polymerization and structure of polymers

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  • 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) HO 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 moveincrease 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


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