Chemistry · Ch 15 — Introduction to Polymer Chemistry
Molecular mass and degree of polymerization of polymers
Molecular mass and degree of polymerization of polymers
A bulk sample of a polymer is never made of chains of one single, exact length -- it is a complex mixture of molecules with a whole range of molecular masses. Because of this, 'the' molecular mass of a polymer is really an average over all its constituent molecules. This average molecular mass depends directly on the degree of polymerization, DP, defined as the number of monomer units strung together in a polymer molecule. Since most of a polymer's mechanical properties depend on its molecular mass, there are two undesirable extremes: a polymer kept at too low a molecular mass is brittle and mechanically weak, while a polymer allowed to reach a very high molecular mass becomes tough but unmanageable to process. A useful polymer must therefore exceed some minimum molecular mass -- corresponding to a critical degree of polymerization -- to show the properties needed for its intended application, which is why the polymerization process is deliberately stopped/controlled once that stage is reached rather than being allowed to run indefinitely. Polymers whose chains are held together by strong forces s …
Worked out. Worked problem: nylon 6 is a polyamide with strong intermolecular hydrogen bonding between its -CO-NH- groups on neighbouring chains, so even relatively short chains already hold together strongly enough for the required mechanical properties -- giving it a low critical degree of polymerization. Polythene chains, by contrast, are held together only by weak van der Waals forces, so much longer chains (a higher critical DP) are needed before enough cumulative intermolecular attraction builds up to give adequate mecha …