DYNAMICS OF CHAIN-LIKE MOLECULES ON SURFACES

We consider the diffusion and spreading of chainlike molecules on soli d surfaces. We first show that the steep spherical cap shape density p rofiles, observed in some submonolayer experiments on spreading polyme r films, imply that the collective diffusion coefficient D-C(theta) mu st be an increasing function of the surface coverage a for small and i ntermediate coverages. Through simulations of a discrete model of inte racting chainlike molecules, we demonstrate that this is caused by an entropy-induced repulsive interaction. Excellent agreement is found be tween experimental and numerically obtained density profiles in this c ase, demonstrating that steep submonolayer film edges naturally arise due to the diffusive properties of chainlike molecules. When the entro pic repulsion dominates over interchain attractions, D-C(theta) first increases as a function of theta but then eventually approaches zero f or theta-->1. The maximum value of D-C(theta) decreases for increasing attractive interactions, leading to density profiles that are in betw een spherical cap and Gaussian shapes. We also develop an analytic mea n-field approach to explain the diffusive behavior of chainlike molecu les. The thermodynamic factor in D-C(theta) is evaluated using effecti ve free-energy arguments and the chain mobility is calculated numerica lly using the recently developed dynamic mean-field theory. Good agree ment is obtained between theory and simulations. [S1063-651X(98)00202- 5].