Role of epoxy groups on clay surface in the improvement of morphology of poly(L-lactide)/clay composites

An organoclay containing epoxy groups, namely twice functionalized organoclay (TFC), was successfully synthesized by reacting (glycidoxypropyl)trimethoxysilane with Cloisite25A (C25A), which had previously been modified with an amine compound. The role of the epoxy groups on the clay surface in improving the morphology of the poly(L-lactide) (PLLA)/clay composites was investigated. The silicate layers in the PLLA/TFC composite were initially intercalated and became fully exfoliated as the epoxy content in the TFC was increased from 0 to 0.359 mmol/g, as confirmed by infrared spectra and TEM observations. However, the exfoliated morphology became an intercalated/exfoliated mixture when the clay content in the composites was in excess of 5 wt %. Not only was the tensile modulus and tensile strength enhanced by compounding with TFC but so was elongation at break of the PLLA. In contrast, the tensile properties of the PLLA/C25A were far inferior to those of the PLLA/TFC, and its elongation at break and tensile strength decreased sharply as the content of C25A increased. The higher degree of exfoliation in the silicate layers in PLLA/TFC and the improved mechanical properties compared with those of PLLA/C25A were attributed to the enhanced interfacial interaction through a chemical reaction between the epoxy groups of the TFC and the end groups of the PLLA.