Computational study of polymer films using a Monte Carlo model of vapor deposition polymerization
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Polymer films are a subject of both technological importance and fundamental scientific interest. Very often polymer films are created under far-from-equilibrium conditions. Polymer film growth is a complex process due to polymer’s complicated structure and interactions that include internal degrees of freedom, limited bonding sites, chain relaxation, chain-chain interactions, etc. My doctoral research focusses on the computational study of polymer films grown by an experimental growth technique referred to as vapor deposition polymerization (VDP), where a 2D substrate is exposed to gas phase monomers from the top and a polymer film grows on the substrate through a polymerization reaction occurring during the growth process. A lattice Monte Carlo (MC) model was used to study polymer film growth and to examine the effects of random angle deposition, monomer diffusion, monomer adsorption, and polymerization reaction in determining polymer film properties. In addition to the aforementioned stochastic processes, our model also implemented the processes of polymer chain initiation, extension, and merger. In our analysis, the spatial and temporal behavior of kinetic roughening were extensively studied using finite-length scaling and height-height correlations. The scaling behaviors at local and global length scales were found to be very different. The global and local scaling exponents for morphological evolution were evaluated for varying system parameters. A systematic study was performed to discover the universality class of our VDP growth model. We also studied the aggregation mechanism of polymers grown by VDP. The behavior of polymer chain length distributions were carefully analyzed and the dynamic scaling approach was employed to highlight the dependence of polymer aggregation on the system parameters. As the ratio of diffusion rate to the deposition rate was increased in the VDP growth, we observed a systematic change in the aggregation mechanism that prevented the manifestation of a unique scaling function for chain aggregates. Finally, we calculated the conformational properties of polymer chains and studied their dependence on system parameters. The structural studies were useful in understanding the bias in the preferred growth direction of the films as diffusion was increased in the system.