Polymers And Molecular Structure PDF

UNIT – 3 POLYMERS AND MOLECULAR STRUCTURE POLYMERS POLYMER: Polymers or Macromolecules can be defined as the large molecules built-up by the linkage together of small molecules. (in Greek languag...

UNIT – 3 POLYMERS AND MOLECULAR STRUCTURE POLYMERS POLYMER: Polymers or Macromolecules can be defined as the large molecules built-up by the linkage together of small molecules. (in Greek language, poly means many and mermeans unit). MONOMER: Monomers are the small molecules which combine with each other to form polymer molecules. POLYMERISATION : The process of chemical combination of small molecules (monomers) to form large sized molecules (polymers) is called polymerization. DEGREE OF POLYMERISATION : The number of monomer units present in a polymer is known as degree of polymerization. The “repeat unit” in a polymer is called “mer”. There may be hundreds or thousands or tens of thousands or more monomer molecules linked together in a polymer molecule. Most of the polymers usually fall into the 5000-200000 molecular mass range. FUNCTIONALITY : The number of bonding sites in a monomer is referred to as its functionality. In an olefin, the double bond can be considered as a site for two free valencies. When the double bond is broken, two single bonds become available for combination. Thus, ethylene is considered to be bifunctional. Other reactive groups are hydroxyl, acid, amino-acid, di-ol, polyalcohols, di-amino acids, di-acids, etc. Depending on functionality of monomeric units, it is possible to obtain different types of structures. In case of a bifunctional monomer, two reactive groups attach side by side to each other forming a linear or straight-chain molecule. When a trifunctional monomer is mixed in small amounts with a bifunctional monomer and polymerized, it results branched-chain polymer. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 1 NOMENCLATURE OF POLYMERS: 1)Homopolymer:A polymer consisting of identical monomers is called Homo polymer. (-M-M-M-M-M-M-M)n Ex: Polyethylene, PVC, Teflon 2)Co-polymer: Polymer formed by two or more monomers of different chemical structures is calledCo-polymers. (M1-M2-M1-M2-M1-M2-M1-M2-)n Ex: Buna-S, Butyl rubber, Thiokol etc. 3)Hetero polymer: If the main chain is made up of different atoms then it is called heterochain polymer. –C-C-O-C-C-O-C-C-O-C-C-O- Ex: Polyester, Nylon-6,6, Kevlar 4)Graft Copolymer: The monomers of the polymer in back bone and branch chain differ. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 2 THERMOPLASTIC POLYMERS:This type of polymers can soften on heating and harden on cooling any number of times. So the desired shape can be obtained many times. Usually thermoplastics have linear or branched structure but not cross-linked. They possess weak forces of attraction (wander vall or dipole-dipole or hydrogen bonding) between the adjacent strands. Consequently, they are flexible and can easily moulded on heating. Ex polythene, polypropylene, pvc, polystyrene, Teflon THERMOSETTING POLYMERS:These types of polymers get hardened during moulding, and once they have solidified, they cannot be softened. Such polymers during moulding acquire three-dimensional cross-linked structure with predominantly strong covalent bonds. Hence, these once set cannot be remoulded. On heating, they first fuse to form hard mass and then burn. Ex: Polyester, Bakelite, epoxyresins, melamine, urea-formaldehyd, nylon-6,6 DIFFERENCES BETWEEN THERMOPLASTICS AND THERMOSETTING PLASTICS: Thermo plastics Thermosetting plastics They are softened on heating and They do not soften on heating. On hardened on cooling. prolong heating, however, they burn. These are formed by additional These are formed by condensation polymerization. Polymerization These are long chain linear Their set molecules have three macromolecules dimensional cross –linked network structure. The adjacent polymer chains are held Polymer chains are held together by together by eithervanderwal forces or by strong covalent bonds. dipole-dipole or H-bonds. They can be remoulded, reshaped and They cannot be remoulded. reused. They can be reclaimed from waste. They cannot be reclaimed from waste. Thermoplasts are soluble in organic These are insoluble in almost all the solvents solvents. These are soft, weak and less brittle. These are hard, strong and more brittle. Ex: PE, PVC andTeflon. Ex: Bakelite and Urea Formaldehyde resins. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 3 TYPES OF POLYMERIZATION: 1. Addition or Chain polymerization: It is a reaction that yields a product, which is an exact multiple of the original monomeric molecule. Such a monomer molecule, usually contains one or more double bonds, which by intermolecular rearrangement, may make the molecule bifunctional. The addition polymerization reaction must be instigated by the application of heat, light, pressure or a catalyst for breaking down the double covalent bonds of monomers. 