a b s t r a c tCrosslinked organic polymers are used in a wide variety of coatings and composites to distribute stress, increase toughness and protect the substrate by limiting the passage of aggressive chemicals. Enhancing performance of crosslinked polymers requires understanding how precursor chemistry and geometry, as well as crosslinking protocol, determine the structure and performance of the resulting network. Previous molecular dynamics studies have indicated that cavitation produces pores in simulated liquids, even metals (and the resultant solids), when there is only a single type of force, usually van der Waals, between particles. Here we show that nano-sized cavitation voids also occur in a system bound by van der Waals (Lennard-Jones) forces that is additionally crosslinked with strong covalent (FENE) bonds to form 3 or 6 functional solid networks. Cavitation was observed in both systems. These voids are not a consolidation of "free volume", nor due to a loss of volatiles, but happen as the solidification/cooling stresses exceed the local tensile strength of the material. At temperatures well above the glass transition temperature, "free volume" is distributed evenly throughout the sample in very small pores. As the system cools through its rubbery phase, a few larger voids form via cavitation. Although the loci of these larger voids is associated with crosslinked nodes, cavitation involves the rupturing of weak van der Waals (Lennard-Jones) bonds between molecular chains in regions not constrained by the strong intramolecular bonds. Voids were observed to form during rapid quenches, as well as during much slower cooling at fixed volume, which emulates adhesion of the network to a more rigid body. The voids are large compared to the dimensions of aggressive ionic species and water molecules, and may potentially reduce the barrier properties of a crosslinked coating or composite. Such pore formation, via cavitation, during network formation and curing is not incorporated in current theories of the crosslinking process.

We examine the applicability of diffusive lattice Boltzmann methods to simulate the fluid transport through barrier coatings, finding excellent agreement between simulations and analytical predictions for standard parameter choices. To examine more interesting non-Fickian behavior and multiple layers of different coatings, it becomes necessary to explore a wider range of parameters. However, such a range of parameters exposes deficiencies in such an implementation. To investigate these discrepancies, we examine the form of higher-order terms in the hydrodynamic limit of our lattice Boltzmann method. We identify these corrections to fourth order and validate these predictions with high accuracy. However, it is observed that the validated correction terms do not fully explain the bulk of observed error. This error was instead caused by the standard finite boundary conditions for the contact of the coating with the imposed environment. We identify a self-consistent form of these boundary conditions for which these errors are dramatically reduced. The instantaneous switching used as a boundary condition for the barrier problem proves demanding enough that any higher-order corrections meaningfully contribute for a small range of parameters. There is a large parameter space where the agreement between simulations and analytical predictions even in the second-order form are below 0.1%, making further improvements to the algorithm unnecessary for such an application.

Fickian diffusion is often used to model moisture transport through barrier coatings, where the goal is to protect an underlying substrate from the onset of corrosion caused by buildup of water or other aggressive species. Such coatings are often exposed to cyclic moisture, either in laboratory testing or in service due to natural environmental fluctuations. In this paper, we use lattice Boltzmann numerical techniques to investigate the effects of reservoir cycling on moisture propagation and concentration at the substrate where corrosion onset occurs. We examine both the simple case of constant diffusivity, representing idealized Fickian diffusion, and diffusivity that depends on concentration via either a step or linear function, representing polymer network swelling. The use of a coating subject to swelling is shown to lead to highly variable equilibrium behavior. We show that the nature of the functional diffusivity has large effects on water concentration at the substrate, and has implications for material design and analysis to avoid corrosion.

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