Crystalline Behavior Mediation And Property Of Poly(Lactic Acid)

Polylactic acid(PLA)is a typical material among the environmental friendly polymers.But its inherent disadvantages such as brittleness,slow crystallization rate,and poor thermal stability seriously prohibit its widespread application.It is well known that the structure of polymers determines the property,therefore how to control the internal structure to achieve improved properties has always been a research hotspot.One of the key questions is how to effectively improve the property by crystallization modification.In this dissertation,the crystallization behavior of PLA is regulated via two kinds of methods:by adding poly(D-lactic acid)triblock copolymers with flexible segments and by irradiation technique.The structural evolution during deformation and its relationship with properties were systematically investigated.The research contents and results were summarized as follows:1.Effect of stereocomplex crystal and flexible segments on the crystallization and tensile behavior of poly(L-lactide)The triblock copolymers with soft chains,PDLA-PEG-PDLA,PLLA-PEG-PLLA,and PDLA were selected as modifiers of PLLA.Those mixtures were prepared via melt blending.The crystallization behavior and tensile behavior of PLLA with different modifiers were systematically studied,in order to clarify the effect of stereocomplex crystal and flexible segment PEG on the crystallization of PLA.The results showed that the stereocomplex crystal could form when the blends contained PDLA,and stereocomplex crystal promoted the nucleation of α/α’ crystal.The soft segment PEG could increase the spherulitic growth rate.Compared to PLLA/PDLA blends,the PLLA/PDLA-PEG-PDLA blends increases the elongation at break and maintained higher strength and modulus at the same time.The results of in-situ WAXD showed that the crystallization rate of PLLA/PDLA-PEG-PDLA was higher than that of PLLA/PDLA and PLLA/PLLA-PEG-PLLA blends during the stretching process.The addition of stereocomplex crystal and soft chains PEG not only accelerated the crystallization of PLLA,but also improved the toughness.The effect was much better than the modification effect of the solely stereocomplex crystal or solely flexible segments.The above results provided a guidance for the development of PLA materials with excellent comprehensive properties.2.Crystallization modification of PLLA/PDLA blends via irradiationPLA with triallyl cyanurate(TAC)as the crosslinking agent was subjected to different irradiation treatments by the simple and controllable irradiation technique.The influence of different irradiation methods on the crystallization behavior of PLLA/PDLA blends,especially the effects on stereocomplex crystal was systematically explored.(1)The PLLA/PDLA blends prepared by melt mixing were irradiated,and the effect of TAC content and irradiation dose on the crystallization behavior of PLLA/PDLA was studied.When the irradiation dose was 100 KGy,the irradiated PLLA/PDLA blends with 0.5wt%TAC had the strongest ability to form stereocomplex crystals in the subsequent heating process.(2)The irradiated PLLA and PDLA were used as additives for preparing irradiated PLLA/PDLA blends.The content of stereocomplex crystals increased to 42%,compared to the PLLA/PDLA blends with prepared under the same conditions(6.43%).In addition,the melting point of stereocomplex crystals moved toward higher temperatures.Moreover,the addition of TAC further promoted the formation of stereocomplex crystals in the blends.(3)The in-situ electron irradiation PLLA,PDLA,and TAC was used to prepare PLLA/PDLA blends.The content of stereocomplex crystals could reach 41.2%.Furthermore,the mechanism caused by the introduction of irradiation and TAC was also explored primary.