Synthesis Of Bio-based Elastomers Based On Disulfide Bond And Toughening In Polylactic Acid

The development of traditional petroleum-based polymer materials is facing problems such as resource crisis and environmental pollution.One of the valid ways to solve these problems is to develop more"sustainable"bio-based polymers.Polylactide(PLA)is a thermoplastic aliphatic polyester derived from plant resources and be considered as a possible material to replace petroleum-based materials in some traditional fields by virtue of its advantage including good biocompatibility,high mechanical strength,easy processibility,and environmental friendliness.However,its inherent brittleness severely limits its application development.Therefore,aiming at that defect of PLA,dynamic vulcanization of elastomers and polylactic acid where the elastomer will crosslink has received plentiful research.However,it often requires the complicated process,long reaction and appropriate amount of vulcanizing agent,which may increase operating costs and not conducive to actual application.Thus,in this context,we use cross-linked elastomers to directly toughen polylactic acid.However,cross-linked elastomers as typical thermoset materials are often not processable,and not easily dispersed in the matrix during the blending process.Recently,the applications of reversible dynamic covalent bonds in the cross-linking polymer to form a dynamic adaptive network and endow the reprocessable property of materials are widely reported.Under these circumstances,based on the design concept of dynamic adaptive network,dynamic disulfide bonds are introduced into the bio-based aliphatic polyester elastomer to construct a bio-based vitrimeric elastomer which is reprocessable and can be topological rearrangement at high temperatures.Furthermore,the elastomer is used to melt blending with polylactic acid.Without adding other components,co-continuous network is successfully formed by virtue of its characteristics of the dynamic adaptive network,and significantly improves the impact toughness of the blending.Firstly,this article uses succinic acid,adipic acid,sebacic acid,and butanediol as raw materials,and introduces a small amount of 3,3-dithiodipropionic acid and glycerin to synthesize the bio-based dynamic covalent adaptive elastomer BEG_xS_y through polycondensation.Meanwhile,glycerol and disulfide bonds provide cross-linking sites and dynamic covalent bonds for elastomers,and are used as variables to adjust the performance of elastomers.The change of glycerol content will affect the cross-linking density and crystallization properties which can adjust the mechanical properties and self-healing properties of the elastomer.During the experiment,it is found that when the glycerol content fixed at 0.5 mol%,the material has the highest elongation at break(1700%),besides the elastomer can finally achieve complete self-healing at room temperature for 20 minutes due to the low cross-linking density and the rapid mobility of the polymer chain.To the best of our knowledge,this is the first case to realize room temperature self-healing polyester elastomer bearing aliphatic disulfide bonds.When adjusting the proportion of glycerol content(0.5-3 mol%)and DTPA fix at 10 mol%,all elastomers maintain good reprocessing ability(mechanical properties can be recovered100%after four times processing),and high elongation at break(500%-1700%).On the other hand,the change of disulfide bonds will affect the crystallization,reprocessing,and self-healing ability of the material.The experimental results show that the existence of disulfide bonds can destroy the regularity of polymer chains and effectively inhibit the crystallization of materials.It is worth noting that,the control(without disulfide bonds)cannot be processed,indicating that the existence of disulfide bonds plays a vital role in the reprocessing ability of the material.At the same time,the content of dynamic covalent bonds will also affect the self-healing ability of the material due to the interface exchange of disulfide bonds.Secondly,we melt blended BEG_xS_y with PLA to toughen polylactic acid after the successful preparation of the bio-based vitrimeric elastomer.It can be found that,the elastomer with a linear structure cannot effectively toughen PLA because the linear elastomer has a small particle size which cannot be availably used as a stress concentration point leading to a poor energy dissipation.Similarly,the cross-linked elastomer without disulfide bond,as a traditional thermosetting material,the agglomerated particles in the PLA matrix are visible to the naked eye,so its toughening effect is unsatisfactory.In contrast,the vitrimeric elastomer with a moderate crosslinking density can be uniformly dispersed in the PLA matrix during the blending process and is conducive to construct co-continuous or pseudo-continuous network which prominently enhances the toughness.Then,we systematically studied mechanism of vitrimeric elastomers in the construction of continuous structure from the perspectives of the viscosity ratio of the matrix and the elastomer during the blending process,the interface modulus and the relaxation of the elastomer at high temperatures.Finally,a co-continuous structure was successfully constructed by adjusting the structure and component content of the elastomer,and the impact performance of the blend was increased to 436 J/m.In a nutshell,we first synthesized a vitrimeric polymer via introducing dynamic disulfide bonds into the cross-linked elastomer,and then blend the vitrimeric polymer directly with PLA.By adjusting the cross-linking density and component content of the vitrimeric polymer,the construction of the co-continuous phase and the improvement of the toughness can be achieved.This novel tactic of forming a co-continuous network in incompatible two phases have the advantages of facile,easy adjustment,short blending time,and no introduction of vulcanizing agent,providing a new perspective for toughening PLA.