Effects of Metal Ion Type on Ionomer-Assisted Reactive Toughening of Poly(lactic acid) Hongzhi Liu, Xiaojie Guo, Wenjia Song, and Jinwen Zhang* Composite Materials and Engineering Center, Washington State University, Pullman, Washington 99164, United States ABSTRACT: In this study, toughening of poly(lactic acid) (PLA) through reactive blending with epoxy-containing elastomer and ionomer was investigated. Four commercial ionomers based on the poly(ethylene-co-methacrylic acid) (EMAA) precursor were evaluated in this study, which contained zinc, magnesium, sodium, an lithium ions, respectively. Effects of metal ion type, elastomer/ionomer weight ratio and blending temperature on impact toughness of PLA ternary blends were studied. The toughening effects of metal ions of the ionomers is in the order of Zn > Mg > Li ≈ Na. High blending temperature and high elastomer/ionomer ratio both promoted the effective toughening of the PLA ternary blends. The reactive compatibilization and cross-linking of the epoxy-containing elastomer were analyzed using FT-IR, SEM, dynamic mechanical analysis, and torque rheology. In addition, the effect of metal ion type on thermal degradation of PLA was also examined. 1. INTRODUCTION Poly(lactic acid) (PLA) is a very promising biodegradable polyester derived from renewable resources. In recent years, PLA toughening has attracted considerable attention. So far, the main routes for PLA toughening involve addition of plasticizers, 1,2 copolymerization, 3,4 addition of rigid fillers, 5 and melt-blending with various flexible polymers. 6-15 Of the aforementioned strategies, reactive blending with rubbers or elastomers has been shown to be especially effective in achieving supertoughness. 10,11,15 In our previous study, a novel PLA ternary blend system consisting of ethylene/n- butyl acrylate/glycidyl methacrylate (EBA-GMA) terpolymer elastomer and zinc ionomer (containing Zn 2+ ions) of ethylene/methacrylic acid copolymer (EMAA-Zn) was re- ported. 15 The unique feature of this ternary blend system lies in the fact that both interfacial compatibilization of PLA/EBA- GMA and cross-linking reaction of EBAGMA triggered by EMAA-Zn occur simultaneously during reactive blending. The balance between both reactions was found to be critical in achieving desired toughening effects. 16 Further studies also showed that reactive blending temperature, 15,17 EBA-GMA/ EMAA-Zn ratio, 16,17 and the characteristics of EMAA-Zn (i.e., neutralization degree of MAA and MAA content of the ionomer precursor), 18 all had significant effects on the impact toughness of the ternary blends. In our study, at the optimum blend composition and elevated blending temperature, a notched Izod impact strength (IS) up to 860 J/m was successfully achieved. 15 The free carboxylic acid groups in the ionomer can initiate the curing of epoxy groups in the EBA- GMA phase, while zinc ions can catalyze the interfacial compatibilization between the pendent epoxy groups of EBA- GMA and end functional groups of PLA at elevated blending temperatures, thereby resulting in improved interfacial wetting. 15 Other investigators have found that some metal salts and complexes can catalyze the curing of epoxy resins, and that the reactivity depends largely on the type of metal ions. 19 It is thus reasonable to anticipate that the kinetics of the two reactions which both involve the epoxy functional groups will also depends on the metal ion type of the ionomers in our ternary PLA blend system. On the other hand, it is known that PLA is susceptible to thermal degradation during molten processing (especially at elevated temperature and long residence time), which adversely affects the mechanical properties of the final products. 20,21 Other studies showed that many residual metal (e.g., Zn, Mg, Al, Fe, Ti, Zr, Sn, and Ca) compounds markedly affected thermal degradation behaviors of PLA, and their catalytic activity on PLA degradation varied with the metal ion involved. 22-24 Therefore, another motivation of our investigation arose from the assumption that the type of metal ions in the ionomers may play an important role in the thermal degradation of the PLA matrix, presumably influencing the phase morphologies and mechanical properties of the PLA ternary blends. In this work, in addition to the previously reported EMAA- Zn ionomer, we selected three other EMAA-derived ionomers respectively containing Li + , Na + , and Mg 2+ ions (which we denoted as “EMAA-Li”, “EMAA-Na”, and “EMAA-Mg”, respectively) to prepare the ternary blends. The impact toughness and phase morphology of the ternary blends were compared. To account for the difference in toughening effect between these ionomers, the influences of metal ion type in the ionomers on interfacial compatibilization, cross-linking of elastomer, thermal degradation, and crystallinity of the PLA component in the ternary blends were investigated. 2. EXPERIMENTAL SECTION 2.1. Materials. All materials used in this work are commercially available. NatureWorks PLA 2002D with a melt index (MI) value of 5-7 g/10 min (210 °C, 2.16 kg) was used for the blends. Ethylene/n-butyl acrylate/glycidyl methacrylate terpolymer pellets (Elvaloy PTW; designated as “EBA-GMA”) with a MI value of 12 g/10 min (190 °C, 2.16 kg), was kindly Received: December 1, 2012 Revised: March 2, 2013 Accepted: March 9, 2013 Published: March 9, 2013 Article pubs.acs.org/IECR © 2013 American Chemical Society 4787 dx.doi.org/10.1021/ie303317k | Ind. Eng. Chem. Res. 2013, 52, 4787-4793