Synthesis Of Degradable Branched Copolymers And Their Applications In Modification Of Poly (L-lactic Acid)

As a biodegradable polymer,polylactic acid（PLA）is the most widely studied and used biopolymers owing to its numerous advantages including its biomass raw materials,good biocompatibility,excellent mechanical properties and thermoplastic processability.Nowadays,it has been widely used in medical and environmentally friendly general-purpose materials However,the extensive application of PLA has been severely limited by its inherent brittleness and its low melt strength which leads to poor processability.Therefore,it is necessary to improve the toughness and the melt strength of PLA.The methods commonly used for toughening PLA mainly include plasticization,copolymerization,nanocomposite and blending.Among them,blending,especially for rubber/elastomer toughening,is the simplest and most effective way.However,in most cases,Blending,especially for rubber/elastomer toughening,is the simplest and most effective way to achieve the high toughness of PLA.However,in most cases,the obvious enhancement of toughness by introduction of rubber/elastomers into PLA matrix may come at the expense of biodegradability,thermoplastic processability and mechanical properties.On the other hand,this kind of toughening agent and the PLA matrix will also form an obvious phase separation in the melt state,and the interaction of the two-phase interface is weak,resulting in the blended toughening system not being able to improve the rheological processing properties of PLA.Therefore,it is still a challenging task to develop a toughening modifier that does not introduce non-degradable components and chemical cross-linking structure,and can improve the toughness and melt strength of the material at the same time.Concerning on the above problems,we synthesized two branched degradable polycaprolactone-polylactic acid block copolymers(LBC_CL-LA and TBC_CL-LA).On the one hand,the chain entanglement of the branched PCL can be stabilized by stereocomplex crystals（SCs）formed beforehand between the copolymers with different optical PLA segments to build a crosslinked network structure like rubbers inside the toughened phase.Meanwhile,the co-crystallization between copolymers and the PLA substrate can improve the compatibility between the toughening phase and PLA substrates.Therefore,while realizing the improvement of material toughness,it still has excellent biodegradability and high mechanical strength.On the other hand,the stereocomplex crystals,which can be stored well in the molten state of modified PLA,which can effectively enhance the interaction between polymer chains in the melt state,thereby enhancing the melt strength of the modified PLA and improving its rheological processing performance.The main contents and results of this thesis are as follows:1.Long-chain branched copolymers LBC_CL-DLA and LBC_CL-LLA with different optical PLA segments were synthesized.The toughening agent（Pseudo-crosslink toughener,abbreviated as PCT）with a rubber cross-linked structure is prepared by the solution blending of LBC_CL-DLA and LBC_CL-LLA,in which the chain entanglement of LB_CL was stabilized by the SCs formed by co-crystallization of PDLA and PLLA segments between copolymers with different optical PLA segments.Then the PCT modified PLA system is prepared by melt blending.With the remaining PDLA segments or the PLLA segment of PCT interacting with the PLA segment in the substrate to improve the compatibility of PCL and PLA,the compatibility between the toughened phase and the PLA matrix is effectively improved,so that the toughened phase can effectively induce plastic deformation of the matrix,thereby achieving the purpose of high-toughness of PLA.The effects of copolymer structure and mass ratios of LBC_CL-DLA and LBC_CL-LLA on the thermal properties of PCT and the mechanical properties,non-isothermal crystallization properties,dynamic mechanical properties,rheological properties and micro morphology of the modified PLA were systematically studied,and the toughening mechanism was proposed.DSC and DMA test results show that a rubber-like network structure is formed in the toughening agent PCT.The mechanical properties test results show that the toughness of the modified PLA is improved:the elongation at break increased from 8.1%to 222.9%,the tensile strength was maintained at 54.4 MPa,and the impact strength increased from 3.2k J/m² to 27.2 k J/m².2.In view of the difficulty of controlling the structure of long-chain branched copolymers and the complicated preparation methods,the star-shaped three-arm copolymers TBC_CL-DLA and TBC_CL-LLA containing PLA segments with different optical isomerism were synthesized through simple polyhydroxy compounds in one step.And then the TBC modified PLA system was prepared by melt blending.On the one hand,The lengths of the PDLA segments and PLLA segments in copolymers with different optical isomers are similar,so the two can selectively and preferentially co-crystallize to form a"long-chain branched molecular chain entanglement network"with stereocomplex crystals as branching/crosslinking points.On the other hand,the remaining PDLA segments in TBC_CL-DLA can interact with the PLA segments of the substrate to improve the compatibility of PCL and PLA and build a"network"which penetrates the PLA matrix and toughing phase.This penetrating network can not only improve the toughness of PLA,but also induce the two phases of the modified PLA to share the stress as a whole,effectively avoiding stress concentration,so that it has almost the same tensile strength as pure PLA.The effects of copolymer structure and copolymer amount on the mechanical properties,non-isothermal crystallization properties,dynamic mechanical properties,rheological properties and micro morphology of modified PLA were systematically studied.The mechanical properties test results show that the total amount of the copolymer exceeds 5 wt%,the tensile toughness of PLA can be significantly increased:the elongation at break increased from 8.1%to 171.5%-221.5%,and the tensile strength only slightly decreased from70.3 MPa to 68.5-56.5 MPa,which greatly retained the tensile strength of PLA.3.The LBC_CL-LLA modified PLA system was prepared by melt extrusion.The long-chain branched copolyme has strong chain entanglements,thereby enhancing the melt strength of the modified PLA.The effect of the amount of copolymer on the non-isothermal crystallization behavior,rheological behavior and blown film processing of PLA were systematically studied.Blown film processing of the modified PLA is good,and their bubble are stable.4.Aiming at the problem that the long-chain branched copolymers modified PLA system crystallizes slowly and easily deforms in the blown film processing,the TBC_CL-DLA modified PLA system was prepared by melt extrusion.The stereocomplex crystals are formed to construct a physical network in the modified system,improving the melt strength of PLA while promoting the fast crystallization of PLA matrix.The effects of the amount of copolymer and the content of PDLA segments on the non-isothermal crystallization behavior,rheological behavior and blown film processing of PLA were systematically studied.Blown film processing of the modified PLA shows that the blown film processing properties of the modified PLA is good,and their bubble are stable.In addition,only adding 5 wt%of TBC_CL30-DLA60%,the bubble of the modified PLA exhibits increased deformation resistance,therefore producing a flat film.