(614d) Altered Clay Percolation in PBAT Nanocomposites Compatibilized by PBAT-g-MA

Poly(butylene adipate-co-terephthalate) (PBAT) was modified through graft copolymerization with maleic anhydride (MA) in a supercritical carbon dioxide medium using a free radical initiator.  This grafting method can be employed at milder reaction conditions than reactive extrusion and without the organic solvents typically employed in solution grafting.  Further, supercritical carbon dioxide swells PBAT significantly without dissolving the polymer chains, giving rise to a lower viscosity reaction medium with reduced cage effects than that of solution or melt grafting.  The swelled polymer phase also provides a larger reactive surface area with enhanced initiator and monomer penetration, thus resulting in more efficient grafting than radiation-induced grafting.

The two-step method of PBAT grafting via a supercritical fluid and subsequent compounding with nanoclay limits the exposure of the biodegradable polymer to free radicals while in the molten state which can lead to significant chain scission, molecular weight reduction, and decreased polymer viscosity and processability.  Maleated PBAT (PBAT-g-MA) was pre-mixed with PBAT and Cloisite 30B, an organically-modified MMT clay, prior to extrusion in a parallel co-rotating twin screw extruder. Neat PBAT nanocomposites were similarly prepared in order to evaluate the compatibilization achieved with the incorporation of PBAT-g-MA.

The development of a secondary network of nanoclay particles was substantiated through X-ray diffraction patterning and transmission electron microscopy.  The dispersion and exfoliation state of the nanoclay was investigated in both the compatibilized and neat nanocomposites.  As an additional tool to characterize the clay structure, water vapor permeation studies were conducted to determine the reduction of water vapor transmission through extruded films.  Melt rheology measurements  helped to estimate the percolation threshold of the nanocomposites through the linear relationship developed between the storage modulus and the clay loading.