Discussion
Thermoplastic Polyurethane and Polyamide 6 both contain amide linkages, but when compared to Polyurethane, Polyamide 6 shows higher melting point due to _______________.
Answer: A
The reason given by the key is higher molecular rigidity.
Polyamide 6 has an amide group after every five methylene units, regularly spaced along an otherwise simple chain. These groups make the chain stiffer and let neighbouring chains line up and hydrogen bond in a regular pattern, so the crystals melt at about 220 °C.
A thermoplastic polyurethane links its urethane groups through an extra oxygen (-NH-CO-O-), which rotates easily, and it contains long flexible soft segments of polyether or polyester. Its chains are therefore more flexible, pack less regularly and melt lower.
Lower rigidity would lower the melting point, not raise it. Branching disrupts packing and also lowers it. Both polymers are thermoplastics, so differences in crosslinking are not what separates their melting points.
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