2. Condensation or step-polymerization: It 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, nylon 6:6 Thus, condensation polymerization is an intermolecular combination, and it takes place through the different functional group in the monomers having the affinity for each other. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 4  3. Copolymerization: It is the joint polymerization of two or more monomer species. High molecular-weight compounds obtained by copolymerization are called copolymers. For example, butadiene and styrene copolymerize to yield Buna-S rubber. (1) PLASTICS POLY ETHYLENE TEREPTHALATE: (P.E.T) “Polyethylene terephthalate is a condensation polymer of ethylene glycol and terephthalic acid.” The by-product of the reaction to create this compound is water so it is an example of condensation or step-growth polymerization. On the basis of mechanism, there are two types of polymerization; step-growth and chain-growth polymerization. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 5 Polyethylene Terephthalate Properties  Crystal clear polymer – It is a crystal clear polymer with good purity and healthy. You must have seen the sparkling PET bottles with brilliant glass-clear presentation attract us.  Purity – The products of PET taste good and comply with international food contact regulations.  Safe – The objects made from PET like bottles are tough and virtually unbreakable therefore can be easily used for storage and transportation. This polymer has a high impact and tensile strength that makes it ideal for carbonated products.  Good barrier – PET products have low permeability to oxygen, carbon dioxide and water, therefore, it maintains the integrity of products with good shelf life.  Lightweight – The lightweight of PET products reduce the shipping costs compared to glass products.  No Leakage and damage – Due to the absence of weld line in the base, PET bottles are free from leakage and damage.  Recyclable – PET polymer is recyclable and can be reshaped in different shapes.  Good resistance power – PET products have good resistance against different chemicals such as acids, bases, etc. Polyethylene Terephthalate Uses: 1. For the manufacturing of shopping bags, water bottles, videotapes 2. For manufacturing of, containers and bags 3. For the manufacturing of clothes and housing material 4. For manufacturing of water bottles 5. For manufacturing of microwaves containers 6. For manufacturing of carpets 7. For the manufacturing of packaging films R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 6 2.BAKELITE:It is a phenol formaldehyde resin. It is prepared by condensing phenol with formaldehyde in presence of acidic or basic catalyst. The initial reaction results in the formation of ortho and para hydroxyl, methyl phenol, which reacts to form linear polymer, Novolac. During moulding, hexamethylenetetramine [(CH2)6N4] are added. It provides formaldehyde, which converts the soluble and fusible novolac into a hard, infusible and insoluble solid of cross-linked structure. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 7 Properties:  Bakelite is set to rigid, hard, scratch-resistant, infusible, water-resistant, insoluble solid which is resistant to non-oxidizing acids, salts and many organic solvents.  It is attacked by alkalis, because of the presence of free hydroxyl group in their structure.  It possesses excellent electrical insulating character. Applications:  It is used for making electric insulator parts like switches, plugs, switch-boards, heater-handles, etc.  For making moulded articles like telephone parts, cabinets for radio and television.  For impregnating fabrics, wood and paper.  As adhesives for grinding wheels.  In paints and varnishes.  As hydrogen-exchanger resins in water softening.  For making bearing, used in propeller shafts for paper industry and rolling mills. FIBERS 1.KEVLAR:It is an aromatic polyamide in which benzene rings linked to the amide groups. It is prepared by condensation between aromatic dichloride and aromatic diamines. Properties:  Kevlar is exceptionally strong, 5 times stronger than steel and 10 times stronger than Al on a weight-for-weight bases.  It has high heat stability and flexibility.  The unique properties ofkevlar are due to the delocalized bonding which causes the benzene rings to be inflexible.  