Polymer depletion interaction between parallel walls--a Monte Carlo study

An off-lattice bead-spring model of self-assembling equilibrium ("living") polymers is used to study the polymer-induced interaction between parallel walls immersed in polydisperse solutions of different concentration by means of Monte Carlo simulation. The two walls form an open slit in contact with an external reservoir so that the confined system may exchange monomers with the surrounding phase and adapt its polydispersity in order to relax the confinement constraint. We find that the properties of the polymers in the constrained system as well as the net force deltaF acting on the walls depend essentially on the polymer concentration in the reservoir which leads to qualitative differences in their behavior with changing inter-planar distance H: In a dilute polymer solution at concentration phi below the semi-dilute threshold phi* the force between the walls is attractive and decreases steadily with growing wall separation H, so that deltaF approximately 0 at H/ Rg> or =3 if H is measured in gyration radii Rg of the unperturbed polymers. The total monomer concentration within the slit is smaller than the concentration in the reservoir and decreases monotonically with H/Rg-->0. The ratio Nin/Nout of mean chain length Nin in the slit to that in the reservoir, Nout, decreases from unity at H-->infinity, goes through a minimum at H/Rg approximately 1, and then rises again to Nin/Nout>1 for wall separations H/Rg<1. In contrast, in a dense solution of equilibrium polymers at phi>phi* one detects no indirect wall-wall interaction, deltaF approximately 0, for H larger than the monomer size. Thus, earlier speculations about the existence of possible depletion interaction between parallel walls even in a dense polymer system cannot be confirmed. Inside the slit the monomer density is found to be always larger than in the reservoir while Nin/Nout<1 and decreases steadily as H/Rg-->0. The depletion force between parallel plates has been determined also in a monodisperse solution of conventional polymers. Qualitatively the force behavior does not differ from that of living polymers.