Influence of the solution cation mobility on the water uptake estimation of PVC Plastisol freestanding films by EIS

In this work, the influence of the outer solution cation mobility on the electrochemical behaviour of PVC Plastisol freestanding films was studied. For that purpose, the paint films were immersed in 0.5 M and 0.05 M chloride solutions with different cations (Na+, K+, Cs+). The electrochemical impedance spectroscopy (EIS) was the technique chosen for this study and estimated values of film capacitance and resistance were calculated. For PVC Plastisol films, it was observed that the ion diffusion depends on the outer solution concentration. To explain such results different mechanisms for the ionic diffusion were considered. For higher concentration solutions the ions of the outer solution must diffuse into the film, while for lower concentration solutions the dissolution inside the film occurs followed by ionic diffusion from the film to the outer solution. The water uptake was determined by EIS and gravimetry techniques. The Brasher and Kingsbury (BK) and the low frequency solution (LF) models were used to estimate this parameter by the EIS technique. A direct dependency between water uptake and solution concentration was obtained by this technique, while by gravimetry an inverse relation was obtained. However, the LF model presented water uptake values better correlated with the gravimetric ones comparatively to the BK equation. For the more concentrated solutions, the cation mobility influenced the water uptake estimated by EIS, whereas, such dependency was not observed with the gravimetric technique. The results obtained in this work suggest that the outer solution resistivity and consequently the film resistance influence the water uptake measurements and should take part in any improved model used for water uptake determination. (C) 2006 Elsevier B.V. All rights reserved.