Preparation And Properties Of PLA/NR-PMMA/SiO₂/NR TPV With Balanced Stiffness-toughness

Polylactic acid(PLA),a kind of bio-based polymer,is derived from renewable resources and has many advantages,such as excellent biodegradation,excellent mechanical strength and good biocompatibility,etc.Due to the environmental pollution caused by petroleum-based polymer materials and the deletion of non-renewable petroleum resources,the application and development of PLA has been widely concerned.However,it's necessary to toughen PLA because of its poor toughness and notch sensitivity,which limited its application to a large extent.Among them,using elastomer to toughen PLA can promote its impact strength effectively,but the tensile strength usually decreased greatly.Thereby,how to improve the toughness of PLA significantly while ensure the rigidity and obtain PLA-based materials with balanced rigidity and toughness is a research hotspot.In this work,a series of PLA/poly(methyl methacrylate)-grafted natural rubber(NR-PMMA)/natural rubber(NR)thermoplastic vulcanizates(TPV)with various DCP content were fabricated first by dynamic vulcanization.The mechanical properties,structure and crystallinity of the TPV were studied,and the shape memory behavior of PLA/NR-PMMA/NR TPV was also studied.On the basis,the mechanical properties of the TPVs were further improved by introducing rigid nano-SiO₂ and regulating its distribution.The PLA/NR-PMMA/SiO₂/NR TPV with high toughness and balanced stiffness-toughness was obtained at a low rubber content.The results of PLA/NR-PMMA/NR TPV with various DCP content show that the toughness and morphology of the TPV can be tailored by changing DCP content through dynamic vulcanization.The micro-structure of the TPV is bi-continuous,in which the rubber phase as a continuous network exists in PLA matrix.With the increase of DCP content,the hole diameter of rubber network decreases and the wall thickness increases.DCP initiates the cross-linking of rubber and also the grafted reaction between PLA and rubber phase during dynamic vulcanization.The impact strength first increased and then decreased with the increase of DCP content,and the optimal DCP content is 2.5phr in the system.The crosslinking density of rubber phase and the interfacial grafting rate are improved,which are beneficial to the improvement of interface compatibility and impact strength,while excessive DCP content will lead to the degradation of PLA and the decrease of mechanical properties.The PLA/NR-PMMA/NR TPV exhibit a excellent shape memory effect with 100%shape fixing rate and shape recovery rate,but repeated shape memory reduce the shape recovery rate.With the increase of NR-PMMA content,the shape recovery rate first increased and then decreased,the PLA/NR-PMMA/NR TPV demonstrates an optimal shape memory effect when the ratio of NR-PMMA/NR is 10/10.The PLA/NR-PMMA/SiO₂/NR TPV still demonstrate a bi-continuous structure,and the rubber phase as a continuous network exists in continuous PLA matrix.The introduction of SiO₂ enhances NR-PMMA and improves the interfacial compatibility,which increases the wall thickness,reduces the hole diameter and makes the hole diameter distribution more uniform.Although SiO₂ tend to migrate into PLA phase,the migration of SiO₂ can be restricted because of strong physical entanglements between NR-PMMA and SiO₂by blending with NR-PMMA first and then melt blending with PLA.SiO₂ is finally regulated to distribute in NR-PMMA and the interface,which reinforces NR-PMMA and promotes the interfacial compatibility.Under the combined action of the above factors,PLA/NR-PMMA/SiO₂/NR TPV show excellent mechanical properties,the impact strength first increased and then decreased,with 2phr SiO₂,the TPVs exhibite a remarkable promoted impact toughness of 843.7J/m,which was 168%that of PLA/NR-PMMA/NR TPVs,meanwhile,the tensile strength was improved simultaneously from 45.2MPa to 47.4MP.Incorporation of SiO₂ and regulation its distribution achieved both toughening and reinforcing effects successfully.What's more,the incorporation of SiO₂ promoted the crystallization properties and thermal stability of the TPV.With the increase of SiO₂ content,the cold crystallization temperature declined,the crystallinity increased,and the initial decomposing temperature increased.SiO₂ dispersed in the interface improved the compatibility,which could transferred energy efficiently between PLA phase and NR phase,and SiO₂ localized in NR-PMMA reinforced NR-PMMA,the suitably enhanced NR-PMMA has a better synergistic effect with NR on dissipating impact energy.These are the two main reasons for the significant improvement of the TPV impact strength.The co-continuous phase toughening theoretical model can predict the impact strength of PLA/NR-PMMA/NR TPV to a certain extent within a certain range of SiO₂content.