Functional Materials Module 5 PDF

Module:5 Functional materials 4 hours  Polymers (ABS and BAKELITE)- synthesis and application  Conducting polymers- polyacetylene and effect of doping  Nanomaterials – introduction, Bulk Vs Nano (Gold)  Top-down and bottom-up approaches for synthesis, and...

Module:5 Functional materials 4 hours  Polymers (ABS and BAKELITE)- synthesis and application  Conducting polymers- polyacetylene and effect of doping  Nanomaterials – introduction, Bulk Vs Nano (Gold)  Top-down and bottom-up approaches for synthesis, and 1  e-Book for Nanomaterials https://drive.google.com/file/d/18GiueZtfwN d5atOi5V8sijg9oqR-LvjM/view?usp=sharing  Many + Parts This name hints at how polymers are made Latin: Plasticus, that This name honors which can be molded plastics useful property of being easily molded 1. Introduction to Polymers Introduction Polymers : Poly + mers Poly means many; mers means units or parts Polymers - Many parts or many units Definition of Polymer Polymers are macromolecules (giant molecules of higher molecular weight) formed by the repeated linking of large number of small molecules called monomers. Example: n CH2 = CH2 (CH2-CH2-CH2-CH2-)n ethylene (monomer) polyethylene (polymer) 6 1. Introduction to Polymers Terminology: mer: a unit Monomer : one unit (A) Dimer : two units (A-A) Trimer : three units (A-A-A) Tetramer : four units (A-A-A-A) Polymer : many units (-A-A-A-A-A-A-A-A-A-A-A-)n 7  What is a polymer? It is a term used since 1866 by Berthelot who, in an article published in the Bulletin of the Chemical Society of France, noted that “styrolene (styrene), heated at 200° during a few hours, transforms itself into a resinous polymer”. It was the first recognized synthetic polymer. But it was Hermann Staudinger, in the year 1920, proposed the concept of polymers in the sense we use today. It led to the Nobel prize in 1953 for his work which is at the base of all science of macromolecules. It is, however, only during the following decade that the “macromolecular” theory definitively replaced the “micellar” theory to which it was opposed. Introduction In 1953, Hermann Staudinger formulated a macromolecular structure for rubber and received the Nobel Prize.  based on the repeating unit 2-methylbuta-1,3-diene isoprene Polymers and Polymerization Polymerization is the process of joining together many small molecules repeatedly to form very large molecules polymerization n momomer units polymer Naturally Occurring Polymers and Synthetic Polymers The most important naturally occurring polymers are:  Proteins  Polysaccharides (e.g. cellulose, starch)  Nucleic acids (e.g. DNA, RNA)  Rubber  Natural Polymers Monomer Polymer Isoprene Polyisoprene: Natural rubber n H OH H OH HO H HO HO O HO OH HO OH H OH Poly(ß-D-glycoside): H OH H H cellulose H H n ß-D-glucose O O O O H H H3N H3N N N OH O Polyamino acid: protein R1 Rn+1 n Rn+2 R Amino Acid O O DNA O P O Base O P O O Base O O O oligonucleic acid OH DNA O Nucleotide DNA Base = C, G, T, A 12  Synthetic Polymers Synthetic polymers are produced commercially on a very large scale  have a wide range of properties and uses Plastics are all synthetic polymers / D2 slot/ 02-05  Well-known examples of synthetic polymers are:  Polyethene (PE)  Polystyrene (PS)  Polyvinyl chloride (PVC)  Nylon  Urea-methanal  Classification One classification divides polymers into condensation and addition polymers and the other divides them in to step and chain growth polymers. Depending on their origin polymers are classified into natural and synthetic polymers. Depending on kind of atoms constituting backbone of the polymer they are classified as organic and inorganic polymers. Depending upon their ultimate use polymers are classified into plastics, elastomers, fibres and liquid resins. Plastics: Thermoplastic & Thermo setting resins Plastics Plastics are high molecular weight organic materials, that can be moulded into any desired shape by the application of heat and pressure in the presence of a catalyst Plastics or resins are classified into two types A). Thermoplastic resins B). Thermosetting resins 16  Types of Plastics Plastics are classified into two types……….. 1. Thermoplastic 2. Thermosetting resins 1. Thermoplastic Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again.  They are prepared by addition polymerisation.  They are straight chain (or) slightly branched polymers and  Various chains are held together by weak vanderwaal‟s forces of attraction.  It can be softened on heating and hardened on cooling reversibly.  They are generally soluble in organic solvents Examples: Polyethylene, Polyvinylchloride  Common thermoplastics range from 20,000 to 500,000 amu  Each polymer chain will have several thousand repeating units. They can be recycled and reused many times by heating and cooling process.  