2 - Structure of Polymers

  • Introduction to Polymers

    • Polymers are made of repeating units called MERs (mer units).

    • Types of polymer structures:

    • One-Dimensional (1D) Chains:

      • Known as thermoplastic polymers.

    • Characteristics: Can be softened and reshaped upon heating.

    • Example: Polyethylene.

    • Example: Polypropylene.

    • Three-Dimensional (3D) Networks:

      • Known as thermoset polymers.
      • Characteristics: Cured into a rigid structure and cannot be remolded.
      • Example: Melamine-formaldehyde resin.

    • Rarely, 2D sheet structures exist, where covalent bonding occurs in-plane with secondary bonding between sheets.

  • Chemical Composition of Polymers

    • The backbone of most polymers is typically a carbon atom due to its four valence electrons.
    • Two electrons bond in the chain.
    • Other electrons bond to side chains or contribute to cross-linking.
    • Silicon can also act as a backbone due to its position in the periodic table, capable of forming silicon polymers.
    • Lewis Structure Example:
    • Representation of carbon showing four valence electrons as dots.
    • A 1D thermoplastic structure arises if all valence electrons bond to side chains; if one bonds to the backbone, it results in a thermoset structure.

  • Types of Organic Polymers

    • Hydrocarbon Polymers:
    • Composed only of carbon and hydrogen.
    • Example: Polyethylene
      • Derived from ethylene gas through breaking the double bond between carbon atoms, leading to long chains.
      • Example of fluctuating molecular weight within polyethylene due to chain length variability.
    • Molecular Weight Characteristics:
      • Distinct starting/ending points for thermoplastics & no defined start/stop for thermosets.
      • Molecular weight influences mechanical properties (strength, resistance).

  • Examples of Hydrocarbon Polymers

    • Polypropylene:
    • Contains a methyl group (CH3) instead of one hydrogen, affecting its folding and structure.
    • Polystyrene:
    • Has benzene rings as side groups instead of methyls, significantly altering its properties.

  • Inorganic Polymers

    • Example: Silicones
    • Structure consisting of silicon and oxygen backbones with side chains typically being methyl groups (CH3).

  • Functional Elements in Polymers

    • Oxygen:
    • Example: Polymethyl methacrylate (PMMA), a copolymer featuring oxygen in side chains.
    • Chlorine:
    • Example: Polyvinyl chloride (PVC), where hydrogen is partially replaced by chlorine atoms.
    • Fluorine:
    • Example: Teflon (PTFE) has all hydrogens replaced with fluorine.
    • Nitrogen:
    • Present in Kevlar (Aramid fiber), consisting of carbon-nitrogen backbones with side groups including oxygen.

  • Copolymers

    • Example: Styrene-acrylonitrile (SAN) is a copolymer of polystyrene and polyacrylonitrile, randomly mixed.
    • Such random distributions impact properties significantly.
    • Example: Mixing SAN with styrene-butadiene creates ABS plastic.

  • Mechanical Property Comparisons

    • Comparison of polyethylene and PVC with close structural similarities.
    • Mechanical Properties:
    • Modulus: Low Density Polyethylene (LDPE) around 170 MPa; High Density Polyethylene (HDPE) nearly 1100 MPa; PVC about 2.6 GPa.
    • Tensile Strength: LDPE has lower strength (6-17 MPa), while PVC is stronger.
    • Impact Strength: LDPE is not breakable in impact tests, while HDPE and PVC are tough and withstand impacts.

  • Conclusion

    • A complexity of interlinked elemental structures in polymers contributes to vastly different physical properties.