Study On The Toughening Mechanism And Shape Memory Performance Of PLA/NBR Thermoplastic Vulcanizates

Along with the depletion of oil resources and the deterioration of environment pollution,polylactic acid(PLA)has recived more and more attention,due to that it is the most promising biodegradable materials to replace the traditional petroleum-based materials.However,the poor toughness,low impact strength and difficulty in film forming have seriously restrained its application.In this paper,we used polar nitrile rubber(NBR)and PLA as the main materials,dicumyl peroxide(DCP)as vulcanizing agent to prepare high toughed PLA/NBR thermoplastic vulcanizates(TPV)via dynamic vulcanization.The mechanical properties,microstructure,rheological properties and shape memory performance of PLA/NBR TPV were investigated systematically.Based on above studies,the toughening mechanism of the PLA/NBR TPVwas discussed and preliminary toughening theory model was also established.(1)FT-IR and GPC results showed that the addition of DCP not only induced the crosslinking of NBR,but also caused the mild crosslinking,branching and degradation of the PLA phase.The grafting reaction of PLA and NBR increased with the increase of NBR content and acrylonitrile content of the NBR.The dynamic rheological properties results showed that the storage modulus-frequency curve was not consistent with the traditional “Sea-Island” structure.The “second platform” did not appear in the low frequency region,and it presented apparent power law behavior(G???n,0<n<1)in low frequency region.This indicated that the PLA/NBR TPV were co-continuous structure,which was consistent with the results of SEM.The apparent crosslink density increases with the content of NBR and the acrylonitrile content of NBR.In addition,DMA,DSC and rheological studies showed that the compatibility between PLA and NBR phase was increased with the content of NBR and acrylonitrile content of NBR.PLA/NBR TPV showed high toughness because of the co-continuous structure,the good interface compatibility and the strong interface binding force.When 10wt% NBR was added,the impact strength of TPV reached 27.1kJ/m2,which was about 10 times that of the pure PLA(2.75kJ/m2).Its mechanical properties also increased with the increase of NBR content and acrylonitrile content of NBR.(2)Based on the study of the mechanical properties,microstructure and toughening mechanism of PLA/NBR TPV.We established the toughening theoretical model for the PLA/NBR TPV by referring to the theoretical model of steel fiber.The basic fracture function method was used to check the theoretical model.It was found that the impact strength deduced by the basic fracture work methodwas basically consistent with the impact strength calculated by the theoretical model,This showed that the theoretical model could reflect the toughness of PLA/NBR TPV.In addition,we found that the theoretical value was greater than the corresponding test value when the theoretical model was applied to the PLA/NR and PLA/ENR TPV,which showed that theoretical model was not suitable for PLA/NR and PLA/ENR TPV systems.This might be due to that we ignored the difference of surface energy and interaction force between molecules when we discussed the influence of rubber plastic ratio,grafting ratio and the hole size(diameter a and wall thickness t)on the toughness of PLA/NBR TPV in this model.(3)The co-continuous structure of PLA/NBR TPV imparted superior shape memory properties to TPV.The transition temperature of was the Tg of the PLA phase(near 65°C),and the continuous crosslinked NBR phase provided strong restoring force for the shape recovery.The continuous PLA phase played role on fixing shape and controlling shape transformation.The shape fixing ratio of all the TPV reached 100%,and the shape recovery ratio of TPV were more than 85%.The main factors affecting the shape recovery ratio were the interfacial bonding force between PLA phase and NBR phase and the restoring force from NBR Phase.