Fundamental Studies of Various Poly(L-lactide)/clay Nanocomposite Systems by in-situ Polymerization

Abstract

Biodegradable plastics are a promising solution to the increasing global concern of persistent plastic wastes on the environment. In order for biodegradable polymers to successfully compete with commodity polymers, mechanical, thermal, and gas barrier properties must be improved. This can be achieved by dispersing inorganic fillers into the polymer matrix, thus forming an exfoliated polymer nanocomposite. In this study, in situ polymerizations ofpoly(L-lactide)-clay nanocomposites were conducted using various montmorillonite organoclays (PLLA-MMT). When using tin (Il) 2-ethylhexanoate (Sn(Oct)2) as the catalyst, the reaction is believed to initiate from stannous alkoxides, formed from hydroxide (-OH) groups present in the reagents. Several quaternary ammoniums were ion-exchanged onto the surface of montmorillonite-Na+ in order to test effects on the color, morphology, and molecular weights of the final composites. Final products were characterized by Gel Permeation Chromatography (GPC), X-ray diffraction (XRD), and Thermal Gravimetric Analysis (TGA). Exhaustive purification of all reagents, especially monomer, proved to be essential for the polymerization of high molecular weight poly(L-lactide) (Mw > 105). Ultrasonic agitation in toluene was found to be the best technique for dispersing organoclays prior to polymerization. The effects of catalyst concentration, clay loading, and reaction time on PLLA-MMTs were determined. For PLLA-nanocomposites, molecular weight decreased as the clay loading increased. There was also a slight correlation found between catalyst concentration and molecular weight. It was confirmed that the optimal [monomer]0/[Sn] molar ratio for pristine PLLA (180°C, 8 h) is 40,000, while for PLLA-MMT is~ 45 times as concentrated. XRD analysis revealed several exfoliated nanocomposites Excessive washing of organoclays for removal of Na+, silane edge treatment, and addition of a linking agent did not significantly improve molecular weight ofPILA-MMT.