Polymers Classification and Properties

Study Notes

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 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

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

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

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

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

Podcast