A newly developed thermoplastic polymer has a hydrocarbon backbone with a pendant amide group (−CONH2) attached to every second carbon atom. Tensile testing of an oriented sample gives the following room-temperature properties and their temperature dependence:
Tensile strength at 25∘C:60MPaTensile strength at 80∘C:35MPaTensile strength at 130∘C:5MPa
No covalent cross-linking is present, and the polymer is fully recyclable by melt processing.
(a) Identify the predominant type(s) of intermolecular force that contribute to the high tensile strength observed at 25∘C and specify the interacting functional groups.
(b) Explain why the tensile strength decreases sharply with increasing temperature even though the covalent backbone bonds remain intact.
(c) Suggest one structural modification to the repeat unit (other than covalent cross-linking) that would be expected to increase the heat resistance of the material. Justify your suggestion by describing how it alters the balance and/or strength of intermolecular forces.
(d) Predict how your proposed modification in part (c) would influence (i) the density of the polymer and (ii) the ease with which the material can be remelted for recycling, commenting on any trade-offs that may result.