Polymers .pdf
- 1. POLYMERS
- 2. The Outline Summery Reactions of polymers Addition Polymerization Step Growth Polymerization Chemical and Physical Structures of Polymers Polymer’s molecular structures Configuration and conformation of polymers Chain structures of polymers Physical Structures of Polymers Polymer crystallinity Crystallinity and amorphousness of polymers
- 3. The Outline Summery Types of Polymers and Polymer Processing Members of Polymers Definition of Thermosets & Thermoplastics Common products and their properties Forming Techniques of Polymers Extrusion of polymers Injection Molding Blow Molding Thermoforming Compression Molding Casting
- 4. A polymer is a large molecule made by linking together repeating units of small molecules called monomers
- 5. Addition Polymerization (Chain Growth) Step Growth Polymerization (Condensation)
- 7. Differences between step-growth polymerization and chain-growth polymerization Step-growth polymerization Chain-growth polymerization Growth throughout matrix Growth by addition of monomer only at one end of chain Rapid loss of monomer early in the reaction Some monomer remains even at long reaction times Same mechanism throughout Different mechanisms operate at different stages of reaction (i.e. Initiation, propagation and termination) Average molecular weight increases slowly at low conversion and high extents of reaction are required to obtain high chain length Molar mass of backbone chain increases rapidly at early stage and remains approximately the same throughout the polymerization Ends remain active (no termination) Chains not active after termination No initiator necessary Initiator required
- 8. Step of Radical Chain Polymerization Initiation Propagation Termination
- 9. INITIATION
- 10. PROPAGATION
- 11. TERMINATION Dead Polymer i.) Coupling or Combination; ii.) Disproportionation
- 12. CHAIN TRANSFER REACTIONS Transfer to monomer reaction Transfer to initiator reaction Transfer to solvent reaction
- 13. IONIC CHAIN POLYMERIZATION Using catalyst, not initiator Highest reaction rate Termination step is just disproportionation Environment must be pure Reaction occurs in the cold
- 14. Anionic Polymerization=Living Polymerization If the starting reagents are pure and the polimerization reactor is purged of all oxygen and traces of water, polimerization can proceed until all monomer is consumed.
- 15. CONDENSATION POLYMERIZATION Using catalyst Minumum two functional groups required Usually linear Molecular weight increases slowly at low conversion High extents of reaction are required to obtain high chain length
- 16. Chemical Structure of Polymers Molecular configuration of polymers Side groups atoms or molecules with free bonds, called free-radicals, like H, O, methyl affects polymer properties. Stereoregularity describes the configuration of polymer chains : Isotactic is an arrangement where all substituents are on the same side of the polymer chain. Syndiotactic polymer chain is composed of alternating groups Atactic the radical groups are positioned at random Figure 2: Isotactic Syndiotactic and Atactic combinations of a stereoisomers of polymer chain (http://www.microscopy-uk.org.uk/mag/imgsep07/atactic.png)
- 17. Molecular configuration of polymers FIGURE.3. Diagrams of (a) isotactic, (b) syndiotactic, and (c) atactic configuration in a vinyl polymer. The corresponding Fischer projections are shown on the right. (Plastic Technolgoy Handbook)
- 18. Table 1. Properties of Polypropylene Stereoisomers (Plastic Technology Handbook)
- 19. Molecular configuration of polymers Geometrical isomerism: The two types of polymer configurations are cis and trans. These structures can not be changed by physical means (e.g. rotation). The cis configuration → substituent groups are on the same side of a carbon- carbon double bond. Trans → the substituents on opposite sides of the double bond. Figure4.cis trans configurations of polyisoprene ( http://openlearn.open.ac.uk/file.php/2937/T838_1_019i.jpg )
- 20. Conformations of a Polymer Molecule Conformation →The two atoms have other atoms or groups attached to them configurations which vary in torsional angle are known as conformations (torsional angle:The rotation about a single bond which joins two atoms ) Polymer molecule can take on many conformations. Different conformation →different potential energies of the molecule→Some conformations: Anti (Trans), Eclipsed (Cis), and Gauche (+ or -)
- 21. Other Chain Structures Copolymers →polymers that incorporate more than one kind of monomer into their chain (nylon) Three important types of copolymers: Random copolymer contains a random arrangement of the multiple monomers. Block copolymer contains blocks of monomers of the same type Graft copolymer contains a main chain polymer consisting of one type of monomer with branches made up of other monomers. Figure 5 :Block Copolymer Graft Copolymer Random Copolymer http://plc.cwru.edu/tutorial/enhanced/FILES/Polymers/struct/struct.htm
- 22. Physical Characteristics of Polymers The melting or softening temperature ↑ molecular weight ↑ The molecular shape of the polymer has influence on the elastic properties. ↑ coils the ↑ elasticity of the polymer The structure of the molecular chains has an effect on the strength and thermal stability. ↑ crosslink and network structure within the molecule ↑ the strength and thermal stability.
