Q5. 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, polyvinyl, polystyrene.
Answer: Polythene, polyvinyls, and polystyrene soften on heating and harden on cooling. These polymers are a linear array of slightly branched long-chain molecules. Such polymers are called thermoplastic polymers. These possess intermolecular forces whose strength lies between the intermolecular forces of elastomers and fibre. At the same time, bakelite urea and formaldehyde resin are thermosetting polymers that undergo extensive cross-linking in moulds and become infusible on heating.
