| Poly(lactic acid)(PLA)is a biobased,diodegradable aliphatic polyester with excellent mechanical strength and stiffness,as well as excellent transparency.In addition,the raw material of poly(lactic acid)can come from biomass materials such as starch,which gives people a new choice in today's oil crisis and white pollution.However,the inherent brittleness of PLA limits its application in many aspects.Therefore,many researchers at home and abroad have toughened poly(lactic acid)in various ways.Such as adding plasticizers,adding nanoparticles,copolymerizing toughening,etc.However,these methods have the disadvantages of high cost and unsuitable for industrial production,greatly reducing the mechanical properties of PLA,and reducing the degradability of PLA.Toughening PLA with flexible degradable polyesters such as polybutylene adipate/terephthalate(PBAT),polybutylene succinate(PBS),polycaprolactone(PCL),etc.However,the common degradable polyesters also have problems such as high cost,insufficient production capacity,poor compatibility with PLA,and poor mechanical properties.properties Poly(4-hydroxybutyrate)(P4HB)is a new generation of PHA,a bio-based biodegradable polyester with high toughness.It has good biocompatibility,can be completely degraded into carbon dioxide and water,good processability and low melting point.At the same time,my research group has realized the synthesis of high molecular weight P4HB based on the self-synthesized organophosphazene base/urea binary catalytic system,and the P4HB synthesized by chemical method has similar mechanical properties to P4HB obtained by biological fermentation.The chemical synthesis method can greatly reduce the cost of P4HB,which promotes the commercial application of P4HB.In this paper,poly(lactic acid)(PLA)was used as the matrix and poly(4-hydroxybutyrate)P4HB was used as the toughening particle to prepare a new fully biodegradable composite material,and its toughening mechanism was studied.At the same time,the toughening effect and mechanism of P4HB on amorphous PLA and crystalline PLA were summarized and analyzed,which provided the basis for further research on the system.The research contents mainly include:(1)The structure and properties of PLA/P4HB composites with different P4HB contents were firstly studied.By studying the thermodynamic properties,it was found that P4HB and PLA were incompatible.By studying the crystallization properties,it was found that PLA was still amorphous with the addition of P4HB.By studying the mechanical properties,it is found that the elongation at break of the composite material increases with the increase of the P4HB content,and the tensile strength of the composite material gradually decreases;when the content of P4HB is 30%,the composite material has the highest impact strength of 6.8 k J/m~2 and the elongation at break exceeds 140%,while the composite still maintains a high tensile strength(48 MPa).In order to study the reasons for the low impact strength of composites,the mechanical properties and compatibility of PLA/PCL and PLA/P4HB composites were studied.The results showed that the viscosity of P4HB would affect the impact properties of composites.The impact strength of the composite prepared by using high viscosity P4HB was increased to 12.2k J/m~2.Further analysis showed that the particle size of the dispersed phase of P4HB was the fundamental reason for the impact strength of the composite.(2)In order to study the effect of P4HB dispersed phase particle size on the impact properties of composites,a series of P4HB composites with different impact properties and different particle sizes were prepared by using 3 different torque speeds(40 r/min,60 r/min,100 r/min),5 different molding methods(M1-M5),and 3 different viscosities of P4HB.Furthermore,the analysis of its microstructure found that the impact strength of amorphous PLA/P4HB composites has a significant relationship with the particle size of P4HB,when the particle size of P4HB is 1.1-1.3?m,the composite material has high impact strength,which can reach more than 20 k J/m~2.Finally,the toughening mechanism of P4HB on amorphous PLA was obtained through analysis.When the P4HB in the composite was impacted,cavitation induced crazing and shear band termination crazing,which absorbed impact energy in the process and toughened.(3)Many studies have shown that toughening of crystallized PLA can obtain composites with higher impact toughness,and crystallized PLA is more heat-resistant,so it has more research significance and practical value.First,PLA/P4HB composites with high crystallinity were prepared by annealing.After annealing,the heat resistance of the composites improved,but the impact strength of the composites decreased as the particle size of P4HB increased and the phase interface of PLA deteriorated.Therefore,the compatibilizer was added to the composite for improvement,and the impact strength of the composite after adding 0.5 wt%DCP was significantly improved to 6 times that of PLA after annealing.Analysis of the microstructure shows that the composite material undergoes shear yielding.Further research shows that the generation of shear yielding is related to the particle size of P4HB and the distance between toughened particles. |
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