The influence of the quality of the solvent on the properties of a polymer

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Abstract
Computer simulations of a polymer immersed in a solvent are presented. The influence of the quality of the solvent on the static and dynamic properties of a polymer has been studied. The quality of the solvent is modelled by varying the depth (ϵmp) of the Lennard-Jones potential, which describes the interactions of the polymer segments with the monomers. Starting from an isolated chain (ϵmp = 0), the introduction of a small polymer solvent interaction gives rise to a large decrease of the radius of gyration. At very low values of ϵmp the interactions between the polymer segments and monomers is mainly repulsive. Such an unattractive solvent causes the polymer chain to collapse. A further increase of ϵmp causes the radius of gyration to increase again. A simple thermodynamic model, the ‘polymer in a sack model’, is introduced to describe this behaviour of the radius of gyration quantitatively. In this model we introduce some assumptions for handling the chain statistic of the polymer. With these assumptions the calculations reduce to a classical phase equilibrium problem. It is also demonstrated that the quality of the solvent can be used as a coupling parameter in Kirkwood's coupling parameter formalism. With this formalism we were able to calculate differences in free energy.