The high electron-density in the chains of Kevlar also results in relatively stronger vanderwaals intermolecular forces between neighbouringpolymer molecules. Applications:  Kevlar is used extensively in the aerospace and aircraft industries, a car parts such as tyres, brakes, clutch linings, etc.  For making ropes, cables, protective clothing, bullet-proof vests, motorcycle helmets and other high performance materials. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 8  ELASTOMERS Elastomer is any rubber like elastic polymer, which can be stretched to at least thrice its length, but it returns to its original shape and dimensions as soon as stretching force is released. 1.BUNA-S (Styrene rubber): It is produced by copolymerization of butadiene and styrene. (about 75% and 25% by weight). Properties:  It resembles natural rubber in processing characteristics as well as quality of finished products.  It possesses high abrasion-resistance, high load-bearing capacity and resilience.  It gets readily oxidized in presence of traces of ozone present in the atmosphere.  It swells in oils and solvents.  It can be vulcanized in the same way as natural rubber either by sulphur or sulphurmonochloride (S2Cl2). However, it requires less sulphur, but more accelerators for vulcanization. Applications:  Mainly used for the manufacture of motor tyres.  As floor tiles, shoe soles, gaskets, foot-wear components, wire and cable insulations, carpet backing, adhesives, tank-lining etc. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 9 3.SILICONE RUBBERS: These are obtained by mixing high molecular-weight linear dimethyl silicon polymers with fillers like finely divided silicon dioxide and peroxide-containing curing agents. Peroxide causes the formation of dimethyl bridge between methyl groups of adjacent chains. Properties:  They possess exceptional resistance to prolonged exposure to sunlight, weathering, most common oils, boiling water, dilute acids and alkalis.  They remain flexible in the temperature range of 90 – 2500C and here, find use in making tyres of fighter aircrafts, since they prevent damage on landing. Applications:  As a sealing material in search-lights and in aircraft engines.  For manufacture of tyres for fighter aircrafts.  For insulating the electrical wiring in ships.  In making lubricants, paints and protective coatings for fabric-finishing and water- proofing.  As adhesive in electronics industry.  For making insulation for washing machines and electric blankets for iron board covers.  For making artificial heart valves, transfusion tubings, and padding for plastic surgery.  For making boots for use at very low temperature, since they are less affected by temperature variation. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 10  CONDUCTING POLYMERS Polymers which show electrical conductivity on par with metallic conductors are known as conducting polymers. CLASSIFICATION OF CONDUCTING POLYMERS: 1.Intrinsically conducting polymers: a)Conjugated Conducting polymers:Conducting polymers have backbones of conjugated -electrons, which are responsible for electrical charge. In an electrical field, conjugated -electrons of the polymer get excited; thereby can be transported through the solid polymeric material. Overlapping of orbitals of conjugated -electrons over the entire backbone results in the formation of valence bands as well as conduction bands, which extend over the entire polymer molecule. Ex: Polyacetylene, poly-p-phenylene, polyquinoline, polyphenylene-co-vinylene, polyaniline, polyanthrylene, polyphenanthrylene, polypyrrole, polythiophene etc. b)Doped Conducting polymers:They are obtained by exposing a polymer to a charged transfer agent in either gas phase or in solution. The conductivity of intrinsically conducting polymers can be increased by creating either positive or negative charge on the polymer backbone by oxidation or reduction. This technique is called doping. (i) P-doping: It involves treating an intrinsically conducting polymer with a lewis acid, thereby oxidation process takes place and positive charge on the polymer backbone are created. Some of the common P-dopants used are I2, Br2, AsF5, PF6, naphthylamine, etc. (ii) N-doping: It involves treating an intrinsically conducting polymer with a lewis base thereby reduction process takes place and negative charges on the polymer backbone are created. Some of the common N-dopants used are Li, Na, Ca, tetrabutyl ammonium, FeCl3, etc. 2.Extrinsically conducting polymers: The conductivity of these polymers is due to the presence of externally added ingredient in them. These are following two types: a) Conductive element filled polymer: It is a resin or polymer filled with conducting elements such as carbon black, metallic fibres, metal oxides, etc. In this, the polymer acts as