Thermoplastics (80%) No cross links between chains. Can change shape. Can be remoulded. 2. Thermosetting resins or Thermosets  Thermosetting resins can melt and take shape once; after they have solidified, they stay solid.  They are prepared by condensation polymerisation.  Various polymer chains are held together by strong covalent bonds (cross links)  These plastics get harden on heating and once harden, they cannot be softened again.  They are almost insoluble in organic solvents. Examples: Bakelite, Polyester Thermoset Polymers whose individual chains have been chemically cross linked by covalent bonds and form a 3-D cross linked structure.  Therefore, they resist heat softening and solvent attack.  These are hardened during the molding process and once they are cured, they cannot be softened and they cannot be recycled and reused Eg. Phenol-formaldehyde resins, urea-formaldehyde paints.  Thermosets Cross-linking formed by covalent bonds. Bonds prevent chains moving relative to each other. Difference between Thermoplastic and Thermosetting Polymers……… Thermoplastic polymers Thermosetting polymers Consists of long-chain linear polymers Have 3-Dimensional network structures with negligible cross-links. joined by strong covalent bonds. Soften on heating readily because Do not soften on heating; On prolonged secondary forces between the individual heating, they are charred. chain can break easily by heat or pressure. By re-heating to a suitable temperature, Retain their shape and structure even on they can be softened, reshaped and thus heating. Hence, cannot be reshaped. reused. Usually soft, weak and less brittle. Usually, hard, strong and brittle. Can be reclaimed from wastes. Cannot be reclaimed from wastes. Usually soluble in some organic solvents. Due to strong bonds and cross-linking, they are insoluble in almost all organic solvents. Properties and engineering applications Types of Thermoplastic resins: Vinyl resins.. Examples: (i) PVC (ii) TEFLON or FLUON (iii) ABS (Acrylonitrile Butadiene Styrene) Types of Thermosetting resins: Phenolic resins or phenoplasts (i) Novolac (ii) Bakelite  Acrylonitrile-Butadiene-Styrene (ABS) Plastics Properties ABS is an opaque thermoplastic and amorphous polymer. It can be easily recycled and Applications relatively non-toxic. Among the most widely identifiable are ABS has a strong resistance to keys on a computer keyboard, power- corrosive chemicals and/or physical tool housing, the plastic face-guard on impacts. wall sockets (often a PC/ABS blend), and It is very easy to mold and has a low LEGO toys. melting temperature making it ABS is used for 3D Printing and particularly simple to use in Prototype Development. injection molding manufacturing Also, it is used in camera housings, processes or 3D printing on an FDM protective housings, and packaging. machine. 23 ABS is also relatively inexpensive.  Thermosetting Plastics: (a) Phenolic resins or Phenoplasts : Novoloc and Bakelite Phenolic resins are condensation polymerization products of phenol derivatives and aldehydes. At first, Phenol reacts with Formaldehyde in presence of acidic / alkaline catalyst to form Monomethylnol phenol. Monomethylol phenol further reacts with Phenol to form a linear polymer “Novolac”. Water is removed as the by- product. Novolac  Thermosetting Plastics: Bakelite  Further addition of HCHO at high temperature and pressure converts Novolac (soft and soluble) into cross- linked “Bakelite” (hard and insoluble). Bakelite  Bakelite Properties:  Bakelite is resistant to acids, salts and most organic solvents, but it is attacked by alkalis because of the presence of –OH groups.  It possesses excellent electrical insulating property.  As thermoset it is difficult to recycle. Uses:  Bakelite is used as an adhesive in plywood laminations & grinding wheels, etc  It is also widely used in paints, varnishes,  It is used for making electrical insulator parts like plugs, switches, heater handles, paper laminated products, thermally insulation foams etc.  Conduction: It is the process by which heat or electricity is directly transmitted through the material of a substance when there is a difference of temperature or of electrical potential between adjoining regions, without movement of the material.  A good example would be heating a tin can of water using a Bunsen burner. Initially the flame produces radiation which heats the tin can. The tin can then transfers heat to the water through conduction. The hot water then rises to the top, in the convection process.  An electrical conductor is a substance in which electrical charge carriers, usually electrons, move easily from atom to atom with the application of voltage. Conductivity, in general, is the capacity to transmit something, such as electricity or heat. Eg; Copper, steel, gold, aluminum, and brass are also good conductors.  