• Polymer- chain of molecules; means “many units”
  • Typical molecular weight: 10,000 - 1,000,000 g/mol
  • Most are hydrocarbons and, thus, organic
• Polymerization- process by which small molecules (monomers or oligomers) are joined to form giant molecules
  • Monomer- one unit
  • Oligomers- multi-unit
• Plastics- polymers that may contain additives such as fibers, fillers, pigments, etc.
  • 3 main classes of plastics:
    1. Thermoplastics
    2. Thermosets
    3. Elastomers
• Branching of polymers:
  1. Linear unbranched- plain chains, straight, strong bonds w/in chain, weak forces (van der Waals) between chains
  2. Linear branched- branches off plain chain, strong bonds w/in chain, weak forces (VDW) between chains (but slightly stronger than linear unbranched)
  3. Thermoset-unbranched- plain chains are strongly chemically bonded together (cross-linked)
     • Strong (probably covalent) bonds mean that the material is brittle
  4. Thermoset-branched- branched chains are strongly chemically bonded together (cross-linked)

Classification of Polymers

1. Thermoplastics

• Linear or branched molecules (a. or b. above)
• Weak van der Waals between chains
  • Chains can slip past each other, causing thermoplastics to exhibit ductile, plastic behavior
  • Soften and melt upon heating
    • Easily recycled
• Usually amorphous

1. Thermosets

• Cross-linked or 3D network structure (c. or d. above); causes structure to be more crystalline and dense and increases strength of intermolecular forces
  • Stronger but more brittle than thermoplastics
  • Decompose upon heating as opposed to melting (because bonds will just break)
    • Difficult to recycle

1. Elastomer

• Coil-like molecules
• Linear, branched with occasional cross-links
  • Allows molecules to sustain large elastic deformation (>200%)
    • Coils straighten when a load is applied and are pulled back into place by the rest of the cross-linked structure when the load is removed
• Ex: natural rubber

Characteristics Among Polymers

• Stress-strain curve
  • Brittle polymers: high yield strength, no plastic regime, low ductility
    • Includes glassy polymers and low temp thermosets
  • Ductile polymers: large plastic deformation region
    • Includes semi-crystalline polymers, plastics, and high-temperature thermoplastics
  • Highly elastic: large, usually non-linear plastic regime
    • Includes elastomer
• Polymers can be represented by solid structures, 3D models, 2D models, Benzene rings, or (most commonly) a repeating unit
• Polymers are so prevalent in everyday life because you can change the repeating monomers to have different properties for different purposes. This range and changing of properties is not available in other materials.
  • Ex: polyethylene (PE), (C2H4)n can be modified in the following ways:
    • Replace hydrogen with chlorine —> polyvinyl chloride (PVC); chlorine forms negative dipole, and hydrogen becomes positive dipole; polymer becomes hydrophobic
    • Replace hydrogen with benzene ring —> polystyrene (PS); benzene is bulky, so polymer has low density
    • Replace hydrogen with branched carbon and hydrogen group —> polypropylene; branching of side group increases strength of polymer

Polymerization