Study On The Toughening Mechanism Of Poly(Ethylene Oxide) To Poly(Lactic Acid)

Bio-based polylactic acid?PLA?has been acknowledged as a potential alternative to traditional petroleum-based polymers in many application fields like packaging,textile and medical appliance industries.Due to the high glass transition temperature(T_g-PLA)of close to 60?,the inherent brittleness of PLA significantly impedes its wide utilization.Introducing partially compatible polymers as plasticizers is an effective strategy to improve its flexibility and toughness,which has drawn increasing attention.However,a great deal of past investigations were mainly focused on the crystallization kinetics and crystalline morphology of PLA affected by flexible component.There is still a lack of fundamental understanding on the toughening mechanism through a deep insight into the formation and evolution of microscopic structures and morphologies under deformation.In the present work,the PLA/polyethylene oxide?PEO?composite films have been studied by combining the wide angle X-ray diffraction?WAXD?,small angle X-ray scattering?SAXS?,Fourier transform infrared spectroscopy?FTIR?,differential scanning calorimetry?DSC?,polarized optical microscopy?POM?and scanning electron microscope?SEM?measurements.The effects of different cooling rates and different PEO contents on the initial structure of composite films were investigated.Thereafter,in situ WAXD measurements were conducted to follow the microscopic structural evolution under tensile deformation,which was associated to the stress-strain curve to understand the physical mechanism behind the enhanced ductility and toughness of PLA/PEO composite flims.The main experimental results and conclusions are summarized as below.The formation of PLA mesophase and its content variation in PLA/PEO composite films were analyzed.Two different melt solidification strategies including ice-water quenching?WQ?and slow cooling?SC?were employed to prepare the PLA/PEO composite films.DSC measurement demonstrated a good compatibility between PLA and PEO molecular chains due to an obvious decline of T_g-PLA with increasing PEO content.From the FTIR and WAXD measurments,the SC-films contained mainly PLA crystal,while the WQ-films were dominated by PLA mesophase that corresponded to infrared absorption peak of 918cm^-1 and X-ray diffraction 2?of 16.8°.The PLA mesophase showed a non-monotonic variation of content with increasing PEO component,which increased first and then decreased,and reached the highest value of15.2%when PEO component was 24 wt%.The formation of PLA mesophase was mainly attributed to the enhancement of molecular mobility of PLA due to the presence of flexible PEO chains,where the PLA crystallization was avoided by melt quenching.The toughening mechanism of PLA mesophase to PLA/PEO composite films was investigated.Tensile mechanical tests showed that the WQ-films containing PLA mesophase exhibited a much higher ductility and toughness than the SC-films containing PLA crystal.According to SAXS,POM and SEM observations,the good ductility and toughness of WQ-films were ascribed to the formation of a large number of microfibers during tensile deformation,which could inhibit the mechanical instabilities like strain localization and microcrack propagation.By performing in situ WAXD measurement,the PLA mesophase was confirmed to directly participate in the formation of microfibers,which occurred through a possible way of block fragmentation,orientation and slipping.At the same time,tensile deformation could induce the further generation of more PLA mesophase,which displayed a content increase highly consistent with the stress enhancement after tensile yielding.These results indicate that the structural basis for the improved plasticity and toughness of PLA/PEO composite films is PLA mesophase rather than PLA crystal.It can be speculated that,in other similar PLA-based polymer composite systems,the mesophase also plays a critical role in acquiring the plasticity and toughness.