The effect of surface chemistry and nanoclay loading on the microcellular structure of porous poly(d,l lactic acid) nanocomposites
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Three series of polymer nanocomposites, based on poly(d,l lactic acid) (PDLLA) and organically modified montmorillonite, were prepared by the melt and the solution intercalation technique. The first series was prepared by extrusion using different clay loadings. The second series of nanohybrids was obtained using montmorillonite modified with different types of alkylammonium surfactants in terms of carbon-chain lengths (i.e., 4, 8, 12, 16 and 18). In the third series of nanocomposites, the organic cation concentration of the surfactant was varying. Microcellular porous materials were, afterwards, fabricated from these three series of nanocomposites. The porous structures of pure and nanocomposite PDLLA were prepared by isothermal pressure quench using supercritical CO2 as foaming agent. The morphology of the produced porous materials was investigated by scanning electron microscopy (SEM). Image processing of the samples revealed that the final cellular structure is strongly related to clay loading and, both, the type and the organic cation concentration of the alkylammonium used for the modification of the clay. The results suggest that the size of the pores decreases and the cell density and bulk foam density increase with the increase of clay loading or the surfactant's carbon chain length or the cation concentration in clay. Clay dispersion seems to be enhanced by the supercritical treatment upon foaming.