Morphology and diffusion properties of whey/montmorillonite nanocomposites

The diffusion of methanol in solution-cast whey-protein-isolate/poly(vinylidene pyrrolidone)-coated montmorillonite composite films was studied. The filler content was 0–4.8 vol.%. Desorption experiments on samples saturated in methanol were conducted in a nitrogen-purged chamber at 294 K or in a thermogravimeter at 333, 348 and 363 K. The desorption curves were fitted using a two-phase diffusion model; where diffusion occurred first in the rubbery and later in the glassy state. A free volume model was used to analyze desorption at 363 K which occurred only within the rubbery phase and it enabled the geometrical impedance factor and free volume constraints from the nanoparticles to be assessed. Transmission electron microscopy showed a partial exfoliation of the nanoparticles. The geometrical impedance factor increased markedly with increasing filler content in accordance with the high aspect ratio of the nanoparticle aggregates. Transmission electron microscopy, in combination with model calculations, revealed that the clay sheets were oriented preferentially in the plane of the film which was an important factor underlying the high geometrical impedance factor obtained for the nanocomposites. A small but significant reduction in the fractional free volume of the polymer matrix was observed for the rubbery polymer in the presence of montmorillonite.