Effect Of Lignin Graft Copolymer On Properties Of PLA Microcellular Foams

As an ideal alternative to traditional petroleum-based polymer foams,polylactic acid(PLA)foams have received widespread attention for their excellent biocompatibility and environmental friendliness.However,due to its slow crystallization speed and poor melt viscoelasticity,making PLA foams with high expansion rates and high cell density is a challenge.In order to improve the foaming properties of PLA,a lignin graft-poly(lactic acid)copolymer(LG-g-PLA)was synthesized by ring-opening polymerization.Using LG-g-PLA as a PLA foaming nucleating agent,a fully degradable PLA microcellular foamed material with high expansion ratio,high cell density,and excellent mechanical properties was prepared by a batch foaming method.The effects of the content of LG-g-PLA graft-poly(lactic acid)and its addition on PLA rheology,crystallinity and foaming performance were systematically studied.By studying PLA microcellular foam materials with different lignin content,it was found that the addition of lignin would reduce the storage modulus and complex viscosity of PLA melts,showing typical pseudoplastic fluid characteristics.Crystallization properties show that the addition of lignin improves the crystallinity and melting temperature of PLA,and the crystals are more compactly packed,but the cold crystallization temperature is higher,which prolongs the crystallization process.Compared with the neat PLA system,when the lignin content is moderate,the expansion ratio and cell density of PLA foam increased,and the apparent density and cell size decreased.However,when the amount of lignin added is excessive,the cell structure of PLA foam will deteriorate.The cells are distributed radially around the lignin,and the cell size gradually increases from small to the outside.Lignin graft-poly(lactic acid)copolymers with different graft-poly(lactic acid)contents were prepared by ring-opening polymerization,and the graft copolymers were characterized.The crystallization properties showed that the addition of LG-g-PLA with different graft-poly(lactic acid)contents can reduce the cold crystallization temperature of PLA and increase the crystallization rate and crystallinity of PLA.Lignin graft-poly(lactic acid)copolymers can improve the crystal packing structure of PLA,but it will not change the crystal structure.The rheological characteristics show that as the content of grafted polylactic acid decreases,the viscoelasticity of the LG-g-PLA/PLA composite system decreases,and the Newtonian platform of complex viscosity moves toward high frequencies.The foaming performance shows that higher graft-poly(lactic acid)content can improve the expansion ratio and cell density of the system and reduce the cell size.When the content of graft-poly(lactic acid)is 90%,the foamability of LG-g-PLA/PLA is the best.By using LG-g-PLA as a foaming nucleating agent,the foamability of PLA is improved.The effects of different amounts of LG-g-PLA on cell nucleation,rheological behavior and foaming properties induced by PLA crystallization were analyzed.The results show that the heterogeneous nucleation of LG-g-PLA can improve the crystallization rate and crystallinity of PLA.Compared with neat PLA foam,LG-g-PLA improved the foam morphology of PLA,obtaining a reduced and uniform cell size as well as increased expansion ratio and cell density.When 3 wt%of LG-g-PLA was added,the cell density of PLA foam was 3.22×109 cells/cm3,the expansion ratio is increased to 36.54 times,and the compressive strength was 1.6 times that of neat PLA foam.