Transcript Module 1
Chapter – 4 chemistry of engineering materials Materials science • It is an interdisciplinary field which deals with the discovery & design of new materials • It involves studying materials through the materials paradigm (synthesis , structure, properties & performance ) • Commonly known as material science & engineering Material science & engineering • Focus relationship between the properties of a material and its microstructure • Allowed designing materials and provided a knowledge base for the engineering applications Structure Atomic level • Arrangement of atoms in different ways • Eg: different properties of graphite & diamond Microscopic level • Arrangement of small grains of material that can be identified by microscopy • Eg: frosted glassdifferent optical properties to transparent properties Properties • Mechanical, electrical , & magnetic properties responses to mechanical , electrical and magnetic forces • Thermal properties( transmission of heat, heat capacity) • Optical properties (absorption, transmission , scattering of light) • Chemical stability(corrosion resistance) Why study material science? • Able to select material for given use based on cost and the performance • Understand the limits & change their properties • Able to create a new material with some desirable properties Classification of materials • • • • • • Metals Semiconductors Ceramics Polymers Composites Biomaterials Metals: • hard, strong, opaque to light, conduct electricity & heat Eg: Al , steel , brass, gold Semiconductors: •Solid substance , conductivity between insulator & metal •Electrical properties depend on contaminants present •Opaque to visible light but transparent to IR Eg: Si, Ge, Ga, As Ceramics: Inorganic nonmetallic materials whose formation is due to action of heat Eg: clays, bricks, cement Polymers: •Large molecule composed of many repeated subunits •Bound by covalent forces & weak vander waals forces •Decompose at moderate temperatures •Light weight Eg: plastics, rubber Composites: 2 or more materials with different physical & chemical properties combined to produce a material with characteristics different from individual components Eg: fiber glass, concrete Biomaterials: Biocompatible, used to replace human body parts ADVANCED MATERIALS •materials used in high tech applications •Designed for maximum performance •Expensive Eg : Ti alloys for supersonic airplanes magnetic alloys for computer disks Nanomaterials: Materials with structure at the nano scale ( 1-100 nm) Polymers Polymers • Polymers defined as macromolecules of high molecular weight consisting of repeating units held together by covalent bond • Poly – many mers – units or parts • Polymerization: chemical process leading to the formation of polymer Monomers are small molecules which may be joined together in a repeating fashion to form polymer Degree of polymerization: no . Of monomeric units contained in the polymer Degree of polymerisation: 104 Classification of polymers HOMOPOLYMER COPOLYMER •Polymer with identical monomeric unit •Polymer with different monomeric unit •Eg: polyethylene, polystyrene, polyvinyl chloride •Eg: acrylonitrile butadiene styrene (ABS) •Styrene butadiene rubber COPOLYMERS • 2 or more different types of monomers undergo polymerization together to give copolymers • Eg: ABS, SBR, Nitrile rubber etc • Process of preparation of copolymer : copolymerisation Styrene butadiene rubber (buna –s) Preparation • Copolymerisation of butadiene(75%) & styrene (25%) • Catalyst :cumene hydroperoxide or Na • temp : 50 0C Types of styrene butadiene rubber Derived from • From solutions (S-SBR) • from emulsification (E-SBR) Emulsion polymerisation • Polymerisation intiated by free radical • Reaction consist of two monomers , free radical genereator ,chain transfer agent & emulsifying agent • Monomer: butadiene & styrene • Free radical generator: pottasium persulphate & hydroperoxide in combination with ferrous salts • Chain transfer agent : alkyl mercaptan • Emulsifying agent: soap Solution polymerisation • Polymerization intiated by anions ( mainly alkyl Li compounds ) • Reactions done at dry conditions • Reactions consist of two monomers & alkyl lithium compounds • Organo lithium compounds added to one monomer • Generate carbanion • This add to another monomer • And chain length increases Properties of Buna - s • • • • • High abrassion resistance High load bearing capacity & elasticity Readily oxidized in presence of ozone Swells in oils & solvents Vulcanized by sulphur or S2Cl2 (less sulphur but more accelerator) • Better durability, reduced shrinkage Uses or applications • Manufacture of motor tyre • Making gaskets ,foot wear, shoesoles • Building applications as sealing and bind pigmented coating • Making adhesives, tank lining Acrylonitrile butadiene styrene ( ABS) Preparation • • • • Ter polymer (polymer made from 3 monomers) Acrylonitrile ( 15- 35%) Butadiene( 5- 30 %) Styrene (40 – 60%) • Nitrile ( CN) group is polar, attract each other & bind the chains together - ABS stronger • Provides elasticity at lower temperature Properties • • • • Tough , hard ,rigid High impact resistance Good chemical & heat resistance Resistant