Impact Of Zn(?)Ions On Crystallization And Thermal Properties Of PLA

Poly(lactic acid)(PLA)is one of the most extensively developed biodegradable polymers.PLA has been considered as a promising substitute for petroleum-based polymers in a wide range of commodity and engineering applications.Although PLA has many unique properties,such as good transparency,glossy appearance,high rigidity and good processing properties,however,PLA suffers from serious drawbacks including low glass transition temperature,low crystallinity and slow crystallization rate.The glass transition temperature(Tg)of PLA is in the range 55-60?.Because of these drawbacks,PLA possesses low service temperature(i.e.low heat deflection temperature,or HDT).Usually the crystallization rate of PLA can be improved by adding nucleating agent,which can reduce the surface free energy barrier towards nucleation.In this paper,non-toxic and non-polluting Zn2+ ion is used as a crystallization accelerator to accelerate the crystallization rate of polylactic acid,increase the crystallinity,and improve the heat resistance of PLA materials.Three kinds of PLA materials doping with Zn2+(PLA/ZnCl2,PLA/ZnSt,PLA/ZnOAc)were prepared by compression molding.After Zn2+is added to the PLA matrix,the results of infrared spectroscopy(FTIR)and variable temperature Raman spectroscopy(Raman)prove the interaction of Zn2+with the ester bond of the polylactic acid matrix.Thermogravimetric analysis(DTG)results show that as the content of Zn2+increases the thermal stability of the materials has decreased.Isothermal crystallization studies show that the crystallization rate of PLA/Zn2+materials is greatly increased,and non-isothermal crystallization studies show that the crystallinity of PLA/Zn2+ materials is increased to a maximum of 24.83%,Tg and Tc are also significantly reduced.X-ray diffraction(XRD)results again confirm the increase in crystallinity.At the same time,the Vicat softening temperature(VST)of the materials has also been increased from 64? to 79.1?,which has expanded the use temperature of the materials.Scanning electron microscope(SEM)results show that Zn2+has agglomerated in PLA matrix.As a comparison,we also prepared PLA materials doping with MgSt and CaSt using the same process conditions.The results prove that infrared spectroscopy show that the position of each characteristic peak is basically unchanged,and no new characteristic peak appears.Similarly,the results of thermogravimetric analysis(DTG)show that as the contents of Mg2+and Ca2+increase,the thermal stability of the materials decrease gradually.The results of differential scanning calorimetry(DSC)and X-ray diffraction(XRD)show that the crystallinity of the materials is not substantially improved,which once again prove the promoting effect of Zn2+on PLA crystallization.The Vicat softening temperature of PLA/MgSt and PLA/CaSt materials is slightly increased,but not obvious.We also studied the effect of different annealing temperatures on the crystallinity of PLA/Zn2+materials.XRD results show that annealing can greatly promoted crystallization.Annealing at 60? near the glass transition temperature Tg,the crystallinity of materials is not increased.When the annealing temperature reached 100?,110?,and 120?,the crystallinity of the materials is greatly improved,and when the temperature is 110?,the material has the highest crystallinity and the most perfect crystallization.