Equilibrium conformations and dynamic relaxation of double-tethered chain molecules at an impenetrable interface

Abstract

In this work, the static conformational properties and dynamic relaxation behavior of monolayers of chain molecules grafted at both ends (polymer ‘‘loops’’) to an impenetrable plane are investigated utilizing off-lattice discontinuous molecular dynamics (DMD) simulations. The conformational properties observed from DMD simulation are compared to the results of a previous on-lattice bond fluctuation (BF) study. This provides a unique opportunity to decouple the effects of excluded volume, chain flexibility and interparticle interactions, since these effects are treated differently in the two simulation methods. Static equilibrium properties of the looped chain layers determined from the DMD and BF models are in qualitative agreement with each other and with self-consistent field (SCF) predictions for brushes of N/2 repeat units. By allowing the chain anchors to move laterally along the interfacial plane, the effect of annealing on layer properties is also investigated. The characteristic relaxation time of the annealed looped chains, discerned from radius of gyration autocorrelation functions, obeys a scaling relationship of the form τ∼N3σ4/3, where σ is the surface density of the tethered chain ends. The lateral self-diffusion coefficient of the chains during annealing is also measured, with lateral diffusivity scaling as Nασβ. The exponents α and β undergo a smooth transition from α≊−4/3, β≊−4/3 at low densities to α≊−2, β≊−2 at high densities. Both relaxation times and lateral diffusivity exhibit different density scaling than brushes with N/2 repeat units.