Question

Thermoplastic Polyurethane and Polyamide 6 both contain amide linkages, but when compared to Polyurethane, Polyamide 6 shows higher melting point due to .................

• A. Higher molecular rigidity
• B. Higher degree of branching
• C. Lower molecular rigidity
• D. Lower degree of crosslinking

Hint:

Polyamides (Nylons) have higher melting points than polyurethanes because of stronger hydrogen bonding and higher molecular rigidity.

Answer 1

The Correct Option is A

Step 1: Compare structures.
- Polyurethane: Contains urethane linkages (–NH–CO–O–). These groups allow hydrogen bonding but the presence of –O– linkages increases flexibility. Hence, rigidity is lower.
- Polyamide 6 (Nylon-6): Contains amide linkages (–NH–CO–) in a highly regular and linear structure. The –CONH– groups strongly hydrogen bond with adjacent chains, giving high chain rigidity.

Step 2: Effect on melting point.
- High molecular rigidity and strong intermolecular hydrogen bonding increase crystallinity and thus the melting point.
- Therefore, Nylon-6 has a higher melting point than Polyurethane.

Step 3: Eliminate incorrect options.
- (B) Higher degree of branching → branching usually lowers melting point.
- (C) Lower molecular rigidity → contradicts the fact.
- (D) Lower degree of crosslinking → irrelevant, since both are thermoplastics. Final Answer: \[ \boxed{\text{(A) Higher molecular rigidity}} \]