fiber reinforced plastics are produced by reinforcing a plastic matrix with a high strength fiber materials suchas glass, Graphite, alumina, carbon, boron, beryllium and aromatic polyamide. Natural fibers such as sisal,asbestos are also used for reinforcement. Depending on the desired properties of the final reinforcedcomposite, the nature of the fiber used is decided. Glass fiber is the most extensively used reinforced fiberbecause of durability, acid proof, water proof, and fire proof nature of glass.Glass is drawn into threads or fibers in the form of filaments fire than cotton or silk thread. Then the filamentsare woven in the form of mats. The fiber material is suitably bonded with plastics materials to be reinforced.The common plastics resins used are polyester, epoxy, silicone, melamine, vinyl derivatives and polyamides.The following are the various processing techniques adopted. a. Matched metal die molding: This is the most efficient and economical method for mass production of high strength parts. It is press moulding under a temperature of 235 – 260 F and 200 – 300 psi pressure. The upper mould containing the resin and reinforcing fiber is pressed on the lower mould. b. Injection Moulding: This method is for reinforced thermoplastics. A mix of short fiber and resin are forced by a screw or plunger through a nozzle into the heated mould and allowed to curve. c. Hand-lay-up: Mostly used thermoset plastic resins the reinforcing mat fiber is cut to fit in a mould and saturated with resin by hand using a brush. Roller or spray gun. Layer built up gives the thickness of the articles.Polymer Science (Unit-III) 18 Prepared by B.SRINIVAS
  19. 19. d. Continuous lamination: reinforcing mats or fabrics are impregnated with resin, run through laminating rolls between cellophane sheets to control the thickness and resin continent. They are then cured in heating chamber. e. Centrifugal casting: Chopped fibers and resin are placed inside a mandrel and uniformly distributed as mandrel is rotated inside an oven. f. Spray up: Short length of reinforcement and resin are projected by a specially designed spray gun so that they are deposited simultaneously on the surface of the mould. Curing is done with a catalyst at room temperature.Applications of FRP: Fiber reinforced plastics find extensive use in space crafts, aeroplanes, boat nulls, acidstorage tanks, motor cars and building materials. Melamine FRP is used for insulation and making baskets.Advantages of FRP: The fiber reinforced plastics have the following advantages. a. Low efficient of thermal expansion b. High dimensional stability c. Low cost of production d. Good tensile strength e. Low dielectric constant f. Non inflammable and non-corrode and chemical resistance.Polymer Science (Unit-III) 19 Prepared by B.SRINIVAS