| Nowadays,poly(L-lactic acid)(PLLA)is a very important bio-based polymer that can bebiodegradable entirely.This material has high strength and modulus,good biocompatibility and processability,which is poisonless and harmless and can be biodegradable entirely.The application of PLLA has suffered from low elongation at break,high brittleness,slow crystallization rate,etc.Under such circumstance,the method to improve the performance of PLLA has been studied.Among all the methods of modification of PLLA,it is useful to toughen polymer by selecting flexible segments as block structure.In the paper,PLLA-PHAD-PLLA triblock copolymers were prepared by the ring-opening copolymerization between flexible segments and lactide.Firstly,the dihydroxyl-terminated PHAD random polyesters with the lowest crystallinity were prepared by adjusting the proportion of hexanediol and decanediol used in esterification and condensation with adipic acid.Then PLLA-PHAD-PLLA triblock copolyesters were synthesized by initiating the ring-opening polymerization of lactide under the condition of the above-mentioned PHAD as a macroinitiator.The obtained PLLA-PHAD-PLLA triblock copolyesters with different segments were investigated in terms of thermal and mechanical properties,thermal degradation and thermostability of which can be correspondingly regulated by adjusting the proportion of PLLA.According to the results of WAXD,it was observed that good crystal structures were formed by the segments of PHDA and PLLA.The thermal and crystal properties of PLLA-PHAD-PLLA can be regulated by adjusting the molar ratio of LLA and HDA in the chains.The mechanical properties of PLLA-PHAD-PLLA were investigated by extension test,instrumented impact test,and observing test sample across section appearance.It was shown that the elongation at break of PLLA-PHAD-PLLA grew with the decrease of the contest of PLLA in the copolymers.With the data of instrumented impact test,the impact strength of two units of PLLA-PHAD-PLLA sample was 357.2 J/m and 259.6 J/m,respectively.Compared with PLLA with 28.6 J/m,correspondingly,the ductility and impact resistance of PLLA-PHAD-PLLA has been much improved and the fracture appearance of material has undergone a transformation from typical features of brittle fracture to those of ductile fracture.Secondly,use amorphous unsaturated polyester to toughen polylactic acid,the toughening polylactic acid can be further functionalized,the dihydroxyl-terminated succinic acid 2-butene-1,4-diester(PCBS)and glutaric acid 2-butene-1,4-diester(PCBG)can be synthesized by esterification and condensation using adipic acid,glutaric acid and2-butene-1,4-diol,which can be used as flexible segments to modify PLLA by the means of block.The synthesized PLLA-PCBS-PLLA and PLLA-PCBG-PLLA triblock copolymerswith different length of PLLA were analyzed according to the results of characterization of DSC,TGA and WAXD.The test results showed PLLA was embedded with PCBS,which changed the properties of PLLA,including slow decline in melting temperature,rise in thermal degradation temperature and increase in thermostability.Compared with PCBS,PCBG had less influence on the crystallization and melting of PLLA,but affected remarkably its thermostability.It was observed by WAXD that PCBS had less effect on the process of crystallization of PLLA because its crystallization capability was difficult,and in contrast,PLLA had a marked impact on PCBS,reducing significantly its crystallization capability.However,the crystallization of PLLA was irrelevant to PCBG because it was amorphous.The experimental results of tensile properties showed the toughness of triblock copolymers were apparently enhanced,but strength of those tended to decrease when PLLA was embedded with PCBS and PCBG.The elongation at break of copolymers reached up to 145.27% and250.53% when block was PCBS and PCBG,respectively.That meaned the toughening effect of PCBG was more obvious than that of PCBS.Moreover,the tensile strength of copolymers decreased evidently because PCBG was amorphous. |
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