Unit 5: Polymers
Course Code: CHM-DSC-351
Paper Name: Advance Materials
1. Classification of Polymers
Polymers are high-molecular-weight compounds (macromolecules) formed by the chemical combination of small repeating units called monomers.
Basis of Classification
- Source: Natural (Starch, Cellulose), Synthetic (PVC, Nylon), and Semi-synthetic.
- Structure: Linear, Branched, and Cross-linked (Network) polymers.
- Mode of Polymerization: Addition (Chain-growth) and Condensation (Step-growth).
- Molecular Forces: Elastomers, Fibres, Thermoplastics, and Thermosetting plastics.
2. Mechanism of Polymerization
The chemical process of linking monomers is governed by two major mechanisms.
Addition Polymerization
Involves the repeated addition of monomers possessing double or triple bonds. No side products are formed. It often follows a Free Radical Mechanism consisting of:
- Chain Initiation
- Chain Propagation
- Chain Termination
Condensation Polymerization
Involves the reaction between bifunctional or polyfunctional monomers with the elimination of small molecules like H2O or NH3. Examples include Nylon-6,6 and Terylene.
3. Molecular Weight of Polymers
Unlike simple molecules, polymers consist of chains of varying lengths, so we calculate average molecular weights.
- Number Average Molecular Weight (Mn): Total weight divided by the total number of molecules.
- Weight Average Molecular Weight (Mw): Based on the mass contribution of each chain size.
- Polydispersity Index (PDI): The ratio Mw/Mn. If PDI = 1, the polymer is monodisperse (all chains are the same length).
4. Conducting Polymers
Traditionally, polymers are insulators. However, certain polymers with conjugated double bonds can conduct electricity.
- Examples: Polyacetylene, Polypyrrole, and Polyaniline.
- Mechanism: Conduction is possible due to the delocalization of pi-electrons along the polymer backbone.
- Doping: The conductivity can be increased by several orders of magnitude through "doping" (adding electron donors or acceptors).
These polymers are engineered to maintain their properties under extreme mechanical and thermal stress.
Kevlar
- Nature: A para-aramid synthetic fibre.
- Properties: High tensile strength-to-weight ratio (5 times stronger than steel).
- Uses: Bulletproof vests, racing tires, and aerospace components.
Nomex
- Nature: A meta-aramid polymer related to Kevlar.
- Properties: Excellent thermal, chemical, and radiation resistance. It does not melt.
- Uses: Firefighter suits, pilot uniforms, and electrical insulation.
6. Exam Focus: Tips and FAQs
Exam Tips:
- Differentiation: Be ready to distinguish between Thermoplastics (can be remolded) and Thermosetting plastics (cannot be remolded once set).
- PDI: Remember that for synthetic polymers, Mw is always greater than or equal to Mn.
- Structural Comparison: Notice that Kevlar has para-linkages while Nomex has meta-linkages on the benzene rings.
Frequently Asked Questions
Q: Why is Polydispersity Index (PDI) important?
A: It indicates the breadth of the molecular weight distribution. A high PDI means a wide variety of chain lengths, which affects the physical properties of the polymer.
Q: What makes Polyacetylene a conducting polymer?
A: The presence of alternating single and double bonds (conjugation) allows electrons to move along the chain.
Q: Give an example of a natural polymer.
A: Cellulose, which is the primary structural component of green plants.