Interaction between undersaturated polymer layers: Computer simulations and numerical mean-field calculations

Lattice Monte Carlo simulations and the contact distribution method are use d to analyze the force of interactions between two undersaturated polymer l ayers in good solvents. The net force between the surfaces, resulting from a competition between bridging attraction and steric repulsion, is obtained unambiguously for coverages Gamma ranging from 0 to the saturation coverag e Gamma (0) The sizes and the number of the bridges are in qualitative agre ement with previous scaling analysis,. but a number of quantitative differe nces important in practice emerge from the simulations. While the bridges m ay be considered as independent elastic tethers at low coverages, steric in teractions begin to exert influence for coverages as low as Gamma approxima te to 0.3 Gamma (0). Comparison of the results of the simulations with nume rical mean-field calculations based on second-order Markov chains (i.e., wi thout backfolding) shows good agreement with some of the structural feature s of the polymer layer, but important discrepancies regarding the force of interaction exist for all values of Gamma/Gamma (0). Whereas the mean-field calculations predict that the crossover from attraction to steric repulsio n occurs only near saturation, the simulations show the crossover to occur at a much higher degree of undersaturation (i.e., Gamma/Gamma (0) approxima te to 0.85). The simulation method used provides an opportunity to scrutini ze systematically the details of the mean-field theory and to refine the th eory for providing better guidance in practice. The results presented here focus on a low molecular weight chain (N = 200). Whether the asymptotic beh avior for very long chains predicted by the scaling/free energy functional approach is obtained still needs to be tested.