Photographic applications rely greatly on the permeability and swelling behavior of multilayer thin films. This behavior is, at least, a function of (I) material properties such as particle size, percent solids and solubility, and (II) processing conditions that affect film formation, such as rate of drying, maximum web temperature and nature of underlying layers.
In order to test permeability of multilayer structures, an alkaline medium is allowed to permeate sample. In the structure under test, the bottom most layer is acidic in nature. This acid neutralizes the alkali as it permeates through the layers. Neutralization causes a change in ionic concentration in the sample and this is measured using a high frequency cavity sensor (commercially available as the conductivity Puntex technique) that changes its resonant frequency in response to change in conductivity of the sysytem. This frequency change can be monitored to to give a time transient for permeability. In the event that frequency changes even before neutralization has occured, it can be correlated to capacitance changes within the layers produced by swelling and water uptake.
For a porous layer within the sample, the permeation proceeds along two paths. One through the porosity and the other through the bulk. In this case the bulk signal is superimposed over the signal from porous component. For such a system, a transient of the time derivative of resonant frequency proved to be a useful tool to study interfacial interactions between the acid layer and the layer above it. Impact of drying was also inferred from similar measurements. Data indicate that the higher temperature of drying of constituent layers resulted in a microstructurally tighter film, that took longer time for the alkali to permeate.