Structure-property relationships in cross-linked polyester-clay nanocomposites

Crosslinked polyester-clay nanocomposites were prepared by dispersing organically modified montmorillonite in prepromoted polyester resin and subsequently crosslinked using methyl ethyl ketone peroxide catalyst at several different clay concentrations (1.0, 2.5, 5.0, and 10.0 wt%). X-ray diffraction studies revealed the formation of a nanocomposite in all cases with the disappearance of the peak corresponding to the basal spacing of the pure clay. Transmission electron microscopy was used to study the morphology at different length scales and showed the nanocomposite to be comprised of a random dispersion of intercalated/exfoliated aggregates throughout the matrix. Thermal degradation of the nanocomposites was found to be slightly but progressively hastened compared to the pure crosslinked polymer, loss and storage modulus were monotonically shifted toward higher frequency values, and the tensile modulus was found to decrease with increasing clay content. These unexpected results were rationalized based on the decrease in the degree of crosslinking of the polyester resin in the nanocomposite, in the presence of clay. In particular, the nanocomposite containing 2.5 wt% clay consistently demonstrated a drop in properties far greater than that observed at other clay concentrations, and was attributed to the greater degree of exfoliation seen in this case which presumably leads to a greater decrease in the degree of crosslinking. Oxygen permeability rates in the polyester nanocomposites decreased with increasing clay content, as expected, and was satisfactorily reproduced using a tortuosity based model. (C) 2002 Elsevier Science Ltd. All rights reserved.