Study On The Effect Of Rigid/flexible Hybrid Modifier On The Mechanical Performance Of PLA-based Composites

Polymers have been widely used in a variety of fields,including aerospace,automobile manufacture and electronic industry etc.,due to their advantages of light weight,high strength,water-proofing and corrosion-resisting.However,the traditional polymers,produced from non-renewable petroleum,are difficult to be degraded in nature,which over-use of these polymers inevitably brings a negative impact on the environment and energy sustainability.Therefore,developing the fully biodegradable polymer with excellent properties has attracted increasing attention in the field of polymers.Poly(lactic acid)(PLA)is currently the most promising biodegradable polymer.Nevertheless,PLA with linear molecular chain structure suffers from the shortcoming of inherent brittleness,and the existing toughening technologies induce the contradiction of strength and tougheness improvement.In addition,the PLA matrix has an inferior strength and stiffness.These impede the wide engineering application of PLA.Accordingly,the rigid/flexible hybrid modifier is applied to improve the strength and toughness of PLA matrix and basalt fiber reinforced PLA-based composite.The mechanical properties of PLA-based composites under static and low-velocity impact loading and the relevant toughening mechanisms are investigated.The main contents of this thesis are follows:The core-shell nanoparticle was synthesized,in which the nano-silica(Si O₂)and poly(butyl acrylate)(PBA)rubber were devised as the rigid inner core and flexible outer shell,respectively.The PLA resin was modified by blending the nanoparticle.The effect of rigid silica and core-shell nanoparticle on strength and toughness of PLA matrix and natural jute fiber reinforced PLA-based green composites were experimentally investigated.It was concluded that the rigid-flexible core-shell nanoparticle can play a more significant effect on strengthening and toughening of the PLA-based composite compared to the rigid particle.The micromechanical models of rigid particle and core-shell particle reinforced polymer-based composites were established.The cohesive zone model and continuous damage model were applied to simulate the interfacial debonding of particle and damage propagation of matrix,respectively.The failure processes of these two composites under uniaxial tensile loading were investigated.It was revealed that grafting rubber shell on surface of rigid particle could evidently change the failure mechanism of composite.Furthermore,the effects of the thickness of outer rubber shell in core-shell particle were explored.The result showed that the damage initiating position of matrix turned to the equator of particle from the two poles with the thickness of rubber shell increasing.The experimental method was applied to further study the effect of rubber shell thickness of core-shell particle on the tensile behavior of PLA and relevant toughening mechanisms.The core-shell nanoparticles consisted of rigid silica core(diameter is 300 nm)and biodegradable flexible poly(?-caprolactone-co-_L-lactide)(PCLLA)shell with different thicknesses(0,6,10,18,25 and 31 nm)were synthetized via ring-opening polymerization.The core-shell nanoparticle modified PLA composites were prepared using the solution-casting method.The variation of tensile properties and toughening mechanisms of nanocomposites with rubber shell thickness were investigated via tensile tests,X-ray scattering(SAXS),diffraction(WAXD)and surface morphology observation.It is found that by tuning rubber shell thickness,the core-shell particles can achieve a trade-off between strengthening and toughening of PLA,and initiate its brittle-ductile transition.In addition,in view of the low strength and stiffness of above obtained PLA matrix,further to broaden the application of biodegradable composites,the natural continuous basalt fiber(BF)with high specific stiffness and strength was used as the reinforcement to prepare the basalt fiber reinforced poly(lactic acid)(BF/PLA)laminated eco-friendly composites by the improved vacuum assisted prepreg process(VAPP).Furthermore,the hybrid nano-coating made of rigid Si O₂ nanoparticles and flexible PCL biopolymer was applied to treat basalt fiber surface to improve the interfacial property of environmental composite.The effects of the mass ratio of rigid/flexible phase(coating type)and coating concentration on tensile,flexural and mode I interlaminar fracture toughness of BF/PLA composites were investigated.The optimized coating type and concentration to significantly improve the strength and fracture toughness of BF/PLA composites were experimentally obtained.Meanwhile,the corresponding reinforcing mechanisms were explored by fracture surface morphology observation and digital image correlation(DIC)technique.The low-velocity impact resistance of BF/PLA composites were investigated.The dynamic response process of various composites impacted at low(20 J),medium(60 J)and high(90 J)impact energy were analysised.The X ray(X-ray)computerized tomography(CT)nondestructive testing method was used to examine the interior damage of specimens.The failure modes of the BF/PLA composite under different impact energy were illustrated,and the effects of fiber surface coating modification on the energy absorption mechanisms of the composites were displayed.