Synthesis Of Polylactic Acid And Study On Its Multi-Component Blending Modification

With the increasingly severe problems of environmental pollution and energy depletion,people have vigorously developed biodegradable materials.Scientists have developed a series of biodegradable polymers through microbial fermentation,artificial synthesis,and other methods,with polylactic acid(PLA)being one of the most focused materials.The monomer for PLA synthesis is lactic acid,which can be obtained from biomass resources such as starch and sugar.PLA can be synthesized by melt condensation of lactic acid or by ring-opening polymerization of lactide.PLA prepared by the latter method often has a higher molecular weight and more substantial mechanical properties.However,this method requires high purity of lactide,and the technology for preparing high-purity lactide has yet to be mastered domestically.PLA can also be prepared by melt condensation and then cross-linked using 1,3propanediol bis(2-oxazoline)(1,3-PBO)and hexamethylene diisocyanate(HDI)to increase its molecular weight.The cross-linking reaction can introduce benzene ring groups into the PLA molecular chain to improve the thermal and mechanical properties of the polymer.PLA is a rigid material with high strength and modulus at room temperature,but its toughness is poor,and its crystallization rate is slow.These shortcomings limit its application scenarios,and modification is needed to meet different application requirements.Polybutylene adipateco-terephthalate(PBAT)is a highly tough rubbery material at room temperature,and blending it with PLA is expected to complement the properties of PLA.However,PBAT and PLA are incompatible two-phase systems.PBAT-PLA copolymers and hydrophobic modified CNC are used as compatibilizers to improve the interfacial compatibility of the two phases,thereby preparing fully biodegradable blends with both strength and toughness.This thesis focuses on the synthesis of PLA and the properties of co-blended modified composites,as follows:(1)Synthesis of high molecular weight PLA by chain extension method:A sample of PLA with Mw of 36400 g/mol was synthesized by melt condensation,and 1,3-PBO and HDI were used as chain extenders to carry out the chain extension reaction.The effects of reaction temperature and reaction time on the chain extension reaction were studied,and according to the experimental results,a chain-extended polymer with a weight average molecular weight of up to 274,000 g/mol could be obtained under the conditions of a reaction temperature of 180℃ and a reaction time of 30 min.The mechanical and thermal properties of the chainextended polymer were improved compared to pure PLA.(2)Compatibilization of PBAT-PLA copolymer binary blends:Different mass ratios of PLA/PBAT binary blends were prepared by melt blending,and morphology observation and interfacial tension calculation results showed that PLA and PBAT were incompatible twophase systems.PLA was dispersed in tiny droplets in the PBAT matrix,and as the PLA content increased,the interface morphology of the blend changed from "sea-island" to a bicontinuous phase structure.Adding copolymers can reduce the size of the dispersed phase and enhance the interfacial adhesion to improve interfacial compatibility.Compatibilizers to the binary blends improved their yield strength,elastic modulus,and elongation at break.(3)Hydrophobic modified CNC and PLA synergistically plasticized PBAT:Hydrophobic modification of CNC was carried out using tannic acid and octadecyl amine.A ternary composite material was prepared by solution blending.The experimental results showed that the ternary composite material with 2%modified CNC had the best overall performance.Compared with the binary blend,its yield strength(11.8 MPa-11.5 MPa)and fracture strength(23 MPa-23.2 MPa)did not decrease significantly,but the elongation at break was greatly improved(538.3%-836.5%).Scanning images showed that the debonding of modified CNC during stretching would promote matrix cavitation and silver marking and ultimately promote in-situ fib erization of the matrix,which is the fundamental reason for the plasticization of the composite material.