Microstructure Tuning And Performances Of PLLA/EPDM And PLLA/PETG Blends

Since the birth of plastic,its characteristics have shown great value,and the development of society is inseparable from it.So finding a substitute that can be recycled and does not pollute the environment has become an urgent problem to be solved.Poly(lactic acid)(PLA),especially L-Poly(lactic acid)(PLLA)has gradually entered people's vision through its excellent degradation performance in recent years.PLA has no pollution to the environment and has high strength and rigidity,but PLA also inevitably has its own defects,such as inherent brittleness,poor heat resistance,etc.Therefore,many parties have actively carried out PLA research work.In this paper,ethylene propylene diene monomer(EPDM)and poly(ethylene terephthalate glycol)(PETG)were used as toughening resins to modify the PLLA matrix.By adjusting the microscopic morphology of the toughening phase,the tensile ductility of the blends can be improved.Then the relationship between the aggregate structure and the properties of the blends were studied.Finally,the compatibilized PLLA/EPDM and PLLA/PETG blend with excellent comprehensive performance were prepared succesfully.(1)PLLA,EPDM,and organic montmorillonite(OMMT)were prepared by meltcompositing in two molding processes: compression-molding and calendering.Then the PLLA/EPDM/OMMT sheets were prepared separately.Microscopic morphology,tensile properties,crystalline properties,heat resistance and crystalline morphology of the samples were studied by using scanning electron microscope(SEM),tensile test,X-ray diffraction(XRD),differential scanning calorimetry(DSC),vicat softening point(VST),polarizing microscope(POM),etc.Specially,the influences of different molding processing methods and the OMMT addition on the micro-morphology,mechanical properties and crystalline properties of the PLLA/EPDM/OMMT samples were researched in detail.In terms of the mechanical properties,the addition of OMMT can significantly improve the tensile ductility of the PLLA/EPDM blend.According to the SEM pictures,compared to the binary PLLA/EPDM blend,the EPDM domain size of the ternary blend significantly reduces with increasing the OMMT addition,indicating that OMMT can effectively enhance the interfacial compatibility of PLLA matrix and EPDM dispersed phase.At the same time,it is found that the calendered samples show a better tensile ductility than those prepared by the compression-molded samples.It can be attributed to the fact that the strong tensile/shear fields generated in the calendering process can cause the EPDM dispersed phase in the PLLA matrix to have a larger degree of orientation and deformation,and then the spherical/ellipsoidal EPDM particles are transformed into some orientation structures,including fibrous,banded,and lameller structures.Compared to the spherical/ellipsoidal EPDM dispersed phase,the oriented EPDM dispersed phase with a higher specific surface area can effectively induce crazing and prevent the crazing developing into cracks in the transverse direction.In addition,the calendered sample also contains a certain amount of oriented PLLA rigid amorphous fraction(RAF).These two reasons make the calendered samples to exhibit excellent tensile plasticity.After heat-treating the calendered/compression-molded samples,the DSC result shows that the crystallinity of the samples significantly increases compared to the unheat-treated samples.However,the tensile ductility of the compression-molded samples with high crystallinity decreases sharply and could only be maintained at a relatively low level(??25%).Although the ? of the calendered samples with high crystallinity also shows a certain reduction,but it can still be maintained at a relatively high level.It shows that compared to the spherical/ellipsoidal EPDM dispersed phase,the oriented EPDM dispersed phase also has a better enhancing effect on the tensile ductility of the PLLA/EPDM/OMMT samples with relatively high crystallinity.(2)PLLA/PETG blends compatibilized by a multi-component epoxy chain extender(ADR)were prepared by melt blending.Then,the obtained blends were prepared into sheets by compression-molding.The effects of ADR content on the thermodynamic properties,tensile properties,micro-morphology and rheological properties of the PLLA/PETG samples were studied.The torque test results show that the torque of neat PLLA,neat PETG,and PLLA/PETG(70/30)blend melts increases significantly with increasing the ADR addition,indicating that ADR has obvious effect of chain extension and viscosity enhancement on PLLA and PETG.In term of the tensile properties,similar to the neat PLLA,the PLLA/PETG(70/30)sample also shows the typical brittle fractured behavior,which is caused by the poor adhesion of the phase interface.With the addition of ADR,the tensile ductility of the PLLA/PETG/ADR samples increased rapidly.When the ADR addition increases to 0.6 phr,the elongation at break of the PLLA/PETG/ADR(70/30/0.6)sample reaches a maximum of 206.6%.This shows that ADR has a significant effect on the reactive compatibilization and toughening of PLLA/PETG blends.SEM and transmission electron microscope(TEM)results show that the introduction of ADR reduces the PETG domain size largly due to the reactive compatibilization of ADR on the PLLA/PETG interface.The higher the amount of ADR,the more significant the compatibilizing effect.The brittleductile transition behavior of the PLLA/PETG samples are discussed and analyzed by using the craze-shear band theory and Wu's equation.The melt mass flow rate test result and capillary rheology test result show that the chain extension/branching reaction of ADR significantly increases the melt viscosity,resulting in a decrease in melt fluidity.Appropriately increasing the melt temperature can improve the flow properties of the blend melt to a certain extent at high ADR contents.At the same time,from the capillary rheological test results,it can be seen that as the ADR content increases,the pseudoplasticity of the PLLA/PETG blend melt has an increasing trend.It shows that the increase of ADR content will increase the degree of chain extension/branching of the melt,and the flow properties of the melt greatly deviate from Newtonian fluid,and the apparent viscosity of the melt is more sensitive to the change of shear rate.During the film blowing process,the PLLA/PETG(70/30)melt cannot form a stable and uniform film bubble.As compared,the film bubble stability of the PLLA/PETG/ADR(70/30/0.6)melt has been significantly improved due to the thickening effect of ADR.The above experimental results show that the PLLA/PETG blends compatibilized by ADR show a relatively good comprehensive performance,which would have a good researching and application prospects in the field of packaging materials and film materials.