to aqueous acids, alkalies, alcohols,animal, vegetable & mineral oils • Swollen by glacial acetic acid , CCl4, aromatic hydrocarbons • High tensile strength & stiffness • Flammable with smoke generation Applications ABS – light weight & easy ability to injection • Used to make automobile parts, building material • Drain -waste -vent pipe , musical instruments, golf club heads • Protective head gear, white water canoes • Small kitchen appliances and toys Conducting polymers polymers conduct electricity eg: polypyrrole polyaniline Conducting polymers Intrinsically conducting polymers :extensive conjugation in the backbone responsible for conductance 1. conductive polymers having conjugated ∏ e- in the backbone 2. Doped conducting polymers Extrinsically conducting polymers :conductivity is due to the externally added ingredients 1. Conductive element filled polymers 2. Blended conducting polymers Conductive polymers having conjugated ∏ e- in the backbone • Overlapping of conjugated ∏ e- over the entire backbone results in the formation of valence bands and conduction bands • They seperated by significant band gap • Upon excitation of e- ,they jump from V.B to C.B Doped conducting polymers • Conjugation is not enough to make a polymer conductive • If some dopants are added, conductivity can be enhanced 1. P- doping 2. N- doping P-doping • • • • Done by oxidation Some e- are removed Holes are created- electrically conductive Process can be done by treated with lewis acids or iodine vapour or iodine in CCl4 • Lewis acid : FeCl3 , AlCl3, BF3 ( CH)X + poly acetylene A lewis acid (CH)X+ A- → p- doped poly acetylene Eg: ( CH)X + 2 Fecl3 → (CH)X+ Fecl 4- + Fecl2 •Then the radical cation formed ( polaron) •Polarons are mobile – move along polymer •Conduct electricity n – doping • Done by reduction • Some e- are added • Process can be done by treated with lewis bases like sodium naphthalide ( CH)X + poly acetylene ( CH)X B → lewis base + Na+ (C10 H8)- B+ (CH)Xn- doped poly acetylene → Na+ (CH)X- • Then the radical anion formed ( polaron) •Polarons are mobile – move along polymer •Conduct electricity Conductive element filled polymers • Polymers filled with conducting element ( carbon black, metallic fibre , metals oxides) • Polmer act as a binder • Minimum concentration of conductive filler so that polymer start conducting : percolation threshold • At this concentration, conducting path is formed Blended conducting polymers • Blending a convential polymer with a conducting polymer • They have better physical , chemical & electrical properties • Addition of carbon black reduce the tensile strength . This can be overcome by blending Polypyrrole (ppy) Synthesis of Polypyrrole 1. Prepared by oxidation of pyrrole using ferric chloride in methanol + 2Fecl3 + 2 Fecl2 + 2 Hcl methanol 2. Electrochemically By passing current of 0.8v 0.8 v Properties of polypyyrole • Films of polypyyrole are yellow but darken in air due to some oxidation, but doped films are blue or black • Undoped & doped films are insoluble in solvents but swellable • Doping makes the material brittle • They are amorphous & stable in air upto 1500c • Oxides derivatives of polypyyrole are good electrical conductors.( 2 to 100 s/cm) • High chemical resistance Applications of polypyrrole • Main applications : electronic devices & chemical sensors • Potential vehicle for drug delivery • Catalyst support for fuel cells and sensitize cathode electrocatalysis • Excellent thermal stability & use in carbon composites • Water resistant polyurethane sponge coated with thin layer of polypyyrole absorb oil & is reusable • Ppy based polymer blends can protect corrosion of metals Poly Aniline (PANI) Synthesis of Polyaniline • Aqueous solution of ammonium per sulphate added slowly to solution of aniline dissolved in dil Hcl at temperature of 3- 40 c • Reaction is exothermic . Hence in order to maintain temperature , kept in ice bath • Precipitate formed washed with NH4 OH & dried. n Poly aniline exists in four main oxidation states 1. 2. 3. 4. Leucoemeraldine – colorless Emeraldine base – blue Emeraldine salt – green Pernigraniline – blue Leucoemeraldine & Pernigraniline are poor conductors Only Emeraldine salt is highly electrically conductive Properties of polyaniline • Relatively inexpensive ,having different oxidation states with different colors. Hence used in sensors & electrochromic devices • More noble than Cu . Hence use in printed circuit manufacturing & in corrosion protection • Light weight , highly flexible • High chemical resistance • Processed in any shapes by using ordinary fabrication methods • By using different dopants , it can be converted into wide range of good conductors Applications of polyaniline • Used to make chemical vapour sensors & biosensors • Manufacture of electrically conducting yarns, antistatic coating, electromagnetic shielding & flexible electrodes • Used to make actuators, supercapacitors & electrochromics • Printed circuit board manufacturing • Corrosion resistant treatment • Light weight rechargeable batteries Applications of conducting polymer • • • • In rechargeable batteries In analytical sensors(O2, NOx,SO2,NH3, glucose ) In electronic display & optical filters In electronics (LED) Advanced polymers polymers with ultra high molecular weight with excellent properties for variety of applications eg: Kevlar, Polybutadiene rubber, Silicones KEVLAR • Aromatic polyamide similar to nylons , but with benzene rings rather than aliphatic chains linked to the amide groups • Prepared by polycondensation of paraphenylene diamine & tere- phthaloyl chloride Synthesis of kevlar Properties of kevlar • • • • • • • 5 times stronger than steel Extremely light weight High heat stability & flexibility Very high chemical resistance Very high tensile strength High structural rigidity Flame resistant Applications of kevlar • • • • Aerospace & aircraft industries Car parts ( tyre, brakes, clutch lining) Ropes, cables Protective clothing, bullet proofs, motor cycle helmet Poly butadiene rubber Synthesis • Polymerisation of monomer- 1,3- butadiene Depending upon catalyst used PBR classified into • • • • High cis poly butadiene Low cis poly butadiene High trans poly butadiene High vinyl poly butadiene High cis- polybutadiene • Ziegler- Natta catalyst (of different transition metal like Co , Nd, Ni, Ti) • 92 % cis form obtained • Co gives branched molecules • Nd gives linear structure Low cis polybutadiene • using Alkyl lithium • 36% cis obtained • Used as an additive in plastics due to low content of gels High trans polybutadiene • Ziegler –Natta catalyst based on transition metals ( Nd, La, Ni) • 90% trans obtained • Melts about 800 c • Used for making outer layer of golf balls High vinyl polybutadiene • Alkyl lithium catalyst • 90% vinyl form • Elastic at room temperature, but a fluid at high temperature : using injection moulding Properties of PBR • Cis form have excellent elasticity, abrassion resistance • trans form tough, hard & thermoplastic • Vinyl form have excellent electrical properties, chemical resistance • Glass transition temperature 1000 c • They can easily blended with other diene rubbers Applications • Automobile tyres • Used to improve the mechanical properties. hence use as additive in plastics ( ABS, HIPS) • Used in inner tube of pipes, railway pad, bridge block,golf balls, cable insulations • Used as fuel in combination with oxidizer Silicone rubber Silicones • High molecular weight - polymers • Siloxane unit ( Si-O-Si) frame work • Each Si atom attached to one or three organic group • Preparation : organosilicon chloride by controlled hydrolysis Using dimethyl silicon dichloride – give long chain polymer Trimethyl silicon chloride - limit the chain length polymer Using monomethyl silicon chloride – cross linking polymer Curing of silicone rubber • They can cured by using curing agents like organic peroxides. • Added peroxide act as a curing agent which forms dimethylene bridges between methyl groups of adjacent chains • As a result hardness, rigidity increases peroxide Cured silicone rubber Properties of silicones • • • • • • • Stability over wide range of temperature Good water repellency Chemically & physically inert Good weather resistance Low vapour pressure Non toxic Specific gravity : 1.03- 2.1 Applications of silicones • • • • • High temperature lubricants Antifoaming agents Water repellent : hence use in leather, textiles Cosmetics, polishes Used in tyres for aircraft Organic LED Organic light emitting diode • It is an light emitting diode in which emissive electroluminescent layer is a film of organic compound which emits light in response to an electric current Structure • • • • • Substrate Anode Hole transport layer (HTL) Electron transport & emitting layer (ETL) Metallic cathode Substrate: Bottom layer Made up of clear plastic or glass Anode : • Remove electrons when current flows through it. • Create holes with respect to it • made up of indium titanium oxide(ITO) or LiF cathode • made up of Ag • Create electrons when a current flows through the device Hole transport layer ( HTL) : • conducting polymer transport holes from anode • made up of poly aniline or poly (pphenylene vinylene) • Plays the role facilitating holes injection from anode , accepting holes, transporting them to the emitting layer. • Block electrons from escaping from the emitting layer to anode Electron transport & emitting layer (ETL) • Emissive layer made of an organic molecule or polymer • eg: polyfluorene , aluminium quinacridone • Place where electron hole recombination occurs and energy emitted in the form of light. Working principle • • • • When voltage is applied between electrodes Electrons leave the cathode Holes move from anode + ve holes are much more mobile than – ve electrons • So holes moves across from HTL to emissive layer • Recombination of electron hole pair leads to creation of a photon with frequency between LUMO & HOMO levels • Electrical power applied to electrodes transformed into light • Different materials & dopants can be used to generate different colours Advantages • • • • • • High efficiency & large area sources High brightness Thin , flat & light weight Low voltage & fast switching technology Flexible displays No back light produced. Power consumption is less Applications For lighting as well as displays Backlight source in LCD Signaling Nanomaterials