Preparation And Properties Of Bio-based Polylactic Acid/polymethyl Methacrylate Grafted Natural Rubber/Natural Rubber TPV

In order to relieve the environmental pollution caused by petroleum-based materials,and get rid of excessive reliance on petroleum resources,the application of bio-based material polylactic acid(PLA)has received close attention from everyone.However,the brittleness of PLA,to a large extent,always limits its applications.Therefore,PLA toughen modification has become a hot topic at home and abroad.However,the traditional toughen modification is usually accompanied with a sacrifice of tensile strength.How to achieve a great balance in mechanical properties has become a difficult problem to break through.In this study,PLA and natural rubber grafted methyl methacrylate(NR-PMMA)were used as matrix materials,and dicumyl peroxide(DCP)was used as free radical initiator.The PLA/NR-PMMA binary bio-based thermoplastic vulcanizates(TPV)were prepared via dynamic vulcanization with balanced stiffness and toughness,and its phase structure,mechanical properties,crystallization and rheological properties were systematically studied.On this basis,NR-PMMA and NR were used to toughen PLA synergistically,thus a PLA/NR-PMMA/NR ternary biobased TPV with high toughness in low rubber content and balanced stiffness and toughness was prepared by the same method.Moreover,three different methods of basic fracture work(EWF),instrumental impact test and double continuous toughening theory model,were used to intensivelystudy the toughening mechanism.(1)Infrared spectroscopy(FT-IR)studies of PLA/NR-PMMA binary TPV show that in the process of dynamic vulcanization,DCP initiates the cross-linking of NR-PMMA and also the grafted reaction between PLA and NR-PMMA phase,due to which the interface compatibility and the impact strength were extraordinarily improved.Among them,D60/40-1.5%has the best mechanical properties,the impact strength(102.7 kJ/m~2)is about37.3 times that of PLA(2.75 kJ/m~2).Meanwhile,the tensile strength and Young's modulus decrease slightly to 34.5 MPa and 1.04 GPa(decrease 50.5%and 46.7%),respectively.The SEM analysis results show that the cross-linked NR-PMMA phase is distributed in the PLA matrix with a continuous network structure rather than particles,finally PLA/NR-PMMA binary TPV formed a bicontinuous phase structure which is different from the traditional“sea-island”structure.When the material is subjected to external forces,this structure can play a supporting role,transmitting and absorbing energy in time,and ultimately giving TPV a stiffness and toughness balanced characteristic.The theoretical model of double continuous toughening takes into account the characteristics of plastic matrix,rubber and plastic ratio,grafting rate,pore size and distribution,and so on.The similarity between the theoretical impact strength and the actual impact strength is very high,suggessting it can reflect the toughness of the TPV accurately.(2)In PLA/NR-PMMA/NR ternary TPV,the ratio of plastic/rubber is 80/20.With 10wt%NR and 10 wt%NR-PMMA,the PLA/NR-PMMA/NR ternary TPV had a considerable tensile strength of 41.7MPa(only 38%loss compared with the neat PLA)and a tremendously improved impact strength of 91.30kJ/m~2(nearly 32 times that of the neat PLA).The interfacial in-situ compatibilization between PLA and rubber phases was confirmed by Fourier transform infrared spectroscopy(FT-IR).The results of contact angle measurement,dynamical rheological measurement and differential scanning calorimetry(DSC)confirmed that due to the difference in polarity between components,the NR was encapsulated by the NR-PMMA in the PLA phase,forming a“soft-hard”core-shell rubber phase structure in the PLA continuous phase.When the ratio of NR-PMMA/NR is greater than 10/10 and the interface between the rubber phase and the PLA matrix is basically the same.Toughening mechanism studies have shown that the outer layer of NR-PMMA provides high impact resistance and strong interfacial bonding force,and the inner layer of NR provides flexibility required for deformation,A synergistic effect will be achieved in terms of energy consumption and stress diffusion,leading to a significant toughening effect.(3)The fracture behavior of the PLA/NR-PMMA/NR ternary TPV was studied by the EWF method.The results demonstrate that the synergistic toughening effect of NR and NR-PMMA is due to the both the improvement of crack resistence ability and crack propagation ability.The results of instrumental impact test show that both the crack initiation energy and the crack propagation energy are contribute to the synergistic toughening of the TPV,and the crack propagation energy plays a decisive role in the impact toughness of the final material.The bi-continuous toughening theoretical model found that the relatively factors of ternary system is very complex.Because the physical factors such as interfacial tension and molecular interaction force have a greater influence,and the effect of synergistic toughening of“soft-hard”core-shell rubber phase structure is not well reflected in the formula,ultimately the theoretical model,can not well applied to PLA/NR-PMMA/NR ternary TPV.