NCERT Books Free Download for Class 12 Chemistry Chapter 15 - Polymers

To differentiate between thermoplastic and thermosetting polymers in an exam, present your answer in a clear, point-wise format. Key differences to mention are:

• Structure: Thermoplastics are linear or slightly branched polymers, whereas thermosetting polymers have a cross-linked or heavily branched network structure.
• Effect of Heat: Thermoplastics soften on heating and harden on cooling, and this process can be repeated. Thermosetting polymers undergo permanent chemical change on heating and become infusible solids that cannot be remoulded.
• Intermolecular Forces: Forces in thermoplastics are intermediate, while in thermosetting polymers, strong covalent bonds form the network.
• Examples: Provide examples like Polythene and Polystyrene for thermoplastics, and Bakelite and Melamine for thermosetting polymers.

The monomers for these two important polymers are:

• Nylon 6,6: It is a condensation copolymer formed from two monomers: Hexamethylenediamine (H₂N−(CH₂)₆−NH₂) and Adipic acid (HOOC−(CH₂)₄−COOH).
• Bakelite: It is a condensation polymer formed from Phenol (C₆H₅OH) and Formaldehyde (HCHO).

Vulcanisation is a chemical process for improving the properties of natural rubber. The process involves heating raw rubber with sulphur at a temperature range of 373 K to 415 K. Sulphur forms cross-links at the reactive sites of the double bonds in the isoprene units, creating a more rigid and stable structure.

Importance: This process is crucial because it enhances the rubber's elasticity, tensile strength, and resistance to abrasion and temperature changes, making it suitable for manufacturing tyres and other durable rubber products.

The fundamental difference lies in how the monomers combine:

• In Addition Polymerisation, monomers (usually unsaturated compounds like alkenes) add to one another in such a way that the polymer is formed without the loss of any small molecules. The empirical formula of the polymer is the same as its monomer. Example: The formation of Polythene from ethene.
• In Condensation Polymerisation, two or more bifunctional monomers react to form a larger structural unit while eliminating small molecules like water (H₂O), ammonia (NH₃), or hydrochloric acid (HCl). Example: The formation of Nylon 6,6 from hexamethylenediamine and adipic acid with the loss of water molecules.

The differences in properties arise directly from their synthesis and resulting structure:

• LDPE is prepared by polymerising ethene under high pressure (1000-2000 atm) and high temperature (350-570 K), leading to a highly branched structure. This branching prevents chains from packing closely, making LDPE flexible, tough, and a poor conductor of electricity.
• HDPE is prepared using a catalyst (like Ziegler-Natta catalyst) at low pressure (6-7 atm) and low temperature (333-343 K). This results in linear chains that pack closely together, giving HDPE a high density, greater toughness, and hardness.

The classification depends on their molecular structure and response to heat. Bakelite is a thermosetting polymer because its monomers (phenol and formaldehyde) form extensive covalent cross-links during heating. This creates a rigid, three-dimensional network that cannot be broken down and reformed by reheating. In contrast, Polythene is a thermoplastic because it consists of long, linear polymer chains held together by weaker intermolecular forces. When heated, these forces weaken, allowing the chains to slide past each other, making the material soft and mouldable. The process is reversible upon cooling.

The primary significance of biodegradable polymers is their ability to be decomposed by microbial action, helping to solve the environmental problems caused by the accumulation of non-biodegradable polymer waste. A common example is PHBV (Poly-β-hydroxybutyrate-co-β-hydroxyvalerate). It is a copolymer formed by the condensation of two monomers: 3-hydroxybutanoic acid and 3-hydroxypentanoic acid.