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

As a kind of bio-based polymers,polylactic acid?PLA?has the advantages of abundance in raw material,little environmental pollution in the preparation process,excellent mechanical strength and biodegradability.However,there is an emergent need to toughen PLA due to its poor toughness and the susceptibility of notch.Rubber,as a common additive,can effectively improve the notched impact strength of PLA and ameliorate its toughness.Unfortunately,as the impact resistance increases,the tensile strength and modulus of the blends will decrease significantly.Therefore,how to effectively improve the toughness of PLA while ensure its rigidity and obtain materials with balanced stiffness-toughness is a crucial hotspot in the research field of PLA toughening modification at present.In this thesis,rigid nanoparticle silica?SiO₂?was introduced into the polylactic acid/natural rubber thermoplastic vulcanizate?PLA/NR TPV?system,and a shape memory polymer with balanced stiffness-toughness was obtained by realizing the selective distribution of nanoparticles using thermodynamic factors and kinetic factors.The results demonstrate that the PLA/NR/SiO₂ TPV still exhibits a co-continouse structure,and the rubber phase is distributed in the PLA matrix as a continuous network structure.The addition of SiO₂ strengthens the network of crosslinked rubber,which reduces the hole diameter and increases the thickness between holes.Although there is a trend for SiO₂ to migrate from the rubber phase to the plastic phase during the blending process,the migration of SiO₂ can be inhibited partly by employing a two-step processing method that pre-mixing SiO₂ with NR first and then melt blending with PLA.Because the strong physical entanglement between the chains of rubber molecular and the reinforcing fillers,SiO₂ is finally selectively distributed in the rubber phase and at the interface,which reinforces the rubber phase and improves the interfacial adhesion as well.At the same time,in the process of dynamic vulcanization initiated by the vulcanizing agent DCP,the grafting reaction between the rubber and plastic molecular chains at the interface was also induced.Under the synergistic effects of the above factors,PLA/NR/SiO₂ TPV exhibits excellent mechanical properties.For example,when the content of SiO₂ was 12.5 phr,the notched impact strength of the TPV reached 184.8 kJ/m²,which is 17times than that of PLA/NR?10.4 kJ/m²?,and the tensile strength was almost unchanged from first?34.9 MPa?to last?34.2 MPa?.Consequently,introducing high-reinforcing SiO₂ to PLA/NR TPV and controlling its selective distribution can significantly improve the impact strength of TPV without reducing its strength and rigidity.Besides,the addition of SiO₂improved the crystallization properties and thermal stability of TPV.The cold crystallization temperature of TPV decreased with the increase of SiO₂ content,and the temperature of the maximum mass loss rate gradually increased.The fracture process of PLA/NR/SiO₂ TPV with different SiO₂ content was analyzed by instrumental impact test.The results show that the energy of crack initiation and propogation both increased with the increase of SiO₂ content,but the propogation stage consumed more energy.This is because that SiO₂ is distributed mostly in the rubber phase,which increases the strength of the rubber network,so that the fracture process can consume more energy.At the same time,the co-continouse structure and good interfacial adhesion also have a positive effect on improving the ability of absorbing more energy during the fracture process,thus,the toughness is significantly improved.The co-continuous structure of PLA/NR/SiO₂ TPV endows it with an impressive shape memory effect,all TPVs achieve a shape fixing ratio of¹00%and a shape recovery ratio of>90%.The PLA phase,exhibiting a glassy state at room temperature,can fix the temporary shape,and its glass transition temperature?T_g?can be used as a transition temperature to induce the shape memory behavior of TPV.During the recovery process,when the ambient temperature is above the transition temperature,the rubber network will provide sufficient resilience to drive the deformed material turn back to its original shape due to its large deformation.The selective distribution of SiO₂ in two-step method can enhance the recovery phase and improve the interface,which is beneficial to the shape recovery process.Adding moderate content of SiO₂can improve the resilience of the rubber phase,which accelerates the shape recovery rate and the repeated shape memory behavior of the TPV.