Monte Carlo simulation of submonolayer vapor-deposition polymerization

In this paper, we propose a Monte Carlo simulation model for the initial growth of polymer films by considering only monomer surface diffusion in the vapor-deposition polymerization process. In the model, monomers are deposited randomly on a two-dimensional square lattice with periodic boundary conditions and are allowed to diffuse with nearest neighbor hops. Whenever monomers meet, they stop diffusing and form a stable dimer. When a diffusing or deposited monomer encounters one of the ends of a polymer (L>1), it stops moving and attaches to the polymer. Attachment of monomers or other polymers is allowed only at the two ends of the polymer. We have shown that there are three distinct growth regimes for surface coverages theta<0.5: a low-coverage initiation regime (I), a chain propagation regime (P), and a saturation regime (S). In both regimes I and P, the growth is similar to the molecular beam epitaxy model. We examine in detail the scaling relations for the chain length distribution, which agree quite well with results of a rate equation. However, in regime S, our model gives very different kinetics. The breakdown of scaling at higher coverages is due to the fact that long-chain polymers have partitioned the lattice with inactive sites. This inhibits further polymer growth and enhances production of dimers, shifting the chain distribution to favor shorter polymers and driving the average molecular weight down. The chain configuration initially is similar to a path taken in a diffusion-limited self-avoiding walk. However, at high coverages, due to the correlation of long polymer chains, the polymer chains tend to be compact.