Synthesis Of Fully Bio-based Citrate And Study On Toughening Of Polylactic Acid

Polylactic acid(PLA)is a biocompatible and biodegradable thermoplastic polyester material with certain transparency,its tensile strength and modulus are comparable to traditional general plastics(such as PS and PP).Therefore,PLA is widely used in biomedical,textile,packaging and other industries.However,the toughness of polylactic acid is relatively poor,which limits its application fields.Therefore,exploring the cost-effective toughening method of polylactic acid has important theoretical and practical value.Blending with flexible polymers is the most commonly used method to increase the toughness of polylactic acid.However,there are two major problems with this type of toughening method.First,most of the flexible polymers used for blending are non-biobased or biomass.After blending,the biocompatibility and biodegradability of the polylactic acid-based material will be destroyed;secondly,the strength and modulus of the blend material will decrease significantly(about 40%?50%).In view of these two problems,using citric acid(CA),itaconic acid(IA),1,3-propanediol or 1,10-decanediol as biomass monomers,two series of bio-based polyesters were synthesized by melt polycondensation,namely poly(1,10-decanediol-co-citrate/itaconate)(PDCI)and poly(1,3-propanediol-co-citrate/itaconate)(PPCI).The blending process of PPCI and its prepolymers with PLA,as well as the structure and properties of blended materials,were mainly investigated.The main work is summarized as follows:(1)Synthesis and properties of fully bio-based citrate elastomer.Using melt polycondensation process,two series of bio-based polyester elastomers-PDCI and PPCI were synthesized.The effect of the ratio of citric acid to itaconic acid,the chain length of diol and the polymerization time on the properties of the polyester were studied.The results show that the polyester is a hydrophilic polyester.The higher the CA content is,the better the hydrophilicity and the degradation performance in vitro show.The gel content of the polyester is directly proportional to the CA content and the polymerization time,and the gel content of PDCI is higher than PPCI.At the same time,the gel content greatly affects the mechanical properties of the polyester.As the CA content or the polymerization time increases,the degree of crosslinking increases,which increases the tensile strength and Young's modulus.However,the elongation at break of the polyester decreases.Among them,the PDCI has a tensile strength of 0.27to 1.23 MPa,a Young's modulus of 0.26 to 4.05 MPa,and an elongation at break of58.4 to 83.8%;the PPCI has a tensile strength of 0.26 to 2.63 MPa,a Young's modulus of 0.44 to 12.27 MPa,and an elongation at break of 226.3 to 307.2%.(2)Preparation and properties of PLA/Citrate elastomer blends.Polylactic acid was blended with synthetic PPCI by a dynamic vulcanization process.The effect of the ratio of CA to itaconic acid IA on the mechanical properties of the blend was investigated.During the blending process,the polyester and PLA successfully crosslinked.From the fractured surfaces morphology of the blends,we can see that the tensile toughness follows the cavitation-shear mechanism.At the same time,polyester has good compatibility with PLA,which causes a large amount of plastic deformation of the matrix during the impact process,improving the impact toughness naturally.The blend of Poly-0.9CA-45h has the best tensile toughness,the elongation at break is17.5%,which is 3.7 times that of pure PLA;The blend of Poly-0.8CA-45h has the best impact toughness,the impact strength is 17.3k J/m~2,which is 6.9 times that of pure PLA.(3)Preparation and properties of PLA/Citrate prepolymer blends.In order to further improve the toughness of the blend,poly(1,3-propanediol-co-citrate/itaconate)(PPCI)prepolymer was used to reactively blend with PLA,and the effect of the ratio of CA to IA on the mechanical properties of the prepolymer blends was studied.When the ratio of CA to IA in the prepolymer is 8/2(0.8CA),the toughness of the blend is the best,the elongation at break and impact strength are 22.0%and 20.8k J/m~2,which is 4.2and 7.8 times that of pure PLA.The tensile strength of all prepolymer blends was maintained at about 43 MPa,a decrease of about 24%compared to pure PLA.The SEM images show that the prepolymer is crosslinked into a granular rubber phase during dynamic vulcanization,and it has good compatibility with the PLA matrix,which improves the toughness of PLA.Cavitation induced matrix shear yielding is the toughening mechanism.Compared to polyester blends,both the tensile and impact toughness of the prepolymer blends are improved.