Discussion
The crystalline melting temperature of the polymers high density polyethylene (HDPE), isotactic polypropylene (iPP), nylon 6 (PA6) and poly(ethylene terephthalate) (PET) can be arranged as
Answer: B
The order is \(T_{PET} \gt T_{PA6} \gt T_{iPP} \gt T_{HDPE}\).
Typical crystalline melting points are about 260°C for PET, 220°C for nylon 6, 165°C for isotactic polypropylene and 130-135°C for HDPE.
The aromatic rings in the PET backbone stiffen the chain and the hydrogen bonding between amide groups holds nylon chains together, so both melt high. Polypropylene's methyl groups stiffen its helix compared with polyethylene, whose flexible, non-polar chains melt lowest. The other orders put HDPE or iPP above the polar or aromatic polymers, which is the reverse of the actual trend.
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