Molecular dynamics simulations are used to study the coil-globule transition for a system composed of a bead-spring polymer immersed in an explicitly modeled solvent.Two different versions of the model are used,which are differentiated by the nature of monomer-solvent,solvent-solvent,and nonbonded monomer-monomer interactions.For each case,a model parameter X determines the degree of hydrophobicity of the monomers by controlling the degree of energy mismatch between the monomers and solvent particles.We consider a X-driven coil-globule transition at constant temperature.The simulations are used to calculate average static structure factors,which are then used to determine the scaling exponents of the system in order to determine the thetha-point values gamma_(thetha) separating the coil from the globule states.For each model we construct coil-globule phase diagrams in terms of gamma and the particle density p.The results are analyzed in terms of a simple Flory-type theory of the collapse transition.The ratio of gamma_(thetha) for the two models converges in the high density limit exactly to the value predicted by the theory in the random mixing approximation.Generally,the predicted values of gamma_(thetha) are in reasonable agreement with the measured values at high p,though the accuracy improves if the average chain size is calculated using the full probability distribution associated with the polymer-solvent free energy,rather than merely using the value obtained from the minimum of the free energy.
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