Polymer Science Neha P. Dave DEFINITION Polymers are very large molecules made when hundreds of monomers join together to form long chains. The word ‘polymer’ comes from the Greek words poly (meaning ‘many’) and meros (meaning ‘parts’). Example: POLYBUTADIENE = (BUTADIENE+ BUTADIENE+......)n Where n = 4,000 INTRODUCTION • Polymers are complex and giant molecules usually with carbons building the backbone, different from low molecular weight compounds. • The small individual repeating units/molecules are known as monomers(means single part). • Imagine that a monomer can be represented by the letter A. Then a polymer made of that monomer would have the structure: -A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A-A • This kind of polymer is known as HOMOPOLYMER. (According to their properties & characteristics.) 1. Natural and Synthetic Polymers Polymers which are isolated from natural materials, are called as ‘natural polymers’. E.g. : Cotton, silk, wool, rubber. natural rubber Polymers synthesized from low molecular weight compounds, are called as, ‘synthetic polymers’. E.g. polyethylene, nylon, terylene. Polyethylene Semi synthetic polymers : • Geletin , fibrinogen chitin &chitoson , dextran, alginate NATURAL RUBBER- Hevea brasiilensis 2)BASED ON PRESENCE OF CARBON ATOM: A Polymer whose backbone chain is essentially made of carbon atoms is termed an ‘Organic polymer’. Examples- cellulose, proteins, polyethylene, nylons. A Polymer which does not have carbon atom in their chain is termed as ‘Inorganic polymer’ . Examples- Glass and silicone rubber 3. Classification by Monomer Composition Homopolymer Copolymer Block Graft Alternating Statistical Homopolymer Consist of only one type of constitutional repeating unit (A) AAAAAAAAAAAAAAA Copolymer Consists of two or more constitutional repeating units (A.B ) 4. Based on Microstructure Statistical copolymer (Random) ABAABABBBAABAABB two or more different repeating unit are distributed randomly Alternating copolymer ABABABABABABABAB are made of alternating sequences of the different monomers Block copolymer AAAAAAAAABBBBBBBBB long sequences of a monomer are followedby long sequences of another monomer Graft copolymer AAAAAAAAAAAAAAAAAA B B B B B B (d) 5. Based on Chain structure (molecular architecture) Linear chains :a polymer consisting of a single continuous chain of repeat units Branched chains :a polymer that includes side chains of repeat units connecting onto the main chain of repeat units Hyper branched polymer :consist of a constitutional repeating unit including a branching groups Cross linked polymer :a polymer that includes interconnections between chains Net work polymer :a cross linked polymer that includes numerous interconnections between chains Linear Branched Cross-linked Network Direction of increasing strength 6. Based on physical property related to heating Some polymer are soften on heating and can be converted into any shape that they can retain on cooling. Such polymer that soften on heating and stiffen on cooling are termed as `thermoplastic’ polymers. Ex. Polyethylene, PVC, nylon, sealing wax. Polymer that become an infusible and insoluble mass on heating are called ‘thermosetting’ polymers. Plastics made of these polymers cannot be stretched, are rigid and have a high melting point. 7. Classification by applications Polymer is shaped into hard and tough utility articles by application of heat and pressure, is known as ‘plastics’. E.g. polysterene, PVC, polymethyl methacrylate. When plastics are vulcanised into rubbery products exhibiting good strength and elongation, polymers are known as ‘elastomers’. E.g. silicone rubber, natural rubber, synthetic rubber, etc. Long filament like material whose length is atleast 100 times it’s diameter, polymers are said to be ‘fibres’. E.g. Nylon, terylene. Polymers used as adhesives, potting compounds, sealants, etc., in a liquid form are described as ‘liquid resins’. E.g. Epoxy adhesives and polysulphides sealants. 8. Classification Based on Kinetics or Mechanism A) Step-growth B) Chain-growth 9. BASED ON DEGRADATION OF POLYMER: . Biodegradable polymers: It can be defined as polymers comprised of monomers linked to one another through functional group and have unstable linkage in the backbone. eg. Collagen, Albumin,Casein etc. .Non biodgradable polymers: Polymerization mechanisms Forming large molecules from small molecules – Polymerization. There are two basic kinds of polymerization reactions: A)condensation(example: curing of concrete) or step growth B)Chain growth (example, formation of PVC pipe) - Step-growth polymerization Step-Growth Polymerization Stage 1 n n Consumption of monomer Stage 2 Combination of