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 with in the canonical ensemble (T,V,N) and the amphiphile and solvent are treate d within the grand canonical ensemble (T,V,mu). Using a range of Monte Carl o moves the phase diagram of polymer-amphiphile-solvent mixtures, as a func tion of solvent quality (parametrized by chi) and relative chemical potenti al, mu, is studied for the dilute polymer limit. The effect of increasing t he 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 t he polymer coil-to-globule transition point increases with increasing amphi phile concentration. The resulting collapsed globule has a solvent core and is surrounded by a layer of amphiphile. These results are in good qualitat ive agreement with experimental results for the poly(N-isopropylacrylamide) (PNIPAM)/sodium dodecyl sulfate (SDS) system. At higher amphiphile concent rations, the polymer and amphiphile form several layered structures dependi ng 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-bo dy amphiphilic interactions, l, on the stability of the polymer-amphiphile structures is investigated. (C) 2000 American Institute of Physics. [S0021- 9606(00)51817-0].