!!!!!!!!!!!!POLYMERS!!!!!!!!!!!!

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POLYMERS

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A polymer has a repeating structure, usually based on a carbon backbone. The repeating structure results in large chainlike molecules. In each polymer molecule, the atoms are bound together by covalent bonds. Polymers Poly = many; Mer = unit -> Polymer = many units.

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Covalent Chain Configurations and Strength Increase in Strength Linear Polymers: Polyethylene, polyvinyl chloride (PVC), polystyrene, polymethyl methacrylate (plexiglass), nylon, fluorocarbons (teflon) Branched Polymers: Many elastomers or polymeric rubbers Cross-linked Polymers: Many elastomers or polymeric rubbers are cross-linked (vulcanization process); most thermosetting polymers Network Polymers: Epoxies, phenol-formaldehydes.

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Physical Properties of Polymers Composed of very large molecules Low modulus of elasticity (low stiffness) Low tensile and compressive strengths Can be crystalline or semi-crystalline structure Deformation is very sensitive to temperature Low thermal and electrical conductivity(good insulator) Creep at room temperatures Low temperatures make plastics brittle Plastic deformation

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Thermoplastics Soften when heated and harden when cooled Varying degree of ductility Can be recycled Can not withstand high temperatures Thermosets Become permanently hard after the initial heating-cooling cycle Brittle bdbbdbbbbdbsb Can not be recycled Can withstand high temperatures

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Industrially Important Polymers About 85% of the world plastics consumption consists of four polymers. These polymers are produced in high volume at very low cost. They are all thermoplastics. Polyethylene (PE) electrical wire insulation, flexible tubing, squeeze bottles Polypropylene (PP) carpet fibers, ropes, liquid containers (cups, buckets, tanks), pipes Polystyrene (PS) packaging foams, egg cartons, lighting panels, electrical appliance components Polyvinyl chloride (PVC) bottles, hoses, pipes, valves, electrical wire insulation, toys, raincoats

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Polymers are formed by many low temperature processes. Extrusion Film Blowing Injection Molding Blow Molding Compression Molding Reaction Injection Molding Polymer Processing Methods

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The polymer is heated to the liquid state and forced through a die under pressure resulting in an endless product of constant cross section. 60% of polymers are prepared in this way. Examples: tubing, pipes, window frames, sheet, insulated wire. Extrusion . Using the same method as extrusion the material coming out of the die is blown into a film. An example is plastic wrap. Film Blowing

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Similar to extrusion, the polymeris heated to the liquid state, but it is prepared in metered amounts, and the melt is forced into a mold to create the part. It is not a continuous process. Many toys are made by injection molding. Injection Molding The melted polymer is put into a mold, then compressed air is used to spread the polymer into the mold. It is used to make many containers such as plastic soda containers and milk jugs. Blow Molding

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Solid polymer is placed in a mold, the mold is heated and puts pressure on the polymer to form the part. Liquid monomers are placed in the mold avoiding the need to use temperature to melt the polymer or pressure to inject it. The monomers polymerize in the mold forming the part. Compression Molding Reaction Injection Molding

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Elastomers Elastomers are long polymer chains above their glass transition temperature. Elastomers are usually lightly crosslinked and are easily deformed. Common elastomers include: polybutadiene (used in shoe soles) polyisobutylene (used in automobile tires polyisoprene (natural rubber).

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Elastomeric polymer chains can be crosslinked, or connected by covalent bonds. This process is sometimes called vulcanization. Crosslinking is initiated by heat, light, or the addition of chemicals. Crosslinking makes elastomers reversibly stretchable for small deformations. When stretched, the polymer chains become elongated and ordered along the deformation direction. When no longer stretched, the chains randomize again. The crosslinks guide the elastomer back to its original shape.

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Crosslinking makes elastomers reversibly stretchable for small deformations. Stretched Returned to randomization

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