Polymers Classification and Properties

Types of Polymers

• Syndiotactic polymers: substituents alternate from one side to another in a regular manner
• Atactic polymers: random arrangement of substituent groups
• Isotactic polymers: substituents are arranged in a regular manner, but not alternating

Tg (Glass Transition Temperature)

• Tg of isotactic > syndiotactic > atactic
• Tg can be used to evaluate the flexibility of a polymer and predict its response to mechanical stress
• Many polymers show an abrupt change in their physical properties at their glass transition temperature
• Properties that change at Tg: coefficient of thermal expansion, heat capacity, refractive index, mechanical damping, modulus of elasticity, and electrical properties
• Knowledge of Tg is useful in choosing appropriate temperature for fabrication of polymer materials

Structure-Property Relationship of Polymers

• Crystallinity:
  • Polymers contain both crystalline and amorphous regions
  • Percentage of crystallinity depends on the structure (linear, branched, polar groups) and configuration (stereo regularity)
  • Linear polymers, homopolymers, and polar groups increase crystallinity, while branched, copolymers, and bulky side groups decrease crystallinity
  • High crystallinity polymers exhibit high tensile strength, impact resistance, high density, and sharp melting point
• Tensile strength:
  • Influenced by molecular weight of polymers
  • Tensile strength and impact strength increase with molecular weight up to ~20,000 and then become constant
  • Every polymer has a threshold molecular weight (~DP=30) value below which it does not possess useful strength
  • High molecular weight polymers are tough, hard, heat resistant, and difficult to process
• Chemical resistivity:
  • Chemical attack on polymers involves softening, swelling, and loss of strength
  • Applications: light fixtures, aircraft windows, paints, adhesives, artificial eyes, and teeth

Urea-Formaldehyde Resins (UF Resin)

• Synthesis: condensation of urea with formaldehyde in presence of a base
• Properties:
  • Clear, water-white products with good tensile strength and electrical insulation
  • Good chemical resistance, hardness, and abrasion resistance
  • Amorphous and transparent plastic with outstanding shape formation
• Applications:
  • Bonding in grinding wheels, as cation-exchange resins, binder for glass fibre, rock, wool, and plywood

Elastomers (Rubbers)

• Definition: high molecular weight polymers with elastic properties
• Properties:
  • Can undergo deformation under stress and regain original shape when stress is released
  • Due to coiled nature of chains in them
• Natural Rubber:
  • Found in several species of rubber trees
  • Virtually no practical utility value due to many deficiencies
• Synthetic Rubbers:
  • Nitrile Rubber (Buna-N or Europrene): synthesized by Emulsion Polymerization of butadiene and acrylonitrile monomers
  • Properties:
    • High tensile strength, excellent resistance to heat, sunlight, oils, fats, and organic solvents
    • Less resistant to alkalis due to presence of base labile -CN group
  • Applications:
    • Nuclear, medical, and aeronautical industry applications
    • Sealant, expanded foam, floor mats, non-latex gloves, automotive transmission belts, gaskets

Polymer Composites

• Ideal structural material should have: low density, high strength and stiffness, corrosion resistance, and abrasion and impact resistance
• No single metal, alloy, ceramic, or polymeric material can offer combination of these properties
• Biodegradable polymers:
  • Polyglycolide (PGA): characterized by hydrolytic instability due to the presence of the ester linkage in its backbone
  • Degradation process: erosive and appears to take place in two steps
  • Applications: tissue engineering, drug delivery, biomedicine, sutures, dental devices, orthopaedic fixation devices, tissue engineering scaffolds, biodegradable vascular stents, and biodegradable soft tissue anchor