Effect Of Physical Aging On Properties Of Polylactic Acid And Evolution Of Heterogeneity During Aging

Poly(lactic acid)(PLA)has the characteristics of bio-base,biodegradability and biocompatibility.It has been widely used in biomedical,food packaging and other fields.However,one of the disadvantages of PLA is that it is prone to physical aging during storage and service,resulting in brittle fracture under stress.Therefore,studying the physical aging behavior and structural evolution mechanism of PLA will help to provide a theoretical basis for overcoming this shortcoming.So far,few studies on the physical aging of PLA have explored the structural evolution of PLA during physical aging from an experimental point of view.Firstly,this thesis systematically studied the effect of physical aging on the properties of PLA,especially the deformation mode of PLA under different physical aging levels.Then,the evolution process of heterogeneity of PLA during physical aging is studied,and the possible physical aging mechanism is put forward.The main results are as follows:(1)The effects of physical aging on the enthalpy relaxation,crystallization behavior and mechanical properties of PLA were studied.The results show that the enthalpy relaxation of PLA changes significantly at the initial stage of physical aging,but tends to flatten with the extension of physical aging time.This process can be well described by Kohlrausch Williams Watts(KWW)equation.In addition,physical aging promotes the cold crystallization of PLA,moves the cold crystallization temperature to low temperature,and the formed crystal size is smaller and the distribution is more dense and uniform.In particular,the tensile test results of PLA with different physical aging levels show that the elastic modulus of PLA increases with the increase of physical aging time,while the elongation at break decreases from 265.6%without aging to 6.8%(aging at 40 ~oC for 4 h).The corresponding atomic force microscope(AFM)results show that the PLA without physical aging deforms in the form of shear band in the tensile process,while the PLA after physical aging for 4 h deforms in the form of craze in the tensile process.The craze parallel to the shear band is observed in the PLA after aging for 1.5 h,which indicates that the internal deformation mode of PLA in the intermediate state of brittle-ductile transition is the coexistence of shear band and craze.(2)The evolution process of heterogeneity of PLA during physical aging was detected by AFM.Combined with the energy loss conversion formula and correlation function analysis,the evolution of local heterogeneity of bulk samples and thin film samples during physical aging was studied.The results show that after 10 h physical aging at 40 ~oC,the correlation length of the heterogeneous region in PLA increases from 1.6 nm to 4.8 nm.This shows that with the increase of physical aging time,the scale range of spatial heterogeneity region in PLA is also increasing.The increased scale is only a few nanometers,which indicates that the scale of the spatial heterogeneous region formed in PLA during physical aging is at the nano-level.This can be attributed to the cohesional entanglement formed by the movement of side groups(methyl groups)on the internal molecular chain of PLA during physical aging,and the density fluctuation of entanglement nodes forms the internal spatial heterogeneous structure with local order on the nano-scale.This local ordered structure is very unstable.Once the temperature exceeds the glass transition temperature of PLA,this heterogeneous structure will disappear immediately.