STRUCTURE AND DYNAMICS OF POLYMER BRUSHES NEAR THE THETA POINT - A MONTE-CARLO SIMULATION

Grafted polymer layers under variable solvent conditions are studied by Monte Carlo simulations using the bond fluctuation model. Structural information such as monomer density profiles, brush thickness, mean-square displacement of monomers, and positions of the monomers along the chain are obtained for temperatures above, at, and below the THETA-point. In particular, the scaling of the brush thickness is formulated and verified by the simulation data. At the THETA-point, more extensive simulations are performed to investigate the structural and dynamical properties. While the brush thickness at the THETA-point agrees very well with the scaling and self-consistent field predictions, the latter deviate from our results for the monomer density profile and the distribution function of the free chain ends. It is not clear whether these deviations result from our chains being too short or from a basic inadequacy of the theory. For the dynamics at the THETA-point, both the relaxation of the chain configurations and the mean-square time displacement of the chains are studied. The relaxation time tau for a polymer brush of chain length N and surface coverage-sigma are found to behave as tau approximately N3-sigma(b) with an apparent exponent b congruent-to 1.6. For poor solvent conditions below the THETA-point, we find that the layer (with randomly fixed grafting sites) develops considerable lateral inhomogenity in its density, which has not been predicted by previous theories.