Model for Anomalous Moisture Diffusion through a Polymer-Clay Nanocomposite

Experimental data are reported on moisture diffusion and the elastoplastic
response of an intercalated nanocomposite with vinyl ester resin matrix and montmorillonite
clay filler at room temperature. Observations in diffusion tests showed that
water transport in the neat resin is Fickian, whereas it becomes anomalous (nonFickian)
with the growth of the clay content. This transition is attributed to immobilization
of penetrant molecules on the surfaces of hydrophilic clay layers. Observations
in uniaxial tensile tests demonstrate that the response of vinyl ester resin is strongly
elastoplastic, whereas an increase in the clay content results in a severe decrease of
plastic strains observed as a noticeable reduction in the curvatures of the stress-strain
diagrams. This is explained by slowing down the molecular mobility in the host matrix
driven by confinement of chains in galleries between platelets. Constitutive equations
are developed for moisture diffusion through and the elastoplastic behavior of a nanocomposite.
Adjustable parameters in these relations are found by fitting the experimental
data. Fair agreement is demonstrated between the observations and the results
of numerical simulation. A striking similarity is revealed among changes in diffusivity,
ultimate water uptake, and the rate of plastic flow with an increased clay content.