Study On The Critical Shell Thickness Of SiO₂-PBA Core-shell Particles With Controllable Shell Thickness In Epoxy Resin Toughening System

Rubber is the mostly used toughener to toughen brittle polymers.Although the toughness of system can be improved significantly when the rubber content exceeds the brittle-ductile transition point,the drop of strength and modulus is inevitable.Rigid inorganic nanoparticles had been considered as a very good choice to balance the strength and modulus of toughened polymers in the past few decades.However,the toughness,especially the impact toughness for most polymers,cannot be improved by adding inorganic rigid particles.Core-shell particles with rigid core and soft rubber shell is becoming an important choice of tougheners to solve the dilemma between the strength and modulus of particle toughened polymer.Theoretical studies show that the modulus of the core shell toughened system decreases with the increase of rubber shell thickness(Ts)under a fixed content of core-shell particles.When the Ts is larger than a critical value Tsm,the modulus of toughening system will be lower than that of the matrix.On the other hand,toughening effect increases with the increase of the rubber shell thickness.With a certain particle size and content,there should be a critical value of the rubber shell thickness Tsbd.Brittle-ductile transition can occur only when Ts is higher than Tsbd.Here raised an important question about the polymers toughened by core-shell particles with rigid core and soft shell:Does high toughness can be obtained with no loss of modulus and strength?If Tsm＞Tsbd,the above question should has a positive answer.However,if Tsm＜Tsbd,core-shell particles with rigid core and soft shell cannot achieve the goal of enhancing the toughness without modulus and strength loss.Although a lot of studies about toughening polymer by core-shell particles with rigid core and soft shell have been reported,the above question cannot be clearly answered so far because the particle core size and the rubber shell thickness of the core shell particles cannot be accurately controlled simultaneously.To answer the above question,in this paper,a series of model system of toughening epoxy containing core-shell particles with SiO2 core and poly(butyl acrylate)(PBA)shell are prepared,in which both the SiO2 core size and PBA shell thickness are precisely controlled.The main research contents and innovation points are as follows:1.The SiO2 nanoparticles with controllable size are prepared by sol-gel method and modified by KH570.By optimizing the grafting process,the highest grafting density of KH570 is 45.67μmol/m2;2.Seed emulsion polymerization is applied to prepare SiO2/PBA core shell particles with controllable PBA shell thickness.When the amounts of BA monomer and SiO2 are fixed,the morphology of core-shell microspheres with SiO2 core(Φ=280nm)and poly(butyl acrylate)(PBA)shell varies along with the emulsifier concentration(Ce).If Ce is lower than a low critical value Cel,aggregated micelles with multiple SiO2 cores are formed.And while Ce is higher than an upper critical value Ceu,isolated PBA micelles without SiO2 core appear in the system.For a given amounts of BA monomer and SiO2 core,core shell particles with single SiO2 core can only be obtained at Cel<Ce<Ceu.The values of Cek or Ceu increase with the BA monomer content when the SiO2 amount is fixed.However,when the number of the emulsifier molecules absorbed onto the surface of the microspheres(∑)is considered,the upper and low limit ∑(∑uc and ∑LC)for the core-shell particles containing single SiO2 core seems has nothing to do with the BA monomer content.In our system,the value of ∑UC is around～3.3×106 μm-2 and ∑LC is about 1.3×106 μm-2.When Σ is fixed at 2×106 μ-2(ΣUC>Σ>ΣLC)and weight ratio of BA monomer to SiO2 core increases from 1:10 to 1:1,the rubber shell thicknesses of core shell particles with single SiO2 core can vary from 8nm to 57nm.3.A series epoxy system toughened by SiO2/PBA core-shell particles are prepared.The existence of the critical shell thickness Tsbd and Tsm is clearly confirmed.When the shell thickness Ts is greater than Tsbd,brittle-ductile transition happens;On the other hand,only when the Ts is less than the Tsm,the modulus of toughening system is higher than that of epoxy matrix.The value of Tsm is greater than Tsbd in our system,which proves that core-shell particle with rigid core and soft shell is an effective toughener to get a balance property of toughness and strength of polymers.In addition,although Tsbd increases with increasing of the core diameter(D),but mass fraction(10%)of rubber shell in the core-shell particles and the ratio of rubber shell thickness to the core size(Tsbd/D=0.0375)remain unchanged at brittle-ductile transition point.4.The relationship between the inter particle distance,the rubber shell thickness,the particle size and the toughening results are also studied.It is found that both the rubber shell thickness and the particle size have significant effects on the value of the critical interparticle distance(Lc)basing on S.Wu’s theory.When Ts＞Tsbd,Lc increases with the decrease of D,which is consistent with the percolation theory.When Ts<Tsbd,brittle-ductile transition cannot happen even though Lc is less than 100 nm,which cannot be explained by the percolation theory so far.