| Polylactic acid(PLA)is a renewable degradable polymer material.Its raw materials mainly come from natural crops(such as corn,potatoes,etc.)and can be completely degraded in nature.The degradation products are water and carbon dioxide,which will not pollute the ecological environment.PLA has high mechanical strength,good biocompatibility and transparency,and is easy to be processed.Therefore,replacing petroleum-based polymer materials with PLA can effectively alleviate environmental pollution and energy crisis.PLA materials are widely used in agriculture,biomedicine,packaging and other fields.However,PLA's inherent brittleness and low heat-resistant deformation temperature seriously limit its application in related fields.In this paper,PLA is modified to overcome the above two defects.Polyhexanedioic acid-butylene succinate(PBSA)and polyhexanedioic acid-butylene terephthalate(PBAT)were melt blended with PLA respectively,and the blend system was compatibilized with polyepoxy chain extender ADR to toughen PLA.Then the crystallinity of PLA-based blends was increased by isothermal crystallization,so as to enhance their heat resistance.PLA blends with high toughness and heat resistance can be prepared by the combination of two modification methods,so as to broaden their application scope.Specific research contents are as follows:(1)Rheological properties,impact properties,heat resistance,micro-morphology and isothermal melt crystallization kinetics of PLA/PBSA blends compatibilized by ADR have been studied.The results of torque and melt mass flow rate(MFR)show that the addition of ADR lead to chain extension reaction of PLA and PBSA in the blends,which results in the increase of molecular weight and melt viscosity of the blends.As a result,the torque value increases and MFR value decreases with increasing ADR loading.SEM photos show that the dispersed phase particles become smaller,and the interface between the two phases become blurred with the increase of ADR loading.The compatibilizing and refining effect of ADR on the blends enhance the PLA/PBSA interfacial bonding force.The refined dispersed phase particles can easily induce the PLA matrix to yield deformation under impact,which may absorb impact energy,and lead to an enhancement in impact strength.But,the impact strength of the blends begins to decrease when the ADR loading is more than 0.3 phr.This is due to the existence of the optimum average particle size of the dispersed PBSA toughened PLA.The impact strength of the materials reaches the maximum value when the average particle size of the dispersed phase is 1.40 ?m.Isothermal melt crystallization results in regular and compact stacking of polymer chain segments,which make the interaction force increase between chain segments.This results in a larger impact strength of crystallized samples compared to that of quenched samples.Isothermal melt crystallization kinetics studies show that crystallinity(Xc)and crystallization rate are related to ADR loading and crystallization temperature(Tc),respectively.With the increase of ADR loading,the crystallized rate increases,and the crystallinity decreases.With the increase of Tc,the crystallinity increases.However,the crystallization rate exhibits a maximum at 105 ?,which is determined by the interaction of the crystallization rate on the two processes of crystallization(nucleation and crystal growth).The crystallinity of the PLA/PBSA/ADR blends increases by isothermal melt crystallization,which effectively improves the heat resistance of the materials.(2)Rheological properties,impact properties,heat resistance,micro-morphology and crystallization kinetics of PLA/PBAT/ADR blends have been investigated.Torque and melt mass flow rate(MFR)tests indicate that ADR's chain extension reaction to PLA and PBAT increases the melt viscosity of the blends.This directly leads to that the torque value of the PLA/PBAT/ADR blends increasing,and the MFR value decreasing when content of ADR increasing.The PLA-ADR-PBAT block polymer produced by the chain extension reaction has a compatibilizing effect on the blending system,so that the interfacial adhesion between the two phases is enhanced.Therefore,the higher the ADR content,the greater the impact strength of the blends.The dependence of isothermal melt Crystallization and isothermal cold crystallization on the two crystallization processes(nucleation and crystal growth)determines the relationship between the two isothermal crystallization methods and ADR content as well as crystallization temperature(Tc).In the isothermal melt crystallization process,low temperature is conducive to nucleation,and high temperature is conducive to crystal growth.The crystallization rate of PLA/PBAT/ADR blends is determined by nucleation and crystal growth,but the effect of them on the crystallization rate is opposite,which will inevitably lead to the maximum crystallization rate at a certain temperature.Study on crystallization kinetics,the crystallization rate of PLA/PBAT/ADR blends is the fastest when crystallization temperature is 105?.High temperature contributes to crystal growth,so the crystallinity of the blends increases with the increase of temperature.The chain extension reaction initiated by the addition of ADR increases the melt viscosity of the blends,which is conducive to the nucleation of polymer segments arranged closely,so the crystallization rate increases with the increase of ADR content.However,high melt viscosity is not conducive to the kinematic rearrangement of polymer segments for crystallization,so the crystallinity of PLA/PBAT/ADR blends decreases with the increase of ADR content.Isothermal cold crystallization produces a large number of crystal nucleus during cooling process,so the crystallization rate is mainly determined by the growth rate of crystals.The crystallization rate increases with the increase of temperature,while the crystallization rate decreases with the increase of ADR content,which is obviously different from that of isothermal melt crystallization.The relationship of crystallinity of isothermal cold crystallization with crystallization temperature and ADR content is similar to that of isothermal melt crystallization.The corresponding samples with high crystallinity will be obtained when the PLA/PBAT/ADR samples were treated by isothermal melt crystallization and isothermal cold crystallization at 105? respectively.A large number of crystals in the crystalline samples can effectively disperse the stress and produce synergistic effect with the dispersed phase to induce a large number of plastic deformation to increase the impact strength of PLA matrix.Therefore,the impact property of the crystalline samples is better than that of the quenched samples.It can be seen from POM photos that isothermal melt crystallization is conducive to the formation of larger and more developed crystalline structure,which can more effectively disperse impact stress.This is also the reason why the impact strength of isothermal melt crystallized samples is slightly greater than that of isothermal cold crystallized samples.By isothermal crystallization treatment,the crystallinity of PLA/PBAT/ADR blends increased significantly,and the vicat softening temperature increased significantly.In addition,the vicat softening temperature of isothermal melt crystallized samples is slightly higher than that of isothermal cold crystallized samples. |
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