Preparations,Structures And Properties Of Toughening Modified Polylactide And Fully Stereo-Complexed Polylactide

Because of the scarcity of petroleum resources and serious white pollution,the development of traditional plastic substitutes becomes a hot research direction.As a green & sustainable material,polylactide(PLA)has great potential to replace the petroleum-based polymer materials.The mechanical strength of high molecular weight PLA materials is similar to that of commonly used petroleum-based plastics.At the same time,PLA comes from regenerable plants,it has good degradability and biocompatibility.However,the high molecular weight PLA has the defect of rapid degradation/hydrolysis at high temperatures,which brings great difficulties to its industrial processing.The inherent brittleness and low softening temperature of PLA also limits the range of applications.In order to expand PLA’s application range by improving its toughness and thermal stability,we modified the PLA matrix through various methods.The main research contents and results are as follows:(1)fully stereocomplex polylactide with high molecular weight(HMW SCPLA)was prepared in green solvent via a thermal induced technique.SCPLA has been proven to be possess more excellent properties than homopolylactide.The stereocomplex has a melting temperature(Tm)approximately 50 °C higher than the Tm of homopolylactide and a better crystallization ability.However,the preparation of SCPLA by blending high molecular weight enantiomers usually leads to low degree of stereocomplexation,and the HC crystallites and SC crystallites coexist in the obtained product.The reason is that high molecular weight PLA rigid molecular chain has poor activity,so that the enantiomeric molecules are difficult to diffuse evenly.The rapid preparation of fully stereocomplex HMW SCPLA without degradation still needs further investigation.Thus,a reproducible and environmentally friendly method for the preparation of HMW SCPLA is achieved.Poly(L-lactide)and poly(D-lactide)were simply dissolved in an environmentally friendly solvent—dibasic ester(DBE)at 110 °C,respectively.Then the two solutions were mixed and cooled to room temperature,the HMW SCPLA spontaneously precipitated in the form of fine powder consequently.The correlation between the concentration of the mixture solution,the degree of stereocomplexation and the thermal properties of the SCPLA powder were investigated.Tensile testing indicated that the mechanical properties of the stereocomplex were enhanced compared to PLLA.The presence of the DBE reduced the reaction temperature and improved the molecular mobility of the polymers,thus the degradation problems and the molecular diffusion issue in the process of the formation of the stereocomplex could be overcome.Relationship among concentration of the mixture,degree of stereocomplexation and thermal properties of SCPLA powders were also established.Moreover,porous membranes and films stereocomplex polylactide material with good thermal properties were also obtained using this thermally induced technique.This method could be a good approach to expand the SCPLA applications.(2)a toughened fully stereocomplex HWM SCPLA block copolymer was prepared by multi-block copolymerization and DBE solvent.The preparation of HMW SCPLA provides a feasible way to improve the mechanical strength and thermal stability of PLA,but it is still a brittle material and is not suitable for melt processing.The HMW SCPLA has a poor recrystallization ability,that is,HC and SC crystals appear simultaneously after complete melting and recrystallization.For the sake of enhancing the toughness and recrystallization stability of SCPLA,the enantiomers of PLA multiblock copolymers with same molecular weight were synthesized.HDI was used as chain extender.Then,the stereocomplex films of the multi-block copolymers(scPLCL/PDCD)were prepared by hot-press molding after uniformly mixing the enantiomers in DBE solvent.Tensile testing showed that the elongation of scPLCL/PDCD exceeded 40%,and the tensile strength remained at a high level due to the formation of stereocomplex.The results of DSC indicated that the sc-PLCL/PDCD exhibited satisfactory melting stability,the re-formation of the SC crystals was completely reversible.The sc-PLCL/PDCD was able to accommodate repeated melt processing.The mechanism of recrystallization stability of sc-PLCL/PDCD was discussed from the molecular structure.