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The Crystallization Behaviour Of Stereocomplex Poly(Lactide) In Blend And Copolymer

Posted on:2019-05-23Degree:MasterType:Thesis
Country:ChinaCandidate:W XiaoFull Text:PDF
GTID:2321330563455009Subject:Materials Physics and Chemistry
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Irreversible destruction to natural environment has been caused by the decrease of fossil resource and the serious plastic pollution.People have paid much attention to the phenomenon of cumulating plastic products and expect to acquire a kind of material to replace fossil resources which is abundant in plants,recyclable and degradable under natural environment.Poly?lactide?is exactly a kind of biologic materials which comes from plants and can be ultimately decomposed into CO2 and H2O.Poly-lactides?PLAs?have special optical isomerism characteristics,and the heat resistance can moderately enhanced by preparation of sterecomplex PLA?SC-PLA?.Furthermore,SC has a higher degradation rate and mechanical strength than Homo-crystals?HC?.Hence,it is necessary to investigate the formation conditions and crystalline promotion methods of SC.Firstly,the effects of soft chain of poly?ethlene glycol??PEG?and polycaprolactone?PCL?on the crystallization behavior of PLLA have been investigated.The results shows that PCL segments have little influence on the Avrami index.With the increasing molecular weight of PLLA block,the melting temperature?Tm?and crystallization temperature?Tc?of copolymer gradually increased.Although PLLA blocks have restriction effects on PCL block,the promoting effect on the crystallization rate of PLLA by PCL still exists.The largest crystallization rate is about 1.702min-11 at 95?.The results of Different Scanning Calorimetry?DSC?of PLLA-PEG-PLLA shows that the incorporating of PEG segments accelerates the diffusion of PLLA chains.The critical cooling rates to inhibit the crystallization and nucleation behaviors are 4K/s and 1000K/s,respectively.Which are obtained by Flash DSC.And the critical heating rate to inhibit the nucleation during heating process is about 50K/s.The ring-banded crystallization pattern can be observed for PLLA-PCL-PLLA copolymer,while the addition of PEG segment has little effect on the morphology of PLLA according to the Polarizing Microscope?POM?.Secondly,the effects of mass ratio and molecular weight on the crystallization behavior of PLLA/PDLA blends have been studied by DSC and X-Ray Diffraction?XRD?.The results indicate that a similar rule has been obtained for all of the blends with different composition,i.e.,the crystallization temperature?Tc?of these blends increases with the increasing of PLLA content during the content range from 10wt%to 50wt%while it decreases during the PLLA content range from 50wt%to 90wt%.The Tm,SC,SC for each of the blends is higher than 200oC.And the exclusive SC can be obtained for the blends with a wide range of composition.SC and HC coexist when the qualities of PLLA and PDLA have much larger difference.Thirdly,in order to study the effect of soft chain on the formation of SC crystals,the three-component blends of PLLA/PDLA/PEG and two-component blends of MPEG-b-PLLA/MPEG-b-PDLA and PDLA-PEG-PDLA/PLLA with different compostion were obtained by solution blending.The results obtained by DSC,XRD and POM shows that the SC of PLLA/PDLA can still obtain with the existence of PEG in all of the three kinds of blends.The crystallization behavior of PEG due to the phase separation between SC and PEG can be observed for the blends of PLLA/PDLA/PEG and MPEG-b-PLLA/MPEG-b-PDLA with equivalent mass for PLLA and PDLA,in which only SC crystals exist.However,the exclusive SC crystals of PDLA18700-PEG-PDLA18700/PLLA690009000 blends can be obtained without the crystallization of PEG.The POM results indicate that the largest growth rate at 155oC can be obtained for the blends of MPEG-b-PLLA36000/MPEG-b-PDLA320002000 with an equal mass for PLLA and PDLA,while the lowest crystallization rate is attributed to the PDLA18700-PEG-PDLA18700/PLLA69000blends.At last,the stereocomplex triblock and multiblock copolymers were prepared through ring opening polymerization by using hydroxyl-terminal PLLA and PDLA-PEG-PDLA as initiators.The DSC results show that the crystallization ability of SC in stereo-triblock copolymers is poor,which can be attributed to the poor mobility of triblock copolymer.And the Tm,SC,SC of block copolymer is lower than that of the PLLA/PDLA blends.For the multiblock copolymer,the exclusive SC crystals can be obtained,but neither the crystallization peak nor the melting peaks can be obtained during the second cooling and heating process.
Keywords/Search Tags:PLA, Stereocomplex, Blends, Stereoblock copolymer, Crystallization behaviour
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