| Polylactic acid(PLA)is a kind of biomass and biodegradable synthetic aliphatic polyester,which is an ideal substitute for traditional petroleum-based plastics.PLA has comparable advantages with some engineering plastics,such as better mechanical properties and relatively low price,so it has attracted more and more researchers’attention.However,the inherent disadvantages of PLA are extremely low toughness,low crystallization rate and poor thermal stability,which seriously limit the wide application and development of PLA.At present,the toughening methods for PLA include copolymerization toughening,plasticizing toughening and blending toughening.Among them,the toughness can be significantly optimized by directly blending rubber or flexible polymer segments in the PLA matrix,but this will sacrifice the high strength and modulus of the PLA material itself.So this method of direct blending of flexible polymers is difficult to achieve the rigid-tough equilibrium of PLA materials.Plasticizers will migrate or even precipitate in the PLA system with time,which makes the original toughened and modified PLA materials gradually change from toughness to brittleness.Therefore,in order to obtain rigid-tough balanced PLA blends,by adjusting the optical activity of lactide(LA)and the feed ratio of HTPB/LA,a series of polylactic acid-polybutadiene-polylactic acid(PLA-b-PB-b-PLA)triblock copolymers were prepared by ring-opening polymerization of lactide monomer(LA)initiated by hydroxyl-terminated polybutadiene(HTPB),and then blended with PLLA matrix.The effects of the optical activity of lactide,the block ratio of the copolymer and the blending ratio on the properties of the blends were investigated.A rigid-tough balanced PLA transparent film material was prepared by constructing a stereo composite crystal between the block copolymer PLA-b-PB-b-PLA and the matrix PLLA,and the specific toughening mechanism,phase separation degree and morphology were deeply studied,which provided a basis for the further study of toughening PLA.The main contents of the study include:(1)Firstly,several different polylactic acid-polybutadiene-polylactic acid(PLA-b-PB-b-PLA)triblock copolymers were synthesized by bulk ring-opening polymerization of lactide initiated by flexible macro-initiator hydroxyl-terminated polybutadiene(HTPB)by adjusting the ratio of HTPB to lactide and the optical isomer of lactide.Afterwards,a chain-extension reaction with hexamethylene diisocyanate(HDI)was carried out to prepare(PLA-b-PB-b-PLA)_m multi-block copolymers with enhanced molecular weight.The structures and properties of these block copolymers were then characterized by gel permeation chromatography(GPC),nuclear magnetic resonance(NMR),differential scanning calorimetry(DSC),atomic force microscope(AFM)and Fourier-transform infrared(FTIR).(2)Through the study on the mechanical properties of different types of PLLA/PLA-b-PB-b-PLA blend films,it is found that different optical activity has different toughening effect on the blend films,among which the PDLA-b-PB-b-PDLA triblock copolymer synthesized by d-lactide(D-LA)has obvious toughening effect on PLLA films.Therefore,the toughening and strengthening effect of PDLA-b-PB-b-PDLA triblock copolymer on the blend film based on PLLA was studied systematically,and the effects of block ratio,molecular weight and blending ratio on PLLA materials were investigated.A series of(PLLA/PDLA-b-PB-b-PDLA)blend films with good mechanical properties and good light transmittance were prepared by blending PDLA-b-PB-b-PDLA triblock copolymers with different molecular weights with PLLA matrix(the contents of PDLA-b-PB-b-PDLA triblock copolymers were 5 wt%and 15 wt%respectively).When the content of PDLA-b-PB-b-PDLA in PLLA/PDLA-b-PB-b-PDLA blend film is 15 wt%,the blend film can maintain good light transmittance,and compared with pure PLLA film,the elongation at break of PLLA/PDLA-b-PB-b-PDLA blend film with 15 wt%PDLA-b-PB-b-PDLA triblock copolymer is significantly increased by 10 times,and the blend film also has higher tensile strength(60 MPa).In the PLLA/PDLA-b-PB-b-PDLA blend film,the blend film was strengthened and toughened by the synergistic action of the stereo composite crystal formed between the PLLA matrix and the PDLA segment and the flexible segment PB.According to the AFM image analysis,there is a nano-scale"Salami-like"dispersed phase in the PLLA/PDLA-b-PB-b-PDLA blend film,which is beneficial to improve the transparency of the blend film.According to the image analysis of SEM spline fracture morphology,the toughening mechanism of PLLA/PDLA-b-PB-b-PDLA blend film is mainly the shear yield and cavitation of the matrix,and the deformation stress is dispersed on a large number of PLLA matrix by shear yield and cavitation,so as to improve the toughness of the blend film.In addition,the toughening effect of(PLA-b-PB-b-PLA)_mmultiblock copolymers on PLLA films was studied.Compared with pure PLLA films,when the mass fraction of(PDLLA-b-PB-b-PDLLA)_m multiblock copolymers in the blend films increased from 5 wt%to 10 wt%,the PLLA/(PDLLA-b-PB-b-PDLLA)_mblend films always had better ductility,but the tensile strength decreased obviously.When the mass fraction of(PDLA-b-PB-b-PDLA)_m multiblock copolymer is 5 wt%,the tensile strength of PLLA/(PDLA-b-PB-b-PDLA)_m blend film is lower than that of pure PLLA film,but when the mass fraction of(PDLA-b-PB-b-PDLA)_m multiblock copolymer increases to 15 wt%,the tensile strength of the blend film is improved obviously.The results show that the addition of(PLA-b-PB-b-PLA)_m multiblock copolymer to the blend system can improve the toughness of PLLA film,and when(PDLA-b-PB-b-PDLA)_m multiblock copolymer is added to the blend system,the stereotactic composite crystal(SC)formed by PDLA segment and PLLA matrix will also strengthen the blend system and compensate the strength loss due to the addition of flexible polymer.It is further confirmed that the stereo composite crystal is effective for strengthening PLA. |
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