Effect Of Physical Aging On Tensile Deformation Mechanism Of PLA/PCL Blends And Their Nanocomposites Probed By AFM Nanomechanical Mapping

Poly(lactic acid)(PLA)is a kind of bio-based environmental-friendly plastic with great development potential,but its brittleness caused by physical aging limits its wide application.Blending with immiscible flexible polymers is an effective method to toughen PLA.However,the mechanism of flexible polymer toughening PLA,especially whether physical aging will affect the initiation and development of crazing is still unclear.Crazing is a localized phenomenon during macro deformation of materials,while traditional macro mechanical test combined with small angle X-ray scattering(SAXS),transmission electron microscope(TEM),scanning electron microscope(SEM)and other characterization methods could not detect the local mechanical properties of PLA and its blends during deformation.In this thesis,the atomic force microscope(AFM)based nanomechanical mapping technology was used to quantitatively probe the on-situ morphology and micro Young’s modulus of PLA/poly(ε-caprolactone)(PCL)blends and PLA/PCL/organic modified montmorillonite(OMMT)nanocomposites under different tensile ratios.In particular,the effect of physical aging on the tensile deformation mechanism of the above systems was addressed.The main results are as follows:(1)The effect of physical aging on the tensile deformation mechanism of PLA/PCL blends was studied.Uniaxial tensile results show that the presence of PCL significantly increases the elongation at break of PLA matrix by 72% and 186% for both unaged and aged samples.The AFM based nanomechanical mapping results show that the obvious increase in the heterogeneity of PLA matrix can still be observed in aged PLA/PCL blends in the presence of PCL.Obviously,the presence of PCL could not prevent the physical aging of PLA matrix.The increase in heterogeneity of matrix caused by physical aging will lead to the high heterogeneity of external stress response of PLA/PCL blends,which is manifested in the uneven distribution of micro Young’s modulus of the matrix.In the unaged PLA/PCL samples,the PCL particles are uniformly deformed,while in the aged PLA/PCL samples,a small part of PCL particles are deformed seriously,while the other part of PCL particles are hardly deformed.The stress state of the two PCL particles in the aged samples completely corresponds to the distribution of the micro Young’s modulus of the matrix,which proves that physical aging makes the toughness of the blends worse by improving the heterogeneity of the matrix.In addition,physical aging also has a significant effect on the development of crazing in PLA matrix.Although PCL could not prevent the physical aging of PLA matrix,it can still disperse stress and toughen by initiating a large number of crazes,which is the reason why the toughness of PLA / PCL blend is better than that of pure PLA.(2)The tensile deformation mechanism of PLA/PCL/OMMT nanocomposites after physical aging was studied.Uniaxial tensile results show that after sufficient physical aging the toughness and strength of PLA/PCL/OMMT nanocomposites with0.4 wt% OMMT are the best,and the mechanical properties and stability of nanocomposites are fully improved compared with PLA/PCL blends.AFM nanomechanical mapping results show that the addition of OMMT blurs the interface between PLA matrix and dispersed PCL phase,and effectively compatibilizes the twophase interface.Compared with PLA / PCL blends,the distribution of micro Young’s modulus in the samples before and after deformation is narrowed,indicating that the addition of OMMT improves the heterogeneity of PLA/PCL blends affected by physical aging.However,the addition of OMMT will change the deformation mechanism of PLA/PCL blends,resulting in the weakening of crazing induced by PCL.This is unfavorable to the toughness of PLA/PCL/OMMT nanocomposites.In addition,the results of differential scanning calorimetry(DSC)show that OMMT has a certain inhibitory effect on the physical aging of PLA.The above results are helpful to understand the failure mechanism of PLA/PCL blends and PLA/PCL/OMMT nanocomposites caused by physical aging.