Glass transition behavior of clay aerogel/poly(vinyl alcohol) composites

In polymer/ clay nanocomposites, the size and morphology of the dispersed clay are crucial in determining the macroscopic composite properties. Clay can be converted into an aerogel structure through a freeze- drying process which results in a reduction of its density from 2.35 to 0.05 g/cm(3). The morphology of these clay aerogels resembles a house of cards structure. Low weight fraction ( 0.5- 4 wt %) composites of clay and clay aerogel with a poly( vinyl alcohol) ( PVOH) matrix polymer were prepared. Glass transition temperature ( Tg) behaviors of clay/ PVOH composites were investigated as a function of size, loading, and dispersion of clay and clay aerogel in the polymer matrix. The trends in Tg measured from both dynamic mechanical analysis and differential scanning calorimetry are similar, exhibiting a maximum value at 1 wt % loading for both clay and clay aerogel and then decreasing with additional filler content. Although the trends are similar, the drop in Tg at 4 wt % clay composite is considerably larger than that for a similar loading of clay aerogel. In comparison with the mesoscale clay aerogel, the nanoscale clay shows better dispersion and higher interfacial interaction with the polymer, which enhances polymer crystallinity at lower weight fractions and increases polymer free volumes at higher weight fractions. The relative changes in Tg are proposed to be the result of two competing effects: ( i) surface interaction which strengthens the interface ( decreasing chain mobility) and ( ii) enhanced interfacial free volume due to the lower bulk crystallinity of polymer chains ( increasing chain mobility).