Weak-to-strong penetration transition of macromolecules into a slit in theta solvent

Partitioning of polymer chains in the theta solution with a confined space of a slit was studied in a wide range of concentrations by using lattice Monte Carlo simulations. The slit width was equal to or greater than the radius of gyration of the theta chains. In the low concentration limit, the partitioning of the theta chains was indistinguishable from that of athermal chains compared at the same chain dimension. At higher concentrations but below the overlap concentration in the solution surrounding the slit, the partition coefficient was nearly independent of the concentration. With a further increase in the concentration, the partition coefficient increased toward unity, as the motional unit in the solution changed from a single chain to the concentration blob of the semidilute theta solution and the blob size decreased with an increasing concentration. The increase in the partition coefficient occurred, however, at concentrations much higher than those for the athermal solutions that showed a similar weak-to-strong penetration transition. We ascribe the delay to the absence of the positive second virial coefficient and the presence of chain clusters. The latter was verified in the density correlation functions of the bulk solutions as well as in the snapshots of the solutions. (C) 2000 American Institute of Physics. [S0021-9606(00)50142-1].