Question

Which of the following polymers soften on heating and harden on cooling? What are the polymers with this property collectively called? What are the structural similarities between such polymers?

Bakelite, urea-formaldehyde resin, polythene, polyvinyls, polystyrene.

Answer 1

Defining thermoplastic polymer

The polymers which soften on heating and harden on cooling are called thermoplastic polymers.

Properties of thermoplastic polymers

• Thermoplastic polymers are the linear or slightly branched long-chain molecules capable of repeatedly softening on heating and hardening on cooling.
• The polymers with this property can be collectively called plastics.
• These polymers possess intermolecular forces of attraction intermediate between elastomers and fibres.
• Some common thermoplastics are polythene, polystyrene, polyvinyl etc.

Structural similarities between thermoplastics.

• Polythene, polyvinyl and polystyrene are thermoplastic polymers and can be reused.
• Thermoplastic polymers do not have an extensive network structure. These are mostly chain type polymers with some branching.
• On the other hand, polymers like Bakelite and urea-formaldehyde resin are thermosetting polymers.
• These polymers are cross-linked or heavily branched molecules, which on heating undergo extensive cross linking in moulds and become infusible. They cannot be reused.