The effect of polydimethylsiloxane (PDMS) content, PDMS molecular weight, and the extent of prepolymerization reactions between hydroxyl functional PDMS and polyisocyanate, were explored in order to understand the influence of these variables on the formation of surface rnicrotopography in a siloxane-polyurethane thermoset. The system is based on a difunctional hydroxyalkyl terminated PDMS, trifunctional poly(caprolactone) polyol, and a polyisocyanate. The effect of PDMS content was studied with PDMS of MW 1000 and 2000. PDMS of MW 1000 was varied from 5% to 15% and with PDMS of MW 2000 the level was varied from 5% to 12%. Tapping mode AFM images of the Surface topography were used to characterize the system. The extent of reaction prior to film formation was studied by FTIR and correlated with microdomain formation in the final polymer film. Two different mixing strategies were explored: one where all of the ingredients were mixed together and a second approach where PDMS and polyisocyanate were mixed first, followed by the addition of the trifunctional polyol. Monte Carlo simulation of the polymerization with 10% PDMS of MW 1000 in the formulation revealed that there was a window of conversion for both PDMS and isocyanate at which the system spontaneously phase separates to form a microtopographical surface. (C) 2007 Elsevier Ltd. All rights reserved.