Toughening Poly(lactic Acid)with Ionomeric Elastomers

Poly(lactic acid)(PLA),100%derived from renewable resources,is a kind of very important commercial bioplastic products.PLA is considered as the most promising alternative to petroleum-based plastics in many applications.However,PLA is brittle with a very low impact strength and elongation at break.This limitation greatly hinder its widespread application in many fields.Physical melt blending with elastomers is one of the most facile and efficient methods for modifying PLA.Yet poor compatilibility between the phases is the key hindrance in toughening PLA/elastomer blends.The introduction of ions or intermolecular interactions can effectively improve the compatibility,resulting in high toughness PLA blends.In this work,a series of imidazolium-based brominated poly(isobutylene-co-isoprene)ionomers with different alkyl chain lengths or PLA segments have been successfully synthesized(i-BIIR-n,n=2,4,8,12;i-BIIR-PLA)through a facile quaternization reaction method.Using the imidazolium-based brominated poly(isobutylene-co-isoprene)ionomers as effective toughening agents,we successfully prepared a series of ionomeric elastomers toughening PLA blend by melt blending.First,long alkyl chain imidazole monomers and a kind of imidazole monomer with PLA segments were prepared by a simple reaction between alkyl bromide and imidazole and ROP of L-lactide initiated by 1-hydroxyethyl imidazole,respectively.Then,a series of brominated butyl rubber ionomer elastomers with a conversion ratio?0.90 were successfully prepared by the quaternization reaction.The ionomer elastomers have good thermal stability,higher viscosity,and strong ionic association between molecules.The imidazolium-based ionomers(i-BIIR)are effective toughening agents for PLA.The DMA and SEM analysis showed that the i-BIIR had better compatibility with PLA compared to the neat BIIR.Moreover,the long alkyl side chain was benefical for enhancing the miscibility between PLA/imidazolium-based ionomer elastomers,because long chains shield the intramolecular interaction between ions of the ionomers.For the PLA/i-BIIR-PLA blends,stronger intermolecular interactions were form between the ionomers and PLA matrix,manifesting as smaller dispersed phase particles and more unclear interface.Tensile and Izod notch impact test showed that the elongation at break of PLA/i-BIIR-12 blend was up to 235%,10 times higher than that of pure PLA,indicating the flexibility is significantly improved.The elongation at break of the PLA/i-BIIR-PLA blend can futher reach up to 250%,and the impact strength is 5.9 KJ/m~2,owing to the good interfacial adhension.Toughening mechanism had been established by systematical analysis of the compatibility,intermolecular interaction and phase structures of the blends.Interfacial cavitations initiating massive shear yielding of the PLA matrix owing to a suitable interfacial adhesion played a key role in the enormous toughening effect in these blends.We believed that introducing imidazolium group into the BIIR elastomer was vital for the formation of a suitable interfacial adhesion.