Synthesis and properties of poly(lactic acid) ionomers

Date of Completion

January 2008


Chemistry, Polymer|Plastics Technology




Poly(lactic acid) (PLA) ionomers with metal-carboxylate groups were synthesized and the effect of the ionic groups on the thermal and mechanical properties of PLA was studied. Telechelic PLA ionomers were synthesized using a chemical recycling process. A transesterification reaction between a commercial PLA and 2-hydroxyethyl methacrylate or ethylene glycol was used to produce a hydroxy-terminated PLA. The hydroxy-terminated PLA was then reacted with itaconic anhydride to produce terminal carboxylic acid groups, which were neutralized with appropriate metal acetates to produce Na-, Li-, K-, Zn-, Ca- and Y-w- and α,ω-telechelic PLA ionomers. 1H-NMR spectroscopy was used to confirm the presence of the itaconic acid end-groups and FTIR spectroscopy was used to quantify the extent of neutralization. The addition of the ionic groups increased the glass transition (Tg), and Tg increased as the strength of the ion-pair increased. The ionic groups suppressed crystallinity, especially when multivalent cations were used. ^ A random poly(lactic acid),PLA, based ionomer was synthesized by copolymerizing a methacrylate-terminated PLA macromonomer and methyl methacrylate. The copolymerization kinetics were studied using 1H-NMR spectroscopy and the copolymer composition was characterized by 13C-NMR spectroscopy. Carboxylic acid groups were introduced into the copolymer by reacting the hydroxyl end-groups of the PLA macromonomer with succinic anhydride, and the acid groups were neutralized with metal acetates to produce Na-, Ca-, and Y-PLA ionomers. Significant increases in Tg were observed for the ionomers and thermomechanical analysis indicated that the ionomers were more resistant to penetration by a weighted probe and an apparent rubbery plateau was observed above T g. The ionomers were more hydrophilic than PLA, but relatively low water absorption could be achieved for the Ca2+-salt ionomer. ^ Sulfonated poly(styrene) ionomer (SPS) ionomers (5.3 mol % sulfonation) that were neutralized with Na and Zn were blended with poly(lactic acid) (PLA). The thermal and mechanical properties of the blends were investigated using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Blends that contained 2% and 5% of SPS ionomers were found to be immiscible. The degradation behavior of the blends was studied using thermogravimetric analysis (TGA) and melt indexer. The SPS ionomers did not decrease the decomposition temperature (Td) significantly. The melt flow rates of the blends were measured in continuous cycles through the melt indexer. ^