A lattice model Monte Carlo study of coil-to-globule and other conformational transitions of polymer, amphiphile, and solvent

A model of polymer-amphiphile-solvent systems on a cubic lattice is used to investigate the phase diagram of such systems. The polymer is treated within the canonical ensemble (T,V,N) and the amphiphile and solvent are treated within the grand canonical ensemble (T,V,mu). Using a range of Monte Carlo moves the phase diagram of polymer-amphiphile-solvent mixtures, as a function of solvent quality (parametrized by chi) and relative chemical potential, mu, is studied for the dilute polymer limit. The effect of increasing the polymer chain length, N, on the critical aggregation concentration (CAC), and the type of polymer-amphiphile complex formed above the CAC are also examined. For some parameters, it is found that the polymer and amphiphile form a polymer-micelle complex at low amphiphile concentrations, and that the polymer coil-to-globule transition point increases with increasing amphiphile concentration. The resulting collapsed globule has a solvent core and is surrounded by a layer of amphiphile. These results are in good qualitative agreement with experimental results for the poly(N-isopropylacrylamide) (PNIPAM)/sodium dodecyl sulfate (SDS) system. At higher amphiphile concentrations, the polymer and amphiphile form several layered structures depending on the strength of the three-body amphiphilic interactions, l. Finally, the effect of the polymer chain length, N, and the strength of the three-body amphiphilic interactions, l, on the stability of the polymer-amphiphile structures is investigated. (C) 2000 American Institute of Physics. [S0021-9606(00)51817-0].