- 23. Polymer Crystallinity Crystallinity is indication of amount of crystalline region in polymer with respect to amorphous content X-ray scattering and electron microscopy have shown that the crystallites are made up of lamellae which,in turn, are built-up of folded polymer chains Figure.6 Schematic representation of (a) fold plane showing regular chain folding, (b) ideal stacking oflamellar crystals, (c) interlamellar amorphous model, and (d) fringed micelle model of randomly distributed crystallites (Plastic Technology Handbook)
- 24. Polymer crystallinity Crystallinity occurs when linear polymer chains are structurally oriented in a uniform three dimensional matrix. Three factors that influence the degree of crystallinity are: i) Chain length ii) Chain branching iii) Interchain bonding Figure 7: Crystalline chain http://plc.cwru.edu/tutorial/enhanced/FILES/Polymers/orient/Orient.htm
- 25. Polymer cristallinity Crystallinity influences: Hardness,modulus tensile, stiffness, crease, melting point of polymers. Most crystalline polymers are not entirely crystalline. The chains, or parts of chains, that aren't in the crystals have no order to the arrangement of their chains Crystallinity makes a polymers strong, but also lowers their impact resistance Crystalline polymers are denser than amorphous polymers, so the degree of crystallinity can be obtained from the measurement of density → Wc=Φcρc/ ρ ρ → density of entire sample ρc → density of the crystalline fraction. Φc→ volume fraction Wc→ mass fraction
- 26. Determinants of Polymer Crystallinity The degree of crystallinity of a polymer depends on the rate of cooling during solidification as well as on the chain configuration. In most polymers, the combination of crystalline and amorphous structures forms a material with advantageous properties of strength and stiffness. Figure 8: Mixed amorphous crystalline macromolecular polymer structure (http://web.utk.edu/~mse/Textiles/Polymer%20Crystallinity.htm)
- 27. Polymer cristallinity Polymer molecules are very large so it might seem that they could not pack together regularly and form a crystal. Regular polymers may form lamellar crystals with parallel chains that are perpendicular to the face of the crystals. A crystalline polymer consists of the crystalline portion and the amorphous portion. The crystalline portion is in the lamellae, and the amorphous portion is outside the lamellae . Figure 9. Arrangement of crystalline and amorphous portions http://pslc.ws/mactest/crystal.htm#structure
- 28. Cristillanity and amorphousness An amorphous solid is formed when the chains have little orientation throughout the bulk polymer. The glass transition temperature is the point at which the polymer hardens into an amorphous solid. In between the crystalline lamellae,regions with no order to the arrangement of the polymer chains → amorphous regions Polyethylene can be crystalline or amorphous. Linear polyethylene is nearly 100% crystalline. But the branched polyethylene is highly amorphous. Figure 10.Linear and Branched Polyethylene (http://pslc.ws/macrog/kidsmac/images/pe03.gif )
- 29. Examples... Highly crystalline polymers: Polypropylene, Nylon, Syndiotactic polystyrene.. Highly amorphous polymers: Polycarbonate, polyisoprene, polybutadiene Polymer structure and intermolecular forces has a major role of a polymer’s crystallinity.
