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 (T g ) 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 T g 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 T g 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 T g 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).

Polymer composites reinforced by hydrophilic clay aerogels were produced and found to possess interpenetrating cocontinuous structures, not the exfoliated structure often observed in clay nanocomposites. An efficient process for producing these clay aerogels was recently reported; in-situ polymerization of N-isopropylacrylamide within the clay aerogels was readily accomplished. The resulting composites have low densities, are stable, and exhibit a new synergistic effect of interpenetrating organic−inorganic phases in which the organic polymer prevents loss of aerogel structure in water by encapsulation, while the inorganic filler increases the structural integrity of the polymer. The composites undergo phase transition and show LCST behavior similar to unmodified poly(N-isopropylacrylamide) despite the presence of reinforcing clay aerogels. Reversible changes in morphology of the aerogel hydrogel composites are observed with varying degrees of hydration.

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ABSTRACT:The improvement of the oxygen-barrier properties of poly(ethylene terephthalate) (PET) via blending with an aromatic polyamide [poly(m-xylylene adipamide) (MXD6)] was studied. The compatibilization of the blends was attempted through the incorporation of small amounts of sodium 5-sulfoisophthalate (SIPE) into the PET matrix. The possibility of a transamidation reaction between PET and MXD6 was eliminated by 13 C-NMR analysis of melt blends with 20 wt % MXD6. An examination of the blend morphology by atomic force microscopy revealed that SIPE effectively compatibilized the blends by reducing the MXD6 particle size. Thermal analysis showed that MXD6 had a nucleating effect on the crystallization of PET, whereas the crystallization of MXD6 was inhibited, especially in compatibilized blends. Blending 10 wt % MXD6 with PET had only a small effect on the oxygen permeability of the unoriented blend when it was measured at 43% relative humidity, as predicted by the Maxwell model. However, biaxially oriented films with 10 wt % MXD6 had significantly reduced oxygen permeability in comparison with PET. The permeability at 43% relative humidity was reduced by a factor of 3 in compatibilized blends. Biaxial orientation transformed spherical MXD6 domains into platelets oriented in the plane of the film. An enhanced barrier arose from the increased tortuosity of the diffusion pathway due to the high aspect ratio of MXD6 platelets. The aspect ratio was calculated from the macroscopic draw ratio and confirmed by atomic force microscopy. The reduction in permeability was satisfactorily described by the Nielsen model. The decrease in the oxygen permeability of biaxially oriented films was also achieved in bottle walls blown from blends of PET with MXD6.