Module 4

• Polymers


  • Characterized by low density, strength, elastic modulus, thermal conductivity, electrical conductivity and high strength-to-weight ratio
• Polymers


  • Many design possibilities, easy to manufacture (formable), relatively low cost (production, maintenance), wide choice of colors and transparencies
• Polymers


  • Useful temperature range is low (max of 350ºC), less dimensional stability in service over a period of time
• Plastics


  Came from the Greek word "plastikos" meaning to be molded or shaped
• Sources of plastics

  • Natural organic materials (wood, rubber, cotton, wool, leather, and silk, cellulose (earliest polymers))
  • Synthetic organic polymers. Manmade; extracted from coal and petroleum products
• First polymer used: phenol-formaldehyde (thermoset), aka Bakelite


  1906
• Monomers


  Basic building block of a polymer; most are organic materials in which carbon atoms are joined with other atoms, such as H, O, N and others
• Polymers


  Long chain of molecules formed by polymerization
• Polymerization


  The process that monomer units are bonded by covalent (electron-sharing) bonds to generate giant molecules in a chain-like structure
• Condensation polymerization


  Polymers are produced by the formation of bonds between two types of reacting mers; a characteristic of this reaction is that reaction by-products (such as water) are condensed out
• Addition polymerization


  Bonding takes place without reaction by-products; it is called "chain reaction" because of the high rate at which long molecules form simultaneously
• Molecular Weight Distribution (MWD)


  Spread of the chain's molecular weight
• Degree of Polymerization (DP)


  Ratio of the molecular weight of the polymer to the molecular weight of the repeating unit (smallest repetitive unit is called a mer)
• Increase in MWD or DP

  Increase in strength, resistance to cracking, viscosity
• Increase in MWD or DP affects ease of shaping and raises overall cost of processing
• Linear polymer chains


  • Linear in structure but not necessarily straight in shape (e.g. thermoplastics such as acrylics and nylons)
• Branched polymer chains


  • Side-branch chains are attached to the main chain; branching interferes with the relative movement of the molecular chains (e.g. polyethylene)
• Cross-linked polymer chains


  • 3D in structure; have adjacent chains linked by covalent bonds (e.g. elastomers)
• Network polymer chains

  • Spatial 3D networks of 3 or more active covalent bonds; highly cross-linked polymers (e.g. thermosets)
• Homopolymer


  The repeating units in a polymer chain are all of the same type
• Copolymers


  Contain two types of polymers (e.g. styrene-butadiene which is widely used for automobile tires)
• Terpolymers


  Contain three types of polymers (e.g. acrylonitrile-butadiene-styrene which is widely used for helmets and telephones)
• Vulcanization

  Cross-linking elastomer chains by introducing sulfur or other chemicals; these additives modify the polymer by forming cross-linking (bridges) between individual polymer chains
• Crystallinity


  Polymers are generally amorphous (disordered structure); may be done either during the synthesis of the polymer or by deformation during processing
• As crystallinity increases

  Polymers become stiffer, harder, less ductile, more dense (crystallization shrinkage), less rubbery, and more resistant to solvents and heat
• Crystallite


  Crystalline regions in polymers
• Reflection of light from the boundaries between the crystalline and amorphous regions

  Causes opaqueness
• The index of refraction is proportional to density


  The greater the density difference between the amorphous and crystalline phases, the greater is the opaqueness
• Polymers that are completely amorphous can be transparent such as polycarbonate and acrylics
• Thermoplastic


  Linear or branched polymers in which chains of molecules can be weakened by increased temperature (until melting point)
• Thermoplastics


  • Polymer can be softened or molten and reformed into new shapes. Polymer returns to original hardness and strength. Process is reversible
• Thermoset


  Polymers that are heavily cross-linked to produce a strong three-dimensional network structure which cannot be softened or molten then reshaped
• Thermosets


  • More durable, harder, tough, light
• Elastomers


  A large family of amorphous polymers that can undergo large elastic deformations without rupture held together by weak intermolecular forces, generally exhibiting low Young's modulus and high yield strength or high failure strain
• Elastomers


  • Material resumes its original shape substantially when stretched or compressed and then released
• Elastomer applications

  • High-friction and nonskid surfaces, protection against corrosion and abrasion, electrical insulation, and shock and vibration insulation
• Polymer additives


  • Fillers
  • Plasticizers
  • Stabilizers
  • Colorants
  • Flame Retardants
  • Lubricants
• Fillers


  Improves mechanical properties and reduces cost
• Plasticizers


  Improves flexibility, ductility, and toughness of polymers. Their presence also produces reductions in hardness
• Stabilizers


  Protects against atmospheric oxidation or UV radiation