small fragments Stage 3 Reaction of oligomers to give high molecular weight polymer Chain-growth polymerization Chain growth polymerization involves an active chain site which reacts with an unsaturated (or heterocyclic) monomer such that the active site is recovered at the chain end. Chain polymerization Radical polym. Ionic polym. The C=C is prefer the Polym. by R.P. and also can be used in Anionic polym. Cationic polym. the steric hindrance of Electron with drawing Electron donating the substituent substituent decreasing substituent increasing the electron density on the electron density on the double bond and the double bond and facilitate the attack of facilitate the attack of anionic species cationic species such as cyano and such as alkoxy, alkyl, alkenyl, carbonyl and phenyl δ+ δ- δ- δ+ CH =CH Y 2 CH2 =CH Y X X X radical cationic anionic COMMON SYNTHESIS STEPS FOR DIFFERENT TECHNIQUES: 1)Initiation ; 2)Propagation; 3)Termination; -Coupling ; -Disproportionation ; -Chain transfer; -Inhibitor; (1)INITIATION: • Initiation in free radical polymerization involves first the generation of free radicals, which then attacks the double bond in the monomer molecule, resulting in the following chemical change; • R + CH2 = CH + R-CH2-CH ! ! free radicals X X monomer molecule • The free radical site is now shifted from the initiator fragment to the monomer unit. • The monomer initiating polymerization is an exothermic process. (2)PROPAGATION: • In these step, the radical site at the first monomer unit attacks the double bond of a fresh monomer molecule. • This results in the linking up of the second monomer unit to the first and the transfer of the radical site from the first monomer unit to the second , by the unpaired electron transfer process. • This process involving a continuing attack in fresh monomer molecules. (3)TERMINATION: • In this process any further addition of the monomer unit to the growing chain is stopped & the growth of the polymer chain is arrested by one of the following reaction. Coupling ; • The coupling of the lone electron present in each chain to form an electron pair and, thus nullify their reactiveness. Disproportionation ; • One H from one growing chain is abstracted by the other growing chain & utilized by the lone electron for getting stabilized. Inhibitor; • MH is known as a chain transfer agent, and addition of controlled amounts of MH to the polymerization reaction can be used to control molecular weight of the polymers. Characteristics of polymer Low Density. Low coefficient of friction. Good corrosion resistance. Good mould ability. Excellent surface finish can be obtained. Economical. Poor tensile strength. Low mechanical properties. Poor temperature resistance. Can be produced transparent or in different colours Polymer properties Physical Properties Mechanical Properties • Specific Gravity • Strength (Tensile and • Mold Shrinkage (in Flexural) flow, cross-flow, and • Modulus (Tensile and thickness directions) Flexural) • Elongation • Hardness • Impact Resistance Environmental Properties Thermal Properties • Chemical Resistance • Heat Deflection • UV Resistance Temperature • Flame Resistance (UL • VICAT Softening Rating) Temperature • Oxygen Index • Glass Transition Temp • Water Absorption • Heat Capacity • Thermal Conductivity STRUCTURAL POLYMER PROPERTIES • Mol. wt of polymer, this affect over all properties. the Mol.wt increases with increased tensile strength & resistance. • Force of attraction between polymer chain is high, crystals are formed. • Secondary interaction between atom on side chain stiffing the chain and increase strength. • Polymer appears translucent. • Heating of crystalline material above their melting point cause individual polymer chain to become mobile and transparent. • Density of polymer is increases by increasing crystalline content. Solubility decreases with the closer and more closer and more regular packing of polymer chain. CHEMICAL PROPERTIES • Polymer undergo significant degradation in body. • High crystallinity can increase polymer stability. • Ingredient can be use to improve polymer formation and enhance the overall properties. • Polymer properties may affect interaction with surrounding. • Polymer has been found into shape suitable for intended, sterilization process. Morphological Properties Crystallinity •A synthetic polymer may be described as crystalline if it contains regions of three-dimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding and/or stacking of adjacent chains. •Synthetic polymers may consist of both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. •The driving force for crystallization is a closer packing of the polymer chains