Preparation And Crystallization Behavior Of Polylactic Acid Based Nanocomposites

Bio-based polymer material Polylactic acid(PLA),which is derived from starch and cellulose of plants,is prepared by polymerization.PLA has the advantages of renewable and degradable,due to its good biocompatibility and good mechanical properties,it has been widely used in packaging,medicine,3D printing and other fields.However,the inherent brittleness,slow crystallization rate and poor heat resistance limit the application of PLA in more fields.It is a common method to increase the crystallization rate and crystallinity of PLA by adding plasticizer and nucleating agents.However,a perfect nucleating agent has not yet been developed to enhance the crystallinity of PLA during processing,it is often necessary to eliminate internal stress and promote crystallization by annealing.In this paper,polyethylene glycol(PEG)was used as a plasticizer,and needlelike halloysite nanotubes(HNTs)and flake graphene nanosheets(GNPs)were used as nucleating agents respectively,PEG/HNTs and PEG/GNPs compounding modifie prepared by self-made ultrasonic assisted dynamic dispersion device,finally,series of PLA-based nanocomposites prepared by melt blending through twin-screw extruder for improving the crystallization rate and crystallinity of PLA,meanwhile,improving the thermal stability and mechanical properties of PLA.In this paper,we researched on the following three aspects mainly,firstly,the HNTs/PEG compounding modifier was prepared by self-made ultrasonic assisted dynamic dispersion device,the PLA-based nanocomposites were prepared by melt-quenching process,the effects of compound modifier HNTs/PEG on PLA crystallization behavior,thermal stability and mechanical properties were investigated;Then,the effect of GNPs/PEG compounding modifier on PLA crystallization behavior was studied,and the isothermal crystallization and non-isothermal crystallization kinetics of PLA/GNPs and PLA/GNPs/PEG nanocomposites were analyzed;Finally,we studied the effects of different cooling rates on the crystallization behavior of PLA,and the PLA/GNPs/PEG nanocomposites were annealed with different annealing temperatures and different annealing time,the effect of annealing on their crystallization behavior was investigated.The results indicat that the PLA-based nanocomposites prepared by the melt-quenching process are not perfect in crystallization,HNTs can play the role of heterogeneous nucleation to improve the crystallinity of PLA,and the effect of compounding modifier is more obvious than HNTs,the crystallinity of PLA/HNTs/PEG composites reach a maximum of 10.7% when the content of HNTs/PEG is 0.4 wt%,compared with pure PLA,the thermal stability and mechanical properties of PLA/HNTs/PEG is improved due to the improvement of crystallinity.The addition of GNPs can also promote the crystallization of PLA,but it is prone to agglomeration when the content is large.PEG can weaken the agglomeration of GNPs and increase the speed of PLA molecular chain,two of them form a good synergistic effect,the crystallinity of PLA increased by 266.32% when the GNPs/PEG content is 0.1 wt%.In addition,isothermal crystallization studies show that 0.1 wt% GNPs and compound modifier GNPs/PEG can effectively increase the isothermal crystallization rate of PLA,and 110 °C is the best isothermal crystallization temperature of PLA.Non-isothermal crystallization studies found that the cold crystallization peaks of pure PLA,PLA/GNPs and PLA/GNPs/PEG composites shifted to the high temperature direction and the cold crystallization enthalpy enhance with the increase of heating rate.The cooling rate has a great influence on the melt crystallization behavior of pure PLA during processing,pure PLA hardly crystallizes during the high-speed cooling process,when adding 0.1 wt% GNPs and GNPs/PEG,the crystallization peak appears at 112 °C;When the annealing temperature reaches 100 °C,the crystallization is gradually improved.Annealing can promote the crystallization of PLA,the crystallization of pure PLA is more perfect when annealing at 110 °C,the annealing temperature and time do not change the crystal form of PLA.