Surface diffusion of long chainlike molecules: The role of memory effects and stiffness on effective diffusion barriers

We study the coverage dependence of surface diffusion for chainlike molecul es by the fluctuating-bond model with a Monte Carlo dynamics. The model inc ludes short-ranged excluded volume interactions between different chains as well as an intrachain bond angle potential to describe the chain stiffness . Our primary aim is to consider the role played by chain stiffness and the resulting memory effects in tracer diffusion, and in particular their role in the effective tracer diffusion barrier E-A(T) extracted from the well-k nown Arrhenius form. We show that the memory effects in tracer diffusion be come more pronounced at an increasing coverage as a result of packing requi rements. Increasing the chain flexibility furthermore has the same overall effect as increasing the chain length, namely, they both increase E-A(T). W e then analyze the influence of memory effects on E-A(T) and find that, for a single diffusing chain, about 20% of E-A(T) arises from temperature vari ations in the memory effects, while only the remaining part comes from ther mally activated chain segment movements. At a finite coverage, the memory c ontribution in E-A(T) is even larger and is typically about 20%-40%. Furthe r studies with chains of different lengths lead to a conclusion that, for a single diffusing chain, the memory contribution in E-A(T) decreases along with an increasing chain length and is almost negligible in the case of ver y long chains. Finally, we close this work by discussing our results in lig ht of recent experimental work as regards surface diffusion of long DNA mol ecules on a biological interface. (C) 2000 American Institute of Physics. [ S0021-9606(00)70110-3].