Which of the following would you expect to be elastomers and which thermosetting polymers at room temperature? Justify each choice. (a) Linear and highly crystalline polyethylene (b) Phenol-formaldehyde (c) Heavily crosslinked polyisoprene having a glass-transition temperature of \(50^{\circ} \mathrm{C}\) \(\left(122^{\circ} \mathrm{F}\right)\) (d) Lightly crosslinked polyisoprene having a glass-transition temperature of \(-60^{\circ} \mathrm{C}\) \(\left(-76^{\circ} \mathrm{F}\right)\) (e) Linear and partially amorphous poly(vinyl chloride)

Elastomers typically have lightly crosslinked structures and low glass-transition temperatures, which means they are highly flexible, stretchable, and recover their shape when the force is removed. In contrast, thermosetting polymers have a highly crosslinked structure, making them rigid and resistant to melting.

(a) Linear and highly crystalline polyethylene: Since it is linear and crystalline, it has no crosslinks and its structure is ordered. This material is neither an elastomer nor a thermosetting polymer. (b) Phenol-formaldehyde: These polymers have a highly crosslinked structure and are generally rigid, which indicates that it is a thermosetting polymer. (c) Heavily crosslinked polyisoprene having a glass-transition temperature of \(50^{\circ}\mathrm{C}\) \(\left(122^{\circ}\mathrm{F}\right)\): Given that it is heavily crosslinked and its glass-transition temperature is above room temperature, it will be a thermosetting polymer. (d) Lightly crosslinked polyisoprene having a glass-transition temperature of \(-60^{\circ}\mathrm{C}\) \(\left(-76^{\circ}\mathrm{F}\right)\): This material is lightly crosslinked and has a low glass-transition temperature, indicating that it is an elastomer at room temperature. (e) Linear and partially amorphous poly(vinyl chloride): This material has a linear structure and partially amorphous nature. This material is neither an elastomer nor a thermosetting polymer.

Based on the analysis: (a) Linear and highly crystalline polyethylene: Neither an elastomer nor a thermosetting polymer. (b) Phenol-formaldehyde: Thermosetting polymer. (c) Heavily crosslinked polyisoprene having a glass-transition temperature of \(50^{\circ} \mathrm{C}\) \(\left(122^{\circ} \mathrm{F}\right)\): Thermosetting polymer. (d) Lightly crosslinked polyisoprene having a glass-transition temperature of \(-60^{\circ} \mathrm{C}\) \(\left(-76^{\circ} \mathrm{F}\right)\): Elastomer. (e) Linear and partially amorphous poly(vinyl chloride): Neither an elastomer nor a thermosetting polymer.