Core/shell Nanoparticles-Their Synthesis And Use For Epoxy Resin And Carbon Fiber Composite Toughening

Carbon fiber reinforced polymer composites(CFRPs)are widely used to manufacture primary and secondary structures in the aerospace,rail,auto and energy industries due to their superior specific strength and rigidity,low coefficient of thermal expansion as well as highly thermal stability.However,due to the lack of fibers in the through thickness direction of the 2D laminate structure and the extensive use of the inherently brittle thermosets,such as epoxy resins,such composites have poor interlaminar toughness and delamination resistance.Because a matrix with high toughness can effectively absorb impact energy,resist damage and crack propagation,this study focuses on matrix toughening to improve the interlaminar fracture toughness of CFRPs.In terms of toughening the matrix resin,how to take into account the rigidity and toughness at the same time has always been the main technical difficulty in this field.Due to its strong designability,polymer core/shell particles with highly-efficiency toughening effects can be synthesized through double-layer design of polymer molecules and particles.In addition,core/shell particles can be independently dispersed in the resin matrix by cross-linking the shell polymer,avoiding the loss of solvent and heat resistance,strength and stiffness,caused by incomplete phase separation.Therefore,adding core/shell particles to the matrix resin is considered to be an effective way to solve this technical difficulty.Although many researchers have studied the effect of core/shell particles on the toughness of epoxy matrix resins and attempted to reveal their toughening mechanisms,there are still many fundamental questions to be answered.For example,what are the main factors affecting the toughening effect of core/shell particles?what is the correlation between the composition of the particles and their compatibility with the matrix resin?what is the role of the core and shell layers of the core/shell particles in toughening epoxy resins?what are the respective influencing factors and the mechanism of synergy?etc.Based on these factors,this research work mainly focused on the following aspects:(1)To investigate the effect of the composition and morphology of polymer particles on the curing behavior,rheological,mechanical properties and heat resistance of epoxy resins,five polymer nanoparticles was precisely prepared from methyl methacrylate,butyl acrylate and ethylene glycol dimethacrylate via aqueous emulsion polymerization.These nanoparticles were transferred from aqueous emulsions into the epoxy resin via a water-oil phase phase transfer process,during which the particle morphology,dispersion state and particle size distribution were preserved.The results showed that the addition of these nanoparticles had little impact on the cure and glass transition temperature of matrix resin.However,mechanical test results demonstrated a clear correlation between nanoparticle structures and their effectiveness on toughening epoxy resins,and the toughening effect of core/shell nanoparticles was better than that of homogeneous nanoparticles.Particularly,a soft core/rigid shell nanoparticle(B/M),represented by a 50/50 poly butyl acrylate core with a polymethyl methacrylate shell,showed an impressive toughening effect on the epoxy matrix.The addition of 10 wt%B/M gave 220%and 851%enhancement in critical stress intensity factor(K_IC)and the critical strain energy release rate(G_IC),respectively.(2)From the perspective of polymer molecular design,Hansen's solubility parameter theory(HSP)was used to explore the interfacial interaction between epoxy resin and the shell polymers,and its effect on the dispersion,rheology and mechanical properties of epoxy resin.The result showed that the B/M particles,with moderate polarity of PMMA as the shell polymer,which had the compatibility matching that of the epoxy resin,would form uniform dispersion,yielding the most significant toughening effect.In the case that the compatibility of the shell polymer did not match that of the epoxy matrix resin,such as the B/S particles with low shell polymer polarity or the B/A particles with high shell polarity,the particles would more or less aggregate,lowering mechanical performance and leading to little or no toughening.Under the guidance of HSP compatibility theory,this study also realized the optimal design and experimental verification of the molecular structure of toughened core/shell nanoparticles,which provided a new practical approach for the formulation design and optimization of highly toughened epoxy resin systems.(3)The effect of core polymer T_g on the rheological and mechanical properties of core/shell particle modified epoxy resins was further investigated from the perspective of molecular design.The results showed that the T_g of the core layer polymer has little effect on the tensile properties of the modified resin system under the condition of keeping the shell polymer unchanged.However,the reduction in the core polymer T_g helps to reduce the viscosity of the blend system.In addition,different from the traditional toughening theory and mechanism,the fracture toughness resulted show that it is not the core/shell particles with lower core polymer T_g that have better toughening effect on epoxy resin.(4)The influence of core-shell particle structure parameters such as particle size and core/shell mass ratio on rheological and mechanical properties of epoxy matrix resin was studied from the aspect of particle design.The optimal core-shell particle size and core-shell ratio for toughening epoxy resin were selected and determined.The results showed that the modified resin systems had the best toughening effect when adding core/shell particles with a particle size of about100 nm and core/shell mass ratio of 1:1.(5)The effect of nanoparticle structure or morphologies on the fracture toughness of epoxy neat resins and their corresponding carbon fiber composites was investigated by preparing nanoparticles with the same polymer composition but different particle structures.In addition,the fracture toughness transfer from resin matrix to carbon fiber composites with different particle structures was studied.Data showed that the effect of particle morphology on composite flexural properties and interfacial shear strength was relatively small.However,the particle morphology had significant effect on the fracture toughness of epoxy neat resins and their carbon fiber composites.Nanoparticles having uniform morphology yielded little composite toughness improvement,while those with core/shell morphology were significantly more effective.Specifically,the addition of 10wt%of B/M particle gave 185%,77%and 43%increase in G_IC,init,G_IC,propand G_IIC of carbon fiber composites,respectively,compared to the control.