The Morphology And Nanomechanical Properties Of The Polylactide Based Blends And Nanocomposites Probed By Atomic Force Microscopy

Polylactic acid(PLA)is an important biodegradable plastic derived from renewable resources with promising to replace traditional petroleum-based non-degradable plastics.However,brittleness is the drawback of PLA.Blending PLA with flexible polymers such as poly(?-caprolactone)(PCL)or poly(ethylene oxide)(PEO)is an important means to improve its toughness.Adding nanofillers in the blends helps to further regulate their morphological structure and properties.This paper focuses on investigating the morphology and nanomechanical properties of PLA/PCL/multi-walled carbon nanotubes(MWCNTs)nanocomposites,and those of deformed PLA/PEO blends by using atomic force microscopy(AFM).The main results of this thesis are as follows:(1)The micro-morphology and nanomechanical properties of PLA/PCL/MWCNTs nanocomposites with the weight ratio of 80/20/0.5 were probed by both the ScanAsyst and force volume(FV)modes of AFM.Under the ScanAsyst mode,the surface topography of the specimen can be clearly obtained.The PLA and PCL samples are amorphous and crystalline,respectively.The morphology of the blends is a typical “sea-island” structure.For PLA/PCL/MWCNTs nanocomposites,MWCNTs are mainly dispersed in the interface and the PCL phase.The localized distribution of MWCNTs reduces the particle size of the dispersed PCL phase,further demonstrating that MWCNTs can increase the compatibility of PLA and PCL.Under the FV mode,nanomechanical technology has been successfully used to identify and characterize the heterogeneity of nanocomposites containing various components.For PLA/PCL/MWCNTs nanocomposites,the macro-modulus from the tensile testing is 2.69 ± 0.10 GPa,which is consistent with the higher peak modulus(2.72 ± 0.03 GPa)from the FV measurement.For PLA/PCL blends,the macro-modulus of the sample and the peak micro-modulus are different significantly.In contrast,the microscopic morphology of this selective distribution of MWCNTs,to a certain extent,affects the macroscopic properties of PLA/PCL blends.Finally,MWCNTs have no effect on the interface width and strength of PLA/PCL,but it can smooth the morphology of the interface.(2)The micro-morphology and nanomechanical properties of PLA/PEO blends with the weight ratio of 80/20 under different strains were probed by both the tapping and FV modes of AFM.Under tapping mode,the underforned PLA/PEO sample presents clearly a “sea-island” structure in its phase images.In addition,it can be concluded from the phase images that,in the blends,PLA is in the continuous phase,while the dispersed phase is a PEO-PLA compatible phase,and the PEO phase exists in the dispersed phase.From the results of the deforemed PLA/PEO blends,the formation of cavitation in the matrix is the key factor for the brittle-ductile transformation of PLA.In the modulus distribution diagram of FV mode,the entire deformation process is visualized,especially through the change of the disperse phase morphology.The morphology has undergone an interesting process: for the dispersed pahse,it deforms as the strain is 50%,aggregates at the strain of 100%,merges at 200%,and fragments at 225% finally.In the modulus distribution plot,the peaks of PEO,PEO-PLA,and PLA phases were fitted.The resultant changes in peak value can be ascribed to the phase separation caused by stress-induced plastic flow.As the strain is high(200% and 225%),the PEO-PLA phase undergoes phase separation.It produces a PEO-PLA phase with high PEO concentration,and the one with high PLA concentration.