Conductivity of a substance is defined as 'the ability or power to conduct or transmit heat, electricity, or sound'. Its units are Siemens per meter [S/m] in SI and millimhos per centimeter [mmho/cm] in U.S. customary units. Its symbol is k or s.  Conducting Polymers Introduction Polymers are typically insulators due to their very high resistivity. Insulators have tightly bound electrons so that nearly no electron flow occurs, and they offer high resistance to charge flow. For conductance free electrons are needed. Conductivity in polymers can be induced by delocalizing their orbitals with conjugated p-electron backbones. Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa discovered that Polyacetylene can be made conductive almost like a metal (Nobel Prize for Chemistry in 2000).  Conducting Polymers Polymers, particularly those with a conjugated p-bond structure often show higher conductivity when doped with conductive materials. But the use of conductive polymers is limited since they have poor mechanical strength. Hence, a combination of mechanical and electrical properties can only find good applications in conductive polymers area. Sometimes, in a polymer blend, a bifunctional linker is doped to increase the conductivity of Polyaniline (PANI) (having conductivity) and polycaprolactum (PCL) (having mechanical strength) blend. Conductive polymers can be made using simple procedures like melt blending, solution blending etc., and can be used for antistatic and electromagnetic shielding applications. 31 Some Examples of Conductive Polymers 32  The game offers a simple model of a doped polymer. The pieces cannot move unless there is at least one empty "hole". In the polymer each piece is an electron that jumps to a hole vacated by another one. This creates a movement along the molecule - an electric current. Watch the video about conducting polymer at https://www.quirkyscience.com/polyacetylene-conductive-polymer/  Mechanism of Conduction in Polymers Conjugation of p-electrons Undoped Insulating Conducting Doped + A- Dopant anion for charge neutrality 35 Different Types of Conducting Polymers: 1. Intrinsically conducting polymers (ICP) 2. Doped Conducting polymers 3. Extrinsically conducting polymers (ECP) Factors that affect the conductivity: 1. Density of charge carriers 2. Their mobility 3. The direction 4. Presence of doping materials (additives that facilitate the polymer conductivity in a better way) 5. Temperature 36 1. Intrinsically Conducting Polymers (ICPs)  Polymer consisting of alternating single and double bonds is called conjugated double bonds.  In conjugation, the bonds between the carbon atoms are alternately single and double. Every bond contains a localised “sigma” (σ) bond which forms a strong chemical bond.  In addition, every double bond also contains a less strongly localised “pi” (π) bond which is weaker.  Conjugation of sigma and pi-electrons over the entire backbone, forms valence bands and conduction bands. Eg: Poly-acetylene, poly-p-phenylene, polyaniline, polypyrrole polymers Polyacetylene 37  Doped Conducting Polymers  ICPs possess low ionisation potential and high electron affinity. So they can be easily oxidised or reduced.  The conductivity of ICP can be increased by creating positive charges (oxidation) or by negative charges (reduction) on the polymer backbone.  This technique is called DOPING.  There are two types of doping: p-doping achieved by oxidation n-doping achieved by reduction  p-Doping  p-doping is achieved by oxidation process. It is also known as the oxidative doping.  It involves treatment of an polyacetylene with a Lewis acid or iodine which leads to oxidation process and positive charges on the polymer backbone are created.  Some of the p-dopants are I2, Br2, AsF5, FeCl3, HClO4, PF5 etc. (CH)x + 2 FeCl3 → (CH)x+. FeCl4- + FeCl2 2 (CH)x + 3 I2 → 2 (CH)x+. I3-  This oxidation process or removal of one electron leads to the formation of delocalized radical ion called polaron  A second oxidation of the polaron, followed by radical recombination generates two mobile positive charge carriers also known as soliton, which are responsible for conduction  n-Doping  n-doping is achieved by reduction process. It is also known as the reductive doping.  It involves treatment of an polyacetylene with a Lewis base which leads to reduction process and negative charges on the polymer backbone are created.  