Polymerization and structure of polymers
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Polymerization and structure of polymers

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    Polymerization and structure of polymers Polymerization and structure of polymers Presentation Transcript

    • • Collagen • Gelatin • Silk • Wool • Natural rubber • DNA NATURAL POLYMER SYNTHETIC POLYMER • Polyethylene terephthalate (PET) • High Density Polyethylene (HDPE) • Polyvinyl Chloride (PVC) • Low Density Polyethylene (LPDE) • Polypropylene (PP) • Polystyrene (PS)
    • • The word polymer comes from the Greek ‘poly’ meaning many, and ‘meros’ is parts or units. • A polymer is organic substance made up of many repeating units or building blocks of molecules called mers. • Combine many monomers to create a polymer. • Polymer is often used as a synonym for ‘plastic’. All plastic are polymers, but not all polymers are plastics. Poly mers are made up of many Mono mer ↓ ↓ ↓ ↓ Many Units One Unit
    • POLYMERIZATION chemical process where monomers linked into polymers in repeating unit to make longer and larger molecules • Also called additional polymerization, with aids of initiators to form benzene or paraffin. Chain – Reaction Polymerization • Also called condensation polymerization, dissimilar monomer joined into short groups that gradually grow with by product released. Step – Reaction Polymerization
    • The straightforward addition of monomers of the same kind Homogeneous type : A +A … → A-A-A-A-… or a different kind Copolymer type : A +B+A+B… → A-B-A-B-…  Rapid chain reaction of chemically activated mers  Each reaction sets up the condition for another to proceed  Each site need a reactive site (a double carbon bond or unsaturated molecules)  Initiator is added to open the double bond between carbon  3 stages : Initiation Propagation Temination  The composition of resultant molecule is a multiple of the individual mers  Most commonly produced linear structure but can produce network structure
    • • Initiation free radical – a single unit that has one unpaired electron (OH‾ molecule) • H₂O₂ break up into 2 OH‾ molecules • Each can act to initiate and to terminate the reaction • Termination - recombination Polymerization of polyethylene
    • Involves a polymerization reaction between two monomers with the expulsion of a simple by product. A+B → AB + simple by product  Individual chemical reactions between reactive mer that occur one step at a time  By products (water or carbon ,oxygen or hydrogen gas) is formed and condensed out  Polymer molecule growth step by step until all of one reactant is consumed  Slower than additional polymerization  Need reactive functional groups  No reactant species has the chemical formula of a mer repeating unit  Most commonly produce network structure but can produce linear structures
    • Condensation polymerization of nylon 6,6
    • The properties of the polymer depends on: i. Structures of individual polymer molecules ii. Molecule shape and size iii. Arrangement of molecules to form a polymer structure Basic structure of polymer molecules: (a) ethylene molecule (b) polyethylene, a linear chain of many ethylene molecule (c) molecular structure of various polymers
    • • Molecular weight of the polymer is the sum of the molecular weights of mers in a representative chain. • Molecular Weight Distribution (MWD) is the spread of the molecular weights in a chain • Strong influence on the properties: Increase in molecular weight will increase: i. Tensile & impact strength ii. Resistance to cracking iii. Viscosity of molten state higher molecular weight, greater average chain length Figure 2 Effect of molecular weight and degree of polymerization on the strength and viscosity of polymers
    • • The ratio of the molecular weight of the polymer to the molecular weight of the mer (repeating unit) • Example: Polyvinyl chloride (PVC) Mer weight: 62.5, thus DP of PVC with 50,000 molecular weight is: 50,000 / 62.5 = 800 Higher DP → Higher viscosity (resistance to flow) → hard to shape → increase cost adversely. Higher DP → stronger polymers
    • • During polymerization, the monomers are linked together by covalent bond forming a polymer chain (high strength at primary bond) • The polymer chains are held together by secondary bonds (low strength): i. Van der Waals bonds ii. Hydrogen bonds iii. Ionic bonds • In polymer, the increase in strength and viscosity → the longer the polymer chain → the greater is energy needed to overcome secondary bonds
    • Linear Polymers (sequential structure) • Generally a polymer consists of more than one type of structure (a linear polymer may contain some branched and cross-linked chains; properties are changed significantly) Branched Polymers • Side-branch chains are attached to the main chain during the synthesis. • Interferes with the relative movement of the molecular chains → increase in resistance to deformation and stress cracking • Interferes with the packing efficiency of chains → density is lower than linear-chain Branched polymers ~pile of tree branches Linear-chain polymers ~ bundle of straight logs Difficult to move branch rather than log. 3D entanglements of branches→ difficult to move→increase in strength Schematic illustration of polymer chains. (a) Linear structure-- thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures. (b) Branched structure, such as in polyethylene
    • Cross-linked polymers • Thermosets or thermosetting plastics 3D structure, adjacent chains linked by covalent bonds • Increase hardness, strength, stiffness, brittleness, better dimensional stability Network polymers • Spatial, 3D networks of three or more active covalent bonds • Highly cross-linked polymers = network polymer • Cross-linking thermoplastics polymers → by high-energy radiation (UV, X-rays, e- beams) → increase in strength
    • Polymers are generally amorphous The chain exist without long-range order (like bowl of spaghetti, or worms in a bucket, all intertwined with each other) Crystallinity in polymers • modify the characteristics • fostered during synthesis or deformation in subsequent process Crystallites → Crystalline region in polymers Formed when long molecules arrange themselves in an orderly manner Semicrystalline polymer 2 phase material (crystalline + amorphous) Different degree of crystallinity can be impart by controlling: • Rate of solidification during cooling • Chain structure Degree of crystallinity affected by branching: The higher the crystallinity, the harder, stiffer, and less ductile the polymer.
    • 1. S. Kalpakjian, S. R. Schmid, “Manufacturing Engineering and Technology”, 6th ed, 2010