Reports: AC745688-AC7: How Does Substrate Geometry Affect the Surface-Initiated 'Living' Polymerization?

Jan Genzer , North Carolina State University

Our goal is to investigate how substrate geometry and solution conditions affect the effectiveness of controlled radical polymerization (CRP) from substrates (“grafting from”) in producing polymers with controlled molecular weight and polydispersity index (PDI). In order to achieve this objective, bulk- and surface-initiated controlled radical polymerizations (CRP) in implicit solvents are simulated using a stochastic Monte Carlo algorithm implementing the bond-fluctuation scheme. We study the effect of system properties such as bulk vs. surface initiation, surface density of initiators and solvent quality on the molecular weight and molecular weight distributions of the resulting polymers. We implement variations in the solvent conditions by means of truncated inter- and intra-molecular potentials between bonded polymer beads and we achieve variations in the surface density of initiators by changing the linear dimensions of the lattice while keeping the total volume of the lattice constant. In order to determine the effect of geometry and steric hindrance on the polymerization we monitor the sizes of the growing polymers, the number of reactive species located near the reactive polymer chain-ends and, for surface-initiated systems, the concentration profiles of monomers, polymers and chain-ends as a function of distance from the substrate. Our results indicate that confinement during polymerization hinders the ability of chain-ends and monomers to approach each other, which has detrimental consequences on the capacity of CRP to yield nearly monodisperse polymers. In addition, surface-initiated polymerizations do not yield polymer brushes with equal tethering densities as those of the precursor initiators because the growing radicals may not all be simultaneously activated and they do not possess equal access to the monomers due to chain crowding. Confinement of polymers to impenetrable substrates and decreases in the solvent quality therefore result in decreases in the rate of polymerization and increases in PDI. During the last year, we have used the kinetic Monte Carlo method, implementing the bond fluctuation model formalism in the canonical (NVT) ensemble, to study living polymerization initiated concurrently in bulk and on flat substrates. Our results reveal that the molecular weights and molecular weight distributions of both classes of polymers depend on the grafting density of the surface-bound polymers and the fraction of polymers on the surface (relative to that in bulk). In general, polymer grafts on the surface posses lower molecular weight and higher polydispersity index relative to their bulk counterparts. The differences between the molecular weight of the two populations of polymers decrease with decreasing the grafting density of macromolecules on the surface and decreasing the number of bulk-initiated polymers. This work provides evidence that the common practice involving using the molecular weight of bulk-initiated polymers in estimating the grafting density of polymeric anchors on flat substrates is not generally valid.
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