Molecular Dynamics Simulation of Chain Collapse of Random Ionomers in a Poor Solvent
Chain collapse and ionic aggregation of sodium salts of lightly sulphonated polystyrene (Na-SPS) in a poor solvent were simulated by molecular dynamics of a single ionomer chain in vacuum. Ionomers with a sulphonation level from 0 to 28 mol% and a polymerisation index of 50 were constructed by virtual copolymerisation of the ionic and nonionic species. Chain collapse occurred by a three-stage process. Local collapse of the ionomer occurred in the first stage, which was marked by sharp decreases in both the radius of gyration (Rg) and the chain end-to-end distance (D). The second stage was a transition to a nearly constant Rg, and it was marked by one or more local maxima of Rg and D as a function of time. During that stage, the ionic groups aggregated and coalesced. The third stage was characterised by a nearly constant Rg. No intrachain ionic aggregation was observed below a critical sulphonation level of 4.0 mol% for Na-SPS. Packing of the ionic groups was highly ordered with a correlation length between Na atoms of 4.3 Å. As the sulphonation level increased, Rg of the collapsed ionomers increased slightly and the conformation became less globule-like.