(3)a series of thermoplastic high molecular weight branched multi-block copolymers were successfully synthesized by hydrolysis and condensation of silicon ethoxyl at room temperature,and blended with the PLLA substrate in different proportions to achieve the purpose of toughening PLA.PLA and flexible segments can be mixed at the molecular level through block copolymerization,thereby effectively improving the compatibility of different components,and achieving the purpose of toughening.However,the chain extension method selected in the previous chapter requires long-time reaction at a higher temperature to obtain a product having a high molecular weight and a narrow distribution,while high-temperature degradation of the PLA pre-polymer cannot be avoided.In order to reduce the polymerization temperature and make the reaction proceed under milder conditions,a series of high molecular weight branched multi-block poly(ε-caprolactone)/poly(D-lactide)(BMCD)were synthesized by in situ hydrolysis at room temperature in one pot.3-Isocyanatopropyltrimethoxysilane(IPTS)was used as chain extender.The branched multiblock copolymers exhibited good thermo plasticity,remarkable toughness and good biocompatibility.The BMCD film also maintained good biocompatibility.On this basis,in order to improve the toughness and thermal stability of the ductile material,reduce the mechanical strength loss,PLLA/ BMCD blends were prepared via a simple solvent evaporation method at various BMCD loadings.Tensile test showed that the elongation at break of PLLA blends increased to 50.97% and 104.55% at the loadings of 5% and 7%(mass fraction)BMCD respectively.SEM photos showed that BMCD was compatible with PLLA and no obvious interfaces were observed.A similar thermal degradation curve of the pure PLLA film and the blend film indicated that the addition of BMCD did not change the thermal degradation mechanism of the PLLA matrix,and the slightly improved thermal stability of the blends was due to stereocomplexation.This approach allows for simultaneous control of mechanical and biodegradable properties of PLLA with a few additives in actual production.Furthermore,UV-vis test showed that the light transmittance of the films at the loadings of 5%(mass fraction)BMCD was almost the same as pure PLLA at 400 nm,with no sacrifice of their transmittance.(4)a high-molecular-weight stereocomplex branched multi-block polylactide(sbPCLDA)was synthesized from D-lactide,L-lactide and poly(ε-caprolactone)diol(PCL)through hydrolysis and condensation of silicon ethoxyl at room temperature.The use of thermal induced technique in(1)and(2)promoted the flexibility and diffusion of molecular chains to form a fully stereocomplex HWM SCPLA and ductile materials,which improved the heat resistance and brittleness of PLA materials,but this method was greatly affected by factors such as solvent selection,mixing time,and temperature.To prepare a toughening HMW SCPLA material that can be spontaneously formed without being affected by external conditions,while satisfying the requirements of melt processing,is a new research direction.Thus,based on our previous research,a branched multi-block HMW SCPLA(sb-PCLDA),which having PCL,PLLA and PDLA segments on one molecular chain simultaneously,was synthesized by polymerization of L-lactide and D-lactide with PCL and in situ hydrolysis condensation with silane coupling agents.sb-PCLDA could formed fully SC crystallites with 60% crystallinity spontaneously,and the formation of the SC crystallites after repeated melting was completely reversible,suggesting much improved processability.Meanwhile,the sb-PCLDA maintained good thermal properties and hydrolysis resistance.The polymer exhibited significant tensile toughness with an elongation at break of 185%.This facile method could simultaneously improve the mechanical property and recrystallization ability of SCPLA.The architecture having both PLLA and PDLA arms in one molecule was also firstly synthesized via coupling reaction of IPTS in 1,4-dioxane.The fully stereocomplex microparticles spontaneously precipitated in the form of spherical microspheres with size of approximately 5μm.In summary,this research improved the brittleness and thermal stability of the PLA material by combining toughening and enhanced crystallization performance,and the softening temperature and hydrolysis resistance were also improved.Mechanical properties,heat resistance,hydrolysis resistance,etc.were tested.The mechanism of toughening and formation of fully stereocomplex were explained.All the synthesizes were based on environmental friendly methods,providing a new idea for PLA processing applications in various fields.