the binder to hold the conducting elements together in the solid entity. These polymers possess reasonably good bulk conductivity and are generally low in cost, R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 11  light in weight, mechanically durable and strong and easily processable in different forms, shapes and sizes. b) Blended conducting polymer: It is obtained by blending a conventional polymer with a conducting polymer either by physical or chemical change. Such polymer can be easily processed and possess better physical, chemically and mechanical properties. POLY ACETYLENE: Polymerization of acetylene with Zeiglar Natta Catalyst yields poly acetylene as a silvery film. When the reaction is carried at room temperature, both cis and trans isomers are formed. But when heated to 1500C, cis form converts to trans form. Mechanism of conduction in polyacetylene: Applications of conducting polymers:Conducting polymers are finding increased use because they are light weight, easy to process and have good mechanical properties.  In rechargeable light weight batteries based on perchlorate doped polyacetylene- lithium system. These are about 10 times lighter than conventional lead storage batteries. Such batteries are sufficiently flexible to fit a variety of designed configuration.  In optically display devices based on polythiophene. When the structure is electrically biased, the optical density of the film changes, i.e., its colour changes. Such electrochromic systems produce coloured displays with faster switching time and better viewing than conventional LED devices.  In wiring aircrafts and aerospace components.  In telecommunication systems.  In antistatic coatings for clothing.  In electromagnetic screening materials.  In electronic devices such as transistors and diodes.  In solar cells.  In drug delivery system for human body. R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 12   In photo voltaic devices.  In non-linear optical materials.  In molecular wires and molecular switches BIODEGRADABLE POLYMERS: Biodegradable polymers are the polymers that breakdown and lose their initial integrity. These are used in medical devices to avoid second operation to remove them, or to gradually release a drug. Polylactic Acid: Poly(lactic acid) or polylactide (PLA) is a thermoplastic aliphatic polyester derived from renewable resources, such as corn starch (in the United States), tapioca roots, chips or starch (mostly in Asia), or sugarcane. Bacterial fermentation is used to produce lactic acid from corn starch or cane sugar. Lactic acid cannot be polymerised to a useful product because each reaction generates one molecule of water, presence of which degrades the forming polymer chain. Due to this low molecular weight polymers will be formed. Instead, two lactic acids are dimerized to di- lactic ester. Although dimerization generates water, it can be separated prior to polymerisation. PLA of high molecular weight is produced from di-lactic ester by ring opening polymerization using stannous octate catalyst. Properties: Polylactic acid can be processed like most thermoplastics into fibre and film. The melting temperature of PLLA can be increased 40-50 °C and its heat deflection temperature can be increased from approximately 60°C to up to 190 °C by physically blending the polymer with PDLA (poly-D-lactide). R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 13 Applications: Poly (lactic acid) can be processed by extrusion, injection moulding, film & sheet casting, and spinning, providing access to a wide range of materials. Being able to degrade into innocuous lactic acid, PLA is used as medical implants in the form of anchors, screws, plates, pins, rods, and as a mesh. Depending on the exact type used, it breaks down inside the body within 6 months to 2 years. In the form of fibres and non-woven textiles, PLA also has many potential uses, for example as upholstery, disposable garments, awnings, feminine hygiene products, and diapers. Question Bank 1. Define the following terms with suitable examples. a) Polymer b) Monomer 2. Explain addition, condensation and copolymerizationreactions withexamples. 3. Differentiate between thermoplastic and thermosetting polymers. 4. Write the preparation, properties and uses of following: i) PET ii) Bakelite iii) Kevlar iv) Nylon-6,6 5. Discuss preparation, properties and uses of following rubbers: a) Buna-S b) Butyl rubber c) Silicone rubber 6. What are conducting polymers? Give their applications? 7. Write mechanism of conduction in polyacetylene? 8. Write preparation, properties and uses of polylactic acid? R.Gangadhara,MSc.MPhil.(Ph.D) Asst.Professor Stanley College of Engg&Tech for Women Page 14