The effects of filler particle size and concentration on the rheological properties of hydroxyl terminated polvdimethylsiloxane (HO-PDMS) filled with calcium carbor ate (CaCO3) were investigated by an advanced rheometric expansion system (ARES). The Casson model was used to describe the relationship between shear stress and shear rate for steady-state measurement. Micron-CaCO, could not afford the CaCO3/HO-PDMS suspensions obvious shear thinning behavior and a yield stress high enough, whereas nanoCaCO, could provide the suspensions with remarkable shear thinning behavior and high yield stress. Incorporation of nano-CaCO3 into HO-PDMS resulted in the transformation of HO-PDMS from a mainly viscous material to a mainly elastic material. With increasing nano-CaCO, content, shear thinning behavior of nano-CaCO,/HO-PDMS suspensions became more obvious. Remarkable yield stress was observed in nano-CaCO3/HO-PDMS suspensions with high filler content, and increased with increasing nano-CaCO3 content. The degree of thixotropy was quantitatively determined using a thixotropic loop method. It was found that nano-CaCO3 favored more the buildup of filler network structure in the suspensions than micron-CaCO, at the same weight fraction. Furthermore, increasing nano-CaCO3 content accelerated the establishment of filler network structure in the nano-CaCO3/HO-PDMS suspensions. An overshoot phenomenon was observed in the nano-CaCO,/HO-PDMS suspensions at high shear rates. (c) 2006 Wiley Periodicals, Inc.