Ammoniated Zinc Oxide Nanoparticles for Preparing High-Melt-Performance Poly(lactic acid) Bearing Dual Hybrid Long-Branched Chains by Degrading and Restructuring Chains

Long-chain branch (LCB) structures are critical for upgrading the poly(lactic acid) (PLA) melt performance. Polymer chains that are restructured by introducing LCBs via melt transesterification show higher reactive efficiencies and are environmentally friendly and gel free. Although PLA is environmentally friendly, its low melt strength hinders film blowing, blow molding, foaming, and other processes involving polymer melts. Therefore, the construction of LCBs is the key to solving this problem. However, because PLA easily thermally degrades, severe excessive transesterification-associated polymer degradation via the drastic chain-scission reaction substantially outcompetes the LCB-branching reaction, which substantially narrows the processing window for forming LCBs. Therefore, in this study, a dual hybrid branching (DHB) strategy was proposed to overcome these challenges. Specifically, surface-aminated ZnO (SAN-ZnO) nanoparticles were applied as an accelerant via ester activation by coordinating zinc atoms in ZnO with ester group oxygen atoms in PLA, which promoted transesterification between PLA and a trifunctional ester to form LCB–PLA. Moreover, amidogens on the SAN-ZnO surface could collect degraded PLA chains and react with their terminal carboxyl groups via amidation in situ to form DHB chains and thus restrain polymer overdegradation. Because DHB both facilitated LCB formation and restrained excessive polymer degradation, the PLA melt performance, especially the melt strength, was obviously improved to over 37 cN.