Rheology of nanosized hairy grain suspensions

Abstract:

Suspensions of nanosized hairy grains have been prepared by grafting long polydimethylsiloxane chains (molecular weight $\cong 160000$) onto silica particles (radius $\cong 15nm$), dispersed into a good solvent of PDMS. Depending on the particle volume fraction, different rheological behaviors are observed. In the very dilute regime, the suspensions are perfectly stable and the particles behave almost as hard spheres: flow penetration inside the corona is then very weak. When the particle volume fraction goes to the close packing volume fraction, the suspension viscosity does not diverge as for hard spheres due to the increase of flow penetration inside the corona and to corona entanglements. The particles have then the same behavior as polymer stars having an intermediate number of arms ($\cong 55$). Finally, in the concentrated regime ($c \ge 0.1g/c{m^3}$), the suspensions form irreversible gels. We shown that this unexpected gelation phenomenon is related to the presence of the silica cores: grafted PDMS chains can adsorb onto different particles and form irreversible bonds between the cores. The viscosity and elastic modulus evolutions during gelation are well described by the scalar percolation model of sol-gel transition.