Hydrogen Bonding between Polymer Chains

The intermolecular forces between polymer chains control properties such as strength and melting point. AQA asks you to explain these forces for condensation polymers.Polyamides (nylon-6,6, Kevlar and proteins) have an N-H group and a C=O group in every amide link. The H is bonded to a very electronegative N atom, so it is δ+, and the O of C=O is δ− with lone pairs. Hydrogen bonds form between the N-H of one chain and the C=O of a neighbouring chain:N-H^δ+ ···· :O^δ−=CPolyesters such as Terylene have polar C=O and C-O bonds, but no O-H or N-H groups within the chain, so the chains cannot hydrogen bond to each other. Instead they are held by permanent dipole-dipole forces (as well as van der Waals forces).Polyalkenes such as poly(ethene) are non-polar and have only van der Waals forces between their chains.So, for chains of similar length, the forces between chains are generally strongest in polyamides, then polyesters, then polyalkenes. This is why polyamides such as nylon make strong fibres, and why Kevlar is exceptionally strong: its flat chains pack closely, so many hydrogen bonds line up along them.Worked example. Explain why nylon-6,6 has a higher melting point than poly(ethene) of similar chain length. Nylon-6,6 chains are held by hydrogen bonds between N-H and C=O groups, as well as van der Waals forces; poly(ethene) has only van der Waals forces. More energy is needed to separate the nylon chains.All polymers also have van der Waals forces, which increase with chain length.