Unit 5 Notes - DSM 351 Chemistry Semester 6

Course Code: CHM-DSM-351

Paper Name: Advance Materials (Minor)

Minor Focus: This final unit explores how small molecules (monomers) link together to form giant chains (polymers) and how these structures define everyday materials like plastics and high-tech fibers.

1. Introduction and Classification

Polymers are macromolecules formed by the repeated joining of small units called Monomers. This process is called Polymerization.

Classification by Structure:

Linear: Long straight chains (e.g., PVC, HDPE).
Branched: Chains with side branches (e.g., LDPE).
Cross-linked: Chains joined together to form a 3D network (e.g., Bakelite, Vulcanized rubber).

[Image showing linear, branched, and cross-linked polymer structures]

2. Thermoplastics vs. Thermosetting Plastics

The way a polymer reacts to heat determines its industrial use:

Feature	Thermoplastics	Thermosetting Plastics
Action of Heat	Soften on heating, harden on cooling.	Do not soften; degrade on excessive heating.
Reusability	Can be remolded and recycled.	Cannot be remolded once set.
Structure	Usually linear or branched.	Heavily cross-linked (network).
Examples	Polythene, PVC, Polystyrene.	Bakelite, Urea-formaldehyde resin.

3. Mechanisms of Polymerization

Addition (Chain-Growth) Polymerization

Monomers with double or triple bonds add to one another without the loss of any molecules. Example: Ethene → Polythene.

Condensation (Step-Growth) Polymerization

Monomers react with the elimination of small molecules like water or ammonia. Example: Hexamethylenediamine + Adipic Acid → Nylon-6,6 + H₂O.

4. Molecular Weight of Polymers

Since polymer chains are not all the same length, we use statistical averages:

Number Average Molecular Weight (Mn): Based on the number of molecules.
Weight Average Molecular Weight (Mw): Based on the total mass contribution.
PDI (Polydispersity Index): The ratio Mw/Mn. For natural polymers (like DNA), PDI is often 1. For synthetic polymers, PDI is always > 1.

5. Special Polymers

Conducting Polymers

Usually, plastics are insulators. However, polymers with conjugated double bonds (alternating single and double bonds) can conduct electricity. Example: Polyacetylene.

High-Performance Fibers

Kevlar: Used in bulletproof vests and helmets due to extreme tensile strength.
Nomex: Excellent heat resistance; used in firefighter suits and racing gear.

6. Exam Focus: Tips and FAQs

Exam Tip: The difference between Thermoplastics and Thermosetting plastics is a "High Probability" 5-mark question. Practice the table!

Frequently Asked Questions

Q: What is a monomer?
A: A small molecule that can react with others of the same type to form a polymer chain.

Q: Why is Vulcanized Rubber stronger than Natural Rubber?
A: Vulcanization adds sulfur cross-links between polymer chains, preventing them from sliding past each other too easily.

Q: What does a PDI of 1 indicate?
A: It means the polymer is "monodisperse," implying all polymer chains in the sample have the exact same length.

Unit 5: Polymers