- 30. Classification of Polymers …with regard to their thermal processing behavior ; Thermoplastic Polymers (Thermoplastics) soften when heated and harden when cooled Thermosetting Polymers (Thermosets) once having formed won’t soften upon heating
- 31. Thermoplastics have linear or branched structure chains are flexible and can slide past each other
- 32. have strong covalent bonds and weak intermolecular van der Waals bonds elastic and flexible above glass transition temperature can be heat softened, remolded into different forms reversible physical changes without a change in the chemical structure
- 33. Thermosets chains chemically linked by covalent bonds hardening involves a chemical reaction which connects the linear molecules together to form a single macromolecule.
- 34. Thermosets once polymerization is complete, cannot be softened, melted or molded non-destructively. have higher thermal, chemical and creep resistance than thermoplastics Thermosets suitable materials for Composites Coatings Adhesive applications
- 35. Common thermoplastics Commodity Polymers POLYETHYLENES POLYPROPYLENE POLYSTYRENE POLYVINYLCHLORIDE-PVC POLYMETHYLMETHACRYLATE-PMMA Engineering Polymers(have a thermal resistance 100-150°C) POLYCARBONATE NYLON(POLYAMIDE) POLYETHYLEN TEREPHATALATE-PET High Performance Polymers (have a thermal resistance >150°C) POLYTETRAFLUOROETHYLENE-teflon POLYARYLETHERKETONES-PEEK
- 36. POLYETHYLENE prepared directly from the polymerization of ethylene (C2H4). two main types are; low-density (LDPE) and high-density polyethylene (HDPE) Advantages cheap good chemical resistance high impact strength
- 37. Limitations low heat resistance (upper temperature limit is 60°) degrade under UV irradiation. high gas permeability, particularly CO2 Applications extensively for piping and packaging chemically resistant fittings, garbage bags containers, cable covering
- 38. POLYPROPLYLENE improved mechanical properties compared to polyethylene; has a low density (900–915 kg/m3), harder, and has a higher strength Good chemical and fatigue resistance Disadvantages Oxidative degradation, high thermal expansion, high creep poor UV resistance Applications medical components, films for packaging (e.g. cigarette packets)reusable containers, laboratory equipment
- 39. POLYSTYRENE a light amorphous thermoplastic Advantages low cost, easy to mould, rigid, transparent no taste, odor, or toxicity, good electrical insulation Disadvantages sensitive to UV irradiation (e.g. sunlight exposure) chemical resistance is poor, brittle Applications CD-DVD cases, electronic housings, food packaging, foam drink cups and egg boxes
- 40. POLYVINYLCHLORIDE-PVC was the first thermoplastic used in industrial applications very resistant to strong mineral acid and bases, good electrical insulators, flame-retardant Two grades of the PVC material are available: rigid PVC is used in the construction industry for piping cold water and chemicals flexible PVC is used in wire and cable coating, paints, signs
- 41. Common thermosets EPOXIES UNSATURATED POLYESTERS PHENOL FORMALDEHYDE (PHENOLIC) POLYURETHANES
- 42. EPOXIES Advantage mechanically strong, highly adhesive good chemical and heat resistance electrical insulators Disadvantage expensive Applications as industrial adhesives, coatings or as matrices in advanced reinforced plastics and also as encapsulation media
- 43. UNSATURATED POLYSTERS Advantage hard, high strength cheap compared to Epoxy good electrical insulator high heat resistance Disadvantage poor solvent resistance compared to other thermosets Applications molding or casting materials for a variety of electrical applications, matrix for composites such as fiberglass boats, fences, helmets, auto body components
- 44. PHENOLICS most commonly used thermosets high hardness, excellent thermal stability; low tendency to creep Applications wiring devices, bottle caps, automotive parts, plugs and switches, as adhesives coatings and molded components for electrical applications
- 45. POLYURETHANES depending on the degree of cross-linking they behave as thermosets or thermoplastics low cost, high impact strength, high adhesion properties be processed into coatings, adhesives, binders, fibers and foams
- 46. Methods of polymer fabrication ▪ Extrusion of polymers ▪ Injection Molding ▪ Blow Molding ▪ Thermoforming ▪ Compression Molding ▪ Casting