气相白炭黑/低分子量聚乙二醇悬浮体系的分子弛豫与流变行为

采用亲水气相二氧化硅(FS)、非缠结聚乙二醇(PEG,重均分子量400)制备悬浮体系,考察FS体积分数(φ)对PEG本体相α-松弛、结晶行为及悬浮体系流变行为的影响.结果表明,FS可延缓PEG本体相α-松弛,提高玻璃化转变温度,并显著增加浮体系黏度,降低本体PEG相结晶与熔融焓.低填充时,FS起成核作用;高填充时,FS延迟PEG分子扩散,并降低结晶温度.FS对PEG结晶的不同作用发生在悬浮体系溶胶-凝胶转变附近,此时悬浮液非线性动态流变行为呈现显著的硬化软化特性,线性动态流变行为呈现最为显著的频率依赖性.通过建立线性动态流变行为叠加曲线,揭示了FS对PEG分子链扩散行为的显著推迟作用.

Molecular Relaxation and Rheological Behaviors of Fumed Silica/Low-molecular Weight Polyethylene Glycol Suspensions

Suspensions were prepared by mixing hydrophilic fumed silica (FS) with low-molecular polyethylene glycol (PEG,weight-averaged molecular weight 400).By differential scanning calorimeter and broadband dielectric spectrometer,influences of FS volume fraction (φ) on α-relaxation and crystallization behaviors of bulk PEG phase were investigated.Furthermore,influences of φ on the linear and nonlinear rheology behaviors of the suspensions were also investigated using stress-controlled rheometer.For the bulk PEG phase,α-relaxation is retarded and glass transition temperature (Tg) is increased due to the addition of FS.On the other hand,FS significantly retard the diffusion of global PEG molecules,which leads to a great increase in viscosity of the suspensions along with significant reductions in crystallization and fusion enthalpy of the bulk PEG phase.The suspensions demonstrate different rheology and crystallization behaviors with varying φ.The low-filled suspensions exhibit an apparent sol-like rheological response,in which FS plays a nucleating role to promote PEG crystallization.However,the high-filled suspensions apparently behave like gels,in which FS restrains the chain diffusion and crystallization.The role of FS changes in the vicinity of φ =0.16,where the system undergoes an apparent sol-to-gel transition.In the close vicinity of this transition,the nonlinear dynamic rheology shows the most obvious strain hardening,while the linear dynamic rheology demonstrates the most significant frequency-dependence.Due to the extremely complicated nonlinear rheological response of the suspensions,it is hard to observe the pure hydrodynamics region experimentally,and in the frequency range achieved,complex viscosity (η*) as a function of φ at given frequencies does not fit classical hydrodynamics equations that do not consider the effect of nanoparticles on the polymer chains diffusion.To account for the effect of nanoparticles on the diffusion of PEG chains,data of liner dynamic rheology of the suspensions are shifted horizontally and vertically to create master curves with pure PEG as the reference.The creation of master curves reveals that FS greatly retards the diffusion of global PEG chains in the constrained bulk phase,which plays a major role in the reinforcement and dissipation of the suspensions.In addition to the dynamically retarded bulk PEG chains,both the inter-particle friction in the low-filled sols and the in-cage rattling motion of nanoparticles in the high-filled gels additionally contribute to the dissipation behavior of the suspensions,resulting in the complicated dynamic rheological behavior.