Lecture 02: Classification and Molecular Structure

Molecular Characteristics

• Chemistry (repeat unit composition)
• Size (molecular weight)
• Shape (chain twisting, entanglement, etc.)
• Structure (Linear, Branched, Crosslinked, Network)

Polymer Molecular Structures

• Linear: Repeat units joined end to end in single chains.
  • Examples: polyethylene, poly(vinyl chloride), polystyrene.
• Branched: Side-branch chains connected to the main chains.
  • Reduces chain packing efficiency and polymer density.
  • Example: Low-density polyethylene (LDPE).
• Crosslinked: Adjacent linear chains joined by covalent bonds.
  • Achieved during synthesis or by a non-reversible chemical reaction.
  • Example: Vulcanized rubbers.
• Network: Multifunctional monomers forming three-dimensional networks.
  • Examples: Epoxies, polyurethanes, phenol-formaldehyde.

Chain Flexibility

• Key factor determining material properties.
• Influenced by:
  • Temperature
  • Types of bonds
  • Side groups
  • Branching
  • Additives

Properties and Structure

• Linear: Flexible, low density, often amorphous.
• Branched: Improved melt viscosity, unique rheological properties.
• Crosslinked: Rigid, high melting points, insoluble, high mechanical strength and thermal stability.
• Network: Extremely hard, rigid, brittle, high melting points, insoluble, exceptional strength and durability.

Thermoplastics vs. Thermosets

• Thermoplastics:
  • Soften when heated, harden when cooled (reversible).
  • Mostly linear or branched structures.
  • Examples: Polyethylene, polystyrene, poly(ethylene terephthalate), poly(vinyl chloride).
• Thermosets:
  • Permanently hard, do not soften upon heating.
  • Cross-linked and network polymers.
  • Examples: Vulcanized rubbers, epoxies, phenolics, some polyester resins.

Curing in Thermosets

• Curing methods:
  1. Temperature-activated
  2. Catalyst-activated
  3. Mixing-activated

Copolymers

• Composed of two repeat units with different sequencing arrangements.
  • Random copolymer: Units randomly dispersed along the chain.
  • Alternating copolymer: Repeat units alternate chain positions.
  • Block copolymer: Identical repeat units clustered in blocks.
  • Graft copolymer: Homopolymer side branches grafted to a different homopolymer main chain.
• Examples:
  • Styrene–butadiene rubber (SBR)
  • Nitrile rubber (NBR)
  • Impact-modified polystyrene