ch14

• Polymer


  A large molecule composed of many smaller, repeating structural units called monomers
• Naturally occurring polymers


  • Derived from plants and animals, include wood, rubber, cotton, wool, leather, silk, proteins, enzymes, starches, cellulose
• Synthetic polymers


  • Produced from small organic molecules, include plastics, rubbers, fiber materials
• Since the conclusion of World War II, the field of materials has been virtually revolutionized by the advent of synthetic polymers</b>
• Synthetic polymers can be produced inexpensively, and their properties may be managed to the degree that many are superior to their natural counterparts
• In some applications, metal and wood parts have been replaced by plastics, which have satisfactory properties and can be produced at a lower cost
• Polymer structure


  Intricately related to the structural elements of the material
• Hydrocarbon


  An organic material composed of hydrogen and carbon
• Covalent bond


  A chemical bond formed by the sharing of one or more pairs of electrons between atoms
• Types of hydrocarbon bonds

  • Single
  • Double
  • Triple
• Unsaturated hydrocarbon


  A hydrocarbon molecule that has double or triple covalent bonds, allowing for the attachment of additional atoms or groups
• Saturated hydrocarbon


  A hydrocarbon molecule with only single covalent bonds, where no new atoms may be joined without the removal of others already bonded
• Examples of paraffin hydrocarbons

  • Methane
  • Ethane
  • Propane
  • Butane
  • Pentane
  • Hexane
• Isomerism


  The phenomenon where hydrocarbon compounds with the same composition have different atomic arrangements
• Isomers of butane


  • Normal butane
  • Isobutane
• Macromolecule


  A very large molecule, such as those found in polymers
• Repeat unit


  The structural entity that is successively repeated along a polymer chain
• Monomer


  The small molecule from which a polymer is synthesized
• Polymerization of ethylene


  1. Reaction of ethylene monomer with initiator/catalyst to form active center

  2. Sequential addition of monomer units to growing chain
• Polytetrafluoroethylene (PTFE)


  Also known as Teflon, belongs to a family of polymers called the fluorocarbons
• Vinyl chloride monomer


  A slight variant of the ethylene monomer, in which one of the four H atoms is replaced with a Cl atom
• Polymerization of vinyl chloride


  Leads to poly(vinyl chloride) (PVC)
• Generalized polymer structure


  1. H-H-H-R-C (where R depicts either an atom [H or Cl] or an organic group such as CH3, C2H5, C6H5)
• Polymers represented by generalized structure

  • Polyethylene, poly(vinyl chloride), polypropylene
• Repeat units for common polymers

  • Nylon, polyester, polycarbonate
• Homopolymer


  A polymer where all the repeating units along a chain are of the same type
• Copolymer


  A polymer composed of two or more different repeat units
• Bifunctional monomer


  A monomer that can form two covalent bonds with other monomers, forming a two-dimensional chainlike molecular structure
• Trifunctional monomer


  A monomer that can form three active bonds, resulting in a three-dimensional molecular network structure
• Extremely large molecular weights are observed in polymers with very long chains
• Number-average molecular weight (Mn)


  Obtained by dividing the chains into a series of size ranges and determining the number fraction of chains within each size range
• Weight-average molecular weight (Mw)


  Based on the weight fraction of molecules within the various size ranges
• Degree of polymerization (DP)


  The average number of repeat units in a polymer chain, related to the number-average molecular weight and the repeat unit molecular weight
• Many polymer properties are affected by the length of the polymer chains
• Polymer molecules can bend, coil, and kink in three dimensions due to the rotational flexibility of the backbone bonds
• Polymer chains extensively intertwine and entangle, leading to important characteristics like large elastic extensions in rubber materials
• Rotational flexibility


  Depends on repeat unit structure and chemistry, e.g. double bonds and bulky side groups restrict rotation
• Polymer chain structure


  • Random coils and molecular entanglements
  • Responsible for large elastic extensions in rubber materials
• Rotational flexibility


  Ability of chain segments to experience rotation in response to applied stresses or thermal vibrations
• Factors affecting rotational flexibility


  • Repeat unit structure and chemistry
  • Presence of double bonds (CC) restricts rotation
  • Introduction of bulky side groups restricts rotation