JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV POLYMERIC MATERIALS Previous Question Paper (March - 2021 - R16)

Polymerisation is the chemical process by which small molecules, known as monomers, react together to form long-chain or three-dimensional network structures called polymers. This process involves the formation of covalent bonds between repeating units.

There are two primary types of polymerisation:

• Addition Polymerisation (Chain-growth): Monomers containing double or triple bonds (unsaturated) add to each other without the loss of any small molecules. The reaction typically proceeds via free radical, cationic, or anionic mechanisms. Example: Formation of polyethylene from ethylene (CH2=CH2).
• Condensation Polymerisation (Step-growth): Monomers containing two or more functional groups react with the elimination of small molecules like water, methanol, or hydrogen chloride. Example: Formation of Nylon 6,6 from hexamethylenediamine and adipic acid.

The Glass Transition Temperature (Tg) is the temperature range at which an amorphous polymer transitions from a hard, glassy, brittle state to a soft, rubbery, or viscous state. Below Tg, polymer chains are essentially frozen in place with only vibrational motion. Above Tg, the polymer gains sufficient thermal energy to allow for long-range segmental motion of the polymer backbone.

The transition is observed by plotting specific volume or enthalpy against temperature. As the temperature increases, the slope of the curve changes significantly at Tg. In the glassy region, the slope is lower due to restricted chain mobility; in the rubbery region, the slope increases as the free volume expands and chain segments gain mobility.

Diagrammatically, this is represented as a plot where the y-axis is Specific Volume and the x-axis is Temperature. The curve starts with a linear increase in the glassy state, undergoes a distinct change in slope at the Tg point, and continues with a steeper linear increase in the rubbery state. This transition is critical for determining the service temperature range of plastics and elastomers.