Functional Materials PDF

Module-5: Functional materials Classification based on heat response and conducting electricity, Polymers (ABS and BAKELITE)- synthesis and application, Conducting polymers- polyacetylene and effect of doping. Nano materials – introduction (classification and properties (Surface area eff...

Module-5: Functional materials Classification based on heat response and conducting electricity, Polymers (ABS and BAKELITE)- synthesis and application, Conducting polymers- polyacetylene and effect of doping. Nano materials – introduction (classification and properties (Surface area effect and Quantum effect), Top-down and bottom-up approaches for synthesis ( Ball mill and sol gel) - bulk vs nano (gold) 1 Introduction to Polymers Polymers are encountered in everyday life and are used for many purposes. Grocery bags, soda and water bottles, textile fibers, phones, computers, food packaging, auto parts, and toys all contain polymers. Module - 5 Functional Materials 51 Introduction to Polymers Polymers: Poly + mers Monomers Poly means many mers means units or parts Polymers - Many repeated parts - or many repeated units Polymer 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: Module - 5 Functional Materials 52 Definition Polymers are materials made of long, repeating chains of molecules. The materials have unique properties, depending on the type of molecules being bonded and how they are bonded. Terminology 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 Module - 5 Functional Materials 53 Polymer – Classification Polymers are classified based on different parameters 1. Based on ‟Occurrence” Natural polymers (e.g. Silk) Synthetic polymers (e.g. Nylon) Semi-synthetic polymers (e.g. vulcanized rubber) 2. Based on ‟Type of polymerization” o Addition polymers (e.g. Polyethylene) o Condensation polymers (e.g. Polyester) 3. Based on ‟Monomeric units” ❖ Homopolymers (e.g. Polypropylene) ❖ Co-polymers (e.g. Styrene butadiene rubber) 4. Based on ‟Thermal Effect” ❖ Plastics (e.g. Polyvinyl chloride) ❖ Rubbers (e.g. Butyl rubber) Module - 5 Functional Materials 55 Classification – Thermal Effect Polymers Plastics Rubbers Thermosetting Thermoplastics Elastomers plastics Crystalline Amorphous Module - 5 Functional Materials 56 Plastics Plastics are high molecular weight organic polymer materials, that can be moulded into any desired shape by applying heat and pressure. Plastics or resins are classified into two types (A) Thermoplastics (B) Thermosetting plastics Module - 5 Functional Materials 57  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 remoulded for repeated cycles. ✓ They are prepared by addition polymerisation. ✓ They are straight chain (or) slightly branched polymers and ✓ Various chains are held together by weak Van der Waal’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. 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. 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 i. TEFLON (or) PTFE – Preparation & Properties TEFLON The trivial name of PTFE (Polytetrafluoroethylene) is Teflon. Teflon is made by the polymerization of tetrafluoroethene. Properties of TEFLON ▪ This polymer is a hard, strong, chemically resistant compound with a high melting point and very low surface friction. ▪ Hydrophobic polymer ▪ Lowest coefficient of friction against any solids Module - 5 Functional Materials 62 i. TEFLON (or) PTFE – Uses Uses of TEFLON In motors, transformers coils, capacitors, pipes, tanks and storage of chemicals. Non-stick appliances. TEFLON is used as a lubricant, it reduces friction, wear and energy consumption of machinery. Module - 5 Functional Materials 63  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. 14 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 Monomethylol 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.  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. 18 Alan MacDiarmid, Alan Heeger, and Hideki Shirakawa received the Nobel Prize in Chemistry in 2000 for their work on “conductive polymers 19  Some Examples of Conductive Polymers CP-struct 20  Mechanism of Conduction in Polymers Conjugation of -electrons Undoped Insulating Conducting Doped + A- Dopant anion for charge neutrality 21 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. Charge carrier mobility 3. Direction of movement of charge carriers 4. Presence of doping materials (additives that facilitate the polymer conductivity in a better way) 5. Temperature 22 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 23  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 anionic 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 28 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.29  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. 30 Introduction - Nanomaterials Nanomaterials can be defined as materials possessing, at minimum, one external dimension measuring 1-100nm. 1 nm = 10-6 mm = 10-9 m Functional Materials Module - 5 99 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 roadmap of 103-104 living cells, human hair Micro nanotechnology. >104 Normal bulk matter Macro 32 Nanoscale Functional Materials Module - 5 100 Categories of Nanomaterials 34 Why do nanoparticles behave differently? Two principal factors cause the properties of nanomaterials different from bulk materials. 1. Surface area effect Surface to volume ratio 2. Quantum effect Quantum confinement 1. Surface area effect Surface to volume ratio By breaking the cube into multiple cubes the amount of surface exposed increases A greater amount of a substance comes in contact with surrounding material This results in better reactivity, since a greater proportion of the material is exposed for potential reaction. 1. Surface area effect 2. Quantum effect 2. Quantum effect Quantum confinement Functional Materials Module - 5 113 2. Quantum effect Quantum confinement Quantum confinement is change of electronic and optical properties when the material sampled is of sufficiently small size - typically 10 nanometers or less. The bandgap increases as the size of the nanostructure decreases. Quantum confinement effects describe electrons in terms of energy levels, potential wells, valence bands, conduction bands, and electron energy band gaps. The quantum confinement effect is observed when the size of the particle is too small to be comparable to the wavelength of the electron. The confinement of an electron and hole in nanocrystals significantly depends on the material properties, namely, on the Bohr radius. These effects take place in bigger nanocrystals and depend on the material properties, namely, on the Bohr radius, which would have Cd-related compounds such as CdTe, CdZnTe, and CdTeSe. Generally, the bandgap of group II–VI semiconductors becomes narrower as the constituent atoms become heavier. In the case of nanoparticles with diameters in the range of 2–10 nm, the bandgap is increased due to the quantum size effect compared with the bulk semiconductor, and it leads to various fluorescent colors reflecting small differences in the particle size. 41  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 42 42  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 43 43  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. Surface plasmon resonance (SPR) is the manifestation of a resonance effect due to the interaction of conduction electrons of metal nanoparticles with incident photons. The interaction relies on the size and shape of the metal nanoparticles and on the nature and composition of the dispersion medium. By understanding the mechanistic aspects of the interaction of altered nanoparticle morphologies together with the associated medium effect, a new technology has been developed for careful spectroscopic monitoring, which lead to sensing applications and imaging events. 45  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) 51 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 52 Synthesis of nanomaterials Functional Materials Module - 5 118 Synthesis of nanomaterials Functional Materials Module - 5 119 Top down approach Begins with a pattern generated on a larger scale, then reduced to nanoscale Relatively expensive and time consuming technique The approach use larger (macroscopic) initial structures The structures can be extremely-controlled in the processing of nanostructures Functional Materials Module - 5 120 Bottom up approach Starts with atoms or molecules and build up to nanostructures Fabrication is much less expensive Includes the miniaturization of materials components (atomic level) leading to formation of nano structures During self assembly the physical forces operating at nanoscale are used to combine basic units into larger stable structure. Functional Materials Module - 5 121 Synthesis & Characterization of nanoparticles Synthesis Functional Materials Module - 5 122 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 58  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 59 DNA Origami  Schematic representation of the principle of mechanical milling High-energy_ball_milling WC coated 50 µm powder ball Ball_mill 220px-8000M_Mixer_Mill_%28open%29_incl_accessories 60  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. 61  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%) 62  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. 63 Sol is a liquid state of colloidal solution whereas gel is a solid or semisolid state of colloidal solution. No definite structure is present for sols whereas generally a honeycomb like structure is present for gel. 64 65  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. 66  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 67 ❖ 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 68