Dynamics of\nPolymer Films on Polymer-Grafted Substrates:\nA Molecular Dynamics Simulation

For substrate-supported polymer films, the tails of adsorbed\nchains\nare generally assumed to play important roles in the propagation of\nthe substrate’s effect inside polymer films. The effects of\nthe grafting density and the rigidity of substrate-grafted polymers,\nthe simplest model for the adsorbed tails, on the diffusivity of film\npolymers are investigated by performing molecular dynamics simulations.\nAn optimal grafting density σ<sub>o</sub>, around the critical\ngrafting density for the transition from “mushroom”\nto “brush”, is found with the most pronounced suppression\nof diffusivity on the film polymers; i.e., the penetration of the\nfilm polymers into the grafting layer reaches the maximum. However,\nat high grafting density, the crowded and vertically stretched brush\nexcludes the coil-like film polymers, and the suppression is thus\nreduced. At σ<sub>o</sub>, with an increase in the rigidity\nof the grafted polymers, the suppression is increased quickly at low\nrigidity but slowly at high rigidity. The dynamic suppression is attributed\nto the combination of the conformation change from stretching at low\nrigidity to tilted orientation at high rigidity and decelerated mobility\ninduced by the rigidity. The stretching conformation enhances, whereas\nthe tilted conformation weakens the interpenetration between the grafted\npolymers and the film polymers. Our results reflect the importance\nof both conformational variation and interchain interaction in the\ninterface region.