Some commonly available n-dopants are Li, Na, Ca, sodium naphthalide, etc. (CH)x + Li → Li+(CH)x-. + C10H8  This reduction process or the donation of one electron leads to the formation of delocalized radical anion, an anioninc polaron  Second reduction, followed by radical recombination generates negatively charged soliton  Doping in ICP p-doping n-doping  What is a soliton? The soliton is a charged or a neutral defect in the polyacetylene chain that propagates down the chain, thereby reducing the barrier for interconversion. In p-type doping, the dopant (Iodine, I2) attracts an electron from the polyacetylene chain to form (I3-) leaving a positive soliton (carbenium ion) in the polymer chain that can move along its length. The lonely electron of the double bond, from which an electron was removed, can move easily. As a consequence, the double bond successively moves along the molecule, and the polymer is stabilized by having the charge spread over the polymer chain. Doping in Trans-Polyacetylene 42 Conductivity Mechanism in Polyacetylene: The mechanism followed by polyacetylene for the transfer of charge from one chain to another is called intersoliton hopping. What is a soliton? The soliton is a charged or a neutral defect in the polyacetylene chain that propagates down the chain, thereby reducing the barrier for interconversion. In n-type doping (This can be done by dipping the film in THF solution of an alkali metal) soliton is a resonance- stabilized polyenyl anion of approximately 29-31 CH units in length, with highest amplitude at the centre of the defect. The solitons (anions) transfer electrons to a neutral soliton (radical) in a neighboring chain through an isoenergetic process. The charged solitons are responsible for making polyacetylene a conductor.43  3. Extrinsically Conducting Polymers These are those polymers whose conductivity is due to the presence of externally added ingredients in them. Two types: (1) Conductive element filled polymer:  It is a resin/polymer filled with carbon black, metallic fibres, metal oxides etc. Polymer acts as a binder to those elements.  These have good bulk conductivity and are low in cost, light weight, strong and durable. They can be in different forms, shapes and sizes. (2) Blended Conducting Polymers:  It is the product obtained by blending a conventional polymer with a conducting polymer either by physical or chemical change.  Such polymers can be processed and possess better physical, chemical and mechanical strength. 44 Conductivity of Polymers and Metals 45 Shield for computer screen against electromagnetic smart" windows "smart" windows radiation Solar cell Light-emitting diodes Photographic Film § Introduction to Nanomaterials A nanoparticle is an entity with a width of a few nanometers to a few hundred, containing tens to thousands of atoms. Their defining characteristic is a very small feature size in the range of 1-100 (nm). Nano size: One nanometre is a millionth part of the size of the tip of a needle. 1 nm = 10-6 mm = 10-9 m Table 1. Some examples of size from macro to molecular Size Examples Terminology (nm) 0.1-0.5 Individual chemical bonds Molecular/atomic Bulk Gold 0.5-1.0 Small molecules, pores in Molecular zeolites 1-1000 Proteins, DNA, inorganic Nano nanoparticles From: USDA’s 103-104 living cells, human hair Micro roadmap of nanotechnology. >104 Normal bulk matter Macro 48  Size and shape dependent colors of Au and Ag nanoparticles Gold NPs in Glass Silver NPs in Glass 25 nm 100 nm Sphere Sphere reflected reflected 50 nm 40 nm Sphere Sphere reflected reflected 100 nm 100 nm prism Sphere reflected reflected 49 49  Size and shape dependent colors of Au and Ag nanoparticles Note: nanomaterials scatter visible light rather than absorb  Distance between particles also effects colour Surface plasmon resonance: Excitation of surface plasmons by light (visible or infra red) is denoted as a surface plasmon resonance Localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures 50 50  What are surface plasmons ?  When an electromagnetic radiation interacts with metal nano particles ( e.g. Au & Ag) present in a dielectric medium, it induces a collective oscillation of conduction electrons called surface plasmons.  It can be studied by the UV-Visible spectrum of the nano particles Applications: diagnostics and analysis of biomolecular interactions etc.  Surface plasmon resonance spectrum can be simulated by Mie theory  It helps to arrive at the particle size of the nano particles.  The adjacent figure shows the experimental spectrum and the calculated one for Ag nano particles. Categories of Nanomaterials 52  Quantum Dots and applications Quantum dots (QDs) are semiconductor particles of few nanometers ( red reflected 50 nm > green reflected 100 nm > orange reflected 4 Is metallic, with a yellow colour when in Are not “metals” but are semiconductors a mass (Band gap energy = 3.4 eV) 5 Good conductors of heat and electricity Are very good catalysts 6 Generally have high densities 7 Have high melting point (~1080oC) Melts at relatively low temp (~940º C) 8 Are often hard and tough with high tensile strength 9 Having high resistance to the stresses of being stretched or drawn out 10 Not easily breakable 11 Inert-unaffected by air & most reagents 58 59  Schematic representation of the „bottom up‟ and top down‟ synthesis processes of nanomaterials High-energy