The permeability of polymeric materials to gasses is determined by the adsorption rate of gas molecules into the matrix at the atmosphere/polymer boundary and the diffusion rate of adsorbed gas molecules through the matrix .
The dispersal of nano-sized fillers into the polymer matrix affects the barrier properties of a homogeneous polymer film in two specific ways.
The first way is by creating a tortuous path for gas diffusion. Because the filler materials are essentially impermeable inorganic crystals, gas molecules must diffuse around them rather than taking a (mean) straight line path that lies perpendicular to the film surface.
The result is a longer mean path for gas diffusion through the film in the presence of fillers, as illustrated in Fig. 2. Essentially, the tortuous path allows the manufacturer to attain larger effective film thicknesses while using smaller amounts of polymer.
The effect of dispersed nanomaterials on the mean path length for gas diffusion has been modeled theoretically.
The simplest model, first proposed by Nielsen, assumes that fillers are evenly dispersed throughout the matrix and take the shape of rectangular platelets of uniform size, and supposes that the tortuosity of the path is the only factor influencing the gas diffusion rate.
In the Nielsen model, the gas permeability is given by