  Molecular dynamics simulations are used to investigate the conformations of a
single polymer chain, represented by the Kremer-Grest bead-spring model, in a
solution with a Lennard-Jones liquid as the solvent when the interaction
strength between the polymer and solvent is varied. Results show that when the
polymer-solvent interaction is unfavorable, the chain collapses as one would
expect in a poor solvent. For more attractive polymer-solvent interactions, the
solvent quality improves and the chain is increasingly solvated and exhibits
ideal and then swollen conformations. However, as the polymer-solvent
interaction strength is increased further to be more than about twice of the
strength of the polymer-polymer and solvent-solvent interactions, the chain
exhibits an unexpected collapsing behavior. Correspondingly, for strong
polymer-solvent attractions, phase separation is observed in the solutions of
multiple chains. These results indicate that the solvent becomes effectively
poor again with very attractive polymer-solvent interactions. Nonetheless, the
mechanism of chain collapsing and phase separation in this limit differs from
the case with a poor solvent rendered by unfavorable polymer-solvent
interactions. In the latter, the solvent is excluded from the domain of the
collapsed chains while in the former, the solvent is still present in the
pervaded volume of a collapsed chain or in the polymer-rich domain that phase
separates from the pure solvent. In the limit of strong polymer-solvent
attractions, the solvent behaves as a glue to stick monomers together, causing
a single chain to collapse and multiple chains to aggregate and phase separate.