By wet ball milling atom-by-atom molecule-by-molecule cluster-by-cluster Less waste More economical 60 Any fabrication technique should provide the followings: Identical size of all particles (also called mono sized or with uniform size distribution Identical shape or morphology Identical chemical composition and crystal structure Individually dispersed or mono dispersed i.e., no agglomeration 61  Nanoparticles preparation:  Top-down approaches  High-energy ball milling/Machining  Chemical Oxidation Process (CNTs to QDs)  Electrochemical Oxidation Process (Graphite rod to QDs)  Lithography (photo- and electrochemical)  Etching/Cutting  Coating  Atomization Bottom-up approaches Gas Condensation Processing (GCP)/Aerosol Based Processes  Chemical Vapour Condensation (CVC)  Atomic or Molecular Condensation  Laser ablation  Supercritical Fluid Synthesis Wet Chemical Synthesis of nanomaterials (Sol-gel process)  Precipitation method  Spinning  Self-Assembly 62  DNA Origami Schematic representation of the principle of mechanical milling WC coated 50 µm powder ball 63  Mineral, ceramic processing, and powder metallurgy industry  Procedure of milling process  Particle size reduction, solid-state alloying, mixing or blending, and particle shape changes  Restricted to relatively hard, brittle materials which fracture and/or deform and cold weld during the milling operation  To produce nonequilibrium structures including nanocrystalline, amorphous and quasicrystalline materials  Users are tumbler mills, attrition mills, shaker mills, vibratory mills, planetary mills etc  Powders diameters of about 50 µm with a number of hardened steel or tungsten carbide (WC) coated balls in a sealed container which is shaken or violently agitated. The most effective ratio for the ball to powder mass is 5 : 10. 64  Shaker mills (e.g. SPEX model 8000) uses small batches of powder (approximately 10 cm3 is sufficient  Advantage: High production rates  Limitation  Severe plastic deformation associated with mechanical attrition due to generation of high temp in the interphase, 100 to 200 oC.  Difficulty in broken down to the required particle size  Contamination by the milling tools (Fe) and atmosphere (trace elements of O2, N2 in rare gases) can be a problem (inert condition necessary like Glove Box)(Fe 10%) 65  Wet Chemical Synthesis of nanomaterials (Sol-gel process) SOL - nanoparticle dispersion GEL - crosslinked network Thermal evaporation Calcine 800oC Schematic representation of sol-gel process of synthesis of nanomaterials. 66 67  Overall Steps: Step 1: Formation of different stable solutions of the alkoxide (the sol). Step 2: Gelation resulting from the formation of an oxide- or alcohol-bridged network (the gel) by a polycondensation or polyesterification reaction Step 3: Aging of the gel, during which the polycondensation reactions continue until the gel transforms into a solid mass, accompanied by contraction of the gel network and expulsion of solvent from gel pores. Step 4: Drying of the gel, when water and other volatile liquids are removed from the gel network. – If isolated by thermal evaporation, the resulting monolith is termed a xerogel. – If the solvent (such as water) is extracted under supercritical or near super critical conditions, the product is an aerogel. Step 5: Dehydration, during which surface- bound M-OH groups are removed, thereby stabilizing the gel against rehydration. This is normally achieved by calcining the monolith at temperatures up to 8000C. Step 6: Densification and decomposition of the gels at high temperatures (T>8000C). The pores of the gel network are collapsed, and remaining organic species are volatilized. The typical steps that are involved in sol-gel processing are shown in the schematic diagram above. 68  Sol/gel transition controls the particle size and shape. Calcination of the gel produces the product (eg. Oxide). Sol-gel processing > hydrolysis and condensation of alkoxide-based precursors such as Si(OEt)4 (tetraethyl orthosilicate, or TEOS). The reactions are as follows: MOR + H2O → MOH + ROH (hydrolysis) MOH+ROM→M-O-M+ROH (condensation) If the aging process of gels exceeds 7 days it is critical to prevent the cracks in gels that have been cast Steps are: Sol Gel Ageing Drying Dehydration Densification & Decomposition Product 69  Advantages Synthesizing nonmetallic inorganic materials like glasses, glass ceramics or ceramic materials at very low temperatures compared to melting glass or firing ceramics Monosized nanoparticles possible by this bottom up approach.  Disadvantages Controlling the growth of the particles and then stopping the newly formed particles from agglomerating. Difficult to ensure complete reaction so that no unwanted reactant is left on the product Completely removal of any growth aids Also production rates of nanopowders are very slow by this process 70