Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): The glass transition temperature of Nylon 6 decreases on absorption of moisture. Reason (R): Nylon 6 fibre shows partial dissolution on absorption of moisture. In the light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Water is the most common plasticizer for natural and some synthetic fibers (like Nylon). This is why your clothes feel softer or "limp" when they are damp.

Answer 1

The Correct Option is C

Concept: The Glass Transition Temperature ($T_g$) is highly sensitive to the presence of plasticizers. Plasticizers are small molecules that fit between polymer chains, increasing the space between them and making them more mobile.

Step 1: Analyzing Assertion (A).
Nylon 6 is a polyamide that contains amide groups capable of forming hydrogen bonds with water molecules. When water is absorbed, it acts as a plasticizer, reducing the intermolecular forces between the nylon chains. This makes the chains more mobile at lower temperatures, effectively decreasing the $T_g$. Thus, (A) is correct.

Step 2: Analyzing Reason (R).
Absorption of moisture causes swelling of the Nylon 6 fiber, not dissolution. Partial dissolution would imply the polymer chains are breaking away and entering a liquid state, which does not happen when nylon simply gets damp or wet. Thus, (R) is incorrect.