(384c) Material Point Method Simulation of Enhanced Penetrant Diffusion in Nanoparticle-Polymer Composite Membranes

Addition of nanofillers to polymeric membranes has been shown to dramatically influence the transport properties of the membrane. Unlike traditional fillers that improve barrier properties by creating more tortuous pathways for penetrant diffusion through polymer membranes, addition of nanofillers can significantly increase penetrant diffusion and improve their selectivity properties. It has been suggested that the addition of impenetrable nanoparticles disrupts polymer chain packing adjacent to the filler surface leading to an increase in free volume for the interfacial polymer. The increase in accessible free volume leads to faster penetrant diffusion through the interfacial polymer layers. Coalescence of interfacial layers with high permeation, due to nanoparticle clustering, may result in the formation of long, high permeability, preferred pathways for penetrant diffusion.

We have utilized 2D material point method (MPM) to study the influence of nanoparticles on the diffusivity of penentrants in model polymer membranes comprised of impenetrable spherical nanoparticles dispersed in a matrix with uniform penetrant solubility and diffusivity. Diffusion in the nanoparticle-polymer composite membrane was enhanced by the presence of a thin ?skin? of matrix material next to the surface of the nanoparticles with a penetrant diffusion coefficient 100 times that of the bulk matrix. The influence of the skin thickness, nanoparticle volume (area) fraction and the manner in which the nanoparticles were distributed in the membrane on penetrant diffusion was studied. For a given skin thickness the penetrant diffusion in the composite membrane was found to increase exponentially with increasing volume (area) fraction of nanoparticles both above and below the percolation threshold. Membranes with nanoparticles experiencing weak attractive interactions were found to have higher penetrants diffusivity than the membranes with fully dispersed particles due of the formation of anisotropic nanoparticle clusters. We found the total volume (area) fraction of particles + enhanced matrix (skin) to be a valid scaling variable for the effective diffusion coefficient of the nanoparticle-polymer composite membranes for the entire range of skin thickness and nanoparticle loadings investigated. Implications of the observed dependencies of effective penetrant diffusivity in the composite membrane on nanoparticle loading, skin thickness and nanoparticle morphology are discussed.

This work is funded by the University of Utah Center for the Simulation of Accidental Fires and Explosions (C-SAFE), funded by the Department of Energy, Lawrence Livermore National Laboratory, under subcontract B341493.