EQUILIBRIUM CONFORMATIONS AND DYNAMIC RELAXATION OF DOUBLE-TETHERED CHAIN MOLECULES AT AN IMPENETRABLE INTERFACE

In this work, the static conformational properties and dynamic relaxat ion behavior of monolayers of chain molecules grafted at both ends (po lymer ''loops'') to an impenetrable plane are investigated utilizing o ff-lattice discontinuous molecular dynamics (DMD) simulations. The con formational properties observed from DMD simulation are compared to th e results of a previous on-lattice bond fluctuation (BF) study. This p rovides a unique opportunity to decouple the effects of excluded volum e, chain flexibility and interparticle interactions, since these effec ts are treated differently in the two simulation methods. Static equil ibrium properties of the looped chain layers determined from the DMD a nd BF models are in qualitative agreement with each other and with sel f-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 investigate d. The characteristic relaxation time of the annealed looped chains, d iscerned from radius of gyration autocorrelation functions, obeys a sc aling relationship of the form tau similar to N-3 sigma(4/3), where si gma is the surface density of the tethered chain ends. The lateral sel f-diffusion coefficient of the chains during annealing is also measure d, with lateral diffusivity scaling as N(alpha)sigma(beta). The expone nts alpha and beta undergo a smooth transition from alpha approximate to-4/3, beta approximate to-4/3 at low densities to alpha approximate to-2, beta approximate to-2 at high densities. Both relaxation times a nd lateral diffusivity exhibit different density scaling than brushes with N/2 repeat units. (C) 1996 American Institute of Physics.