Phase separation in polymer solutions with annealed excluded volume interactions

On the basis of an analytical mean-field model, we consider phase separatio n and coilglobule transitions in solutions of polymer chains with annealed excluded-volume interactions. In our model, the chain monomers are able to reversibly change their state from a hydrophilic to a hydrophobic one. This may mimic the behavior of polymers in solutions that contain amphiphilic m olecules, e.g., globular proteins or surfactants, that are capable of coope rative association with the polymer chains. While the bare polymer chains a re either hydrophobic or weakly hydrophilic, the formation of polymer/surfa ctant or polymer/protein complexes may strongly enhance the solubility of t he polymers in water. We predict different phase behavior for solutions of originally hydrophilic or hydrophobic polymers mixed with amphiphiles. In t he former case, the solution remains homogeneous at low concentrations of a mphiphiles, whereas in the latter case it separates into a dilute and conce ntrated phase. The Theta -transition of the complexed polymers may be induc ed by variation either of the concentration of amphiphiles, of the temperat ure, or of the ionic strength of the solution. At low ionic strength the co llapse of an individual, complexed chain with decreasing solvent strength o r decreasing amphiphile concentration acquires the character of a first-ord er phase transition. At high concentrations of amphiphiles and high ionic s trengths, we predict the possibility of the coexistence of two semidilute p olymer phases, both for hydrophilic and for hydrophobic chains.