Parameters effecting the rheology of particle filled polymeric suspensions

Official URL: http://risc01.sabanciuniv.edu/record=b1534375 (Table of Contents)

In this work, the rheology of colloidal suspensions composed of low weight fraction, agglomerated, anisotropic particles with flocculated microstructure have studied. Firstly, viscosity and electrical resistance measurements were utilized together with cryo-TEM for conductive particles (CNT and graphene) dispersed in non-conducive media (PEG) so as to address the theoretical gap regarding the shear thickening phenomenon in these suspensions, our results revealed that in the shear thickening region, particles dispersed well due to the instability of the microstructure. Secondly, particles with different morphologies (clay, organoclay, halloysite, CNT, graphene, fumed silica) were also studied to understand the effect of morphology to the rheological behavior of these suspensions. Steady and dynamical rheological measurements were performed to analyze the microstructure formation during flow. Halloysite, clay and organoclay particles and graphene in this research experienced shear thickening behavior under large deformation frequencies; however, suspensions containing CNT showed shear thinning. The results point out that the dispersion of CNTs was hindered due to agglomeration which results in shear thinning behavior. Thirdly, to investigate particle-particle and particle-polymer interactions, the surface of fumed silica particles was modified with silane coupling agents with three different organo-functional groups (epoxy, amine and quaternary ammonium compound). Dynamic light scattering (DLS) results showed that the smaller particle size reflects improved dispersion of modified silica particles in continuous media when compared with that of fumed silica. Shear thickening behavior at earlier critical shear rates (27 s-1) was evidenced by particles covered with epoxy end groups (EPPTMSi); however particles with amine (AEAPTMSi) and quaternary ammonium compound (QuadSi) end functional groups demonstrates shear thinning behavior. This positive outcome indicates that surface modification of nanoparticles enables designing of ”tunable materials” depending on the application with the ease of dispersion. Better dispersion occurs with tunable materials containing easily dispersible nanoparticles. Fourthly, effect of temperature on the rheological response of suspensions was investigated. Temperature sweep results with constant stress showed that gel formation is observed when the temperature reaches to a critical point where sol-gel transition takes place. It is also observed that, the viscosity of all suspensions increase with increasing temperature; because the Brownian motion of silica particles in polymeric media increase with increasing temperature and the hydrodynamic forces induce dilatancy phenomenon. Finally, our results showed that polymeric fluids containing agglomerated, anisotropic particles with low weight fraction (20 wt %) also exhibit shear thickening behavior. This observation differs from relevant research about the rheology of filled polymer systems which are composed of monodispersed, isotropic particles with high volume fraction (40 wt%). Low particle loading provides reduction in weight in terms of shear thickening fluid applications, such as liquid armor, shock absorbent and seismic devices.