Dynamics of Poly(methyl methacrylate) Chains Adsorbed on Aluminum Surfaces

We present a stochastic model for the dynamics of poly (methyl methacrylate) (PMMA) chains adsorbed on an aluminum surface. The model is able to investigate the relevant length scales and time scales of adsorbed polymers by employing a coarse-grained representation of the polymer chain (in terms of discrete adsorption states) and stochastic dynamics. The dynamics of the adsorbed polymers are governed by a master equation. The transition probabilities that enter the master equation have been derived by applying transition-state theory to the potential energy surface that describes the interactions of the PMMA segments with the aluminum surface. The potential energy surface has been obtained from previous quantum mechanical calculations for oligomers of PMMA near an aluminum surface. By incorporating the details of the segment– surface interactions, we are able to examine the effects of tacticity on the dynamics of adsorbed chains. We find qualitative similarities and quantitative differences between the dynamics of isotactic and syndiotactic PMMA adsorbed on aluminum. The dynamics of PMMA chains share several features with the dynamics of glass-forming liquids. Both systems exhibit nonexponential relaxation and non-Arrhenius temperature dependences of relaxation times. The physical sources of the glasslike dynamical behavior are a rugged energy surface and strong kinetic constraints. The microscopic basis for the kinetic constraints is discussed in detail.