Preparation Of Poly (Lactic Acid) With Layered Micro-and Nano-Pore Structures And The Study Of The Mechanism Of Toughness

Poly(lactic acid)(PLA)has attracted much attention due to its extensive plant origin and biodegradability,which can replace some petroleum-based polymers,alleviate the shortage of petroleum resources and reduce environmental pollution.However,the brittleness of PLA limits its application fields.In this work,we firstly prepared high-strength and ultra-tough PLA samples by pressure-induced flow(PIF)processing,and investigated the effects of PLA initial state,processing pressure and compression ratio on the microstructure and properties of PLA respectively.The results showed that both amorphous PLA(A-PLA)and crystalline PLA(C-PLA)exhibited a layered internal structure after PIF:the PIF-A-PLA sample exhibited a spherulite structure in the skin,and a spherulite and shish-kebab coexistence in the core;the PIF-C-PLA sample exhibited a compressed and deformed spherulite structure in the skin and a microfiber oriented“brick-mud”structure in the core.The orientation of both PIF-A-PLA and PIF-C-PLA samples increased and showed various trends with processing pressure and compression ratio.The results of mechanical properties showed that the mechanical properties of PIF-A-PLA samples were not improved,while the impact strength,tensile strength and elongation at break of PIF-C-PLA samples were improved by 36.8 folds,2.1 folds and 12.2 folds,respectively,achieving the brittle-tough transition.In order to describe the deformation of spherulites in C-PLA during PIF processing,a compression model was established to predict the degree of deformation of spherulites(?_spherulite),and the model prediction was basically consistent with the experimental results,and a critical?_spherulite of around 2.6 was obtained.Due to the large internal stress within the PIF-C-PLA samples,this paper proposed the use of supercritical carbon dioxide(sc-CO₂)foaming technique to treat the PIF-C-PLA samples to reduce their internal stress.The effects of foaming temperature on the microstructure and properties of foamed PLA(FOAM-PLA)were investigated.The results showed that FOAM-PLA formed a special bimodal distribution structure of micro-nanopores in the core layer,which effectively reduced the internal stress of PIF-C-PLA samples.With the raising of foaming temperature,the cell size decreased and the cell density increased.Although sc-CO₂ foaming reduced the mechanical properties of PIF-C-PLA,the impact strength could still reach 32.3 k J/m² with reduced internal stress,which was 10 times that of C-PLA.The evolution of microstructure of PLA after PIF processing and sc-CO₂ foaming treatment was proposed,and the mechanism of sc-CO₂ foaming to reduce the internal stress of PIF-C-PLA was discussed.